Table of Contents

Robert's Perl Tutorial

Version 4.1.1


The last hack was made on: 20th April 1999
so Henry can concentrate

Introduction


This tutorial is...

A basic Perl course primarily for use on Win32 platforms. It assumes that the reader knows nothing of programming whatsoever, but needs a solid grounding for further work. After you finish this course you'll be ready to specialise in CGI, sysadmin or whatever you want to do with Perl.


This tutorial is not...


The Table of Contents

I've had a fair amount of requests for a ToC, so here it is:



Introduction

This tutorial is...
This tutorial is not...
How to Use...
The Table of Contents
Conventions used in this Tutorial
What you need to know
Use of this document

Personal Printouts
Intranet usage
Mirroring
Translations


A Short Introduction To Perl

What is Perl?
What is ActivePerl? Are the other Perls inactive?
Can I run Perl on my computer?
What can I do with Perl ?

The Internet
Systems Administration

What can't I do with Perl ?
Support


Setup

1. Getting the Software
2. Installation
3. Testing - Your First Perl Script


The Tutorial: The Journey Begins

Your First Time
What if it doesn't...?
Assuming it's now all right...
Shebang
Variables

Scalars
$ % @ are Good Things
Typing
Variable Interpolation
Changing Variables

Auto(de|in)crements

Escaping
Context: About Perl and @^$%&~`/?
Strings and Increments
Print: A List Operator
Subroutines -- A First Look
Comments


Comparisons

An iffy start
The Truth According to Perl
Equality and Perl

All Equality is Not Equal: Numeric versus String

An interlude -- The Perl Motto
The Comparison Operators Listed

The Golden Rule of Comparisons

More About If: Multiples

elsif


User Input

STDIN and other filehandles
Chop

Safe Chopping with Chomp


Arrays

Lists, herds -- what are arrays?

Basic Array Work

Elements of Arrays

How to refer to elements of an array

More ways to access arrays
For Loops

A for Loop demonstrated
For loops with .. , the range operator
foreach
The infamous $_
A Premature End to your loop

A little more control over the premature ending: Labels

Changing the Elements of an Array
Jiggerypokery with Arrays

A table of array hacking functions
Splice


Deleting Variables

False values versus Existence: It is, therefore...


Basic Regular Expressions

An introduction
Senstivity -- regexes in touch with their inner child
Character Classes
Matching at specific points
Negating the regex
Returning the Match
* + -- regexes become line noise
The Difference Between + and *
Re-using the match -- \1, $1...
How to Avoid Making Mountains while Escaping Special Characters


Subsitution and Yet More Regex Power

Basic changes
\w
Replacing with what was found
x

More Matching

Parentheses Again: OR

(?: OR Efficiency)

Matching specific amounts of...
Pre, Post, and Match
RHS Expressions

/e
/ee

A Worked Example: Date Change


Split and Join

Splitting
A very FAQ
What Humpty Dumpty needs : Join


A recap, but with some new functions

Randomness
Concatenation


Files

Opening
An unforgivable error
\\ or / in pathnames -- your choice
Reading a file
Writing to a File

A simple write
Appending
@ARGV: Command Line Arguments
Modifying a File with $^I
$/ -- Changing what is read into $_
HERE Docs


Reading Directories

Globbing
readdir : How to read from directories


Associative Arrays

The Basics
A Hash in Action
When you should use hashes
Hash Hacking Functions
More Hash Access: Iteration, keys and values


Sorting

A Simple Sort
Numeric Sorting -- How Sort Really Works
Sorting Multiple Lists


Grep and Map

Grep
Map
Writing your own grep and map functions


External Commands

Some ways to...
Exec
System
Backticks
When to use external calls
Opening a Process
Quote execute


Oneliners

A short example
File access
Modifying files with a oneliner and $^I


Subroutines and Parameters

Parameters
Namespaces
Variable Scope
my Variables
Multiple Returns
Local
Returning arrays


Modules

An introduction
File::Find -- using a module
ChangeNotify
Your Very Own Module


Bondage and Discipline

-w

Shebang

use strict;


Debugging

Logical Operators

or
Precedence: What comes First
And
Other Logical Operators


Last words

Thanks to...



What you need to know

You need to be able to differentiate between a PC and a toaster. No programming experience is necessary. You do need to understand the basics of PC operation. If you don't understand what directories and files are then you'll find this difficult. You might find it difficult even if you do :-)

You do need to exercise the brain cells, and you need time.

What you need to have

Note: You don't even need a Win32 PC if you are comfortable installing Perl under other operating systems like Linux, but not all the information here will be relevant.

You don't need a complier. Perl is an interpreted language, which means you run code directly, not compile it then run it.


How to use this tutorial...

Just work through from start to finish.

Generally, the explanation follows the code sample. Before you read the explanation, try and work out what the code does. Then check if you're right. In this way, you'll derive maximum value from the tutorial and exercise the old grey cells a little.

When you finish, please send me a critique. In fact, send one even if you don't finish. I appreciate all feedback! Please note -- I am not a source of free technical support. Do not email me your general Perl problems. If you want support, ask on Usenet or the ActiveState mailing lists. That said, I welcome problems related to the tutorial itself.


Conventions used in this Tutorial

The humour is non-conventional. I think. Of more importance, the text is coloured strangely in places. My intention is to aid your comprehension, not attempt beautification. The meaning of the colours:

All the code examples have been tested, and you can just cut'n'paste (brave statement). I haven't listed the output of each example. You need to run it and see for yourself. Consider this course interactive. Consider it any which way you like.


Use of this document


Personal Printouts

Fine by me, feel free print to a copy for your own use.

Intranet usage

Just email me and let me know.

Mirroring

Again, all I ask is an email.

Translations

Every so often someone offers to translate the tutorial. Nobody has actually done so. If you want to, the conditions are:

Remember this document is copyrighted and all associated rights are strictly reserved.

--

Robert Pepper
mailto:Robert@netcat.co.uk



A Short Introduction To Perl

If you already understand what Perl is designed to do, know its features and limitations then you can skip this very small but highly informative section, over which I laboured long and hard for those that didn't know. If you are really sure, jump to the Setup Section.


What is Perl?

Perl is a programming language. Perl stands for Practical Report and Extraction Language. You'll notice people refer to 'perl' and "Perl". "Perl" is the programming language as a whole whereas 'perl' is the name of the core executable. There is no language called "Perl5" -- that just means "Perl version 5". Versions of Perl prior to 5 are very old and very unsupported.

Some of Perl's many strengths are:


What is ActivePerl? Are the other Perls inactive?

A company named ActiveState exists to provide Perl tools for the Win32 environment. ActiveState used to be ActiveWare, and before that it was sort of a part of Hip Communications. It now appears to be happy with its current name, having not changed it for over a year. Win32 means, at the time of writing, Windows 95, Windows 98 and Windows NT. It does not mean Windows 3.11, even with Win32s installed.

Prior to Perl version 5.005, there was one version of Perl for Win32, and another for all the other systems. The other version was known as the "native version".

The Win32 version was developed by ActiveState, called "Perl for Win32" and typically lagged slightly behind the native version. As of the 5.005 release, Perl for Win32 and the native version have merged -- the native version now supports Win32 directly and doesn't need any tweaking by ActiveState.

ActiveState have dropped "Perl for Win32" and renamed their distribution, which comes with an InstallShield installer, "ActivePerl".

Incidentally, a few months before 5.005 merge the native Perl version was changed so it would run on Win32 directly. This version was best known by the creator's name, "Gurusamy Sarathy". However, there were still quite a few differences between it and Perl for Win32, so many people ran both. The merge brought the best of both worlds together.


Can I run Perl on my computer?

Probably. Perl runs on everything from Amigas to Macintoshes to Unix boxen. Perl also runs on Microsoft operating systems, namely Windows 95, Windows 98 and Windows NT 3.51 and later. There are versions of Perl that run on earlier versions of these operating systems but they are no longer developed or supported. See http://www.perl.com/ for full details.


What can I do with Perl ?

Just two popular examples :

The Internet

Go surf. Notice how many websites have dynamic pages with .pl or similar as the filename extension? That's Perl. It is the most popular language for CGI programming for many reasons, most of which are mentioned above. In fact, there are a great many more dynamic pages written with perl that may not have a .pl extension. If you code in Active Server Pages, then you should try using ActiveState's PerlScript. Quite frankly, coding in PerlScript rather than VBScript or JScript is like driving a car as opposed to riding a bicycle. Perl powers a good deal of the Internet.


Systems Administration

If you are a Unix sysadmin you'll know about sed, awk and shell scripts. Perl can do everything they can do and far more besides. Furthermore, Perl does it much more efficiently and portably. Don't take my word for it, ask around.

If you are an NT sysadmin, chances are you aren't used to programming. In which case, the advantages of Perl may not be clear. Do you need it? Is it worth it?

After you read this tutorial you will know more than enough to start using Perl productively. You really need very little knowledge to save time. Imagine driving a car for years, then realising it has five gears, not four. That's the sort of improvement learning Perl means to your daily sysadminery. When you are proficient, you find the difference like realising the same car has a reverse gear and you don't have to push it backwards. Perl means you can be lazier. Lazy sysadmins are good sysadmins, as I keep telling my boss.

A few examples of how I use Perl to ease NT sysadmin life:


What can't I do with Perl ?

The question is, "what shouldn't I do with Perl". Write office suites is one answer. Perl, like most scripting languages, is a glue language designed for short and relatively simple tasks. Just don't equate this philosophy with a lack of power or "serious" features.


Support

See the FAQs at www.perl.com. Of course there are Usenet groups, but also many mailing lists. Microsoft Windows users will be interested in those hosted by http://www.activestate.com/ which discuss all things Perl and Windows.

Please, before you ask any question, anywhere:

  1. Make sure you read the group charter. Many people put time and effort into the creation of those charter in the interests of efficient discussion, so don't degrade the discussion quality and insult us by ignoring the guidelines.
  2. Read the FAQs at least twice. Try and find related FAQs. Try hard. You won't be popular if you post a question starting "I've looked at all the FAQs..." and then ask something that actually is in the FAQs. Or the manual for that matter. Believe me, it will be patently obvious to all on the list if you haven't done your homework.
  3. Carefully phrase the questions and provide source code because if you do that, you may well end up solving the problem yourself because you have thought it through a little more.

Think to yourself -- honestly -- if I was a busy Perl Professional, would I want to answer my own question?

Does it clearly state what I want an answer to? Preferably just one question at a time. Am I being unreasonable, for example asking for someone to code it for me? Have I shown evidence that I have tried to help myself? Have I made any mistakes in grammar? Is it polite? Is there enough information in there for the answer to be given?

Why should you care? Well, if you ask poorly-formed questions or those already answered in the FAQ...let's just say you won't get the answers you want. If you care about your online reputation and wasting other people's time -- two more reasons.




Setup

There are four stages:

  1. Get the software.
  2. Install it.
  3. Run a test Script.
  4. Celebrate or troubleshoot.


1. Getting the Software

An old version of Perl for Win32 is included with the Windows NT Resource Kit. It is sadly out of date. Follow the steps below to get a newer version. Having said that, you can complete the tutorial with the Resource Kit version but you should upgrade as soon as you can.

Go to http://www.activestate.com/ and follow the links to download ActivePerl. It will be a single file, and the name will be something like api508e.exe. The i stands for Intel. If you have an Alpha, download apaXXXe.exe. If you're not sure, download the Intel version.

The 508e is the version number, so expect this to change quite rapidly. The file size will be just over 5Mb, so it will take a while to download via modem. If you know how to use FTP, try ftp.activestate.com/activeperl/.

When you find ActivePerl, save the file into any directory you please. I like to organise my downloads into c:\downloads but that is just personal preference. As long as ActivePerl ends up on your hard disk somewhere it doesn't matter.

2. Installation

So you now have apixxxx.exe. If you forget where you saved it, don't panic, just run Windows Explorer and search for api*e.exe

  1. Double-click the apixxxx.exe. You'll see the fantastic ActivePerl graphic and be advised to close all open applications before proceeding. The lizard thing is a gecko, which adorns the famous O'Reilly book "Learning Perl on Win32 Systems". This tutorial is aimed at a more basic level than that book, in terms of the author's knowledge, intended audience and quality of humour.
  2. Agree to the license agreement or cancel the install, stop this tutorial and deny yourself any hope of hackership.
  3. Destination directory is whatever you want. I usually install Perl in c:\progs\perl rather than c:\program files\perl because many Win32 programs don't properly handle long filenames, let alone those with spaces in. Or you could accept the default. Your choice.
  4. Select Components. All you'll need for this tutorial is "Perl for Win32 Core", but installing the "Online Help and Documentation" and "Example Files" is highly recommended. If you run Internet Information Server (IIS) 3 or later, or Personal Web Server (PWS), then install "Perl for ISAPI" and "PerlScript" too, although don't try either of these until you are proficient with the basics. The phrase running before walking comes to mind.
  5. Select Options.
    • "Associate '.pl' with Perl.exe". If you select this option then you can just type in the name of a script at the command line, or double-click it and the script will run. If you don't, then in order to get a script to execute you'll need to type:
      perl myscript.pl
      to execute myscript.pl. Personally, I prefer double-clicking to allow me to edit the file so I do not select this option. Also, perl has a plethora of command line arguments which are difficult to pass to a script if you run it by association. For the purposes of this tutorial I'm assuming that you haven't associated .pl with perl.
    • "Add the Perl bin directory to your path". Do this, otherwise you'll have to specify the full path to perl.exe every time you use it. Not fun.
    • "Standard I/O redirection for IIS". If you run IIS or PWS, select this. It is a Good Thing. Understand it later.
  6. IIS Options If you use IIS or PWS you'll have this screen -- just accept both options.
  7. Program Folder whatever your preference is. This is just a link to the documentation, to the perl.exe itself.
  8. Confirmation make sure that what is displayed is what you have selected...
  9. The install program will now copy files. At the end it will run a few perl scripts itself, which briefly appear as DOS boxes. Don't worry, it is all quite normal.
  10. Release notes. Well worth a read.
  11. Reboot! Just so the path statement takes effect. In any case, it is always good practice to reboot after a new install.


3. Testing - Your First Perl Script

So you know what this tutorial is designed to do. You know what Perl is designed to do, and you have even installed it. It is now time to start the tutorial proper, and actually hack some code.



The Tutorial: The Journey Begins


Your First Time

Assuming all has gone to plan, you can now create your first Perl script. Follow these instructions, but before you start read them through once, then begin. That's a good idea with any form of computer-related procedure. So, to begin:

  1. Create a new directory for your perl scripts, separate to your data files and the perl installation. For example, c:\scripts\, which is what I'll assume you are using in this tutorial.
  2. Start up whatever text editor you're going to hack Perl with. Notepad.Exe is just fine. If you can't find Notepad on your Start menu, press the Start button, then select Run, type in 'notepad' and click OK.
  3. Type the following in Notepad
    print "My first Perl script\n";     
  4. Save the to c:\scripts\myfirst.pl. Be careful! Notepad will may save files with a .txt extension, so you will end up with myfirst.txt.pl by default. Perl won't mind, it'll still execute the file. If your version of Notepad does this, select "All files" before saving or rename the file then load it again. Better yet, use a decent text editor!
  5. You don't need to exit Notepad -- keep it open, as we'll be making changes very soon.
  6. Switch to your command prompt. If you don't know how to start a command prompt, click 'Start' and then 'Run'. If using Windows 9x, type in 'command' and press enter. If using NT, type in 'cmd' and press Enter.
  7. Change to your perl scripts directory, for example cd \scripts .
  8. Hold your breath, and execute the script: perl myfirst.pl

and you'll see the output. Welcome to the world of Perl ! See what I mean about it being easy to start ? However, it is difficult to finish with Perl once you begin :-)


What if it doesn't...?

So you typed in perl myfirst.pl and you didn't see My first Perl script on the screen. If you saw "bad command or filename" then either you haven't installed Perl or perl.exe is not in your path. Probably the latter. Reboot, then try again.

If you saw Can't open perl script "xxxx.pl": No such file or directory then perl is defintely installed, but you have either got the name of the script wrong or the script is not in the same directory as where you are trying to run it from. For example, maybe you saved in script in c:\windows and you are in c:\scripts so of course Perl complains it can't find the script. Could you? Well, don't expect Perl to then. You don't have to run the script from the directory in which it resides, but it is easier.

Assuming it's now all right...

W need to analyse what's going on here a little. First note that the line ends with a semicolon ; . Almost all lines of code in Perl have to end with semicolons, and those that don't have to will accept semicolons anyway. The moral is -- use semicolons. Sorry; the moral is; use semicolons.

Oh, one more thing -- if you haven't already done so, continue breathing.

Also note the \n . This is the code to tell Perl to output a newline. What's a newline? Delete the \n from the program and run it again:

print "My first Perl script";

and all should become clear.    You have now written your first Perl script.   


Shebang

Almost every Perl book is written for UN*X, which is a problem for Win32. This leads to scripts like:

#!c:/perl/perl.exe

print "I'm a cool Perl hacker\n";
   

The function of the 'shebang' line is to tell the shell how to execute the file. Under UNIX, this makes sense. Under Win32, the system must already know how to execute the file before it is loaded so the line is not needed.

However, the line is not completely ignored, as it is searched for any switches you may have given Perl (for example -w to turn on warnings).

You may also choose to add the line so your scripts run directly on UNIX without modification, as UNIX boxes probably do need it. Win32 systems do not. We shall continue with the lesson.

Variables


Scalars

So Perl is working, and you are working with Perl. Now for something more interesting than simple printing. Variables. Let's take simple scalar variables first. A scalar variable is a single value. Like $var=10 which sets the variables $var to the value of 10. Later, we'll look at lists like arrays and hashes, where @var refers to more than one value. For the moment, remember that Scalar is Singular. If weird metaphors help, think of lots of scaly snakes at a singles bar. If that didn't help, I apologise for putting the thought into your mind.


$ % @ are Good Things

If you have any experience with other programming languages you might be surprised by the code $var=10. With most languages, if you want to assign the value 10 to a variable called var you'd write var=10.

Not so in Perl. This is a Feature. All variables are prefixed with a symbol such as $ @ % . This has certain advantages, like making programs easier to read. Honestly, I'm serious! It just takes some getting used to. The prefixes mean that you can see where the variables are quite easily. And not only that, what sort of variable it is. The human language German has a similar principle (except nouns are capitalised, not prefixed with $ and Perl is easier to pronounce). You'll agree later, I think.

So, ever onwards. Time to try some more variables:

$string="perl";
$num1=20;
$num2=10.75;
print "The string is $string, number 1 is $num1 and number 2 is $num2\n";

   

Typing

A closer look...notice you don't have to say what type of variable you are declaring. In other languages you need to say if the variable is a string, array, what sort of number it is and so on. You might even have to declare what type of number it is. As an example, in Java you'd been saying things like int var=10 which defines the variable var as an integer, with the value 10.

So, why do these other programming languages force you to declare exactly what your variables are? Wouldn't it be easier if we could just not bother?

For short programs, yes. For really big projects with many programmers working on the same application, no. That's because forcing variable type declaration also forces a certain discipline and rigour which is what you need on big projects.

As you know, Perl is not designed for gigantic software engineering efforts. It is all about small, quick programs. For these purposes you don't need the rigour of variable controls as much, so Perl doesn't bother.

This idea of forcing a programmer to declare what sort of variable is being created is called typing. As Perl doesn't by default enforce any rules on typing, it is said to be a loosely typed language, as opposed to something like C++ which is strongly typed.


Variable Interpolation

We still haven't finished learning from that humble bit of code. To refresh your memory, here it is again:

$string="perl";
$num1=20;
$num2=10.75;
print "The string is $string, number 1 is $num1 and number 2 is $num2\n";

Notice the way the variables are used in the string. Sticking     variables inside of strings has a technical term - "variable     interpolation". Now, if we didn't have the handy $ prefix for we'd have to do something like     the example below, which is pseudocode. Pseudocode is code to demonstrate a    concept, not designed to be run. Like certain Microsoft software.   

print "The string is ".string." and the number is ".num."\n";

which is much more work. Convinced about those prefixes yet ?

Try running the following code:

$string="perl";
$num=20;
print "Doubles: The string is $string and the number is $num\n";
print 'Singles: The string is $string and the number is $num\n';
   

Double quotes allow the aforementioned variable interpolation. Single quotes do not. Both have their uses as you will see later, depending on whether you wish to interpolate anything.


Changing Variables


Auto(de|in)crements

If you want to add 1 to a variable you can, logically, do this; $num=$num+1 . There is a shorter way to do this, which is $num++. This is an autoincrement. Guess what this is; $num-- . Yes, an autodecrement.

This example illustrates the above:

$num=10;
print "\$num is $num\n";

$num++;
print "\$num is $num\n";

$num--;
print "\$num is $num\n";

$num+=3;
print "\$num is $num\n";
   

The last example demonstrates that it doesn't have to be just 1 you can add or decrease by.


Escaping

There's something else new in the code above. The \ . You can see what this does -- it 'escapes' the special meaning of $ .

Escaping means that just the $ symbol is printed instead of it referring to a variable.

Actually \ has a deeper meaning -- it escapes all of Perl's special characters, not just $ . Also, it turns some non-special characters into something special. Like what ? Like n . Add the magic \ and the humble 'n' becomes the mighty NewLine ! The \ character can also escape itself. So if you want to print a single \ try:

print "the MS-DOS path is c:\\scripts\\";
   

Oh, '\' is also used for other things like references. But that's not even covered here.

There is a technical term for these 'special characters' such as @ $ %. They are called metacharacters. Perl uses plenty of metacharacters. In fact, you'll wear your keyboard pretty evenly during a night's perl hacking. I think it is safe to say that Perl uses every possible keystroke and shifted keystroke on a standard US PC keyboard.

You'll be working with all sorts of obscure characters in your Perl hacking career, and I also mean those on your keyboard. This has earned perl a reputation for being difficult to understand. That's entirely true. Perl does have such a reputation, no doubt about it.

Is the reputation justified? In my opinion, Perl does have a short but steep learning curve to begin with simply because it is so different. However, once you learn the character meanings reading perl code becomes much easier precisely because of all these strange characters.


Context: About Perl and @^$%&~`/?

Perl uses so many weird characters that there aren't enough to go round. So sometimes the same character has two or more meanings, depending on its context. As an example, the humble dot . can join two variables together, act as a wildcard or become a range operator if there are two of them together. The caret ^ has different effects in [^abc] as opposed to [a^bc] .

If this sounds crazy, think about the English language. What do the following mean to you ?

Mean is, in one context, is a word to used describe the purpose of something. It is also another word for average. Furthermore, it describes a nasty person, or a person who doesn't like spending money, and is used in slang to refer to something impressive and good.

That's five different uses for 'mean', and you don't have any trouble understanding which one I mean due to context.

Polish, when capitalised, can either mean pertaining to the country Poland, or the act of making something shiny. And 'like' can mean similar to, or affection for.

So, when you speak or write English (think of two, to and too) you know what these words mean by their context. It is exactly the same way with Perl. Just don't assume a given metacharacter always means what you first thought it did.

To finish off this section, try the following:

Strings and Increments

$string="perl";
$num=20;
$mx=3;

print "The string is $string and the number is $num\n";

$num*=$mx;
$string++;
print "The string is $string and the number is $num\n";
Note the easy shortcut *= meaning 'multiply $num by $mx' or, $num=$num*$mx .
Of course Perl supports the usual + - * / ** % operators. The last two are
exponentiation (to the power of) and modulus (remainder of x divided by y).
Also note the way you can increment a string ! Is this language flexible or what ? 
   

Print: A List Operator

The print function is a list operator. That means it accepts a list of things to print, separated by commas. As an example:

print "a doublequoted string ", $var, 'that was a variable called var', $num," and a newline \n";
Of course, you just put all the above inside a single 
doublequoted string: 
print "a doublequoted string $var that was a variable called var $num and a newline \n";
to achieve the same effect. The advantage of using the print function in list context
is that expressions are evaluated before being printed. For example, try this: 
$var="Perl";
$num=10;
print "Two \$nums are $num * 2 and adding one to \$var makes $var++\n";
print "Two \$nums are ", $num * 2," and adding one to \$var makes ", $var++,"\n";
   

You might have been slightly surprised by the result of that last experiment. In particular, what happened to our variable $var ? It should have been incremented by one, resulting in Perm. The reason being that 'm' is the next letter after 'l' :-)

Actually, it was incremented by 1. We are postincrementing $var++ the variable, rather than preincrementing it.

The difference is that with postincrements, the value of the variable is returned, then the operation is performed on it. So in the example above, the current value of $var was returned to the print function, then 1 was added. You can prove this to yourself by adding the line print "\$var is now $var\n"; to the end of the example above.

If we want the operation to be performed on $var before the value is returned to the print function, then preincrement is the way to go. ++$var will do the trick.


Subroutines -- A First Look

Let's take a another look at the example we used to show how the autoincrement system works. Messy, isn't it ? This is Batch File Writing Mentality. Notice how we use exactly the same code four times. Why not just put it in a subroutine?

$num=10;		# sets $num to 10
&print_results;		# prints variable $num

$num++;
&print_results;

$num*=3;
&print_results;

$num/=3;
&print_results;

sub print_results {
        print "\$num is $num\n";
}
   

Easier and neater. The subroutine can go anywhere in your script, at the beginning, end, middle...makes no difference. Personally I put all mine at the bottom and reserve the top part for setting variables and main program flow.

A subroutine is just some code you want to use more than once in the same script. In Perl, a subroutine is a user-defined function. There is no difference. For the purposes of clarity I'll refer to them as subroutines.

A subroutine is defined by starting with sub then the name. After that you need a curly left bracket { , then all the code for your subroutine. Finish it off with a closing brace } . The area between the two braces is called a block. Remember this. There are such things as anonymous subroutines but not here. Everything here has a name.

Subroutines are usually called by prefixing their name with an ampersand, that is one of these -- & , like so &print_results; . It used to be cool to omit the & prefix but all perl hackers are now encouraged to use it to avoid ambiguity. Ambiguity can hurt you if you don't avoid it.

If you are worrying about variable visibility, don't. All the variables we are using so far are visible everywhere. You can restrict visibility quite easily, but that's not important right now. If you weren't worrying about variable visibility, please don't start. I'd tell you it's not important but that'll only make you worried. (paranoid ?) We'll cover it later.


Comments

Did you see a # crept in there. That's a comment. Everything after a # is ignored. You can't continue it onto a newline however, so if your comment won't fit on one line start a new one with # . There are ways to create Plain Old Documentation (POD) and more ways to comment but they are not detailed here.


Comparisons


An iffy start

An if statement is simple. if the day is Sunday, then lie in bed. A simple test, with two outcomes. Perl conversion (don't run this):

if ($day eq "sunday") {
	&lie_in_bed;
}

You already know that &lie_in_bed is a call to a subroutine. We assume $day is set earlier in the program. If $day is not equal to 'Sunday' &lie_in_bed is not executed (pity). You don't need to say anything else. Try this:

$day="sunday";

if ($day eq "sunday") {
        print "Zzzzz....\n";
}
Note the syntax. The if statement requires something to test for Truth. This expression must
be in (parens), then you have the braces to form a block. 
   

The Truth According to Perl

There are many Perl functions which test for Truth. Some are if, while, unless . So it is important you know what truth is, as defined by Perl, not your tax forms. There are three main rules:

  1. Any string is true except for "" and "0".
  2. Any number is true except for 0. This includes negative numbers.
  3. Any undefined variable is false. A undefined variable is one which doesn't have a value, ie has not been assigned to.

Some example code to illustrate the point:

&isit;                   # $test1 is at this moment undefined

$test1="hello";         # a string, not equal to "" or "0"
&isit;

$test1=0.0;             # $test1 is now a number, effectively 0
&isit;

$test1="0.0";           # $test1 is a string, but NOT effectively 0 !
&isit;

sub isit {
        if ($test1) {                           # tests $test1 for truth or not
                print "$test1 is true\n";
        } else {                                # else statement if it is not true
                print "$test1 is false\n";
        }
}

The first test fails because $test1 is undefined. This means it has not been created by 
assigning a value to it. So according to Rule 3 it is false. The last two tests are
interesting. Of course, 0.0 is the same as 0 in a numeric context. But it is not the
same as 0 in a string context, so in that case it is true.    

So here we are testing single variables. What's more useful is testing the result of an expression. For example, this is an expression; $x * 2 and so is this; $var1 + $var2 . It is the end result of these expressions that is evaluated for truth.

An example demonstrates the point:

$x=5;
$y=5;

if ($x - $y) {
        print '$x - $y is ',$x-$y," which is true\n";
} else {
        print '$x - $y is ',$x-$y," which is false\n";
}
   

The test fails because 5-5 of course is 0, which is false. The print statement might look a little strange. Remember that print is a list operator? So we hand it a list. First item, a single-quoted string. It is single quoted because it we do not want to perform variable interpolation on it. Next item is an expression which is evaluated, and the result printed. Finally, a double-quoted string is used because we want to print a newline, and without the doublequotes the \n won't be interpolated.

What is probably more useful than testing a specific variable for truth is equality testing. For example, has your lucky number been drawn?

$lucky=15;
$drawnum=15;

if ($lucky == $drawnum) {
        print "Congratulations!\n";
} else {
        print "Guess who hasn't won!\n";
}
   

The important point about the above code is the equality operator, == .


Equality and Perl

Now pay close attention, otherwise you'll end up posting an annoying question somewhere. This is a FAQ, as in a Frequently Asked Question.

The symbol = is an assignment operator, not a comparison operator. Therefore:

So far we have been testing numbers, but there is more to life than numbers. There are strings too, and these need testing too.

$name	 = 'Mark';

$goodguy = 'Tony';

if ($name == $goodguy) {
        print "Hello, Sir.\n";
} else {
        print "Begone, evil peon!\n";
}
   

Something seems to have gone wrong here. Obviously Mark is different to Tony, so why does perl consider them equal?

Mark and Tony are equal -- numerically. We should be testing them as strings, not as numbers. To do this, simply substitute == for eq and everything will work as expected.


All Equality is Not Equal: Numeric versus String

There are two types of comparison operator; numeric and string. You've already seen two, == and eq. Run this:

$foo=291;
$bar=30;

if ($foo < $bar) { 
        print "$foo is less than $bar (numeric)\n"; 
}

if ($foo lt $bar) { 
        print "$foo is less than $bar (string)\n"; 
}
   

The lt operator compares in a string context, and of course < compares in a numeric context.

Alphabetically, that is in a string context, 291 comes before 30. It is actually decided by the ASCII value, but alphabetically is close enough. Change the numbers around a little. Notice how Perl doesn't care whether it uses a string comparison operator on a numeric value, or vice versa. This is typical of Perl's flexibility.

Bondage and discipline are pretty much alien concepts to Perl (and the author). This flexibility does have a drawback. If you're on a programming precipice, threatening suicide by jumping off, Perl won't talk you out of your decision but will provide several ways of jumping, stepping or falling to your doom while silently watching your early conclusion. So be careful.


An interlude -- The Perl Motto

The Perl Motto is; "There is More Than One Way to Do It" or TIMTOWTDI. Pronounced 'Tim-Toady'. This tutorial doesn't try and mention all possible ways of doing everything, mainly because the author is far too lazy. Write your Perl programs the way you want to.


The Comparison Operators Listed

The rest of the operators are:

Comparison Numeric String
Equal == eq
Not equal != ne
Greater than > gt
Less than < lt
Greater than or equal to >= ge
Less than or equal to <= le


The Golden Rule of Comparisons

They may be odious, but remember the following:


More About If: Multiples

More about if statements. Run this:

$age=25;
$max=30;

if ($age > $max) {
        print "Too old !\n";
} else {
        print "Young person !\n";
}
It is easy to see what else does. If the expression is false then whatever is in 
the else block is evaluated (or carried out, executed, whatever term you choose to use).
Simple. But what if you want another test ? Perl can do that too. 
   

elsif

$age=25;
$max=30;
$min=18;

if ($age > $max) {
        print "Too old !\n";
} elsif ($age < $min) { 
        print "Too young !\n"; 
} else { 
        print "Just right !\n"; 
}
If the first test fails, the second is evaluated. This carries on until there are no 
more elsif statements, or an else statement is reached. An else statement is optional,
and no elsif statements should come after it. Logical, really.    

There is a big difference between the above example the one below:

if ($age > $max) {
        print "Too old !\n";
} 

if ($age < $min) {
        print "Too young !\n";
}
   

If you run it, it will return the same result - in this case. However, it is Bad Programming Practice. In this case we are testing a number, but suppose we were testing a string to see if it contained R or S. It is possible that a string could contain both R and S. So it would pass both 'if' tests. Using an elsif avoids this. As soon as the first statement is true, no more elsif statements (and no else statement) are executed.

You don't need to take up a whole three lines:

print "Too old\n" if     $age > $max;
print "Too old\n" unless $age < $max;
   

I added some whitespace there for aesthetic beauty. There are other operators that you can use instead of if and unless , but that's for later on.

Incidentally, the two lines of code above do not do exactly the same thing. Consider a maximum age of 50 and input age of 50. Therefore, you should be very careful about your logic when writing code (nice obvious statement there).

For those that were wondering, Perl has no case statement. This is all explained in the FAQ, which is located at http://www.perl.com/.


User Input


STDIN and other filehandles

Sometimes you have to interact with the user. It is a pain, but sometimes necessary, especially for the live ones. To ask for input and do something with it try this:

print "Please tell me your name: ";
$name=<STDIN>;
print "Thanks for making me happy, $name !\n";

New things to learn here. Firstly, <STDIN> . STDIN is a filehandle. Filehandles are what 
you use to interact with things such as files, console input, socket connections and more.    

You could say STDIN is the standard source for input. Guess what STDIN stands for. In this case the STDIN filehandle is reading from the console.

The angle brackets <> read data from a filehandle. Exactly how much is dependent on what you do, but in this case it is whatever was input at the prompt.

So we are reading from the STDIN filehandle. The value is assigned to $name and printed. Any idea why the ! ends up on a new line ? on a new line on a newline ????

As you pressed Enter, you of course included a newline with your name. The easy way to get rid of it is to chop it off:

Chop

print "Please tell me your name: ";
$name=<STDIN>;
chop $name
print "Thanks for making me happy, $name !\n"

and that fails with a syntax error. Can you spot why? Look at the error code, look at the 
line number and see where the syntax is wrong. The answer is a missing semicolon 
( ; ) on the end of the last two lines.    

If you add a ; to the end of line 3, but not to the last line, then the program works as it should. This is because Perl doesn't need a semicolon to end the last statement of a block. However, I'd advise ending all your statements with semicolons because you may well be adding more code to them and it is only one little keystroke.

When you add the semicolon(s), the program runs correctly. The chop function removes the last character of whatever it is given to chop, in this case removing the newline for us. In fact, that can be shortened:

print "Please tell me your name: ";
chop ($name=<STDIN>);
print "Thanks for making me happy, $name !";
   

The parentheses ( ) force chop to act on the result of what is inside them. So $name=<STDIN> is evaluated first, then the result from that, which is $name , is chopped. Try it without.

You can read from STDIN as much as you like. For your entertainment I have created a sophisticated multinational greeting machine:

print "Please tell me your name: ";
chop ($name=<STDIN>);

print "Please tell me your nationality: ";
chop ($nation=<STDIN>);

if ($nation eq "British" or $nation eq "New Zealand") {
	print "Hallo $name, pleased to meet you!\n";

} elsif ($nation eq "Dutch" or $nation eq "Flemish") {
	print "Hoi $name, hoe gaat het met u vandaag?!\n";

} else {
	print "HELLO!!!  SPEAKEEE ENGLIEESH???\n";
}
   

Aside from demonstrating the native English speaker's linguistic talents, this script also introduces the or logical operator. We'll cover or and its associates in more detail later on. First, a word of warning.

Chopping is dangerous, as my friend One Hand Harold will tell you. Everyone is concerned about various forms of safety these days, and your perl code should be no exception.


Safe Chopping with Chomp

Rather than just wantonly remove the last character regardless of whatever it is, without a care in the world, just simply consigning the poor little thing to the Great Bit Bucket in the Sky, you can remove the last character only if it is a newline with chomp :

chomp ($name=<STDIN>);
   

At this point the perl gurus are screaming "I found an error !". Well, chomp doesn't always remove the last character if it is a newline but if it doesn't, you have set a special variable, namely $/ , to something different. I presume that if you do set $/ you know what it does. It is explained later in this very document. Of course, being a good pupil, you wouldn't experiment with the unknown, blindly changing things just for the hell of it to see what happens.

If you don't, you'll never learn anything useful.


Arrays


Lists, herds -- what are arrays?

Perl has two types of array, associative arrays (hashes) and arrays. Both types are lists. A list is just a collection of variables referred to as the collection, not as individual elements.

You can think of Perl's lists as a herd of animals. List context refers to the entire herd, scalar context refers to a single element. A list is a herd of variables. The variables don't have to be all of the same type -- you might have a herd of ten sheep, three lions and two wolves. It would probably be just three lions and one wolf before long, but bear with me. In the same way, you might have a Perl list of three scalar variables, two array elements and ten hash elements.

Certain types of lists are known by certain names. Just as a herd of sheep is called a flock, a herd of lions is called a pride, a herd of wolves is called a pack and a herd of managers a confusion, some types of Perl list have a special names.


Basic Array Work

For example, an array is an ordered list of scalar variables. This list can be referred to as a whole, or you can refer to individual elements in the list. The program below defines a an array, called @names . It puts five values into the array.


@names=("Muriel","Gavin","Susanne","Sarah","Anna");

print "The elements of \@names are @names\n";
print "The first element is $names[0] \n";
print "The third element is $names[2] \n";
print 'There are ',scalar(@names)," elements in the array\n";

Firstly, notice how we define @names . As it is in a list context, we are using parens. Each value is comma separated, which is Perl's default list delimiter. The double quotes are not necessary, but as these are string values it makes it easier to read and change later on.

Next, notice how we print it. Simply refer to it as a whole, that is in list context.. List context means referring to more than one element of a list at a time. The code print @names; will work perfectly well too. But....

I usually learn something about Perl every time I work with it. When running a course, a student taught me this trick which he had discovered:


@names=("Muriel","Gavin","Susanne","Sarah","Anna","Paul","Trish","Simon");

print @names;
print "\n";
print "@names";
When a list is placed inside doublequotes, it is space delimited when interpolated. Useful.    

If we want to do anything with the array as a list, that is doing something with more than one value, then refer to the array as @array . That's important. The @ prefix is used when you want to refer to more than one element of a list.

When you refer to more than one, but not all elements of an array that is known as a slice . Cake analogies are appropriate. Pie analogies are probably healthier but equally accurate.


Elements of Arrays

Arrays are not much use unless we can get to individual elements. Firstly, we are dealing with a single element of the list, so we cannot use @ which refers to multiple elements of the array. It is a single, scalar variable, so $ is used. Secondly, we must specify which element we want. That's easy - $array[0] for the first, $array[1] for the second and so forth. Array indexes start at 0, unless you do something which is so highly deprecated ('deprecated' means allowed, usually for backwards compatibility, but disapproved of because there are better ways) I'm not even going to mention it.

Finally, we force what is normally list context (more than one element) into scalar context (single element) to give us the amount of elements in the array. Without the scalar , it would be the same as the second line of the program.

How to refer to elements of an array

Please understand this:

$myvar="scalar variable";
@myvar=("one","element","of","an","array","called","myvar");

print $myvar;        # refers to the contents of a scalar variable called myvar
print $myvar[1];     # refers to the second element of the array myvar
print @myvar;        # refers to all the elements of array myvar

   

The two variables $myvar and @myvar are not, in any way, related. Not even distantly. Technically, they are in different namespaces.

Going back to the animal analogy, it is like having a dog named 'Myvar' and a goldfish called 'Myvar'. You'll never get the two mixed up because when you call 'Myvar !!!!' or open a can of dog food the 'Myvar' dog will come running and goldfish won't. Now, you couldn't have two dogs called 'Myvar' and in the same way you can't have two Perl variables in the same namespace called 'Myvar'.

More ways to access arrays

The element number can be a variable.

print "Enter a number :";
chomp ($x=<STDIN>);

@names=("Muriel","Gavin","Susanne","Sarah","Anna");

print "You requested element $x who is $names[$x]\n";

print "The index number of the last element is $#names \n";

This is useful. Notice the last line of the example. It returns the index number of 
the last element. Of course you could always just do this $last=scalar(@names)-1; but 
this is more efficient. It is an easy way to get the last element, as follows: 
print "Enter the number of the element you wish to view :";
chomp ($x=<STDIN>);

@names=("Muriel","Gavin","Susanne","Sarah","Anna","Paul","Trish","Simon");

print "The first two elements are @names[0,1]\n";
print "The first three elements are @names[0..2]\n";
print "You requested element $x who is $names[$x-1]\n";		# starts at 0
print "The elements before and after are : @names[$x-2,$x]\n";
print "The first, second, third and fifth elements are @names[0..2,4]\n";

print "a) The last element is $names[$#names]\n";	# one way
print "b) The last element is @names[-1]\n";		# different way 

It looks complex, but it is not. Really. Notice you can have multiple values separated 
by a comma. As many as you like, in whatever order. The range operator .. gives you
everything between and including the values. And finally look at how we print the last 
element - remember $#names gives us a number ? Simply enclose it inside square brackets
and you have the last element.    

Do also note that because element accesses such as [0,1] are more than one variable, we cannot use the scalar prefix, namely the $ symbol. We are accessing the array in list context, so we use the @ symbol. Doesn't matter that it is not the entire array. Remember, accessing more than one element of an array but not the entire array is called a slice. I won't go over the food analogies again.


For Loops


A for Loop demonstrated

All well and good, but what if we want to load each element of the array in turn ? Well, we could build a for loop like this:

@names=("Muriel","Gavin","Susanne","Sarah","Anna","Paul","Trish","Simon");

for ($x=0; $x <= $#names; $x++) {
        print "$names[$x]\n"; 
}

which sets $x to 0, runs the loop once, then adds one to $x , checks it is less than 
$#names , if so carries on. By the way, that was your introduction to for loops. Just 
to go into a little detail there, the for loop has three parts to it:    

In this case, the variable $x is initialised to 0. It is immediately tested to see if it is smaller than, or equal to $#names . If that is true, then the block is executed once. Critically, if it is not true the block is not executed at all.

Once the block has been executed, the modification expression is evaluated. That's $x++ . Then, the test condition is checked to see if the block should be executed or not.

For loops with .. , the range operator

There is a another version:

for $x (0 .. $#names) {
        print "$names[$x]\n";
}

which takes advantage of the range operator .. (two dots together). This simply gives $x the 
value of 0, then increments $x by 1 until it is equal to $#names . 
   

foreach

For true beauty we must use foreach .

foreach $person (@names) {
        print "$person";
}

This goes through each element ('iterates', another good    technical word to use) 
of @names , and assigns each element in turn to the variable $person . Then you can do 
what you like with the variable. Much easier. You can use 
for $person (@names) {
        print "$person";
}

if you want. Makes no difference at all, aside from a little  clarity. 
   

The infamous $_

In fact, that gets shorter. And now I need to introduce you to $_ , which is the Default Input and Pattern Searching Variable.

foreach (@names) {
        print "$_";
}

If you don't specify a variable to put each element into, $_ is used instead as it is 
the default for this operation, and many, many others in Perl. Including the print function : 
foreach (@names) {
        print ;
}

As we haven't supplied any arguments to print , $_ is printed as default. You'll be seeing 
a lot of $_ in Perl. Actually, that statement is not exactly true. You will be seeing lot
of places where $_ is used, but quite often when it is used, it is not actually written. 
In the above example, you don't actually see $_ but you know it is there. 
   

A Premature End to your loop

A loop, by its nature, continues. If that didn't make sense, start reading this sentence again.

The old jokes are the best, aren't they?

The joke above is a loop. You continue re-reading the sentence until you realise I'm trying to be funny. Then you exit the loop. Or maybe somebody doesn't exit it. Whatever, loops always run until the expression they are testing returns false. In the case of the examples above, a false value is returned when all the elements of the array have been cycled through, and the loop ends.

If you want an everlasting loop, just test an condition you know will always be true:

while (1) {
	$x++;
        print "$x:  Did you know you can press CTRL-C to interrupt a perl program?\n";
}

Another way to exit a loop is a simple foreach over the elements, as we have seen. But if we 
don't know when we want to exit a loop? For example, suppose we want to print out a list of 
names but stop when we find one with a particular title? You are throwing a huge party, 
someone is allergic to vodka, and this person has drunk from the punch bowl despite being 
assured by someone holding two empty bottles of Absolut that he was just using the bottles 
to convey yet more orange juice into said punch bowl. So you need a doctor, and so you write
a Perl script to find one from the list of attendees, wanting the doctor's name to be the last 
item printed: 
@names=('Mrs Smith','Mr Jones','Ms Samuel','Dr Jansen','Sir Philip');

foreach $person (@names) {
	print "$person\n";
	last if $person=~/Dr /;
}

The last operator is our friend. Don't worry about the /Dr / business -- that is a regular 
expression which we cover next. All you need to know is that it returns true if the name begins 
with 'Dr '. When it does return true, last is operated and the loop ends early.    


A little more control over the premature ending: Labels

So that's easy enough. But wait! We need a medical, human-fixer type doctor, not just anyone with a PhD. So, the same principle applies in this example here:

@names  =('Mrs Smith','Mr Jones','Ms Samuel','Dr Jansen','Sir Philip');
@medics =('Dr Black','Dr Waymour','Dr Jansen','Dr Pettle');

foreach $person (@names) {
	print "$person\n";
	if ($person=~/Dr /) {
		foreach $doc (@medics) {
			print "\t$doc\n";
			last if $doc eq $person;
		}
	}
}

Aside from showing one way to indent your code, this also demonstrates a nested loop. A nested 
loop is a loop within a loop. What happens is that the @names array is searched for a 'Dr ', 
and if it is found then the @medics array is searched to make sure the doctor is a human-fixing 
doctor not a professor of physics or something. The regular expression has been shifted into 
an if statement, where it works nicely as it only returns true or false.    

The problem with the code is that after we find our medical doctor we want it to stop. But it doesn't. It only stops the loop it is in, so Dr Pettle never gets printed. However, the code just carries on with Sir Philip who is terribly sorry old chap, but can't be of any bally use at all, what ho! What we need is a way to break out of the entire loop from within a nest. Like so:

@names  =('Mrs Smith','Mr Jones','Ms Samuel','Dr Jansen','Sir Philip');
@medics =('Dr Black','Dr Waymour','Dr Jansen','Dr Pettle');

LBL: foreach $person (@names) {
	print "$person\n";
	if ($person=~/Dr /) {
		foreach $doc (@medics) {
			print "\t$doc\n";
			last LBL if $doc eq $person;
		}
	}
}

Only two changes here. We have defined a label, namely LBL. Instead of breaking out from 
the current loop, which is the default, we specify a label to break out to, which is in 
the outer loop. This works with as many nested loops as your brain can handle. You don't have 
to use uppercase names but for namespace reasons it is recommended, and you can call your 
labels whatever you please. I was just being unimaginative with the name of LBL, feel free 
to invent labels called DORIS or MATILDA if that's what floats your personal boat.    


Changing the Elements of an Array

So we have @names . We want to change it. Run this:

print "Enter a name :";
chomp ($x=<STDIN>);

@names=("Muriel","Gavin","Susanne","Sarah");

print "@names\n";

push (@names, $x);

print "@names\n";

Fairly self explanatory. The push function just adds a value on to the end of the array. 
Of course, Perl being Perl, it doesn't have to be just the one value: 
print "Enter a name :";
chop ($x=<STDIN>);

@names=("Muriel","Gavin","Susanne","Sarah");
@cities=("Brussels","Hamburg","London","Breda");

print "@names\n";

push (@names, $x, 10, @cities[2..4]);

print "@names\n";

This is worth looking at in more detail. It appears there is no fifth element of 
@cities , as referred to by @cities[2..4] .    

Actually, there is a fifth element. Add this to the end of the example :

print "There are ",scalar(@names)," elements in \@names\n";

There appear to be 8 elements in @names . However, we have just proved there are in fact 9. 
The reason there are 9 is that we referred to non-existent elements of @cities , and Perl 
has quite happily extended @names to suit. The array @cities remains unchanged. Try poping 
the array if you don't believe me.    

So that's push . Now for some...

Jiggerypokery with Arrays

@names=("Muriel","Gavin","Susanne","Sarah");
@cities=("Brussels","Hamburg","London","Breda");

&look;

$last=pop(@names);
unshift (@cities, $last);

&look;

sub look {
        print "Names : @names\n";
        print "Cities: @cities\n";
}

Now we have two arrays. The pop function removes the last element of an array and returns it,
which means you can do something like assign the returned value to a variable.    
The unshift function adds a value to the beginning of the array. Hope you didn't forget that 
&subroutinename calls a subroutine. Presented below are the functions you can use to work with arrays: 
   

A table of array hacking functions

push Adds value to the end of the array
pop Removes and returns value from end of array
shift Removes and returns value from beginning of array
unshift Adds value to the beginning of array

Now, accessing other elements of arrays. May I present the splice function ?

Splice

@names=("Muriel","Sarah","Susanne","Gavin");

&look;

@middle=splice (@names, 1, 2);

&look;

sub look {
        print "Names : @names\n";
        print "The Splice Girls are: @middle\n";
}

The first argument for splice is an array. Then second is the offset. The offset is the index    
number of the list element to begin splicing at. In this case it is 1. Then    
comes the number of elements to remove, which is sensibly 1 or more in this    
case. You can set it to 0 and perl, in true perl style, won't complain.    
Setting to 0 is handy because splice can add elements to the middle of an array, and if you don't    
want any deleted 0 is the number to use. Like so: 
@names=("Muriel","Gavin","Susanne","Sarah");
@cities=("Brussels","Hamburg","London","Breda");

&look;

splice (@names, 1, 0, @cities[1..3]);

&look;

sub look {
        print "Names : @names\n";
        print "Cities: @cities\n";
}

Notice how the assignment to @middle    
has gone -- it is no longer relevant.    

If you assign the result of a splice to a scalar then:

@names=("Muriel","Sarah","Susanne","Gavin");

&look;

$middle=splice (@names, 1, 2);

&look;

sub look {
        print "Names : @names\n";
        print "The Splice Girls are: $middle\n";
}

then the scalar is assigned the last element removed, or undef if    
it doesn't work at all.    

The splice function is also a way to delete elements from an array. In fact, a discussion of :

Deleting Variables

is in order. Suppose we want to delete Hamburg from the following array. How do we do it ? Perhaps:

@cities=("Brussels","Hamburg","London","Breda");

&look;

$cities[1]="";

&look;

sub look {
	print "Cities: ",scalar(@cities), ": @cities\n";
}
   

would be appropriate. Certainly Hamburg is removed. Shame, such a great lake. But note, the array element still exists. There are still four elements in @cities. So what we need is the appropriate splice function, which removes the element entirely.

splice (@cities, 1, 1);
   

Now that's all well and good for arrays. What about ordinary variables, such as these:

$car ="Porsche 911";
$aircraft="G-BBNX";

&look;

$car="";

&look;

sub look {
	print "Car :$car: Aircraft:$aircraft:\n";
	print "Aircraft exists !\n" if $aircraft;
	print "Car exists !\n" if $car;
}
   

It looks like we have deleted the $car variable. Pity. But think about it. It is not deleted, it is just set to the null string "". As you recall (hopefully) from previous ramblings, the null string evaluates to false so the if test fails.

False values versus Existence: It is, therefore...

Just because something is false doesn't mean to say it doesn't exist. A wig is false hair, but a wig exists. Your variable is still there. Perl does have a function to test if something exists. Existence, in Perl terms, means defined. So:

print "Car is defined !\n" if defined $car;
   

will evaluate to true, as the $car variable does in fact exist.

This begs the question of how to really wipe variables from the face of the earth, or at least your Perl script. Simple.

$car 	 ="Porsche 911";
$aircraft="G-BBNX";

&look;

undef $car; # this undefines $car

&look;

sub look {
	print "Car :$car: Aircraft:$aircraft:\n";
	print "Aircraft exists !\n"  if $aircraft;
	print "Car exists !\n" 	     if defined $car;
}
   

This variable $car is eradicated, deleted, killed, destroyed.

And now for something completely different....

Basic Regular Expressions


An introduction

Or regex for short. These can be a little intimidating. But I'll bet you have already used some regex in your computing life so far. Have you even said "I'll have any Dutch beer ?" That's a regex which will match a Grolsch or Heineken, but not a Budweiser, orange juice or cheese toastie. What about dir *.txt ? That's a regular expression too, listing any files ending in .txt.

Perl's regex often look like this:

$name=~/piper/
   

That is saying "If 'piper' is inside $name, then True."

The regular expression itself is between / / slashes, and the =~ operator assigns the target for the search.

An example is called for. Run this, and answer it with 'the faq'. Then try 'my tealeaves' and see what happens.

print "What do you read before joining any Perl discussion ? ";
chomp ($_=<STDIN>);

print "Your answer was : $_\n";

if ($_=~/the faq/) {
        print "Right !  Join up !\n";
} else {
        print "Begone, vile creature !\n";
}

So here $_ is searched    
for 'the faq'. Guess what we don't need ! The =~ .
This works just as well: 
if (/the faq/) {

because if you don't specify a variable, then perl searches $_ by default. 
In this particular case, it would be better to use 
 if ($_ eq "the faq") { as we are testing for exact matches.    
   

Senstivity -- regexes in touch with their inner child

But what if someone enters 'The FAQ' ? It fails, because the regex is case sensitive. We can easily fix that:

if (/the faq/i) {

with the /i switch, which    
specifies case-insensitivity. Now it works for all variations, such as "the    
Faq" and "the FAQ".    

Now you can appreciate why a regular expression is better in this situation than a simple test using eq . As the regex searches one string for another string, a response of "I would read the FAQ first !" will also work, because "the FAQ" will match the regex.

Study this example just to clarify the above. Tabs and spaces have been added for aesthetic beauty:

$_="perl for Win32";                            # sets the string to be searched

if ($_=~/perl/) { print "Found perl\n" };       # is 'perl' inside $_ ?  $_ is "perl for Win32".
if (/perl/)     { print "Found perl\n" };       # same as the regex above.  Don't need the =~ as we are testing $_
if (/PeRl/)     { print "Found PeRl\n" };       # this will fail because of case sensitivity
if (/er/)       { print "Found er\n" };         # this will work, because there is an 'er' in 'perl'
if (/n3/)       { print "Found n3\n" };         # this will work, because there is an 'n3' in 'Win32'
if (/win32/)    { print "Found win32\n" };      # this will fail because of case sensitivity
if (/win32/i)   { print "Found win32 (i)\n" };  # this will *work* because of case insensitivity (note the /i)

print "Found!\n"  if      / /;                  # another way of doing it, this time looking for a space

print "Found!!\n" unless $_!~/ /;		# both these are the same, but reversing the logic with unless and !
print "Found!!\n" unless    !/ /;		# don't do this, it will always never not confuse nobody :-)
						# the ~ stays the same, but = is changed to ! (negation)

$find=32;                                       # Create some variables to search for
$find2=" for ";                                 # some spaces in the variable too

if (/$find/)  { print "Found '$find'\n" };      # you can search for variables like numbers
if (/$find2/) { print "Found '$find2'\n" };     # and of course strings !

print "Found $find2\n" if /$find2/;           # different way to do the above

As you can see from the last example, you can embed a variable in    
the regex too. Regular expressions could fill entire books (and they have    
done, see the book critiques at http://www.perl.com/) but here are some useful    
tricks: 
   

Character Classes

@names=qw(Karlson Carleon Karla Carla Karin Carina Needanotherword);

foreach (@names) {                      # sets each element of @names to $_ in turn
        if (/[KC]arl/) {                # this line will be changed a few times in the examples below
                print "Match !  $_\n";
        } else {
                print "Sorry.   $_\n";
        }
}

This time @names is    
initialised using whitespace as a delimiter instead of a comma. qw refers to 
'quote words', which means split the list by words. A word ends with whitespace 
(like tabs, spaces, newlines etc).    

The square brackets enclose single characters to be matched. Here either Karl or Carl must be in each element. It doesn't have to be two characters, and you can use more than one set. Change Line 4 in the above program to:

if (/[KCZ]arl[sa]/) {
   

matches if something begins with K, C, or Z, then arl, then either s or a. It does not match KCZarl. Negation is possible too, so try this :

if (/[KCZ]arl[^sa]/) {

which returns things beginning with K, C or Z, then arl, and then anything EXCEPT s or a. The caret ^ has to be the first character, otherwise it doesn't work as the negation. Having said [ ] defines single characters only, I should mention than these two are the same:

/[abcdeZ]arl/;
/[a-eZ]arl/;

if you use a hyphen then you get the list of characters including the start and finish characters. And if you want to match a special character (metacharacter), you must escape it:

/[\-K]arl/;

matches Karl or -arl. Although the - character is represented by two characters, it is just the one character to match.

Matching at specific points

If you want to match at the end of the line, make sure a $ is the last character in the regex. This one pulls out all those names ending in a. Slot it into the example above :

if (/a$/) {
   

And there is a corresponding character, the caret ^ , which in this context matches at the beginning of the string. Yes, the caret also negates a character class like this [^KCZ]arl but in this case it anchors the match to the beginning of the string.


if (/n/i)  {
if (/^n/i) {

The first one is true if the word contains an 'n' anywhere in it.    
The second specifies that the 'n' must be at the beginning of the string to be    
matched. Use this anchor where you can, because it makes the whole regex    
faster, and safer if you know what the first character must be. 
   

Negating the regex

If you want to negate the entire regex change =~ to !~ (Remember ! means 'not equal to'.)

if ($_ !~/[KC]arl/) {

Of course, as we are testing $_    
this works too: 
if (!/[KC]arl/) {

   

Returning the Match

Now things get interesting. What if we want pull something out of a string ? So far all we have done is test for truth, that is say yea or nay if a string matches, but not return what we found. Run this:

$_='My email address is <Robert@NetCat.co.uk>.';

/(<robert\@netcat.co.uk>)/i;

print "Found it ! $1\n";

Firstly, note the single quotes when $_    
is assigned. If there were double quotes, we'd need \@ instead of @ .
Remember, double quotes "" allow variable interpolation, so Perl looks for an 
array called @NetCat which does not exist.    

Secondly, look at the parens around the entire regex. If you use parens, a side effect is that the first match is put into a variable called $1 . We'll get to the main effect later. The second match goes into $2 and so on. Also note that the \@ has been escaped, so perl doesn't think it is an array. Remember \ either escapes a special character, or gives a special meaning. Think of it as Superman's telephone box. Imagine Clark Kent walking around with his magic partner Back Slash.

Notice how we specify in the regex case-insensitivity with /i and the regex returns the case-sensitive string - that is, exactly what it found.

Try the regex without parens. Then try this one:

/<(robert)\@netcat.co.uk>/i;

You can put the parens anywhere. More or less. Now, run this :    
$_='My email address is <Robert@NetCat.co.uk>.';

/<(robert)\@(netcat.co.uk)>/i;

print "Found it ! $1 at $2\n";

See, you can have more than one ! Look at the above regex. Looks    
easy now, don't you think ? What about five minutes ago ? It would have looked    
like a typing mistake ! Well, there are some hairier regex to come, but you'll    
have a good barber. 
   

* + -- regexes become line noise

What if we didn't know what the email address was going to be ?

$_='My email address is <webslave@work.com>.';

print "Found it ! :$1:" if /(<.*>)/i;

When you see an if    
statement like this, read it right to left. The print statement is only executed if code on    
the right of the expression is true.    

We'll discuss this. Firstly, we have the opening parens ( . So everything from ( to ) will be put into $1 if the match is successful. Then the first character of what we are searching for, < . Then we have a dot, or period . . For this regex, we can assume . matches any character at all.

So we are now matching < followed by any character. The * means 0 or more of the previous character. The regex finishes by requiring > .

This is important. Get the basics right and all regex are easy (I read somewhere once). An example best illustrates the point. Slot this regex in instead:

$_='My email address is <webslave@work.com>.';

print "Found it ! :$1:" if /(<*>)/i;

What's happening here ?    

The regex starts, logically, at the start of the string. This doesn't mean it starts a 'M', it starts just before M. There is a 'nothing' between the string start and 'M'.

The regex is searching for <* , which is 0 or more < .

The first thing it finds is not < , but the nothing in between the start of the string and the 'M' from 'My email...". Does this match ?

As the regex is looking for "0 or more" < , we can certainly say that there are 0 < at the start of the string. So the match is, so far, successful. We have dealt with <* .

However, the next item to match is > . Unfortunately, the next item in the string is 'M', from 'My email..". The match fails at this point. Sure, it matched < without any problem, but the complete match has to work.

The only two characters that can match successfully at this point are < or > . The 'point' being that <* has been matched successfully, and we need either > to complete the match or more of < to continue the '0 or more' match denoted by * .

'M' is neither of them, so it fails at this point, when it has matched

Quick clarification - the regex cannot successfully match < , then skip on ahead through the string until it matches > . The characters in the string between < > also need to match the regex, and they don't in this case.

All is not lost. Regexes are hardy little beasts and don't give up easily. An attempt is made to match the regex wherever possible. The regex system keeps trying the match at every possible place in the string, working towards the end.

Let's look at the match when it reaches the 'm' in 'work.com'.

Again, we have here 0 < . So the match works as before. After success on <* the next character is analysed - it is a > , so the match is successful.

But, be warned. The match may be successful but your job is not done. Assuming the objective of was to return the email address within the angle brackets then that regex is a miserable failure. Watch for traps of this nature when regexing.

That's * explained. Just to consolidate, a quick look at:

$_='My email address is <webslave@work.com>.';
print "Match 1 worked :$1:" if /(<*)/i;

$_='<My email address is <webslave@work.com>.';
print "Match 2 worked :$1:" if /(<*)/i;

$_='My email address is <webslave@work.com<<<<>.';
print "Match 3 worked :$1:" if /(<*>)/i;

Match 1 is true. It doesn't return anything, but it is true    
because there are 0 < at the very    
start of the string.    

Match 2 works. After the 0 < at the start of the string, there is 1 < so the regex can match that too.

Match 3 works. After the failing on the first < , it jumps to the second. After that, there are plenty more to match right up until the required ending.

Glad you followed that. Now, pay even closer attention ! Concentrate fully on the task at hand ! This should be straightforward now:

$_='HTML <I>munging</I> time !.';

/<I>(.*)<\/I>/i;

print "Found it ! $1\n";

Pretty much the same as the above, except the parens are moved so    
we return what's only inside the tags, not including the tags themselves. Also    
note how / is escaped like so; \/ otherwise Perl thinks that's the end of    
the regex.    

Now, suppose we change $_ to :

$_='HTML <I>munging</I> time is here <I>again</I> !.';

and run it again. Interesting effect, eh ? This is known as    
Greedy Matching. What happens is that when Perl finds the initial match, that    
is <I> it jumps right to the end    
of the string and works back from there to find a match, so the longest string    
matches. This is fine unless you want the shortest string. And there is a    
solution: 
/<I>(.*?)<\/I>/i;
Just add a question mark and Perl does stingy matching. No    
nationalistic jokes. I have Dutch and Scottish friends I don't want to offend.     


The Difference Between + and *

You know what * means, namely match 0 or more. If you want to match 1 or more, then use + . The difference is important.

$_='The number is 2200 and the day is Monday';

($star)=/([0-9]*)/;

($plus)=/([0-9]+)/;

print "Star is '$star' and Plus is '$plus'\n";

You'll note that $star has no value. The match was    
successful though. It managed to match 0 or more characters from 0 to 9 at the    
very start of the regex.    

The second regex with $plus worked a little better, because we are matching one or more characters from 0 to 9. Therefore, unless one 0 to 9 is found the match will fail. Once a 0-9 is found, the match continues as long as the next character is 0-9, then it stops.

Now we know this, there is another way to remove an email address from within angle brackets:

$_='My email address is <robert@netcat.co.uk> !.';

/<([^>]+)/i;

print "Found it ! $1\n";
This regex matches <. Then the capturing parens    
start. They have no effect on this regex other than to capture the match.    
After that, there is a character class, containing one character. As ^    
is the first character is the class, it negates the class. That's why    
we are using a character class with only one character in it, because it can    
be negated.    

So far we have matched < and anything that is not >. The + ensures we match as many characters that are not <'s as we can. This has the same effect as .*? but is more efficient. It may also suit your purposes, as .*? relies on you knowing what you want to match up to, whereas [^>]+ simply contines matching until it finds something that fails its criteria. Just make sure you understand the difference because it is a crucial part of regexery.


Re-using the match -- \1, $1...

Suppose we didn't know what HTML tag we had to match ? It could be B, I, EM or whatever, and we want everything that is in between. Well, HTML container tags like B and EM have end tags which are the same as the start tag, except for the / . So what we could do is:

Can this be done ? Of course. This is perl, all things are possible. Now, remember the side effect of parens. I promise I'll explain the primary effect at some point. If whatever is in (parens) matches, the result is stored in a variable called $1 . So we can use <(.*?)> which will find us < then as many anythings (the . and * ) up to the next, not last > (the ? forces stingy matching).

The result is stored in $1 because we used parens. Next, we need everything up to the closing tag. That's easy : (.*?) matches everything up until the next character or set of characters. And how exactly do we define where to stop ?

We can use $1 even in the same regex it was found in. However, it is not referred to within a regex as $1 , but \1 .

So we want to match </$1> which in perl code is <\/\1> . The / must be escaped because it is the end of the regex, and 1 is escaped so it refers to $1 instead of matching the number 1.

Still here ? This is what it looks like:

$_='HTML <I>munging</I> time is here <I>again</I> !.';
/<(.*?)>(.*?)<\/\1>/i;

print "Found it ! $2\n";

If you want to know how to return all the matches above, read on.    
But before that: 
   

How to Avoid Making Mountains while Escaping Special Characters

You want to match this; http://language.perl.com/faq/ . That's a real (useful) URL by the way. Hint. To match it, you need to do this:

/http:\/\/language\.perl\.com\/faq\//;

which should make the awful metaphor above clearer, if not    
funnier. The slash, / , is not    
normally a metacharacter but as it is being used for the regular expression    
delimiters, it needs to be escaped. We already know that . is special.    

Fortunately for our eyes, Perl allows you to pick your delimiter if you prefix it with 'm' as this example shows. We'll use a #:

m#http://language\.perl\.com/faq/#; 
Which is a huge improvement, as we change / to # .
We can go further with readability by quoting everything:    
m#\Qhttp://language.perl.com/faq/\E#;
The \Q escapes everything    
up until \E or the regex delimiter (so    
we don't really need the \E above). In this case #    
will not be escaped, as it delimits the regex.    

Someone once posted a question about this to the Perl-Win32-Users mailing list and I was so intrigued about this apparently undocumented trick I spent the next twenty minutes figuring it out by trial and error, and posted a reply. Next day I found lots of messages telling the poster to read the manual because it was clearly documented. <face colour='red' intensity='high'> My excuse was I didn't have the docs to hand....moral of the story - RTFM and RTF FAQs !

Subsitution and Yet More Regex Power


Basic changes

Suppose you want to replace bits of a string. For example, 'us' with 'them'.

$_='Us ? The bus usually waits for us, unless the driver forgets us.';

print "$_\n";

s/Us/them/;   # operates on $_, otherwise you need $foo=~s/Us/them/;

print "$_\n";

What happens here is that the string 'Us' is searched for, and    
when a match is found it is replaced with the right side of the expression, in    
this case 'them'. Simple.    

You'll notice that only one substitution was made. To match globally use /g which runs through the entire string, changing wherever it can. Try:

s/Us/them/g;

which fails. This is because regexes are not, by default,    
case-sensitive. So: 
s/us/them/ig;

would be a better bet. Now, everything is changed. A little too    
much, but one problem at a time. Everything you have learn about regex so far    
can be used with s/// , like parens,    
character classes [ ] , greedy and    
stingy matching and much more. Deleting things is easy too. Just specify    
nothing as the replacement character, like so s/Us//; .    

So we can use some of that knowledge to fix this problem. We need to make sure that a space precedes the 'us'. What about:

s/ us/them/g;

An small improvement. The first 'Us' is now no longer changed,    
but one problem at a time ! We'll first consider the problem of the regex    
changing 'usually' and other words with 'us' in them.    

What we are looking for is a space, then 'us', then a comma, period or space. We know how to specify one of a number of options - the character class.

s/ us[. ,]/them/g;

Another tiny step. Unfortunately, that step wasn't really in the    
right direction, more on the slippery slope to Poor Programming Practice. Why? 
Because we are limiting ourselves. Suppose someone wrote ' send it to us;    
when we get it'.    

You can't think of all the possible permutations. It is often easier, and safer, to simply state what must not follow the match. In this case, it can be anything except a letter. We can define that as a-z. So we can add that to the regex.

s/ us[^a-z]/ them/g;

the caret ^ negates the    
character class, and a-z represents    
every alphabet from a to z inclusive. A space has been added to the    
substitution part - as the original space was matched, it should be replaced    
to maintain readability.    


\w

What would be more useful is to use a-zA-Z instead. If we weren't using /i we'd need that. As a-zA-Z is such a common construct, Perl provides an easy shorthand:

s/ us[^\w]/ them/g;

The \w construct actually means 'word' - equivalent to a-zA-Z_0-9 . So we'll use that instead.

To negate any construct, simply capitalise it:

s/ us[\W]/ them/g;

and of course we don't need the negating caret now. In fact, we don't even need the character class!

s/ us\W/ them/g;

So far, so good. Matching the first 'us' is going to be difficult though. Fortunately, there is an easy solution. We've seen Perl's definition of a word - \w . Between each word is a boundary. You can match this with \b .

s/\bus\W/ them/g;

that's \b followed by 'us', not 'bus' :-)

Now, we require a word boundary before 'us'. As there is a 'nothing' at the start of the string, we have a match. There is a space after the first 'Us', so the match is successful. You might notice an extra space has crept in - that's the space we added earlier. The match doesn't include the space any more - it matches on the word boundary, that is just before the word begins. The space doesn't count.

Did you notice the final period and the comma are replaced? They are part of the match - it is the \W that matches them. We can't avoid that. We can however put back that part of the match.

Replacing with what was found

s/\bus(\W)/them\1/g;

We start with capturing whatever the \W matches, using parens. Then, we add it to the replacement string. The capture is of course in $1 , but as it is in a regex we refer to it as \1 .

The final problem is of course capitalising the replacement string when appropriate. Which in old versions of the tutorial I left as an exercise to the reader, having run out of motivation. A reader by the name of Paul Trafford duly solved the problem, and I have just inserted his excellent explanation for the elucidation of all concerned:

#         Solution to the us/them problem...
#
#   The program works through the text assigning the 
#   variable $1 to 'U' or 'u' for any words where this 
#   letter is followed by 's' and then by non 'word' 
#   characters.   The latter is assigned to variable $2.
#
#   For each such matching occurrence, $1 is replaced by 
#   the letter that precedes it in the alphabet using 
#   operations 'ord' and 'chr' that return the ASCII value 
#   of a character and the character corresponding to a 
#   given natural number.  After this 'hem' is tacked on 
#   followed by $2, to retain the shape of the original 
#   sentence.  The '/e' switch is used for evaluation.
#
#   NOTES
#   1. This solution will not replace US (short for 
#   United States) with Them or them.
#
#   2. If a 'magical' decrement operator '--' existed for 
#   strings then the solution could be simplified for we 
#   wouldn't need to use the 'chr' and 'ord' operators.
$_='Us ? The bus usually waits for us, unless the driver forgets us.';

print "$_\n";

s/\b([Uu])s(\W)/chr(ord($1)-1).hem.$2/eg;

print "$_\n";
   

An excellent solution, thanks Paul.

There are several more constructs. We'll take a quick look at \d which means anything that is a digit, that is 0-9 . First we'll use the negated form, \D , which is anything except 0-9 :

print "Enter a number :";
chop ($input=<STDIN>);

if ($input=~/\D/) {
        print "Not a number !!!!\n";
} else {
        print 'Your answer is ',$input x 3,"\n";

}

this checks that there are no non-number characters in $x . It's not perfect because it'll choke on decimal points, but it's just an example. Writing your own number-checker is actually quite difficult, but it is an interesting exercise. Try it, and see how accurate yours is.


x

I hope you trusted me and typed the above in exactly as it is show (or pasted it), because the x is not a mistake, it is a feature. If you were too smart and changed it to a * or something change it back and see what it does.

Of course, there is another way to do it :

unless ($input=~/\d/) {
        print 'Your answer is ',$input x 3,"\n";
} else {
        print "Not a number !!!!\n";
}

which reverses the logic with an unless statement.

More Matching

Assume we have:

$_='HTML <I>munging</I> time is here <I>again</I> !.';

and we want to find all the italic words. We know that /g will match globally, so surely this will work:

$_='HTML <I>munging</I> time is here <I>again</I> ! What <EM>fun</EM> !';

$match=/<i>(.*?)<\/i>/ig;

print "$match\n";

except it returns 1, and there were definitely two matches. The match operator returns true or false, not the number of matches. So you can test it for truth with functions like if, while, unless Incidentally, the s/// operator does return the number of substitutions.

To return what is matched, you need to supply a list.

($match) = /<i>(.*?)<\/i>/i;

which handily puts all the first match into $match . Note that an = is used (for assignment), as opposed to =~ (to point the regex at a variable other than $_.

The parens force a list context in this case. There is just the one element in the list, but it is still a list. The entire match will be assigned to the list, or whatever is in the parens. Try adding some parens:

$_='HTML <I>munging</I> time is here <I>again</I> ! What <EM>fun</EM> !';

($word1, $word2) = /<i>(.*?)<\/i>/ig;

print "Word 1 is $word1 and Word 2 is $word2\n";

In the example above notice /g has been added so a global replacement is done - this means perl carries on matching even after it finds the first match. Of course, you might not know how many matches there will be, so you can just use an array, or any other type of list:

$_='HTML <I>munging</I> time is here <I>again</I> ! What <EM>fun</EM> !';

@words = /<i>(.*?)<\/i>/ig;

foreach $word (@words) {
        print "Found $word\n";
}

and @words will be grown to the appropriate size for the matches. You really can supply what you like to be assigned to:

($word1, @words[2..3], $last) = /<i>(.*?)<\/i>/ig;

you'll need more italics for that last one to work. It was only a demonstration.

There is another trick worth knowing. Because a regex returns true each time it matches, we can test that and do something every time it returns true. The ideal function is while which means 'do something as long the condition I'm testing is true'. In this case, we'll print out the match every time it is true.

$_='HTML <I>munging</I> time is here <I>again</I> ! What <EM>fun</EM> !';

while (/<(.*?)>(.*?)<\/\1>/g) {
        print "Found the HTML tag $1 which has $2 inside\n";
}

So the while operator runs the regex, and if it is true, carries out the statements inside the block.

Try running the program above without the /g . Notice how it loops forever? That's because the expression always evaluates to true. By using the /g we force the match to move on until it eventually fails.

Now we know this, an easy way to find the number of matches is:

$_='HTML <I>munging</I> time is here <I>again</I> ! What <EM>fun</EM> !';

$found++ while /<i>.*?<\/i>/ig;

print "Found $found matches\n";

You don't need braces in this case as nothing apart from the expression to be evaluated follows the while function.

Parentheses Again: OR

The real use for them. Precedence. Try this, and yes you can try it at home:

$_='One word sentences ? Eliminate. Avoid clichés like the plague.  They are old hat.';

while (/o(rd|ne|ld)/gi) {
        print "Matched $1\n";
}

Firstly, notice the subtle introduction of the or operator, in this case | , the pipe. What I really want to explain however, is that this regex matches o followed by rd, ne or ld. Without the parens it would be /ord|ne|ld/ which is definitely not what we want. That matches just plain ord, or ne or ld.

(?: OR Efficiency)

In the interests of efficiency, consider this:

print "Give me a name :";
chop($_=<STDIN>);

print "Good name\n" if /Pe(tra|ter|nny)/;

The code above functions correctly. If you were wondering what a good name is, Petra, Peter and Penny qualify. The regex is not as efficient as it could be though. Think about what Perl is doing with the regex, that you are just ignoring. Simply throwing away casually. Without consideration as to the effort that has gone into creating it for you. The resources squandered. The little bytes of memory whose sole function in life is to store this information, which will never be used.

What's happening is that because parens are used, perl is creating $1 for your usage and abusage. While this may not seem important, a fair amount of resources go into creating $1, $2 and so on. Not so much the memory used to store them, more the CPU effort involved. So, if you aren't going to use the parens for capturing purposes, why bother capturing the match?

print "Give me a name :";
chop($_=<STDIN>);

print "Good name\n" if /Pe(?:tra|ter|nny)/;

print "The match is :$1:\n";

The second print statement demonstrates that nothing is captured this time. You get the benefits of the paren's precedence-changing capabilities, but without the overhead of the capturing. This benefit is especially worthwhile if you are writing CGI programs which use parens in regex -- with CGI, every little of bit efficiency counts.

Matching specific amounts of...

Finally, take a look at this :

$_='I am sleepy....zzzz....DING ! Wake Up!';

if (/(z{5})/) {
        print "Matched $1\n";
} else {
        print "Match failed\n";
}
The braces { } specify how many of the preceding character to match. So z{2} matches exactly two 'z's and so on. Change z{5} to z{4} and see how it works. And there's more...

/z{3}/ 3 z only
/z{3,}/ At least 3 z
/z{1,3}/ 1 to 3 z
/z{4,8}/ 4 to 8 z

To any of the above you may suffix an question mark, the effect of which is demonstrated in the following program. Run it a couple of times, inputting 2, 3 and 4:

print "How many letters do you want to match ? ";
chomp($num=<STDIN>);

# we assign and print in one smooth move
print $_="The lowest form of wit is indeed sarcasm, I don't think.\n";

print "Matched \\w{$num,} : $1 \n"  if /(\w{$num,})/;

print "Matched \\w{$num,?}: $1 \n"  if /(\w{$num,}?)/;
   

The first match is 'match any word (that's a-Z0-9_) equal to or longer than $num character, and return it.' So if you enter 4, then 'lowest' is returned. The word 'The' doesn't match.

The second match is exactly the same, but the ? forces a minimal match, so only the part actually matched is returned.

Just to clear this up, amend the program thus:

print "\nMatched \\w{$num,} :";
print "$1 " while /(\w{$num,})/g;

print "\nMatched \\w{$num,?} :";
print "$1 " while /(\w{$num,}?)/g;
   

Note the addition of /g . Try it without - notice how the match never moves on ?


Pre, Post, and Match

And now on the Regex Programme Today, we have guest stars Prematch, Postmatch and Match. All of whom are going to slow our entire programme down, but are useful anyway :

$_='I am sleepy....snore....DING ! Wake Up!';

/snore/;	# look, no parens !

print "Postmatch: $'\n";
print "Prematch: $`\n";
print "Match: $&\n";

If you are wondering what the difference between match and using parens is you should remember than you can move the parens around, but you can't vary what $& and its ilk return. Also, using any of the above three operators does slow your entire program, whereas using parens will just slow the particular regex you use them for. However, once you've used one of the three matches you might as well use them all over the place as you've paid the speed penalty. Use parens where possible.

RHS Expressions

/e

RHS means Right Hand Side. Suppose we have an HTML file, which contains:

<FONT SIZE=2> <FONT SIZE=4> <FONT SIZE=6>

and we wish to double the size of each font so 2 becomes 4 and 4 becomes 8 etc. What about :

$data="<FONT SIZE=2> <FONT SIZE=4> <FONT SIZE=6>";

print "$data\n";

$data=~s/(size=)(\d)/\1\2 * 2/ig;

print "$data\n";

which doesn't really work out. What this does is match size=x, where x is any digit. The first match, size=, goes into $1 and the second match, whatever the digit is, goes into $2 . The second part of the regex simply prints $1 and $2 (referred to as \1 and \2 ), and attempts to multiply $2 by 2. Remember /i means case insensitive matching.

What we need to do is evaluate the right hand side of the regex as an expression - that is not just print out what it says, but actually evaluate it. That means work it through, not blindly treat it as string. Perl can do this:

$data=~s/(size=)(\d)/$1.($2 * 2)/eig;

A little explanation....the LHS is the same as before. We add /e so Perl evaluates the RHS as an expression. So we need to change \1 into $1 and so on. The parens are there to ensure that $2 * 2 is evaluated, then joined to $1 . And that's it !

/ee

It is even possible to have more than one /e . For example:

$data='The function is <5funcA>';

$funcA='*2+4';

print "$data\n";

$data=~s/<(\d)(\w+)>/($1+2).${$2}/;	# first time
# $data=~s/<(\d)(\w+)>/($1+2).${$2}/e;	# second time
# $data=~s/<(\d)(\w+)>/($1+2).${$2}/ee;	# third time

print "$data\n";

To properly appreciate this you need to run it three times, each time commenting out a different line. Only one regex line should be uncommented when the program is run.

The first time round the regex is a dumb variable interpolation. Perl just searches the string for any variables, finds $1 and $2, and replaces them.

Second time round the expression is evaluated, as opposed to just plain variable-interpolated. This means that $1+2 is evaluated. $1 has a value of 5, pl, plus 2 == 7. The other part of the replacement, ${$2} is evaluated only so far as working out that the variable named $2 should be placed in the string.

Third time round and Perl now makes a second pass through the string, looking for things to do. After the first pass, and just before that second pass the string looks like this; 7*2+4 . Perl evaluates this, and prints the result.

So the more /e 's you add on the end of the regex, the more passes Perl makes through the replacement string trying to evaluate the code.

This is fairly advanced stuff here, and it is probably not something you will use every day. But knowing it is there is handy.


A Worked Example: Date Change

Imagine you have a list of dates which are in the US format of month, day, year as opposed to the rest of the world's logical notion of day, month year. We need a regex to transpose the day and month. The dates are:

@dates=(
'01/22/95',
'05/15/87',
'8-13-96',
'5.27.78',
'6/16/1993'
);

The task can be split into steps such as:

  1. Match the first digit, or two digits. Capture this result.
  2. Match the delimiter, which appears to be one of / - .
  3. Match the second two digits, and capture that result
  4. Rebuild the string, but this time reversing the day and month.

That may not be all the steps, but it is certainly enough for a start. Planning regex is important. So, first pass:

@dates=(
'01/22/95',
'5/15/87',
'8-13-96',
'5.27.78',
'6/16/1993'
);

foreach (@dates) {
	print;
	s#(\d\d)/(\d\d)#$2/$1#;
	print " $_\n";
}

Hmm. This hasn't worked for the dates delimited with - . , and the last date hasn't worked either. The first problem is pretty easy; we are just matching / , nothing else. The second problem arises because we are matching two digits. Therefore, 5/15/87 is matched on the 15 and 87, not the 5 and 15. The date 6/16/1993 is matched on the 16 and the 19 of 1993.

We can fix both of those. First, we'll match either 1 or 2 digits. There are a few ways of doing this, such as \d{1,2} which means either 1 or two of the preceding character, or perhaps more easily \d\d? which means match one \d and the other digit is optional, hence the question mark. If we used \d+ then that would match 19988883 which is not a valid date, at least not as far as we are concerned.

Secondly, we'll use a character class for all the possible date delimiters. Here is just the loop with those amendments:

foreach (@dates) {
	print;
	s#(\d\d?)[/-.](\d\d?)#$2/$1#;
	print " $_\n";
}

which fails. Examine the error statement carefully. The key word is 'range'. What range? Well, the range between / and . because - is the range operator within a character class. That means it is a special character, or a metacharacter. And to negate the special meaning of metacharacters we have to use a backslash.

But wait! I don't hear you cry. Surely . is a metacharacter too? It is, but not within a character class so it doesn't need to be escaped.

foreach (@dates) {
	print;
	s#(\d\d?)[/\-.](\d\d?)#$2/$1#;
	print " $_\n";
}

Nearly there. However, we are always replacing the delimiter with / which is messy. That's an easy fix:

foreach (@dates) {
	print;
	s#(\d\d?)([/\-.])(\d\d?)#$3$2$1#;
	print " $_\n";
}

so that fixes that. In case you were wondering, the . dot does not act as '1 of anything' inside a character class. It would defeat the object of the character class if it did. So it doesn't need escaping. There is a further improvement you can make to this regex:

$m='/.-';

foreach (@dates) {
	print;
	s#(\d\d?)([$m])(\d\d?)#$3$2$1#;
	print " $_\n";
}

which is good practice because you are bound to want to change your delimiters at some point, and putting them inside the regex is hardcording, and we all know that ends in tears. You can also re-use the $m variable elsewhere, which is good pratice.

Did you notice the difference between what we assign to $m and what we had before?

    /\-.
$m='/.-';

The difference is that the - is no longer escaped. Why not? Logic. Perl knows - is the range operator. Therefore, there must be a character to the immediate left and immediate right of it in order for it to work, for example e-f. When we assign a string to $m, the range operator is the last character and therefore has no character to the right of it, so Perl doesn't interpret as a range operator. Try this:

$m='/-.';

and watch it fail.

Something else that causes heartache is matching what you don't mean to. Try this:

@dates=(
'01/22/95',
'5/15/87',
'8-13-96',
'5.27.78',
'/16/1993',
'8/1/993',
);

$m='/.-';

foreach (@dates) {
	print;
	s#(\d\d?)([$m])(\d\d?)#$3$2$1# or print "Invalid date! ";
	print " $_\n";
}

The two invalid dates at the end are let through. If you wanted to check the validity of every possible date since the start of the modern calendar then you might be better off with a database rather than a regex, but we can do some basic checking. The important point is that we know the limitations of what we are doing.

What we can do is make sure of two things; that there are three sets of digits seperated by our chosen delimiters, and that the last set of digits is either two digits, eg 99, 98, 87, or four digits, eg 1999, 1998, 1987.

How can we do this? Extend the match. After the second digit match we need to match the delimter again, then either 2 digits or four digits. How about:

$m='/.-';

foreach (@dates) {
	print;
	s#(\d\d?)([$m])(\d\d?)[$m](\d\d|\d{4})#$3$2$1$2# or print "Invalid date! ";
	print " $_\n";
}

which doesn't really work out. The problem is it lets 993 through. This is because \d\d will match on the front of 993. Furthermore, we aren't fixing the year back on to the end result.

The delimiter match is also faulty. We could match / as the first delimiter, and - as the second. So, three problems to fix:

foreach (@dates) {
	print;
	s#(\d\d?)([$m])(\d\d?)\2(\d\d|\d{4})$#$3$2$1$2$4# or print "Invalid!";
	print " $_\n";
}

This is now looking like a serious regex. Changes:

  1. We are re-using the second match, which is the delimiter, further on in the regex. That's what the \2 is. This ensures the second delimiter is the same as the first one, so 5/7-98 gets rejected.
  2. The $ on the end means end of string. Nothing allowed after that. So the regex now has to find either 2 or 4 digits at the end of the string, or it fails.
  3. Added the match of the year ($4) to the rebuild section of the regex.

Regex can be as complex as you need. The code above can be improved still further. We could reject all years that don't begin with either 19 or 20 if they are four-digit years. The other problem with the code so far is that it would reject a date like 02/24/99 which is valid because there are characters after the year. Both can be fixed:

@dates=(
'01/22/95',
'5/15/87',
'8-13-96',
'5.27.78',
'/16/1993',
'8/1/993',
'3/29/1854',
'! 4/23/1972 !',
);

$m='/.-';

foreach (@dates) {
	print;
	s#(\d\d?)([$m])(\d\d?)\2(\d\d|(?:19|20)\d{2})(?:$|\D)#$3$2$1$2$4# or print "Invalid!";
	print " $_\n";
}

We have now got a nested OR, and the inner OR is non-capturing for reasons of efficiency and readability. At the end we alternate between letting the regex match either an end of line or any non-digit, symbolised with \D.

We could go on. It is often very difficult to write a regex that matches anything of even minor complexity with absolute certainity. Think about IP addresses for example. What is important is to build the regex carefully, and understand what it can and cannot do. Catching anything supposedly invalid is a good idea too. Test your regex with all sorts of invalid data, and you'll understand what it can do.

Split and Join


Splitting

While you are in the regex mood, a quick look at split and join . Destruction is always easier (just ask your car mechanic), so lets start with split .

$_='Piper:PA-28:Archer:OO-ROB:Antwerp';

@details=split /:/, $_;

foreach (@details) {
        print "$_\n";
}

Here we give split is given two arguments. The first one is a regex specifying what to split on. The next is what to split. Actually, I could leave $_ out because as usual it is the default if nothing is specified.

The assignment can either be a scalar variable or a list like an array (or hash, but at this time 'hash' to you means what you think the Dutch do or a silly drinking event spoilt by some running). If it's a scalar variable you get the number of elements the split has splut. Should that be 'the split has splittered' or 'the split has splat'. Hmmm. Probably 'the split has split'. You know what I mean. I think I just generated a Fatal Error in English.dll. Whoops. In any case, splitting to a scalar variable is not always a Good Thing, as we'll see later.

If the assignment is an array, then as you can see in the above example the array is created with the relevant elements in order. You can also assign to scalars, for example :

$_='Piper:PA-28:Archer:OO-ROB:Antwerp';

($maker,$model,$name,$reg,$location) = split /:/, $_;
(@aircraft[0..1],$aname,@regdetails) = split /:/, $_;

$number=split /:/ ;             # not bothering with the $_ at the end, as it is the default

print "Using the first 'split'\n";
print "$reg is a $maker $model $name based in $location\n";
print "There are $number details available on this aircraft\n\n";

print "Using the second 'split'\n";
print "You can find $regdetails[0], an $aircraft[1], $regdetails[1]\n";

This demonstrates that a list can be a list of scalar variables (which is basically what an array is anyway), and that you can easily see how many elements the expression can be split into.

The example below adds a third parameter to split, which is how many elements you want returned. If you don't want the extra stuff at the end pop it.

$_='Piper:PA-28:Archer:OO-ROB:Antwerp';

@details=split /:/, $_, 3;

foreach (@details) {
        print "$_\n";
}

In the example below we split on whitespace. Whitespace, in perl terms, is a space, tab, newline, formfeed or carriage return. Instead of writing \t\n\f\r for each of the above, you can simply use \s , or the negated version \S which means anything except whitespace. Think of whitespace as anything you know is there, but you can't see.

The whitespace split is specially optimised for speed. I've used spaces, double spaces, a tab and a newline in the list below. Also note the + , which means one or more of the preceding character, so it will split on any combination of whitespace. And I think the final split is useful to know. The split function does not return the delimiter, so in this case the whitespace will not be returned.

$_='Piper       PA-28  Archer           OO-ROB
Antwerp';

@details=split /\s+/, $_;

foreach (@details) {
        print "$_\n";
}

@chars=split //, $details[0];

foreach $char (@chars) {
        print "$char !\n";
}

   

A very FAQ

The following question has come up at least three times in the Perl-Win32-Users mailing list. Can you answer it ?

"My data is delimited by |, for example:
name|age|sex|height|
Why doesn't
@array=split /|/, $line;
work ?"

Why indeed. If you don't already know the answer, some simple troubleshooting steps can be applied. First, create a sample program and run it.

$line='name|age|sex|height';

@array=split /|/,$line;

foreach (@array) { print "$_\n" }

The effect is to split each character. The | is returned. As it is the delimiter, | should be ignored, not returned.

At this point you should be thinking 'metacharacter'. A little research (looking at the documentation) will reveal that | is indeed a metacharacter, which means 'or', when inside a regex. So, in effect, the regex /|/ means 'nothing, or nothing'. The split is therefore performed on 'nothings', and there are 'nothings' in between each character. The solution is easy ; /\|/ .

$line='name|age|sex|height';

@array=split /\|/,$line;

foreach (@array) { print "$_\n" }
   

So that's the fun stuff, destruction. Now to put it back together again with join .

What Humpty Dumpty needs : Join

$w1="Mission critical ?";
$w2="Internet ready modems !";
$w3="J(insert your cool phrase here)";	# anything prefixed by 'J' is now cool ;-)
$w4="y2k compatible.";
$w5="We know the Web.";
$w6="...the leading product in an emerging market.";

$cool=join ' ', $w1,$w2,$w3,$w4,$w5,$w6;

print $cool;

Join takes a 'glue' operator, which is not a regular expression. It can be a scalar variable however. In this case it is a space. Then it takes a list, which can either be a list of scalar variables, an array or whatever as long as its a list. And you can see what the result is. You could assign it to an array, but you'd end up with everything in the first element of the array.

The example below adds an array into the list, and demonstrates use of a variable as the delimiter.

$w1="Mission critical ?";
$w2="Internet ready modems !";
$w3="J(insert your cool phrase here)"; 	# anything prefixed by 'J' is now cool ;-)
$w4="y2k approved, tested and safe !";
$w5="We know the Web.";
$w6="...the leading product in an emerging market.";
@morecool=("networkable","compatible");

$sep=" ";

$cool=join $sep, $w1,$w2,$w3,@morecool,$w4,$w5,$w6;

print $cool;

   

A recap, but with some new functions


Randomness

Aren't you wishing you could mix and match randomly so you too could get a job marketing vapourware ? Heh.

@cool=(
"networkable directory services",
"legacy systems compatible",
"Mission critical, Business Ready",
"Internet ready modems !",
"J(insert your cool phrase here)",
"y2k approved, tested and safe !",
"We know the Web. Yeah.",
"...the leading product in an emerging market."
);

srand;

print "How many phrases would you like (max ",scalar(@cool),") ?";
while (1) {
        chop ($input=<STDIN>);
        if ($input <= scalar(@cool) and $input > 0) {
                last;
        }
        print 'Sorry, invalid input, try again :';
}

for (1..$input) {
        $index=int(rand $#cool);
        print "$cool[$index] ";
        splice @cool, $index, 1;
}

A few things to explain. Firstly, while (1) { . We want an everlasting loop, and this one way to do it. 1 is always true, so round it goes. We could test $input directly, but that wouldn't allow last to be demonstrated.

Everlasting loops aren't useful unless you are a politician being interviewed. We need to break out at some point. This is done by the last function. When $input is between 1 and the number of elements in @cool then out we go. (You can also break out to labels, in case you were wondering. And break out in a sweat. Don't start now if you weren't.)

The srand operator initialises the random number generator. Works ok for us, but CGI programmers should think of something different because their programs are so frequently run (they hope :-).

rand generates a random number between 0 and 1, or 0 and a number it is given. In this case, the number of elements of @cool -1, so from 0 to 7. There is no point generating numbers between 1 and 8 because the array elements run from 0 to 7.

The int function makes sure it is an integer, that is no messy bits after the decimal point.

The splice function removes the printed element from the array so it won't appear again. Don't want to stress the point.

Concatenation

There is another joining operator, this time the humble dot, or period: . . This concatanates (joins) variables:

$x="Hello";
$y=" World";
$z="\n";

print "$x\n";           # print $x and a newline

$prt=$x.$y.$z;          # make a new var $prt out of $x, $y and $z

print $prt;

$x.=$y." again ".$z;    # add stuff to $x

print $x;

   

Files


Opening

Perl is very good at handling files. Create, in your perl scripts directory c:\scripts, a file called stuff.txt. Copy the following into it :

The Main Perl Newsgroup:comp.lang.perl.misc
The Perl FAQ:http://www.perl.com/faq/
Where to download perl:http://www.activestate.com/

Now, to open and do things with this file. First, we must open the file and assign it to a filehandle. All operations will be done on the file via the filehandle. Earlier, we used <STDIN> as a filehandle - we read from it.

$stuff="c:\scripts\stuff.txt";

open STUFF, $stuff;

while (<STUFF>) {
        print "Line number $. is : $_";
}

What this script does is fail. What is should do is open the file defined in $stuff , assign it to the filehandle STUFF and then, while there are still lines left in the file, print the line number $. and the current line.


An unforgivable error

It fails. That's not so bad, everything fails sometimes. What is unforgivable is NOT CHECKING THE ERROR CODE !

This is a better version:

open STUFF, $stuff or die "Cannot open $stuff for read :$!";

If the open operation fails, the or means that the code on the RHS (right hand side) is evaluated. Perl dies. This means it exits the script, performs a post-mortem which it writes up into $! and tells you the line number at which it died. Just because $! contains useful information doesn't mean to say it is automagically printed, in true perl fashion. Usually you will wish to avail yourself of the information inside as it is of great help when working out why something is not going according to plan. The moral of the chapter is:

Always check your return codes !

\\ or / in pathnames -- your choice

The problem should now be apparent. The backslashes, being escape characters, are not displayed. There are two ways to fix this:

The forward slashes are the preferred option, even under Win32, because you can then port the script direct to Unix or other platforms (assuming you don't use drive letters), and it is less typing. If you wish to use Perl to start external processes then you must use the \\ method, but this variable will be used only in a Perl program, not as a parameter to start an external program. Changing the $stuff variable results in a working script. Always check your return codes !

Reading a file

$stuff="c:/scripts/stuff.txt";

open STUFF, $stuff or die "Cannot open $stuff for read :$!";

while (<STUFF>) {
        print "Line $. is : $_";
}

A little more detail on what is happening here. The file is opened for read. You can append and write too. You don't have to use a variable, but I always do because it is then easy to change and easy to insert into the or die section, and it is easy to change later on. Hardcoding things is not the best way to write a maintainable and flexible program. Just ask the Year 2000 people about code that lived a little longer than the authors imagined :-).

open STUFF, "c:/scripts/stuff.txt" or die "Cannot open stuff.txt for read :$!";

is just as good but more work if you want to change anything.

The line input operator (that's the angle brackets <> reads from the beginning of the file up until and including the first newline. The read data goes into $_ , and you can do what you want with it there. On the next iteration of the loop data is read from where the last read left off, up to the next newline. And so on until there is no more data. When that happens the condition is false and the loop terminates. That's the default behaviour, but we can change this.

This means that you can open a 200Mb file in perl and run through it without having to load the entire file into memory. 200Mb of memory is quite a bit. If you really want to load the entire 200Mb file into one variable, Perl lets you. Limits are not the Perl Way.

The special variable $. is the current line number, starting at 1.

As usual, there is a quicker way to do the previous program.

$STUFF="c:/scripts/stuff.txt";

open STUFF or die "Cannot open $STUFF for read :$!";

while (<STUFF>) {
        print "Line $. is : $_";
}

This saves a little bit of typing, but does tie your filehandle to the variable name. In fact, that entire program could be compressed further, but that's for later.

If you are really into shortness, try this:

$STUFF="c:/scripts/stuff.txt";

open STUFF or die "Cannot open $STUFF for read :$!";

print "Line $. is : $_" while (<STUFF>);
        


   

Writing to a File


A simple write

$out="c:/scripts/out.txt";

open OUT, ">$out" or die "Cannot open $out for write :$!";

for $i (1..10) {
        print OUT "$i : The time is now : ",scalar(localtime),"\n";
}

Note the addition of > to the filename. This opens it for writing. If we want to print to the file we now just specify the filehandle name. You print to the filehandle, which is a gateway to the file.

Filehandles don't have to be capitalised, but it is wise. All Perl functions are lowercase, and Perl is case-sensitive. So if you choose uppercase names they are guaranteed not to conflict with current or future function words.

And a neat way to grab the date sneaked in there too. You should be aware that writing to a file overwrites the file. It does not append data! However, you may append:

Appending

$out="c:/scripts/out.txt";

&printfile;

open OUT, ">>$out" or die "Cannot open $out for append :$!";

print OUT 'The time is now : ',scalar(localtime),"\n";

close OUT;

&printfile;

sub printfile {
        open IN, $out or die "Cannot open $out for read :$!";
        while (<IN>) {
                print;
        }
        close IN;
}

This script demonstrates subroutines again, and how to append to a file, that is write additional data at the end. The close function is introduced here. This, well, closes a filehandle. You don't have to close a filehandle - just leave it open until the script finishes, or the next open command to the same filehandle will close it for you.

@ARGV: Command Line Arguments

Perl has a special array called @ARGV . This is the list of arguments passed along with the script name on the command line. Run the following perl script as:

perl myscript.pl hello world how are you
foreach (@ARGV) {
        print "$_\n";
}

Another useful way to get parameters into a program -- this time without user input. The relevance to filehandles is as follows. Run the following perl script as:

perl myscript.pl stuff.txt out.txt

while (<>) {
        print;
}

Short and sweet ? If you don't specify anything in the angle brackets, whatever is in @ARGV is used instead. And after it finishes with the first file, it will carry on with the next and so on. You'll need to remove non-file elements from @ARGV before you use this.

It can be shorter still:

perl myscript.pl stuff.txt out.txt

print while <>;

Read it right to left. It is possible to shorten it even further !

perl myscript.pl stuff.txt out.txt

print <>;

This takes a little explanation. As you know, many things in Perl, including filehandles, can be evaluated in list or scalar context. The result that is returned depends on the context.

If a filehandle is evaluated in scalar context, it returns the first line of whatever file it is reading from. If it is evaluated in list context, it returns a list, the elements of which are the lines of the files it is reading from.

The print function is a list operator, and therefore evaluates everything it is given in list context. As the filehandle is evaluated in list context, it is given a list !

Who said short is sweet? Not my girlfriend, but that's another story. The shortest scripts are not usually the easiest to understand, and not even always the quickest. Aside from knowing what you want to achieve with the program from a functional point of view, you should also know wheter you are coding for maximum performance, easy maintenance or whatever -- because chances those goals may be to some extent mutually exclusive.

Modifying a File with $^I

One of the most frequent Perl tasks is to open a file, make some changes and write it back to the original filename. You already have enough knowledge to do this. The steps would be:

  1. Make a backup copy of the file
  2. Open the file for read
  3. Open a new temporary file for write
  4. Go through the read file, and write it and any changes to the temp file
  5. When finished, close both files
  6. Delete the original file
  7. Rename the temp file to the original filename

If you have managed to get this far and assiduously work through the examples, the above will be child's play. Play if you want, but there is a Better Way.

Make sure you have data in c:\scripts\out.txt then run this:

@ARGV="c:/scripts/out.txt";

$^I=".bk";              # let the magic begin

while (<>) {
        tr/A-Z/a-z/;    # another new function sneaked in
        print;          # this goes to the temp filehandle, ARGVOUT, 
			# not STDOUT as usual, so don't mess with it !
}

So, what's happening? First, we load up @ARGV with the name of a file. It doesn't matter how @ARGV is loaded. We could have shifted the code from the command line.

The $^I is a special variable. You knew that just by looking at it. It's name is the Inplace Edit variable, and when it has a value the effects are:

  1. The name of the file to be in-placed edited is taken from the first element of @ARGV. In this case, that is c:/scripts/out.txt. The file is renamed to its existing name plus the value of $^I, ie out.txt.bk.
  2. The file is read as usual by the diamond operator <>, placing a line at a time into $_.
  3. A new filehandle is opened, called ARGVOUT, and no prizes for guessing it is opened on a file called out.txt. The original out.txt is renamed.
  4. The print prints automatically to ARGVOUT, not STDOUT as it would usually.

At the end of the operation you have neatly edited the file and made a backup. If you don't want a backup, assign a null string to $^I but don't go crying on any mailing lists if you lose data.

The usual method of in-place editing would involve just printing everything back where it came from until your regex finds whatever needs changing. You could of course slurp the whole file into memory and play with it there, which could be a lot easier but if you are dealing with files of more than a few megabytes this is probably not a feasible approach.

Now take a look at out.txt . Notice how all capital letters have been transliterated into lowercase. This is the tr operator at work, which is more efficient than regex for changing single characters. But that's only a small part of the tr function's value to the world. More later.

You should also have an out.txt.bk file. And finally, notice the way @ARGV has been created. You don't have to create it from the command line arguments -- it can be treated like an ordinary array, for that is what it is.


$/ -- Changing what is read into $_

On a different note, what if your input file is doesn't look like this:

Beer
Wine
Pizza
Catfood

which is nicely delimited with a newline each time, but like this:

shorts
t-shirt
blouse

pizza
beer
wine
catfood

Viz
Private Eye
The Independent
Byte

toothpaste
soap
towel

which is delimited by TWO newlines, not one. You don't have to save the above as shop.txt, but if you don't, the examples will be difficult to follow.

Now, if you want each set of items as elements in an array you'll have to do something like this:

$SHOP="shop.txt";
$x=0;

open SHOP or die "Can't open $SHOP for read: $!\n";

while (<SHOP>) {
        if (/^\n/) {            # does line begin with newline ?
                $x++;           # if so, increment $x.  Rest of if statement not executed.
        } else {
                $list[$x].=$_;  # glue $_ on the end of whatever is in $list[$x], using a .
        }               
}

foreach (@list) {
        print "Items are:\n$_\n\n";
}

which works, but there is a much easier way to do it. You knew I was going to say that.

$SHOP="shop.txt";
$/="\n\n";

open SHOP or die "Can't open $SHOP for read: $!\n";

while (<SHOP>) {
        push (@list, $_);
}

foreach (@list) {
        print "Items are:\n$_\n\n";
}

The $/ variable is a special variable (it even looks special). It is the Default Input Record Separator. Remember the operation of the angle brackets being to read a file in up until the next newline ? Time to come clean. What the angle bracket actually do is read up until whatever $/ is set to. It is set to a newline by default.

So if we set it to two newlines, as above, then it reads up until it finds two consecutive newlines, then puts the data into $_ This makes the program a lot shorter and quicker. You can set $/ to just about anything, not just a newline. If you want to hack this list for example:

Tea:Beer:Wine:Pizza:Catfood:Coffee:Chicken:Salmon:Icecream

you could just leave $/ as a newline and slurp it into memory in one go, but imagine the above items are a list of clothes that your girlfriend wants to buy or a list of clothes your boyfriend should have thrown away by now. Either are going to be really big files, and you might not want to read it all into memory in one go. So set $/=":"; and all will be well. There are also read and seek functions, but they aren't covered here. Those are useful for files where you read in a precise number of bytes.

We'll go back to the last example for a moment. It is useful to know how to read just one line (well, up to $/ ) at a time:

$SHOP="shop.txt";
$/="\n\n";

open SHOP or die "Can't open $SHOP for read: $!\n";

$clothes=<SHOP>;        # everything up until the first occurrence of $/ into $clothes

$food=<SHOP>;   # everything from first occurrence of $/ to the second into $food

print "We need...\n",$clothes,"...and\n",$food;

And now we know that, there is a even quicker way to achieve the aim of the original program :

$SHOP="shop.txt";
$/="\n\n";

open SHOP or die "Can't open $SHOP for read: $!\n";

@list=<SHOP>;   # dumps *all* of $SHOP into @list, not just one line.

foreach (@list) {
        print "Items are:\n$_\n\n";
}

and you don't need to grab it all :

@list[0..2]=<SHOP>;

We haven't mentioned list context for a while. Whether the line input operator <> returns a single value or a list depends on the context you use it in. When you supply @xxxxx then this must be a list. If you supply $xxxxx then that's a scalar variable. You can force it into list context by using parens.

The two lines below are provided so you can paste them into the above program. They demonstrate how parens force list context. Remember to replace the foreach with something that prints the variables.

($first, $second) = <SHOP>;
$first,  $second  = <SHOP>;
   


HERE Docs

The problem:

print "This is a long line of text which might be too long to fit on just one line\n";
print "and I was right, it was too long to fit on one line.  In fact, it looks like it\n";
print "might very well take up to FOUR, yes FOUR lines to print.  That's four print\n";
print "statements, which takes up even more room.  But wait! I'm wrong!  It will take\n";
print "FIVE lines to print this statement!  Or is that six lines? I'm not sure....\n";

The solution:

$var='variable interpolated';

print <<PRT;
This is a long line of text which might be too long to fit on just one line
and I was right, it was too long to fit on one line.  In fact, it looks like
it might very well take up to FOUR, yes FOUR lines to print.  

That's four print statements, which takes up even more room.  But wait! I'm 
wrong!  It will take FIVE lines to print this statement!  Or maybe six lines? 
I'm not sure....but anyway, just to prove this can be $var.
PRT

That's called a 'here' document and you don't need to use PRT, you can use whatever you like within reason. You don't need to put in explicit newlines, although if you do they perform as usual. Now you know about here docs you can stop wearing the print function out by calling it every couple of lines. You don't have to use here docs to print to files, just anywhere you'd normally put a more than one print statement.

Reading Directories


Globbing

For this exercise, I suggest creating another directory where you have at least two text files and two or more binary files. Copy a couple of .dll files from your WINDIR directory if you need to, those will do for the binaries, and save a couple of random text files. Size doesn't matter, in this case.

Then run this, giving the directory as the command line argument:

$dir=shift;	# shifts @ARGV, the command line arguments after the script name

chdir $dir or die "Can't chdir to $dir:$!\n" if $dir;

while (<*>) {
	print "Found a file: $_\n" if -T;
}
   

The chdir function changes perl's working directory. You should, as ever, test to see if it worked or not. In this case we only try and change directory if $dir is true.

The <*> construct reads all files from a given directory, and prints if it passes the file test -T , which returns true if the file is a non-binary, ie text file. You can be more specific:

$dir =shift;
$type='txt';

chdir $dir or die "Can't chdir to $dir:$!\n" if $dir;

while (<*.$type>) {
	print "Found a file: $_\n";
}

like so. But, there is a better way to read from directories. The method above is rather slow and inflexible.

readdir : How to read from directories

Instead, there is readdir . Another version of the previous example:

$dir= shift || '.';

opendir DIR, $dir or die "Can't open directory $dir: $!\n";

while ($file= readdir DIR) {
	print "Found a file: $file\n";
}

The first difference is the first line, which essentially says if shift is false, then $dir = ., which is of course the current directory. Then, the directory is opened and we have the chance to trap the error. It is assigned a filehandle. The readdir function reads each file into $file. There is no while (<WDIR>) { construct.

We can also apply the text file test. Run this, once without entering a directory and the second time with entering a directory path other than the one the script is in:

$dir= shift || '.';

opendir DIR, $dir or die "Can't open directory $dir: $!\n";

while ($file= readdir DIR) {
	print "Found a file: $file\n" if -T $file ;
}

Firstly, because the filename is now not in $_ we have to explicitly apply the -T test to it with -T $file.

Why did this not work the second time? Look at the code carefully. You are testing $file. If perl doesn't get a fully qualified pathname, it assumes you are still in the directory the script was run from, or that of the last successful chdir . Not necessarily where you are readdir'ing from. So, to fix it:

        print "Found a file: $dir/$file\n" if -T "$dir/$file" ;

where we now specify the pathname, both in the printout and in the file test itself. The "" are used because otherwise perl tries to divide $file by $dir.

Try running this on a directory with only a few files in it:

$dir= shift || '.';

opendir DIR, $dir or die "Can't open directory $dir: $!\n";

while ($file= readdir DIR) {
	print "Found a file: '$file'\n";
}

Notice that two files are found which have interesting names, namely . and .. . These two files are the current, and lower directory respectively. Nothing new, they have always been there -- run the DOS command dir if you don't believe me. You don't usually want to know about them, so:

while ($file= readdir DIR) {
	next if $file=~/^\./;
	print "Found a file: '$file'\n";
}

is the usual workaround. You can use scalar context to dump everything to a list of some description:

$dir= shift || '.';

opendir DIR, $dir or die "Can't open directory $dir: $!\n";

@files=readdir(DIR);

print "@files";

but that includes the . files, so it is best to ensure they aren't included:

@files=grep !/^\./, readdir(DIR);

We haven't met -T yet, but for the moment just remember it searches a list and if it returns true, lets the variable pass. In this case, if it doesn't begin with . then that's true so it goes into @files.

There are other commands associated with reading directories, which tell you where in a directory you are, and then where to go to return. You should be aware of their existence, because you never know when you might need them. The one other command of use is closedir , which closes a directory. Optional, but recommended for clarity.

Associative Arrays


The Basics

Very, very useful. First, a quick recap on arrays. Arrays are an ordered list of scalar variables, which you access by their index number starting at 0. The elements in arrays always stay in the same order.

Hashes are a list of scalars, but instead of being accessed by index number, they are accessed by a key. The tables below illustrate the point:

@myarray
Index No. Value
0 The Netherlands
1 Belgium
2 Germany
3 Monaco
4 Spain
%myhash
Key Value
NL The Netherlands
BE Belgium
DE Germany
MC Monaco
ES Spain

So if we want 'Belgium' from @myarray and also from %myhash , it'll be:

print "$myarray[1]";
print "$myhash{'BE'}";

Notice that the $ prefix is used, because it is a scalar variable. Despite the fact it is part of a list, it is still a scalar variable. The hash syntax is simply to use braces { } instead of square brackets.

So why use hashes ? When you want to look something up by a keyword. Suppose we wanted to create a program which returns the name of the country when given a country code. We'd input ES, and the program would come back with Spain.

You could do it with arrays. It would be messy however. One possible approach:

  1. create @country , and give it values such as 'ES,Spain'
  2. Itierate over the entire array and
  3. split each element of the array, and check the first result to see if it matches the input
  4. If so, return the index
@countries=('NL,The Netherlands','BE,Belgium','DE,Germany','MC,Monaco','ES,Spain');

print "Enter the country code:";
chop ($find=<STDIN>);

foreach (@countries) {
        ($code,$name)=split /,/;
        if ($find=~/$code/i) {
                print "$name has the code $code\n";
        }
}

Complex and slow. We could also store a reference to another array in each element of @countries , but that is not efficient. Whatever way we choose, you still need to search the whole thing. And what if @countries is a big array ? See how much easier a hash is:

A Hash in Action

%countries=('NL','The Netherlands','BE','Belgium','DE','Germany','MC','Monaco','ES','Spain');

print "Enter the country code:";
chop ($find=<STDIN>);

$find=~tr/a-z/A-Z/;
print "$countries{$find} has the code $find\n";

Very easy. All we need to do is make sure everything is in uppercase with tr and we are there. Notice the way %countries is defined - exactly the same as a normal array, except that the values are put into the hash in key/value pairs.

When you should use hashes

So why use arrays ? One excellent reason is because when an array is created, its variables stay in the same order you created them in. With a hash, perl reorders elements for quick access. Add print %countries; to the end of that program above and run it. See what I mean ? No recognisable sequence at all. It's like trying to herd cats. If you were writing code that stored a list of variables over time and you wanted it back in the order you found it in, don't use a hash.

Finally, you should know that each key of a hash must be unique. Stands to reason, if you think about it. You are accessing the hash via keys, so how can you have two keys named 'NL' or something ? If you do define a certain key twice, the second value overwrites the first. This is a feature, and useful. The values of a hash can be duplicates, but never the keys.

If you want to assign to a hash, there is of course no concept of push , pop and splice etc. Instead:

Hash Hacking Functions

Assigning $countries{PT}='Portugal';
Deleting delete $countries{NL};

Accessing Your Hash

Assuming you keep the same %countries hash as above, here are some useful ways to access it:

All the keys print keys %countries;
All the values print values %countries;
A Slice of Hash :-) print @countries{'NL','BE'};
How many elements ? print scalar(keys %countries);
Does the key exist ? print "It's there !\n" if exists $countries{'NL'};

Well, that last one is not an access as a such but useful anyway.


More Hash Access: Iteration, keys and values

You may have noticed that keys and values return a list. And we can iterate over a list, using foreach :

foreach (keys %countries) {
        print "The key $_ contains $countries{$_}\n";
}

which is useful. Note how any list can be fed to foreach , and off it goes. As usual, there is another way to do the above:

while (($code,$name)=each %countries) {
        print "The key $code contains $name\n";
}
The each function returns each key/value pair of the hash, and is slightly faster. In this example we assign them to a list (you spotted the parens ?) and away we go. Eventually there are no more pairs, which returns false to the while loop and it stops.

If you are into brevity, both the above can be accomplished in a single line:

print "The key $code contains $name\n" while ($code,$name)=each %countries;

print "The key $_ contains $countries{$_}\n" foreach keys %countries;
   

Note -- this won't win any prizes for easily readable code by non-programmers of Perl.


Sorting


A Simple Sort

If I was reading this I'd be wondering about sorting. Wonder no more, and behold:

foreach (sort keys %countries) {
        print "The key $_ contains $countries{$_}\n";
}

Spot the difference. Yes, sort crept in there. If you want the list sorted backwards, some cunning is called for. This is suitably foxy:

foreach (reverse sort keys %countries) {
        print "The key $_ contains $countries{$_}\n";
}

Perl is just so difficult at times, don't you think ? This works because:

This is a quick example to make sure the meaning of reverse is clear:

print "Enter string to be reversed: ";
$input=<STDIN>;

@letters=split //,$input;	# splits on the 'nothings' in between each character of $input

print join ":", @letters;	# joins all elements of @letters with \n, prints it
print reverse   @letters;	# prints all of @letters, but sdrawkcab )-:

Perl's list operators can just feed directly to each other, saving many lines of code but also decreasing readability to those that aren't Perl-literate:

print "Enter string to be reversed: ";
print join ":",reverse split //,$_=<STDIN>;

This section is about sorting, so enough of reverse . Time to go forwards instead.

Numeric Sorting -- How Sort Really Works

That's easy alphabetical sorting by the keys. If you had a hash of international access numbers like this one:

%countries=('976','Mongolia','52','Mexico','212','Morocco','64','New Zealand','33','France');

foreach (sort keys %countries) {
        print "The key $_ contains $countries{$_}\n";
}

You might want to sort numerically. In that case, you need to understand how Perl's sort function works.

The sort function compares two variables, $a and $b . They must be called $a and $b otherwise it won't work. One chap published a book with stolen code, and he changed $a and $b to $x and $y. He obviously didn't test the program because it would have failed and he would have noticed. And this book was really published ! Don't believe everything you read in books -- but web tutorials are always 100% truthful :-)

Back to sorting. $a and $b are compared, and the result is:

So as long as the sort function gets one of those three values back it is happy. This means we can write our own sort routines, and feed them to sort. For example, we know the default sort is alphabetical. But if we write this:

%countries=('976','Mongolia','52','Mexico','212','Morocco','64','New Zealand','33','France');

foreach (sort supersort keys %countries) {
        print "$_ $countries{$_}\n";
}

sub supersort {
        if ($a > $b) {
                return 1;
        } elsif ($a < $b) { 
		return -1;
	} else { 
		return 0; 
	}
}

then it works correctly. Of course, there is an easier way. The 'spaceship' operator <=> . It does exactly what the supersort subroutine does, namely return 1, -1 or 0 depending on the comparison of two given values.

So we can write the above much more easily as:

%countries=('976','Mongolia','52','Mexico','212','Morocco','64','New Zealand','33','France');

foreach (sort { $a <=> $b } keys %countries) {
        print "$_ $countries{$_}\n";
}

Notice the { } braces, which define the contents as the subroutine sort must use. Pretty short subroutine. There is a companion operator to <=> , namely cmp which does exactly the same thing but of course compares the values as strings, not numbers.Remember, if you are comparing numbers, your comparison operator should contain non-alphas, if you are comparing strings the operator should contains alphas only. And don't talk to strangers.

Anyway, you now have enough knowledge to sort a hash by value instead of keys. Suppose your pointy haired manager bounced up to you and demanded a hash sorted by value ? What would you do ? OK, what should you do ?

Well, we could just sort the values.

foreach (sort values %countries) {

But Pointy Hair wants the keys too. And if you have a value you can't find the key.

So we have to iterate over the keys. But just because we are iterating over the keys doesn't mean to say we have to hand the keys over to sort . What about:

%countries=('976','Mongolia','52','Mexico','212','Morocco','64','New Zealand','33','France');

foreach (sort { $countries{$a} cmp $countries{$b} } keys %countries) {
        print "$_ $countries{$_}\n";
}

beautifully simple. If you want a reverse sort transpose $a and $b .


Sorting Multiple Lists

You can sort several lists at the same time:

%countries=('976','Mongolia','52','Mexico','212','Morocco','64','New Zealand','33','France');
@nations=qw(China Hungary Japan Canada Fiji);

@sorted= sort values %countries, @nations;

foreach (@nations, values %countries) {
        print "$_\n";
}

print "#----\n";

foreach (@sorted) {
        print "$_\n";
}

This sorts @nations and the values from %countries into a new array.

The example also demonstrates that you can foreach over more than one list value -- each list is processed in turn. How I discovered that particular trick with Perl is instructive. I just tried it. If you think you should be able to do something with Perl, try it. Adhere to the syntax and conventions you will be familiar with from experience, in this case delimiting a list with commas, and try it. I'm always finding new shortcuts just by experimentation.


Grep and Map


Grep

If you want to search a list, and create another list of things you found, grep is one solution. This is an example, which also demonstrates join again :

@stuff=qw(flying gliding skiing dancing parties racing);	# quote-worded list

@new = grep /ing/, @stuff;	# Creates @new, which contains elements of @stuff 
				# matching with 'ing' in them.

print join ":",@stuff,"\n";	# first makes one string out of the elements of @stuff, joined
				# with ':' , then prints it, then prints \n

print join ":",@new,"\n";

Remember qw means 'quote words', so word boundaries are used as delimiters instead. The grep function must be fed a list on the right hand side. On the left side, you may assign the results to a list or a scalar variable. Assigning to a list gives you each actual element, and to a scalar gives you the number of matches found:

@stuff=qw(flying gliding skiing dancing parties racing);

$new = grep /ing/, @stuff;

print join ":",@stuff,"\n";

print "Found $new elements of \@stuff which matched\n";

If you decide to modify the elements on their way through grep , you actually modify the original list. Be careful out there.

@stuff=qw(flying gliding skiing dancing parties racing);

@new = grep s/ing//, @stuff;

print join ":",@stuff,"\n";
print join ":",@new,"\n";

To determine what actually matches you can either use an expression or a block. Up to now we've been using expressions, but when things become more complicated use a block:

@stuff=qw(flying gliding skiing dancing parties racing);

@new = grep { s/ing// if /^[gsp]/ } @stuff;

print join ":",@stuff,"\n";
print join ":",@new,"\n";

Try removing the braces and you'll get an error. Notice that the comma before the list has gone. It is now obvious where the expression ends, as it is inside a block delimited with { } . The regex says if the element begins with g, s or p, then remove ing. The result is only assigned to @new if the expression is completely true - 'parties' does begin with p, so that works, but s/ing// fails so the overall result is false, and the value is not assigned to @new .

Map

Map works the same way as grep , in that they both iterate over a list, and return a list. There are two important differences however:

As usual, an example will assist the penny in dropping, clear the fog and turn on the light (if not make my metaphors easier to understand):

@stuff=qw(flying gliding skiing dancing parties racing);

print "There are ",scalar(@stuff)," elements in \@stuff\n";
print join ":",@stuff,"\n";

@mapped  = map  /ing/, @stuff;
@grepped = grep /ing/, @stuff;

print "There are ",scalar(@stuff)," elements in \@stuff\n";
print join ":",@stuff,"\n";

print "There are ",scalar(@mapped)," elements in \@mapped\n";
print join ":",@mapped,"\n";

print "There are ",scalar(@grepped)," elements in \@grepped\n";
print join ":",@grepped,"\n";

You can see that @mapped is just a list of 1's. Notice that there are 5 ones whereas there are six elements in the original array, @stuff. This is because @mapped contains the true results of map -- in every case the expression /ing/ is successful, except for 'parties'.

In that case there the expression is false, so the result is discarded. Contrast this action with the grep function, which returns the actual value, but only if it is true. Try this:

@letters=(a,b,c,d,e);

@ords=map ord, @letters;
print join ":",@ords,"\n";

@chrs=map chr, @ords;   
print join ":",@chrs,"\n";

This uses the ord function to change each letter into its ASCII equivalent, then the chr function convert ASCII numbers to characters. If you change map to grep in the example above, you can see that nothing appears to happen. What is happening is that grep is trying the expression on each element, and if it succeeds (is true) it returns the element, not the result. The expression succeeds for each element, so each element is returned in turn. Another example:

@stuff=qw(flying gliding skiing dancing parties racing);

print join ":",@stuff,"\n";

@mapped  = map  { s/(^[gsp])/$1 x 2/e } @stuff;
@grepped = grep { s/(^[gsp])/$1 x 2/e } @stuff;

print join ":",@stuff,"\n";
print join ":",@mapped,"\n";
print join ":",@grepped,"\n";

Recapping on regex, what that does is match any element beginning with g, s or p, and replace it with the same element twice. The caret ^ forces a match at the beginning of the string, the [square brackets] denote a character class, and /e forces Perl to evaluate the RHS as an expression.

The output from this is a mixture of 1 and nothing for map , and a three-element array called @grepped from grep. Yet another example:

@mapped  = map  { chop } @stuff;
@grepped = grep { chop } @stuff;

The chop function removes the last character from a string, and returns it. So that's what you get back from ^ , the result of the expression. The grep function gives you the mangled remains of the original value.

Writing your own grep and map functions

Finally, you can write your own functions:

@stuff=qw(flying gliding skiing dancing parties racing);

print join ":",@stuff,"\n";

@mapped  = map  { &isit } @stuff;
@grepped = grep { &isit } @stuff;

print join ":",@mapped,"\n";
print join ":",@grepped,"\n";

sub isit {
        ($word)=/(^.*)ing/;

        if (length $word == 3) {
                return "ok";
        } else {
                return 0;
        }
}

The subroutine isit first grabs everything up until 'ing', puts it into $word , then returns 'ok' if the there are three characters in $word . If not, it returns the false value 0. You can make these subroutines (think of them as functions) as complex as you like.

Sometimes it is very useful to have map return the actual value, rather than the result. The answer is easy, but not obvious. Remember that subroutines return the value of the last expression evaluated? So, in this case, do blocks. What if the expression was, very simply:

@grepstuff=@mapstuff=qw(flying gliding skiing dancing parties racing);

print join " ",map  { s/(^[gsp])/$1 x 2/e } @mapstuff;
print "\n";
print join " ",grep { s/(^[gsp])/$1 x 2/e } @grepstuff;

Now, make sure $_ is the last thing evaluated:

@grepstuff=@mapstuff=qw(flying gliding skiing dancing parties racing);

print join " ",map  { s/(^[gsp])/$1 x 2/e;$_} @mapstuff;
print "\n";
print join " ",grep { s/(^[gsp])/$1 x 2/e } @grepstuff;

and there you have it. Now you understand that you can go and impress your friends, but please don't count on success.

External Commands


Some ways to...

Perl can start external commands. There are five main ways to do this:

We'll compare system and exec first.

Exec

Poor old exec is broken on Perl for Win32. What it should do is stop running your Perl script and start running whatever you tell it to. If it can't start the external process, it should return with an error code. This doesn't work properly under Perl for Win32. The exec function does work properly on the standard Perl distribution.


System

This runs an external command for you, then carries on with the script. It always returns, and the value it returns goes into $? . This means you can test to see if the program worked. Actually you are testing to see if it could be started, what the program does when it runs is outside your control if you use system .

This example demonstrates system in action. Run the 'vol' command from a command prompt first if you are not familiar with it. Then run the 'vole' command. I'm assuming you have no cute furry executables called vole on your system, or at least in the path. If you do have an executable called 'vole', be creative and change it.

system("vole");

print "\n\nResult: $?\n\n";

system("vol");

print "\n\nResult: $?\n\n";

As you can see, a successful system call returns 0. An unsuccessful one returns a value which you need to divide by 256 to get the real return value. Also notice you can see the output. And because system returns, the code after the first system call is executed. Not so with exec, which will terminate your perl script if it is successful. Perl's usual use of single and double quotes applies as per variable interpolation.

Backticks

These `` are different again to system and exec. They also start external processes, but return the output of the process. You can then do whatever you like with the output. If you aren't sure where backticks are on your keyboard, try the top left, just left of the 1 key. Often around there. Don't confuse single quotes '' with backticks `` .

$volume=`vol`;

print "The contents of the variable \$volume are:\n\n";

print $volume;

print "\nWe shall regexise this variable thus :\n\n";

$volume=~m#Volume in drive \w is (.*)#;

print "$1\n";

As you can see here, the Win32 vol command is executed. We just print it out, escaping the $ in the variable name. Then a simple regex, using # as a delimiter just in case you'd forgotten delimiters don't have to be / .

When to use external calls

Before you get carried away with creating elaborate scripts based on the output from NT's net commands, note there are plenty of excellent modules out there which do a very good job of this sort of thing, and that any form of external process call slows your script. Also note there are plenty of built in functions such as readdir which can be used instead of `dir` . You should use Perl functions where possible rather than calling external programs because Perl's functions are:

Don't start using backticks all over the place when system will do. You might get a very large return value which you don't need, and will consequently slurp lots of memory. Just use them when you actually want to check the returned strings.

Opening a Process

The problem with backticks is that you have to wait for the entire process to complete, then analyse the entire return code. This is a big problem if you have large return codes or slow processes. For example, the DOS command tree. If you aren't familiar with this command, run a DOS/command prompt, switch to the root directory (C:\ ) and type tree. Examine the wondrous output.

We can open a process, and pipe data in via a filehandle in exactly the same way you would read a file. The code below is exactly the same as opening a filehandle on a file, with two exceptions:

  1. We use an external command, not a filename. That's the process name, in this case, tree.
  2. A pipe, ie | is appended to the process name.
open TRIN, "tree c:\\ /a |" or die "Can't see the tree :$!";

while (<TRIN>) {
	print "$. $_";
}

Note the | which denotes that data is to be piped from the specified process. You can also pipe data to a process by using | as the first character.

As usual, $. is the line number. What we can do now is terminate our tree early. Environmentally unsound, but efficient.

open TRIN, "tree c:\\ /a |" or die "Can't see the tree :$!";

while (<TRIN>) {
	printf "%3s $_", $.;
	last if $. == 10;
}

As soon as $. hits 10 we shut the process off by exiting the loop. Easy.

Except, maybe it won't. What if this was a long program, and you forgot about that particular line of code which exits the loop? Suppose that $. somehow went from 9 to 11, or was assigned to? It would never reach 10. So, to be safe

open TRIN, "tree c:\\ /a |" or die "Can't see the tree :$!";

while (<TRIN>) {
	printf "%3s $_", $.;
	last if $. >= 10;
}

exit your loops in a paranoid manner, unless you really mean only to exit when at line ten. For maximum safety, maybe you should create your own counter variable because $. is a global variable. I'm not necessarily advocating doing any of the above, but I am suggested these things are considered.

You might notice the presence of a new keyword - printf . It works like print , but formats the string before printing. The formatting is controlled by such parameters as %3s , which means "pad out to a total of three spaces". After the doublequoted string comes whatever you want to be printed in the format specified. Some examples follow. Just uncomment each line in turn to see what it does. There is a lot of new stuff below, but try and work out what is happening. An explanation follows after the code.

$windir=$ENV{'WINDIR'};		# yes, you can access the environment variables !

$x=0;

opendir WDIR, "$windir" or die "Can't open $windir !!! Panic : $!";

while ($file= readdir WDIR) {
	next if $file=~/^\./;		# try commenting this line to see why it is there

	$age= -M "$windir/$file";	# -M returns the age in days
	$age=~s/(\d*\.\d{3}).*/$1/;	# hmmmmm

	#### %4.4d - must take up 4 columns, and pad with 0s to make up space
	####         and minimum width is also 4
	#### %10s  - must take up 10 columns, pad with spaces
	# printf "%4.4d %10s %45s \n", $x, $age, $file;

	#### %-10s - left justify
	# printf "%4.4d %-10s %-45s \n", $x, $age, $file;

	####  %10.3 - use 10 columns, pad with 0s if less than 3 columns used
	# printf "%4.4d %10.3d %45s \n", $x, $age, $file;

	$x++;

	last if $x==15;			# we don't want to go through all the files :-)
}

There are some intentionally new functions there. When you start hacking Perl (actually, you already started if you have worked through this far) you'll see a lot of example code. Try and understand the above, then read the explanation below.

Firstly, all environment variables can be accessed and set via Perl. They are in the %ENV hash. If you aren't sure what environment variables are, refer to your friendly Microsoft documentation or books. The best known environment variable is path, and you can see its value and that of all other environment variables by simply typing set at your command prompt.

The regex /^\./ bounces out invalid entries before we bother do any processing on them. Good programming practice. What it matches is "anything that begins with '.'". The caret anchors the match to the beginning of the string, and as . is a metacharacter it has to be escaped.

Perl has several tests to apply on files. The -M test returns the age in days. See the documentation for similar tests. Note that the calls to readdir return just the file, not the complete pathname. As you were careful to use a variable for the directory to be opened rather than hardcoding it (horrors) it is no trouble to glue it together by using doublequotes.

Try commenting out $age=~s/(\d*\.\d{3}).*/$1/ and note the size of $age . It could do with a trim. Just for regex practice, we make it a little smaller. What the regex does is:

Easy !

Mention should also be made of sprintf , which is exactly like printf except it doesn't print. You just use it to format strings, which you can do something with later. For example :

open TRIN, "tree c:\\ /a |" or die "Can't see the tree :$!";

while (<TRIN>) {
	$line= sprintf "%3s $_", $.;
	print $line;
	last if $. == 10;
}

   

Quote execute

@opts=qw(w on ad oe b);

for (@opts) {
	$result=qx(dir /$_);
	print "dir /$_ resulted in:\n$result",'-' x 79;
	sleep 1;
}

Anything within qx( ) is executed, and duly variable interpolated. This sample also demonstrated qw which is 'quote words', so the elements of @opts are delimited by word boundaries, not the usual commas. You can also use for instead of foreach if you want to save typing four character for the sake of legibility.

You may have noticed that system outputs the result of the command to the screen whereas qx does not. Each to its own.



Oneliners


A short example

You'll have noticed Perl packs a lot of power into a small amount of code. You can feed Perl code directly on the command line. This is known as a oneliner, for obvious reasons. An example:

perl -e"for (55..75) { print chr($_) }"

The -e switch tells Perl that a command is following. The command must be enclosed in doublequotes, not singles as on Unix. The command itself in this case simply prints the ASCII code for the number 55 to 75 inclusive.


File access

This is a simple find routine. As it uses a regex, it is infinitely superior to NT's findstr :

perl -e"while (<>) {print if /^[bv]/i}" shop.txt

Remember, the while (<>) construct will open whatever is in @ARGV . In this case, we have supplied shop.txt so it is opened and we print lines that begin with either 'b' or 'v'.

That can be made shorter. Run perl -h and you'll see a whole list of switches. The one we'll use now is -n , which puts a while (<>) {     } loop around whatever code you supply with -e . So:

perl -ne"print if /^[bv]/i" shop.txt

which does exactly the same as the previous program, but uses the -n switch to put a while (<>) loop around whatever other commands are supplied.

A slightly more sophisticated version:

perl -ne"printf \"$ARGV : %3s : $_\",$. if /^[bv]/i" shop.txt

which demonstrates that doublequotes must be escaped.


Modifying files with a oneliner and $^I

If you don't remember $^I then please review the section on Files before proceeding. When you're ready, copy shop.txt to shop2.txt .

perl -i.bk -ne"printf \"%4s : $_\",$." shop2.txt

The -i switch primes the inplace edit operator. We still need -n .

If you had a typical quoted email message such as:

>> this is what was said
>> blah blah
> blaaaaahhh

The new text

and you wanted to remove the >, then:

perl -i.bk -pe"s/^>+ ?//" email.txt

does the trick. Regex recap -- the caret matches what follows to the beginning of the string, the + means one or more (no, we do not use * which means 0 or more), then we will match one space with \s , but it is not necessary for the space to be there for the match to be successful, hence ? .

What is new in terms of oneliners is the use of -p , which does exactly the same thing as -n except that it adds a print statement too. In case you were wondering why the previous example used -n and this one uses -p -- the previous example uses prints data with printf, whereas this example doesn't have an explicit print statement so we provide one with -p .

Some other useful oneliners -- a calculator and a ASCII number lookup:

perl -e"print 50/200+2"
perl -e"for (50..90) { print chr($_) }"

There are plenty more oneliners, and they are an essential part of any sysadmin's toolbox. The two examples below are functionally equivalent but the lower one is perhaps a little more readable:

perl -e"for $i (50..90) { print chr($i),\" is $i\n\" }"

perl -e"for $i (50..90) { print chr($i),qq| is $i\n| }

Whatever follows qq is used as a delimiter, instead of having to escape the backslash. I learnt this from the Perl-Win32-Users mailing list (see top) - I think it was Lennart Borgman who pointed it out. He also mentioned that you don't need the closing doublequote. Saves a little typing.


Subroutines and Parameters

In Perl, subroutines are functions are subroutines. If you like, a subroutine is a user defined function. It's a bit like calling a script a program, or a program a script. For the purposes of this tutorial we'll refer to functions as subroutines, except when we call them functions. Hope that's made the point.

For the purposes of this section we will develop a small program which, by the end, will demonstrate how subroutines work. It also serves to demonstrate how many programs are built, namely a little at a time, in manageable sections. At least, that method works for me. engines.

The chosen theme is gliding. That's aeroplanes without engines. A subject close to every glider pilot's heart is how far they can fly from the altitude they are at. Our program will calculate this. To make it easy we'll assume the air is perfectly calm. Wind would be a complication we don't need, especially when in a crowded lift.

What we need in order to calculate the distance we can fly is:

Obviously input is needed. We can either prompt the user or grab the input from the command line. The latter is easier so we'll just look at @ARGV for the command line parameters. Like so:

($height,$angle)=@ARGV;		# @ARGV is the command line parameters

$distance=$height*$angle;	# an easy calculation

print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

The above should be executed thus:

perl yourscript.pl 5000 24

or whatever your script is called, with whatever parameters you choose to use. I'm a poet and I don't even know it.

That works. What about a slight variation? The pilot does have some control over the glide ratio, for example he can fly faster but at a penalty of a lesser glide ratio. So we should perhaps give a couple of options either side of the given parameters:

($height,$angle)=@ARGV;

$distance=$height*$angle;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

$angle++;			# add 1 to $angle
$distance=$height*$angle;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

$angle-=2;			# subtract 2 from $angle so it is 1 less than the original
$distance=$height*$angle;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

That's cumbersome code. We repeat exactly the same statement. This wastes space, and if we want to change it there are three changes to be made. A better option is to put it into a subroutine:

($height,$angle)=@ARGV;

&howfar;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

$angle++;
&howfar;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

$angle-=2;
&howfar;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

sub howfar {				# sub subroutinename
	$distance=$height*$angle;
}

This is a basic subroutine, and you could stop here and have learnt a very useful technique for programming. Now, when changes are made they are made in one place. Less work, less chances of errors. Improvements can always be made. For example, pilots outside Eastern Europe generally measure height in feet, and glider pilots are usually concerned with how many kilometres they travel over the ground. So we can adapt our program to accept height in feet and output the distance in kilometres:

($height,$angle)=@ARGV;

$height/=3.2;			# divide feet by 3.2 to get metres

&howfar;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

$angle++;
&howfar;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

$angle-=2;
&howfar;
print "With a glide ratio of $angle:1 you can fly $distance from $height\n";

sub howfar {
	$distance=$height*$angle;
}

When you run this you'll probably get a result which involves a fair few digits after the decimal point. This is messy, and we can fix this by the int function, which in Perl and most other languages returns a number as an integer, ie without those irritating numbers after the decimal point.

You might have also noticed a small bit of Bad Programming Practice slipped into the last example. It was the evil Constant, the '3.2' used to convert feet to metres. Why, I don't hear you ask, is this bad? Surely the conversion will never change?

It won't change, but our use of it might. We may decide that it should be 3.208 instead of 3.2. We may decide to convert from feet to nautical miles instead. You don't know what could happen. Therefore, code with flexibility in mind and that means avoiding constants.

The new improved version with int and constant removed:

($height,$ratio)=@ARGV;
$cnv1=3.2;			# now it is a variable.  Could easily be a cmd line 
				# parameter too.  We have the flexibility.
$height  =int($height/$cnv1);	# divide feet by 3.2 to get metres

&howfar;
print "With a glide ratio of $ratio:1 you can fly $distance from $height\n";

$ratio++;
&howfar;
print "With a glide ratio of $ratio:1 you can fly $distance from $height\n";

$ratio-=2;
&howfar;
print "With a glide ratio of $ratio:1 you can fly $distance from $height\n";

sub howfar {
	$distance=int($height*$ratio);
}

We could of course build the print statement into the subroutine, but I usually separate output presentation from the calculation. Again, that means it is easier to modify later on.

Something else we can improve about this code is the use of the $ratio variable. We are having to keep track of what we do to it -- first add one, then subtract two in order to subtract one from the original input. In this case it is fairly easy, but with a complex program it can be difficult, and you don't want to be creating lots of variables just to track one input, for example $ratio1 , $ratio2 etc.


Parameters

One solution is to pass the subroutine parameters. In the best tradition of American columnists, who seem to have a particular affection for this phrase, 'Here's how:'

($height,$ratio)=@ARGV;
$cnv1=3.2;			

&howfar($height,$ratio);
print "With a glide ratio of $ratio:1 you can fly $distance from $height\n";

&howfar($height,$ratio+1);
print "With a glide ratio of ",$ratio+1,":1 you can fly $distance from $height\n";

&howfar($height,$ratio-1);
print "With a glide ratio of ",$ratio-1,":1 you can fly $distance from $height\n";

sub howfar {
	print "The parameters passed to this subroutine are @_\n";
	($ht,$rt)=@_;
	$ht  =int($ht/$cnv1);
	$distance=int($ht*$rt);
}

Quite a few things have changed here. Firstly, the subroutine is being called with parameters. These are a comma-delimited list in parens after the subroutine call. The two parameters are $height and $ratio.

The parameters end up in the subroutine as the @_ array. Being an array, they are in the same order as passed. All the usual array operations work. All we will do is assign the contents of the array to two variables.

We have also moved the conversion function into the subroutine, because we want to put all the code for generating the distance into one place.


Namespaces

We cannot use the variable names $height and $ratio because we modify them in the subroutine and that will affect the main program. So we choose new ones to do the operation on. Finally, a small change is made to the print output.

This approach works well enough for our small program here. For larger programs, having to think of new variable names all the time is difficult. It would be even more difficult if different programmers were working on different sections of the program. It would be impossible if a program were written, then an extension created by another person somewhere else, and that same extension had to be used by many people in many different programs. Obviously, the risk of using the same variable name is too great. There are only so many logical names out there.

There is a solution. Imagine you own a house with two gardens. You have two identical dogs, one in the front garden, one in the back garden. Bear with me, this is relevant. Both dogs are called Rover, because their owner lacks imagination.

When you go to the front garden and call 'Rover!!!' or open a can of dog food, the dog in the front garden comes running. Similarly, you go to the back garden, call your dog and the other dog bounces up to you.

You have two dogs, both called Rover, and you can change either one of them. Wash one, neuter the other -- it doesn't matter, but both are dogs and both have the same name. Changes to one won't affect the other. You don't get them confused because they are in different places, in two different namespaces.


Variable Scope

To bring things back to Perl, a short diversion is necessary to illustrate the point with actual Perl code instead of canine metaphors:

$name='Rover';
$pet ='dog';
$age =3;

print "$name the $pet is aged $age\n";

{
	my $age =4;	  # run this again, but comment this line out
	my $name='Spot';  # and this one
	$pet    ='cat';

	print "$name the $pet is aged $age\n";
}

print "$name the $pet is aged $age\n";

This is pretty straightforward until we get to the { . This marks the start of a block. One feature of a block is that it can have its own namespace. Variables declared, in other words initialised, within that block are just normal variables, unless they are declared with my .

When variables are declared with my they are visible inside the block only. Also, any variable which has the same name outside the block is ignored. Points to note from the example above:


my Variables

So there we have it. Namespaces. They work for all the other types of variable too, like arrays and hashes. This is how you can write code and not care about what other people use for variable names -- you just declare everything with my and have your own private party. Our original program about gliding can be improved now:

($height,$ratio)=@ARGV;
$cnv1=3.2;			

&howfar($height,$ratio);
print "With a glide ratio of $ratio:1 you can fly $distance from $height\n";

&howfar($height,$ratio+1);
print "With a glide ratio of ",$ratio+1,":1 you can fly $distance from $height\n";

&howfar($height,$ratio-1);
print "With a glide ratio of ",$ratio-1,":1 you can fly $distance from $height\n";

sub howfar {
	my ($height,$ratio)=@_;
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio);
}

The only change is that the parameters to the subroutine, ie the contents of the array @_ , are declared with my . This means they are now only visible within that block. The block happens to also be a subroutine. Outside of the block, the original variables are still accessible. At this point I'll introduce the technical term, which is lexical scoping. That means the variable is confined to the block -- it is only visible within the block.

We still have to be concerned with what variables we use inside the subroutine. The variable $distance is created in the subroutine and used outside of it. With larger programs this will cause exactly the same problem as before -- you have to be careful that the subroutine variables you use are the same ones as outside the subroutine. For all the same reasons as before, like two different people working on the code and use of custom extensions to Perl, that can be difficult.

The obvious solution is to declare $distance with my , and thus lexically scope it. If we do this, then how do we get the result of the subroutine? Like so:

($height,$ratio)=@ARGV;
$cnv1=3.2;			

$distance=&howfar($height,$ratio);  # run this again and delete '$distance='
print "With a glide ratio of $ratio:1 you can fly $distance from $height\n";

$distance=&howfar($height,$ratio+1);
print "With a glide ratio of ",$ratio+1,":1 you can fly $distance from $height\n";

$distance=&howfar($height,$ratio-1);
print "With a glide ratio of ",$ratio-1,":1 you can fly $distance from $height\n";

sub howfar {
	my ($height,$ratio)=@_;
	my $distance;
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio/1000); 	# output result in kilometres not metres
}

First change -- $distance is declared with my . Secondly, the result of the subroutine is assigned to a variable, which is also named $distance. However, it is a $distance in a different namespace. Remember the two gardens. You may wish to delete the $distance= from the first assignment and re-run the code. The only other change is one to change the output from meters to kilometres.

We have now achieved a sort of Black Box effect, where the subroutine is given input and creates output. We pass the subroutine two numbers, which may or may not be variables. We assign the output of the subroutine to a variable. We care not what goes on inside the subroutine, what variables it uses or what magic it performs. This is how subroutines should operate. The only exception is the variable $cnv1. This is declared in the main body of the program but also used in the subroutine. This has been done in case we need to use the variable elsewhere. In larger programs it would be a good idea to pass it to subroutines along with the other parameters too.


Multiple Returns

That's all the major learning out the way with. The next step is relatively easy, but we need to add new functionality to the program in order to demonstrate it. What we will do is work out how long it will take the glider pilot to fly the distance. For this calculation, we need to know his airspeed. That can be a third parameter. The actual calculation will be part of howfar. An easy change:

($height,$ratio,$airspeed)=@ARGV;
$cnv1=3.2;			
$cnv2=1.8;

($distance,$time)=&howfar($height,$ratio,$airspeed);
print "Glide ratio $ratio:1, $distance from $height taking $time\n";

($distance,$time)=&howfar($height,$ratio+1,$airspeed);
print "Glide ratio ",$ratio+1,":1, $distance from $height taking $time\n";

($distance,$time)=&howfar($height,$ratio-1,$airspeed);
print "Glide ratio ",$ratio-1,":1, $distance from $height taking $time\n";

sub howfar {
	my ($height,$ratio,$airspeed)=@_;
	my ($distance,$time);			    # how to 'my' multiple variables
	$airspeed*=$cnv2;		 	    # convert knots to kmph
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio/1000);
	$time	 =int($distance/($airspeed/60));    # simple time conversion
	# print "Time:$time, Distance:$distance\n"; # uncomment this later
}

This doesn't work correctly. First, the changes. The result from howfar is now assigned to two variables. Subroutines return a list, and so assigning to some scalar variables between parens separated by commas will work. This is exactly the same as reading the command line arguments from @ARGV .

We are also passing a new parameter, $airspeed. There is a another conversion and a one-line calculation to provide the amount of minutes it will take to fly $distance.

If you look carefully, you can perhaps work out what the problem is. There was a clue in the Regex section, when /e was explained.

The problem is that Perl returns the result of the last expression evaluated. In this case, the last expression is the one calculating $time, so the value $time is returned, and it is the only value returned. Therefore, the value of $time is assigned to $distance, and $distance itself doesn't actually get a value at all.

Re-run the program but this time uncomment the line in the subroutine which prints $distance and $time. You'll noticed the value is 1, which means that the expression was successful. Perl is faithfully returning the value of the last expression evaluated.

This is all well and good, but not what we need. What is required is a method of telling Perl what needs to be returned, rather than what Perl thinks would be a good idea:

($height,$ratio,$airspeed)=@ARGV;
$cnv1=3.2;			
$cnv2=1.8;

($distance,$time)=&howfar($height,$ratio,$airspeed);
print "Glide ratio $ratio:1, $distance from $height taking $time\n";

($distance,$time)=&howfar($height,$ratio+1,$airspeed);
print "Glide ratio ",$ratio+1,":1, $distance from $height taking $time\n";

($distance,$time)=&howfar($height,$ratio-1,$airspeed);
print "Glide ratio ",$ratio-1,":1, $distance from $height taking $time\n";

sub howfar {
	my ($height,$ratio,$airspeed)=@_;
	my ($distance,$time);			 # how lexically scope multiple variables
	$airspeed*=$cnv2;			 # convert knots to kmph
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio/1000); 	 # output result in kilometres not metres
	$time	 =int($distance/($airspeed/60)); # simple time conversion
	return ($distance,$time);		 # explicit return
}

A simple fix. Now, we tell Perl what to return, with the aptly named return function. With this function we have complete control over what is returned and when. It is quite usual to use if statements to control different return values, but we won't bother with that here.

There is a subtle flaw in the program above. It is not backwards compatible with the old method of calling the subroutine. Run this:

($height,$ratio,$airspeed)=@ARGV;
$cnv1=3.2;			
$cnv2=1.8;

($distance,$time)=&howfar($height,$ratio,$airspeed);
print "Glide ratio $ratio:1, $distance from $height taking $time\n";

($distance,$time)=&howfar($height,$ratio+1,$airspeed);
print "Glide ratio ",$ratio+1,":1, $distance from $height taking $time\n";

$distance=&howfar($height,$ratio-1);	# old way of calling it
print "With a glide ratio of ",$ratio-1,":1 you can fly $distance from $height\n";

sub howfar {
	my ($height,$ratio,$airspeed)=@_;
	my ($distance,$time);			 
	$airspeed*=$cnv2;			 
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio/1000); 	 
	$time	 =int($distance/($airspeed/60)); 
	return ($distance,$time);
}

A division by 0 results third time around. This is of course because $airspeed doesn't exist, so of course it will effectively be 0. Making your subroutines backwards compatible is important in large programs, or if you are writing an add-in module for other people to use. You can't expect everyone to retrofit additional parameters to their subroutine calls just because you decided to be a bit creative one day.

There are many ways to fix the problem, and this is just one:

($height,$ratio,$airspeed)=@ARGV;
$cnv1=3.2;			
$cnv2=1.8;

($distance,$time)=&howfar($height,$ratio,$airspeed);
print "Glide ratio $ratio:1, $distance from $height taking $time\n";

($distance,$time)=&howfar($height,$ratio+1,$airspeed);
print "Glide ratio ",$ratio+1,":1, $distance from $height taking $time\n";

$distance=&howfar($height,$ratio-1);
print "With a glide ratio of ",$ratio-1,":1 you can fly $distance from $height\n";

print "Direct print: ",join ",",&howfar(5000,55,60)," not bad for no engine!\n";

sub howfar {
	my ($height,$ratio,$airspeed)=@_;
	my ($distance,$time);			 # how to 'my' multiple variables
	$airspeed*=$cnv2;			 # convert knots to kmph
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio/1000); 	 # output result in kilometres not metres
	if ($airspeed > 0) {
		$time	 =int($distance/($airspeed/60));
		return ($distance,$time);
	} else {
		return $distance;
	}
}

Here we just test the $airspeed to ensure we won't be doing any divisions by 0. It also affects what we return. There is also a new print statement, which shows that you don't need to assign to intermediate variables, or even pass variables as parameters. Constants, evil things that they are, work just as well. I already mentioned this, but a demonstration doesn't hurt. Unless you work for an electric chair manufacturer.

The astute reader.....:-) Every time I read that I wonder what I've missed. Usually something obscure which the author knows nobody will ever notice, but likes to belittle the reader. No exception here! Anyway, you may be wondering why this would not have sufficed instead of the if statement:

sub howfar {
	my ($height,$ratio,$airspeed)=@_;
	my ($distance,$time);			 # how to 'my' multiple variables
	$airspeed*=$cnv2;			 # convert knots to kmph
	$height  =int($height/$cnv1);
	$distance=int($height*$ratio/1000); 	 # output result in kilometres not metres
	$time	 =int($distance/($airspeed/60)) if $airspeed > 0;
	return ($distance,$time);
}

After all, the first item returned is $distance, so therefore it should be the first one assigned via:

$distance=&howfar($height,$ratio-1);

and $time should just disappear into the bit bucket.

The answer lies with scalars and lists. We are returning a list, but assigning it to a scalar. What happens when you do that? The scalar takes on the last value of the list. The last value of the list being returned is of course $time, which is has been declared but not otherwise touched. Therefore, it is nothing and appears as such on the printed statement. A small program to demonstrate that point:

$word=&wordfunc("Greetings");
print "The word is $word\n";

(@words)=&wordfunc("Bonjour");
print "The words are @words\n";

sub wordfunc {
my $word=shift;		# when in a subroutine, shifts @_ if no target specified
	my @words;				# how to my an array
	@words=split //,$word;			# splits on the nothings between each letter
	($first,$last)=($words[0],$words[$#words]);  # see section on Arrays if required
	return ($first,$last);			# Returns just the first and last
}

As you can see, the first call prints the letter 's', which is the last element of the list that is returned. You could of course use a list consisting of just one element:

($word)=&wordfunc("Greetings");

Now we are assigning a list to a list, so perl starts at the first element and keeps assigning till it runs out of elements. The parens turns a lonely scalar into an element of a list. You might consider always assigning the results of subroutines this way, as you never know when the subroutine might change. I know I've just evangelised about how subroutines shouldn't change, but if you take care and the subroutine write takes care, there definitely won't be any problems!

That's about it for good old my . There is a lot more to learn about it but that's enough to get started. You now know about a little about variable visibility, and I don't mean changeable weather.


Local

There is one more function that I'd like to draw to your attention, and we'll launch straight into the demonstration:

@words=@ARGV;

print "Output Field Separator is :$,:\n";
print '1. Words:', @words, "\n";

&change;

$,='_';

print "\nOutput Field Separator is :$,:\n";
print '2. Words:', @words, "\n";

&change;

sub change {
	print '   Words:', @words, "\n";
}

which should be executed something like this:

perl test.pl sarcasm is the lowest form of wit

The special variable $, defines what Perl should print in between lists it is given. By default, it is nothing. So the first two prints should have no spaces between the words. Then we assign '_' to $, so the next prints have underscores between the words.

If we want to use a different value for $, in the change subroutine, and not disturb the main value, we have a little problem. This problem cannot be solved by my because global variables like $, cannot at this time be lexically scoped. So, we could manually do it:

@words=@ARGV;

print "Output Field Separator is :$,:\n";
print '1. Words:', @words, "\n";

&change;

$,="_";

print "\nOutput Field Separator is :$,:\n";
print '2. Words:', @words, "\n";

&change;

sub change {
	$save=$,;
	$,='*';
	print '   Words:', @words, "\n";
	$,=$save;
}

That works, but it is messy. Perl has a special function for occasions of this nature, called local . An example of local in action:

@words=@ARGV;

print "Output Field Separator is :$,:\n";
print '1. Words:', @words, "\n";

&change;

$,="_";

print "\nOutput Field Separator is :$,:\n";
print '2. Words:', @words, "\n";

&change;

sub change {
        local $,="!-!";
	print '   Words:', @words, "\n";
}

You can try it with my instead but it won't work. I'm sure you'll try it anyway, I know you learn things the hard way otherwise you a) wouldn't be programming computers and b) wouldn't be using this tutorial to do it.

The local function works in a similar way to my , but assigns temporary values to global variables. The my function creates new variables that have the same name. The distinction is important, but the reasons require perl proficiency beyond the scope of this humble tutorial. In practice, the difference is:


Returning arrays

So that's the end of subroutines and parameters. Would you believe I have only scratched the surface? There are closures, prototypes, autoloading and references to learn. Not, however, in this tutorial. At least not yet. I'll finish with one last demonstration. You may have noticed that Perl returns one long list from subroutines. This is fine, but suppose you want two separate lists, for example two arrays? This is one way to do it:

($w1,$w2)=&wordfunc("Hello World");	# Assign the array references to scalars

print "@$w1 and @$w2\n";		# deference, ie access, the arrays referred to
#print "$w1 and $w2\n";			# uncomment this next time round

sub wordfunc {
        my $phrase=shift;
	my (@words,@word1,@word2);	# declare three variables lexically
	@words=split /\s+/,$phrase;	# split the phrase on whitespace
	@word1=split //,$words[0];	# create array of letters from the first word
	@word2=split //,$words[1];	# and the second
	return (\@word1,\@word2);	# return references to the two arrays -- scalars
}

There is a lot going on there. It should be clear up until the return statement. As we know, Perl only returns a single list. So, we make Perl return a list of the arrays it has just created. Not the actual arrays themselves, but references to the arrays. A bit like a shopping list is a just a bit of paper, not the actual goods itself. The reference is created by use of the \ backslash.

Having returned two array references they are assigned to scalar variables. If you uncomment the second print line you'll see two references to arrays.

The next problem is how to dereference the references, or access the arrays. The construct @$xxx does that for us. I know I said I wouldn't cover references, and I haven't -- that is just a useful trick.

This little section is not designed as a complete guide, it is just a taster of things to come. Perl is immensely powerful. If you think something can't be done, the problem is likely to be it is beyond your ability, not that of Perl.


Modules


An introduction

Subroutines are oft-used pieces of code. They exist so you can re-use the code and not have to constantly rewrite it.

A module is, in principle, similar to a subroutine. It is also an oft-used piece of code. The difference is that modules don't live in your program, they are their own separate script outside your code. For example, you might write a routine to send email. You could then use this code in ten, a hundred, a thousand different programs just by referencing the original program.

As you would expect, the basic Perl package includes a large number of modules. These have been written by people who had a need for the code, made it a module and released it into the big wide world. Many of these modules have been debugged, improved and documented by yet more people. To quote the OpenSource mantra, all bugs are shallow under the scrutiny of every programmer.

Aside from the many modules included with Perl there are hundreds more available on CPAN, the Comprehensive Perl Archive Network. Refer to your documentation for details.


File::Find -- using a module

An example of a module included with Perl is File::Find. There are several modules under the File::Find section, such as File::Basetree, File::Compare and File::Stat.

This is an example of how File::Find can be used:

use File::Find;

$dir1='/some/dir/with/lots/of/files';
$dir2='/another/directory/';

find(\&wanted, $dir1,$dir2);

sub wanted {
	print "Found it $File::Find::dir/$_\n" if /^[a-d]/i;

}

The first line is the most important. The use function loads the File::Find module. Now, all the power and functionality of File::Find is available for use. Such as the find function. This accepts two basic parameters:

The subroutine wanted simply prints the directory the file was found in if the filename begins with a,b,c or d. Make your own regex to suit. The line $File::Find::dir means the $dir variable in the module $File::Find. This is explained further in the next section.

Note -- the \&wanted parameter is a reference to a subroutine. Essentially, this means that the code in File::Find knows where to find the &wanted subroutine. It is basically like shortcuts under Windows 9x and NT4, instead of actual files (but the UNIX Perl people would slaughter me for that, so be quiet).


ChangeNotify

Another example is Win32::ChangeNotify. As you might expect there are a number of Win32-specific modules, and ChangeNotify is one of them. It waits until a something changes in a directory, then acts. What it waits for and what it does are up to you, for example:

use Win32::ChangeNotify;

$Path='/downloads';
$WatchSubTree=0;
$Events='FILE_NAME';
$browser='E:/progs/netscape/Communicator/program/netscape.exe';
$changes=0;

$notify = Win32::ChangeNotify->new($Path,$WatchSubTree,$Events);

while (1) {
	print "- ",scalar(localtime)," $changes so far to $Path.\n";
	$notify->wait;
	++$changes;
	print "- ",scalar(localtime), " Launching $browser...\n";
	system("$browser $Path");
	$notify->reset;
}


Again, the module is incorporated into the program with use . An object referred to by the variable $notify is created. The parameters passed are the path to be watched, whether we want to watch subtrees, and what sort of events we want to be notified about, in this case only filename changes.

Then, we enter a loop which continues while 1 is true -- which will be forever.

The program pauses when the wait method of the $notify notify object is called. Only when there is a change to the directory, then the rest of the subroutine completes, launching the browser. We have to reset the $notify object.

There is some pretty frightening stuff about objects in the explanation. But you don't actually need to understand anything about objects. Just read the documentation, and experiment.

You can use as many modules as you like in one program. As they are all written with carefully scoped variables you need not worry about programmers using the same variable names in different modules. Now you *really* appreciate scoping!


Your Very Own Module

You too can write your own modules. It is easy. First, we will create the fantastic bit of code that we want to re-use everywhere. First, we'll write a normal Perl program:

$name=shift;

print &logname($name);

sub logname {
	my $name=shift;
	my @namebits;
	my ($logon,$inital);
	@namebits=split /\s+/,$name;
	($inital)=$name=~/(\w)/;
	$logon=$inital.$namebits[$#namebits];
	$logon=lc $logon;
	return $logon;
}

Execute like so; perl script.pl "Nick Bladon"

The script itself is nothing amazing. The lc function stands for LowerCase, or probably lOWERcASE -- you can see what it does.

In order to turn it into a module carry out the following steps:

  1. Find out where your copy of Perl is installed, for example c:\progs\perl.
  2. Within that directory there should be a lib directory.
  3. Make a directory within lib, for example c:\progs\perl\lib\RMP\

Now we'll make the module. Remember, a module is just code you are going to reuse. So we don't need all of the above example. Just this bit:

sub logname {
        my $name=shift;
        my @namebits;
        my ($logon,$inital);
        @namebits=split /\s+/,$name;
        ($inital)=$name=~/(\w)/;
        $logon=$inital.$namebits[$#namebits];
        $logon=lc $logon;
        return $logon;
}

1;

The bit that has been added is the 1 at the bottom. Why? Perl requires that all modules return true. We know that a subroutine always returns the value of the last expression evaluated. As 1 evaluates to true, that'll do.

You need to save this as logon.pm in your newly created directory under lib. The pm stands for Perl Module.

That's it. A module created. To use, just make a normal Perl script such as:

use RMP::logon;

$name=shift;

print logname($name);

and hey presto! Module power is yours!

You don't have to create your own subdirectory within lib but I would advise it for the sake of neatness. And as you might expect, there is a lot more to learn about modules but this is supposed to be a basic tutorial, so that's enough for the time being.


Bondage and Discipline

Perl is a very flexible language. It is designed as a hacking tool, for quick sysadmin magic. It can do quite a bit more besides, but being small and powerful is a core Perl feature. Earlier on I said Perl is not a bondage and discipline language -- to qualify that, it doesn't have to be. However, there is a time and place for everything.

For tiny scripts you don't want to be declaring variables, typecasting and generally spending more time obeying rules than you do getting the job done. So, Perl doesn't force you to do all of these good programming practices. However, not all your programs are going to be five-minute hacks. Some will be pretty large. Therefore, some Discipline is in order.

Perl has two primary methods of enforcing discipline. They are:


-w

Consider for a moment this little program:

@input=@ARGV;

$outfile='outfile.txt';
open OUT, ">$outfile" or die "Can't open $outfile for write:$!\n";

$input2++;
$delay=2 if $input[0] eq 'sleep';

sleep $delay;

print "The first element of \@input is $input[0]\n";
print OUY "Slept $delay!\n";

It doesn't do much. Just prints out the first argument supplied, and demonstrates the uninspiring sleep function. The program itself is full of holes, and it is only a few lines. How many errors can you spot? Try and count them. When you are finished, execute the program with error-checks enabled:

perl -w script.pl hello

Perl finds quite a few errors. The -w switch finds, among other heinous sins:

So, generally, -w is a Good Thing. It forces you to write cleaner code. So use it, but don't be afraid not to for very short programs.


Shebang

You know that you can turn warnings on with -w on the command line. You can also turn them on within the script itself. For that matter, you can give perl any command line option within the script itself. For example:

perl script.pl hello

to execute this:

#!perl -w

@input=@ARGV;

$outfile='outfile.txt';
open OUT, ">$outfile" or die "Can't open $outfile for write:$!\n";

$input2++;
$delay=2 if $input[0] eq 'sleep';

sleep $delay;

print "The first element of \@input is $input[0]\n";
print OUY "Slept $delay!\n";

has the same effect as:

perl -w script.pl hello

It may be more convenient for you to put the flag inside the script. It doesn't have to be just -w , it can be any argument Perl supports. Run

perl -h

for a full list.

The first line, #!perl -w is the shebang line. This is derived from UNIX, where Perl was first developed. UNIX systems make a script executable by changing an attribute. The operating system then loads the file and works out how to execute it -- in this case by looking at the first line, then loading the perl interpreter. Windows systems know that all files with a certain extension must be passed to a certain program for execution, eg all .bat files are passed to command.com, and all .xls files are passed to Excel. The point of all this being that you don't need a shebang line, but it doesn't hurt.


use strict;

So what's strict and how do you use it? The module strict restricts 'unsafe constructs', according to the perldocs. The strict module is a pragma, which is a hint that must be obeyed. Like when your girlfriend says 'oh, that ring is *far* too expensive'.

There is no need to be frightened about unsafe code if you don't mind endless hours of debugging unstructured programs. When you enable the strict module, the three things that Perl becomes strict about are:

This tutorial doesn't presently cover references (and let's hope I remember to remove this sentence if I do cover it in later versions) so we won't worry about refs.

Strict variables are useful. Essentially, this means that all variables must be declared, that is defined before use rather than springing into existence as required. Furthermore, each variable must be defined with my or fully qualified. This is an example of a program that is not strict, and should be executed something like this:

perl script.pl "Alain James Smith";

where the "" enclose the string as a single parameter as opposed to three separate space-delimited parameters.

#use strict;			# uncomment after running a couple of times

$name=shift;			# shifts @ARGV if no arguments supplied

print "The name is $name\n";
$inis=&initials($name);

$luck=int(rand(10)) if $inis=~/^(?:[a-d]|[n-p]|[x-z])/i;

print "The initials are $inis, lucky number: $luck\n";

sub initials {
        my $name=shift;
        $initials.=$1 while $name=~/(\w)\w+\s?/g;
        return $initials;
}

By now you should be able to work out what the above does. When you uncomment the use strict; pragma, and re-run the program, you will get output something like this:

Global symbol "$name" requires explicit package name at n1.pl line 3.
Global symbol "$inis" requires explicit package name at n1.pl line 6.
Global symbol "$luck" requires explicit package name at n1.pl line 8.
Global symbol "$initials" requires explicit package name at n1.pl line 14.
Execution of n1.pl aborted due to compilation errors.

These warnings mean Perl is not exactly clear about what the scope of variables is. If Perl is not clear, you might not be either. So you need to be explicit about your variables, which means either declaring them with my so they are restricted to the current block, or referring to them with their fully qualified name. An example, using both methods:

use strict;

$MAIN::name=shift;			# shifts @ARGV if no arguments supplied

print "The name is ",$MAIN::name,"\n";
my $inis='';
my $luck='';

$inis=&initials($MAIN::name);

$luck=int(rand(10)) if $inis=~/^(?:[a-d]|[n-p]|[x-z])/i;

print "The initials are $inis, lucky number: $luck\n";

sub initials {
        my $name=shift;
	my $initials;
        $initials.=$1 while $name=~/(\w)\w+\s?/g;
        return $initials;
}

The my variables in the subroutine are nothing new. The my variables outside the subroutine are. If you think about it, the main program itself is also a kind of block, and therefore variables can be lexically scoped to be visible only within the block.

The other interesting bit is the $MAIN::name business. This, as you might expect, is the fully qualified name of the variable. The first part is the package name, in this case MAIN. The second part is the actual variable name. Personally, I've never needed to refer to a variable this way. I'm not saying you'll never use the syntax, but I would suggest that knowing this is not on a perl students Top 10 list of Things to Master.

The important thing about use strict is that it does enforce more discipline than you have been used to, and for all but the smallest of programs, that is most definitely a Good Thing.


Debugging

Sooner or later you'll need to do some fairly hairy debugging. It will be later if you are using strict , -w and writing your subroutines properly, but the moment will come.

When it does you'll be poring over code, probably late at night, wondering where the hell the problem is. Some techniques I find useful are:

Eventually, you'll be stuck. Such is the price of progress. In this case, Perl's own debugger can be invaluable. Run this code as normal first:

$name=shift;

print "Logon name creation program\n:Converting '$name'\n";

print &logname($name),"\n\n";

print "Program ended at", scalar(localtime),"\n";

sub logname {
        my $name=shift;
        my @namebits;
        my ($logon,$inital);
        @namebits=split /\s+/,$name;
        ($inital)=$name=~/(\w)/;
        $logon=$inital.$namebits[$#namebits];
        $logon=lc $logon;
        return $logon;
}

We'll run it with the debugger so you can watch perl at work while it runs:

perl -d script.pl "Peter Dakin";

and you are into the debugger, which should look something like this:

c:\scripts\db.pl>perl -d db.pl "Peter Dakin"

Loading DB routines from perl5db.pl version 1.0401
Emacs support available.

Enter h or `h h' for help.

main::(db.pl:1):        $name=shift;
  DB<1>
db.pl Name of script being executed
1 Line number of script that is just about to be executed.
$name=shift; The code that is just about to be executed.

Type s for a single step and press enter. The code $name=shift; will be executed, and perl waits for your next command. Keep inputting s until the program terminates.

This by itself is useful as you see the subroutine flow, but if you enter h for help you'll see a bewildering range of debug options. I won't detail them all here, but some of the ones I find most useful are:

n Executes main program, but skips subroutine calls. The subroutine is executed, but you aren't stepped through it. Try using n instead of s .
/xx/ Searches through program for xx
p Prints, for example p @namebits, p $name
Enter Pressing the Enter key (inputting a carriage return) repeats the last n or s command.
perlcode You can type any perl code in and it will be evaluated, and have a effect on your program. In the example below I remove spaces from $name. Inputs in bold:
main::(db.pl:1):        $name=shift;
  DB<1> s
main::(db.pl:3):        print "Logon name creation program\n:Converting '$name'\n";
  DB<1> $name=~s/\s//g;

  DB<2> print $name
MarkGray
  DB<3>

There are many, many more debugger options which are worth becoming familiar with. Type h for a full list.


Logical Operators

Logical operators are such things as OR, NOT, AND. They all evaluate expressions. The expression evaluates to true, or false. Exactly what criteria for evaluation are used depends on the operator.

or

The or operator works as follows:

open STUFF, $stuff or die "Cannot open $stuff for read :$!";

This line means -- if the operation for opening STUFF fails, then do something else. Another example:

$_=shift;

/^R/ or print "Doesn't start with R\n";

If the regular expression is false, then whatever is on the left side of the or is printed. As you know, shift works on @ARGV if no target is given, or @_ inside a subroutine.

Perl has two OR operators. One is the now familiar or and the other is || .


Precedence: What comes First

To understand the difference between the two we need to talk about precedence. Precedence means priority, order, importance. A good example is:

perl -e"print 2+8

which we know equals 10. But if we add:

perl -e"print 2+8/2

Now, will this be 2+8 == 10, divided by 2 == 5? Or maybe 8/2 == 4, plus 2 == 6?

Precedence is about what is done first. In the example above, you can see that the division is done first, then the addition. Therefore, division has a higher precedence that addition.

You can force the issue with parens:

perl -e"print ((2+8)/2)

which forces Perl, kicking and screaming, to evaluate 2+8 then divide the result by 2.

So what has this to do with logical operators? Well, the main difference between or and || is precedence.

In the example below, we attempt to assign two variables to non-existent elements of an array. This will fail:

@list=qw(a b c);

$name1 =  $list[4] or "1-Unknown";

$name2 =  $list[4] || "2-Unknown";

print "Name1 is $name1, Name2 is $name2\n";

print "Name1 exists\n" if defined $name1;
print "Name2 exists\n" if defined $name2;

The output is interesting. The variable $name2 has been created, albeit with a false value. However, $name1 does not exist. The reason is all about precedence. The or operator has a lower precedence than || .

This means or looks at the entire expression on its left hand side. In this case, that is $name1 = $list[4] . If it is true, it gets done. If it is false, it is not and the right hand side is evaluated, and the left hand side is ignored as if it never existed. In the example above, once the left side is found to be false, then all the right side evaluates is "1-Unknown" which may be true but doesn't produce any output.

In the case of || , which has a higher precedence, the code immediately on the left of the operator is evaluated. In this case, that is $list[4]. This is false, so the code immediately to the right is evaluated. But, the original code on the left which was not evaluated, $name2 = is not forgotten. Therefore, the expression evaluated to $name2 = "2-Unknown".

The example below should help clarify things:

@list=qw(a b c);

$ele1 = $list[4] or print "1 Failed\n";
$ele2 = $list[4] || print "2 Failed\n";

print <<PRT;
ele1 :$ele1:

ele2 :$ele2:

PRT

The two failure codes are both printed, but for different reasons. The first is printed because we are assigning $ele1 a false value, so the result of the operation is false. Therefore, the right hand side is evaluated.

The second is printed because $list[4] itself false. Yet, as you can see, $ele2 exists. Any idea why?

The reason is that the result of print "2-Failed\n" has been assigned to $ele2. This is successful, and therefore returns 1.

Another example:

$file='not-there.txt';

open FILE, $file   || print "1: Can't open file:$!\n";

open FILE, $file   or print "2: Can't open file:$!\n";

In the first example, the error message is not printed. This is because $file is evaluating to true. However, in the second example, or looks at the entire expression, not just what is immediately to the left and takes action on the result of evaluating the entire left hand side, not just the expression immediately to its left.

You can fix things with parens:

$file='not-there.txt';

open FILE, $file   || print "1: Can't open file:$!\n";

open FILE, $file   or print "2: Can't open file:$!\n";

open (FILE, $file) || print "3: Can't open file:$!\n";

like so, but why bother when you have a perfectly good operator in or ? You could apply parens elsewhere:

@list=qw(a b c);

$name1 =  $list[4]   or "1-Unknown";

($name2 =  $list[4]) || "2-Unknown";

print "Name1 is $name1, Name2 is $name2\n";

print "Name1 exists\n" if defined $name1;
print "Name2 exists\n" if defined $name2;

Now, ($name2 = $list[4]) is evaluated as a complete expression, not just as $list[4] is evaluated as a complete expression, not just as $list[4], so we get exactly the same result as if we used or .

And

now for something similar. And. Logical AND operators evaluate two expressions, and return true only if both are true. Contrast this with OR, which returns true only of one or more of the two expressions are true. Perl has a few AND operators.

The first type of AND we will look at is && :

@list=qw(a b c);

print "List is:@list\n";

if ($list[0] eq 'x' && $list[2]++ eq 'd') {
	print "True\n";
	} else {
	print "False\n";
}

print "List is:@list\n";

The output here is false. It is clear that $list[0] does not equal x . As AND statements can only return true if both expressions being evaluated are true, then as the first statement is false this is an obvious non-starter and perl decides it need not continue to the second statement. Entirely sensible.

The second type of AND statement is & . This is similar to && . See if you can work out what the difference is using this example:

@list=qw(a b c);

print "List is:@list\n";

if ($list[0] eq 'x' & $list[2]++ eq 'd') {
	print "True\n";
	} else {
	print "False\n";
}

print "List is:@list\n";

The difference is that the second part of the expression is evaluated no matter what the result of the first part is. Despite the fact that the AND statement cannot possibly return true, perl goes ahead and evaluates the second part of the statement anyway, hence $list[2] ends up as d .

The third AND which we will look at is and . This behaves in the same way as && but is lower precedence. Therefore, all the guidelines about || and or apply.

Other Logical Operators

Perl has not , which works like ! except for low precedence. If you are wondering where you have seen ! before, what about:

$x !~/match/;

if ($t != 5) {

as two examples. There is also Exclusive OR, or XOR. This means:

This needs an example. Jane and Sonia are two known troublemakers, with a reputation for throwing good beer around, going topless at inappropriate moments and singing out of tune to the karaoke machine. You only want to let one of them into your party, and instead of a big muscle-bound bouncer you have this perl script on the door:

($name1,$name2)=@ARGV;

if ($name1 eq 'Jane' xor $name2 eq 'Sonia') {
	print "OK, allowed\n";
} else {
	print "Sorry, not allowed\n";
}

I would suggest running it thus:

perl script.pl Jane Karen (one true, one false)
perl script.pl Jim Sonia (one true, one false)
perl script.pl Jane Sonia (both true)
perl script.pl Jim Sam (both false)
   

Well, the script is not perfect as a doorman, as all Jane and Sonia have to do is type their names in lowercase, but hopefully it demonstrated xor .

One thing to beware of is:

$_=shift;

print "OK\n" unless not(!/r/i || /o/i & /p/ or /q/);

over-complication, and believe me the above is not as complicated as it could be. Take the time to understand what you want to do. Perl provides a plethora of logical operands so you really don't have any excuse for not writing legible code. The above can be written a lot more concisely and clearly. As well as a lot more obscurely :-)

@ARGV

Last words

I hope you have enjoyed this tutorial and learnt something from it. I would appreciate an email letting me know how it could be improved. What you have learnt is just a fraction of Perl's functionality, but you'll find skills like regexes can be applied in many other places than Perl.

Good luck.

--
Robert


Thanks to...

Everyone that helped in the development of this tutorial. I do read all the feedback emails, but don't always action them the same year. What you have just read is better because of the people below. They fix the bugs, scream when they don't understand and I rewrite whole sections. Documents like this are written by the authors, but polished by the readers.

The roll of honour is, in a semi-chronological order:

The original location of this document is:
http://www.netcat.co.uk/rob/perl/win32perltut.html


This tutorial is copyright 1997, 1998, 1999 by Robert Pepper. Reproduction in whole or part is prohibited. Please contact me if you want to use this information anywhere. Thank you.
--
Robert Pepper     mailto:Robert@netcat.co.uk


Free Visitor Maps at VisitorMap.org
Get your own visitor map!