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NAME    [Toc]    [Back]

       perlintro -- a brief introduction and overview of Perl

DESCRIPTION    [Toc]    [Back]

       This document is intended to give you a quick overview of
       the Perl programming 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 it's 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 perltoc.

       Throughout this document you'll see references to other
       parts of the Perl documentation.  You can read that documentation
 using the "perldoc" command or whatever method
       you're 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 system administration, web development,
 network programming, GUI development, and more.

       The language is intended to be practical (easy to use,
       efficient, complete) rather than beautiful (tiny, elegant,
       minimal).  Its major features are that it's easy to use,
       supports both procedural and object-oriented (OO) programming,
 has powerful built-in support for text processing,
       and has one of the world's 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 it's at least worth writing

       Running Perl programs    [Toc]    [Back]

       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, it'll 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    [Toc]    [Back]

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

       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

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

       However, only double quotes "interpolate" variables and
       special characters such as newlines ("0):

           print "Hello, $name0;     # works fine
           print 'Hello, $name0;     # prints $nameliterally

       Numbers don't 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, world0);
           print "Hello, world0;

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

       Perl variable types    [Toc]    [Back]

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

           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 predeclare
 your variable types.

           Scalar values can be used in various ways:

               print $animal;
               print "The animal is $animal0;
               print "The square of $answer is ", $answer *  $answer, "0;

           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 variable".
           It's used as the default argument to a number of functions
 in Perl, and it's 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.  Here's how you get at elements
 in an array:

               print $animals[0];              # prints "camel"
               print $animals[1];              # prints "llama"
           The special variable $#array tells you the index of
           the last element 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.  Don't bother.
           As it happens, using @array where Perl expects to find
           a scalar value ("in scalar context") will give you the
           number of elements in the array:

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

           The elements we're getting from the array start with a
           "$" because we're 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",
               @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 command 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 nicely:

               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 "values()".
               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 environment variables.  Read all about
           it (and other special variables) in perlvar.

       Scalars, arrays and hashes are documented more fully in

       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 element, 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
                            sigil => '@',
               hash    =>  {
                            description => "key/value pairs",
                            sigil => '%',

           print     "Scalars     begin     with     a     $variables->{'scalar'}->{'sigil'}0;

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

       Variable scoping    [Toc]    [Back]

       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 programming errors.  For instance, in
       the example above, the final "print $b" would cause a compile-time
 error and prevent you from running the  program.
       Using "strict" is highly recommended.

       Conditional and looping constructs    [Toc]    [Back]

       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 {

           There's also a negated version of it:

               unless ( condition ) {

           This is provided as a more readable version of "if

           Note that the braces are required in Perl, even if
           you've 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 ) {

           There's also a negated version, for the same reason we
           have "unless":

               until ( condition ) {

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

               print "LA LA LA0 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"

               foreach (@array) {
                   print "This element is $_0;

               # you don't have to use the default $_ either...
               foreach my $key (keys %hash) {
                   print "The value of $key is $hash{$key}0;

       For more detail on looping constructs (and some that
       weren't mentioned in this overview) see perlsyn.
       Builtin operators and functions

       Perl comes with a wide selection of builtin functions.
       Some of the ones we've 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 don't have special variable types,
           and Perl needs to know whether to sort numerically
           (where 99 is less than 100) or alphabetically (where
           100 comes before 99).

       Boolean logic
               &&  and
               ||  or
               !   not

           ("and", "or" and "not" aren't just in the above table
           as descriptions of the operators -- they're also supported
 as operators in their own right.  They're 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 .= "0;     # same as $a = $a . "0;

       Files and I/O

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

           open(INFILE,   "input.txt")    or  die "Can't open input.txt: $!";
           open(OUTFILE, ">output.txt") or die "Can't  open  output.txt: $!";
           open(LOGFILE,  ">>my.log")     or die "Can't 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  = <INFILE>;
           my @lines = <INFILE>;

       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 Perl's
       looping constructs.

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

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

       We've 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.0;
           print OUTFILE $record;
           print LOGFILE $logmessage;

       When you're 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

       Perl's regular expression support is both broad and deep,
       and is the subject of lengthy documentation in perlrequick,
 perlretut, and elsewhere.  However, in short:

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

           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 don't just have to match on fixed strings.  In
           fact, you can match on just about anything you could
           dream of by using more complex regular expressions.
           These are documented at great length in perlre, but
           for the meantime, here's a quick cheat sheet:

               .                   a single character
                                a  whitespace  character  (space,
tab, newline)
                                non-whitespace character
                                 a digit (0-9)
                               a non-digit
               0                 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 character, one of the metacharacters
 listed above, or a group of characters or
           metacharacters in parentheses.
               *                   zero or more of  the  previous
               +                    one  or  more of the previous
               ?                   zero or one  of  the  previous
               {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:

               /^+/              string starts with one  or  more
               /^$/                 nothing  in the string (start
and end are adjacent)
               /({3}/         a three digits, each followed by  a
                                   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
               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 $10;
                   print "Hostname is $20;

       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    [Toc]    [Back]

       Writing subroutines is easy:

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

       What's 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    [Toc]    [Back]

       OO Perl is relatively simple and is implemented using references
 which know what sort of object they are based on
       Perl's 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

       Using Perl modules    [Toc]    [Back]

       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 itself.

       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 perlmodinstall

       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 modules which came with your Perl  installation.
       If you feel the urge to write Perl modules, perlnewmod
       will give you good advice.

AUTHOR    [Toc]    [Back]

       Kirrily "Skud" Robert <skud@cpan.org>

perl v5.8.5                 2002-11-06                         13
[ Back ]
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