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PERLINTRO(1)	       Perl Programmers Reference Guide 	  PERLINTRO(1)

       perlintro -- a brief introduction and overview of Perl

       This document is intended to give you a quick overview of the Perl pro
       gramming language, along with pointers to further documentation.  It is
       intended as a "bootstrap" guide for those who are new to the language,
       and provides just enough information for you to be able to read other
       peoples Perl and understand roughly what its doing, or write your own
       simple scripts.

       This introductory document does not aim to be complete.	It does not
       even aim to be entirely accurate.  In some cases perfection has been
       sacrificed in the goal of getting the general idea across.  You are
       strongly advised to follow this introduction with more information from
       the full Perl manual, the table of contents to which can be found in

       Throughout this document youll see references to other parts of the
       Perl documentation.  You can read that documentation using the "perl
       doc" command or whatever method youre using to read this document.

       What is Perl?

       Perl is a general-purpose programming language originally developed for
       text manipulation and now used for a wide range of tasks including sys
       tem administration, web development, network programming, GUI develop
       ment, and more.

       The language is intended to be practical (easy to use, efficient, com
       plete) rather than beautiful (tiny, elegant, minimal).  Its major fea
       tures are that its easy to use, supports both procedural and object-
       oriented (OO) programming, has powerful built-in support for text pro
       cessing, and has one of the worlds most impressive collections of
       third-party modules.

       Different definitions of Perl are given in perl, perlfaq1 and no doubt
       other places.  From this we can determine that Perl is different things
       to different people, but that lots of people think its at least worth
       writing about.

       Running Perl programs

       To run a Perl program from the Unix command line:

	   perl progname.pl

       Alternatively, put this as the first line of your script:

	   #!/usr/bin/env perl

       ... and run the script as "/path/to/script.pl".	Of course, itll need
       to be executable first, so "chmod 755 script.pl" (under Unix).

       For more information, including instructions for other platforms such
       as Windows and Mac OS, read perlrun.

       Basic syntax overview

       A Perl script or program consists of one or more statements.  These
       statements are simply written in the script in a straightforward fash
       ion.  There is no need to have a "main()" function or anything of that

       Perl statements end in a semi-colon:

	   print "Hello, world";

       Comments start with a hash symbol and run to the end of the line

	   # This is a comment

       Whitespace is irrelevant:

	       "Hello, world"

       ... except inside quoted strings:

	   # this would print with a linebreak in the middle
	   print "Hello

       Double quotes or single quotes may be used around literal strings:

	   print "Hello, world";
	   print Hello, world;

       However, only double quotes "interpolate" variables and special charac
       ters such as newlines ("\n"):

	   print "Hello, $name\n";     # works fine
	   print Hello, $name\n;     # prints $name\n literally

       Numbers dont need quotes around them:

	   print 42;

       You can use parentheses for functions arguments or omit them according
       to your personal taste.	They are only required occasionally to clarify
       issues of precedence.

	   print("Hello, world\n");
	   print "Hello, world\n";

       More detailed information about Perl syntax can be found in perlsyn.

       Perl variable types

       Perl has three main variable types: scalars, arrays, and hashes.

	   A scalar represents a single value:

	       my $animal = "camel";
	       my $answer = 42;

	   Scalar values can be strings, integers or floating point numbers,
	   and Perl will automatically convert between them as required.
	   There is no need to pre-declare your variable types.

	   Scalar values can be used in various ways:

	       print $animal;
	       print "The animal is $animal\n";
	       print "The square of $answer is ", $answer * $answer, "\n";

	   There are a number of "magic" scalars with names that look like
	   punctuation or line noise.  These special variables are used for
	   all kinds of purposes, and are documented in perlvar.  The only one
	   you need to know about for now is $_ which is the "default vari
	   able".  Its used as the default argument to a number of functions
	   in Perl, and its set implicitly by certain looping constructs.

	       print;	       # prints contents of $_ by default

	   An array represents a list of values:

	       my @animals = ("camel", "llama", "owl");
	       my @numbers = (23, 42, 69);
	       my @mixed   = ("camel", 42, 1.23);

	   Arrays are zero-indexed.  Heres how you get at elements in an

	       print $animals[0];	       # prints "camel"
	       print $animals[1];	       # prints "llama"

	   The special variable $#array tells you the index of the last ele
	   ment of an array:

	       print $mixed[$#mixed];	    # last element, prints 1.23

	   You might be tempted to use "$#array + 1" to tell you how many
	   items there are in an array.  Dont bother.  As it happens, using
	   @array where Perl expects to find a scalar value ("in scalar con
	   text") will give you the number of elements in the array:

	       if (@animals < 5) { ... }

	   The elements were getting from the array start with a "$" because
	   were getting just a single value out of the array -- you ask for a
	   scalar, you get a scalar.

	   To get multiple values from an array:

	       @animals[0,1];		       # gives ("camel", "llama");
	       @animals[0..2];		       # gives ("camel", "llama", "owl");
	       @animals[1..$#animals];	       # gives all except the first element

	   This is called an "array slice".

	   You can do various useful things to lists:

	       my @sorted    = sort @animals;
	       my @backwards = reverse @numbers;

	   There are a couple of special arrays too, such as @ARGV (the com
	   mand line arguments to your script) and @_ (the arguments passed to
	   a subroutine).  These are documented in perlvar.

	   A hash represents a set of key/value pairs:

	       my %fruit_color = ("apple", "red", "banana", "yellow");

	   You can use whitespace and the "=>" operator to lay them out more

	       my %fruit_color = (
		   apple  => "red",
		   banana => "yellow",

	   To get at hash elements:

	       $fruit_color{"apple"};		# gives "red"

	   You can get at lists of keys and values with "keys()" and "val

	       my @fruits = keys %fruit_colors;
	       my @colors = values %fruit_colors;

	   Hashes have no particular internal order, though you can sort the
	   keys and loop through them.

	   Just like special scalars and arrays, there are also special
	   hashes.  The most well known of these is %ENV which contains envi
	   ronment variables.  Read all about it (and other special variables)
	   in perlvar.

       Scalars, arrays and hashes are documented more fully in perldata.

       More complex data types can be constructed using references, which
       allow you to build lists and hashes within lists and hashes.

       A reference is a scalar value and can refer to any other Perl data
       type. So by storing a reference as the value of an array or hash ele
       ment, you can easily create lists and hashes within lists and hashes.
       The following example shows a 2 level hash of hash structure using
       anonymous hash references.

	   my $variables = {
	       scalar  =>  {
			    description => "single item",
			    sigil => $,
	       array   =>  {
			    description => "ordered list of items",
			    sigil => @,
	       hash    =>  {
			    description => "key/value pairs",
			    sigil => %,

	   print "Scalars begin with a $variables->{scalar}->{sigil}\n";

       Exhaustive information on the topic of references can be found in perl
       reftut, perllol, perlref and perldsc.

       Variable scoping

       Throughout the previous section all the examples have used the syntax:

	   my $var = "value";

       The "my" is actually not required; you could just use:

	   $var = "value";

       However, the above usage will create global variables throughout your
       program, which is bad programming practice.  "my" creates lexically
       scoped variables instead.  The variables are scoped to the block (i.e.
       a bunch of statements surrounded by curly-braces) in which they are

	   my $a = "foo";
	   if ($some_condition) {
	       my $b = "bar";
	       print $a;	   # prints "foo"
	       print $b;	   # prints "bar"
	   print $a;		   # prints "foo"
	   print $b;		   # prints nothing; $b has fallen out of scope

       Using "my" in combination with a "use strict;" at the top of your Perl
       scripts means that the interpreter will pick up certain common program
       ming errors.  For instance, in the example above, the final "print $b"
       would cause a compile-time error and prevent you from running the pro
       gram.  Using "strict" is highly recommended.

       Conditional and looping constructs

       Perl has most of the usual conditional and looping constructs except
       for case/switch (but if you really want it, there is a Switch module in
       Perl 5.8 and newer, and on CPAN. See the section on modules, below, for
       more information about modules and CPAN).

       The conditions can be any Perl expression.  See the list of operators
       in the next section for information on comparison and boolean logic
       operators, which are commonly used in conditional statements.

	       if ( condition ) {
	       } elsif ( other condition ) {
	       } else {

	   Theres also a negated version of it:

	       unless ( condition ) {

	   This is provided as a more readable version of "if (!condition)".

	   Note that the braces are required in Perl, even if youve only got
	   one line in the block.  However, there is a clever way of making
	   your one-line conditional blocks more English like:

	       # the traditional way
	       if ($zippy) {
		   print "Yow!";

	       # the Perlish post-condition way
	       print "Yow!" if $zippy;
	       print "We have no bananas" unless $bananas;

	       while ( condition ) {

	   Theres also a negated version, for the same reason we have

	       until ( condition ) {

	   You can also use "while" in a post-condition:

	       print "LA LA LA\n" while 1;	    # loops forever

       for Exactly like C:

	       for ($i=0; $i <= $max; $i++) {

	   The C style for loop is rarely needed in Perl since Perl provides
	   the more friendly list scanning "foreach" loop.

	       foreach (@array) {
		   print "This element is $_\n";

	       # you dont have to use the default $_ either...
	       foreach my $key (keys %hash) {
		   print "The value of $key is $hash{$key}\n";

       For more detail on looping constructs (and some that werent mentioned
       in this overview) see perlsyn.

       Builtin operators and functions

       Perl comes with a wide selection of builtin functions.  Some of the
       ones weve already seen include "print", "sort" and "reverse".  A list
       of them is given at the start of perlfunc and you can easily read about
       any given function by using "perldoc -f functionname".

       Perl operators are documented in full in perlop, but here are a few of
       the most common ones:

	       +   addition
	       -   subtraction
	       *   multiplication
	       /   division

       Numeric comparison
	       ==  equality
	       !=  inequality
	       <   less than
	       >   greater than
	       <=  less than or equal
	       >=  greater than or equal

       String comparison
	       eq  equality
	       ne  inequality
	       lt  less than
	       gt  greater than
	       le  less than or equal
	       ge  greater than or equal

	   (Why do we have separate numeric and string comparisons?  Because
	   we dont have special variable types, and Perl needs to know
	   whether to sort numerically (where 99 is less than 100) or alpha
	   betically (where 100 comes before 99).

       Boolean logic
	       &&  and
	       ||  or
	       !   not

	   ("and", "or" and "not" arent just in the above table as descrip
	   tions of the operators -- theyre also supported as operators in
	   their own right.  Theyre more readable than the C-style operators,
	   but have different precedence to "&&" and friends.  Check perlop
	   for more detail.)

	       =   assignment
	       .   string concatenation
	       x   string multiplication
	       ..  range operator (creates a list of numbers)

       Many operators can be combined with a "=" as follows:

	   $a += 1;	   # same as $a = $a + 1
	   $a -= 1;	   # same as $a = $a - 1
	   $a .= "\n";	   # same as $a = $a . "\n";

       Files and I/O

       You can open a file for input or output using the "open()" function.
       Its documented in extravagant detail in perlfunc and perlopentut, but
       in short:

	   open(INFILE,  "input.txt")	or die "Cant open input.txt: $!";
	   open(OUTFILE, ">output.txt") or die "Cant open output.txt: $!";
	   open(LOGFILE, ">>my.log")	or die "Cant open logfile: $!";

       You can read from an open filehandle using the "<>" operator.  In
       scalar context it reads a single line from the filehandle, and in list
       context it reads the whole file in, assigning each line to an element
       of the list:

	   my $line  = ;
	   my @lines = ;

       Reading in the whole file at one time is called slurping. It can be
       useful but it may be a memory hog. Most text file processing can be
       done a line at a time with Perls looping constructs.

       The "<>" operator is most often seen in a "while" loop:

	   while () {	  # assigns each line in turn to $_
	       print "Just read in this line: $_";

       Weve already seen how to print to standard output using "print()".
       However, "print()" can also take an optional first argument specifying
       which filehandle to print to:

	   print STDERR "This is your final warning.\n";
	   print OUTFILE $record;
	   print LOGFILE $logmessage;

       When youre done with your filehandles, you should "close()" them
       (though to be honest, Perl will clean up after you if you forget):

	   close INFILE;

       Regular expressions

       Perls regular expression support is both broad and deep, and is the
       subject of lengthy documentation in perlrequick, perlretut, and else
       where.  However, in short:

       Simple matching
	       if (/foo/)	{ ... }  # true if $_ contains "foo"
	       if ($a =~ /foo/) { ... }  # true if $a contains "foo"

	   The "//" matching operator is documented in perlop.	It operates on
	   $_ by default, or can be bound to another variable using the "=~"
	   binding operator (also documented in perlop).

       Simple substitution
	       s/foo/bar/;		 # replaces foo with bar in $_
	       $a =~ s/foo/bar/;	 # replaces foo with bar in $a
	       $a =~ s/foo/bar/g;	 # replaces ALL INSTANCES of foo with bar in $a

	   The "s///" substitution operator is documented in perlop.

       More complex regular expressions
	   You dont just have to match on fixed strings.  In fact, you can
	   match on just about anything you could dream of by using more com
	   plex regular expressions.  These are documented at great length in
	   perlre, but for the meantime, heres a quick cheat sheet:

	       .		   a single character
	       \s		   a whitespace character (space, tab, newline)
	       \S		   non-whitespace character
	       \d		   a digit (0-9)
	       \D		   a non-digit
	       \w		   a word character (a-z, A-Z, 0-9, _)
	       \W		   a non-word character
	       [aeiou]		   matches a single character in the given set
	       [^aeiou] 	   matches a single character outside the given set
	       (foo|bar|baz)	   matches any of the alternatives specified

	       ^		   start of string
	       $		   end of string

	   Quantifiers can be used to specify how many of the previous thing
	   you want to match on, where "thing" means either a literal charac
	   ter, one of the metacharacters listed above, or a group of charac
	   ters or metacharacters in parentheses.

	       *		   zero or more of the previous thing
	       +		   one or more of the previous thing
	       ?		   zero or one of the previous thing
	       {3}		   matches exactly 3 of the previous thing
	       {3,6}		   matches between 3 and 6 of the previous thing
	       {3,}		   matches 3 or more of the previous thing

	   Some brief examples:

	       /^\d+/		   string starts with one or more digits
	       /^$/		   nothing in the string (start and end are adjacent)
	       /(\d\s){3}/	   a three digits, each followed by a whitespace
				   character (eg "3 4 5 ")
	       /(a.)+/		   matches a string in which every odd-numbered letter
				   is a (eg "abacadaf")

	       # This loop reads from STDIN, and prints non-blank lines:
	       while (<>) {
		   next if /^$/;

       Parentheses for capturing
	   As well as grouping, parentheses serve a second purpose.  They can
	   be used to capture the results of parts of the regexp match for
	   later use.  The results end up in $1, $2 and so on.

	       # a cheap and nasty way to break an email address up into parts

	       if ($email =~ /([^@]+)@(.+)/) {
		   print "Username is $1\n";
		   print "Hostname is $2\n";

       Other regexp features
	   Perl regexps also support backreferences, lookaheads, and all kinds
	   of other complex details.  Read all about them in perlrequick,
	   perlretut, and perlre.

       Writing subroutines

       Writing subroutines is easy:

	   sub log {
	       my $logmessage = shift;
	       print LOGFILE $logmessage;

       Whats that "shift"?  Well, the arguments to a subroutine are available
       to us as a special array called @_ (see perlvar for more on that).  The
       default argument to the "shift" function just happens to be @_.	So "my
       $logmessage = shift;" shifts the first item off the list of arguments
       and assigns it to $logmessage.

       We can manipulate @_ in other ways too:

	   my ($logmessage, $priority) = @_;	   # common
	   my $logmessage = $_[0];		   # uncommon, and ugly

       Subroutines can also return values:

	   sub square {
	       my $num = shift;
	       my $result = $num * $num;
	       return $result;

       For more information on writing subroutines, see perlsub.

       OO Perl

       OO Perl is relatively simple and is implemented using references which
       know what sort of object they are based on Perls concept of packages.
       However, OO Perl is largely beyond the scope of this document.  Read
       perlboot, perltoot, perltooc and perlobj.

       As a beginning Perl programmer, your most common use of OO Perl will be
       in using third-party modules, which are documented below.

       Using Perl modules

       Perl modules provide a range of features to help you avoid reinventing
       the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ).  A
       number of popular modules are included with the Perl distribution

       Categories of modules range from text manipulation to network protocols
       to database integration to graphics.  A categorized list of modules is
       also available from CPAN.

       To learn how to install modules you download from CPAN, read perlmodin

       To learn how to use a particular module, use "perldoc Module::Name".
       Typically you will want to "use Module::Name", which will then give you
       access to exported functions or an OO interface to the module.

       perlfaq contains questions and answers related to many common tasks,
       and often provides suggestions for good CPAN modules to use.

       perlmod describes Perl modules in general.  perlmodlib lists the mod
       ules which came with your Perl installation.

       If you feel the urge to write Perl modules, perlnewmod will give you
       good advice.

       Kirrily "Skud" Robert 

perl v5.8.8			  2008-04-25			  PERLINTRO(1)

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