3 perlintro -- a brief introduction and overview of Perl
7 This document is intended to give you a quick overview of the Perl
8 programming language, along with pointers to further documentation. It
9 is intended as a "bootstrap" guide for those who are new to the
10 language, and provides just enough information for you to be able to
11 read other peoples' Perl and understand roughly what it's doing, or
12 write your own simple scripts.
14 This introductory document does not aim to be complete. It does not
15 even aim to be entirely accurate. In some cases perfection has been
16 sacrificed in the goal of getting the general idea across. You are
17 I<strongly> advised to follow this introduction with more information
18 from the full Perl manual, the table of contents to which can be found
21 Throughout this document you'll see references to other parts of the
22 Perl documentation. You can read that documentation using the C<perldoc>
23 command or whatever method you're using to read this document.
27 Perl is a general-purpose programming language originally developed for
28 text manipulation and now used for a wide range of tasks including
29 system administration, web development, network programming, GUI
30 development, and more.
32 The language is intended to be practical (easy to use, efficient,
33 complete) rather than beautiful (tiny, elegant, minimal). Its major
34 features are that it's easy to use, supports both procedural and
35 object-oriented (OO) programming, has powerful built-in support for text
36 processing, and has one of the world's most impressive collections of
39 Different definitions of Perl are given in L<perl>, L<perlfaq1> and
40 no doubt other places. From this we can determine that Perl is different
41 things to different people, but that lots of people think it's at least
44 =head2 Running Perl programs
46 To run a Perl program from the Unix command line:
50 Alternatively, put this as the first line of your script:
54 ... and run the script as C</path/to/script.pl>. Of course, it'll need
55 to be executable first, so C<chmod 755 script.pl> (under Unix).
57 (This start line assumes you have the B<env> program. You can also put
58 directly the path to your perl executable, like in C<#!/usr/bin/perl>).
60 For more information, including instructions for other platforms such as
61 Windows and Mac OS, read L<perlrun>.
65 Perl by default is very forgiving. In order to make it more robust
66 it is recommended to start every program with the following lines:
72 The two additional lines request from perl to catch various common
73 problems in your code. They check different things so you need both. A
74 potential problem caught by C<use strict;> will cause your code to stop
75 immediately when it is encountered, while C<use warnings;> will merely
76 give a warning (like the command-line switch B<-w>) and let your code run.
77 To read more about them check their respective manual pages at L<strict>
80 =head2 Basic syntax overview
82 A Perl script or program consists of one or more statements. These
83 statements are simply written in the script in a straightforward
84 fashion. There is no need to have a C<main()> function or anything of
87 Perl statements end in a semi-colon:
91 Comments start with a hash symbol and run to the end of the line
95 Whitespace is irrelevant:
101 ... except inside quoted strings:
103 # this would print with a linebreak in the middle
107 Double quotes or single quotes may be used around literal strings:
109 print "Hello, world";
110 print 'Hello, world';
112 However, only double quotes "interpolate" variables and special
113 characters such as newlines (C<\n>):
115 print "Hello, $name\n"; # works fine
116 print 'Hello, $name\n'; # prints $name\n literally
118 Numbers don't need quotes around them:
122 You can use parentheses for functions' arguments or omit them
123 according to your personal taste. They are only required
124 occasionally to clarify issues of precedence.
126 print("Hello, world\n");
127 print "Hello, world\n";
129 More detailed information about Perl syntax can be found in L<perlsyn>.
131 =head2 Perl variable types
133 Perl has three main variable types: scalars, arrays, and hashes.
139 A scalar represents a single value:
141 my $animal = "camel";
144 Scalar values can be strings, integers or floating point numbers, and Perl
145 will automatically convert between them as required. There is no need
146 to pre-declare your variable types, but you have to declare them using
147 the C<my> keyword the first time you use them. (This is one of the
148 requirements of C<use strict;>.)
150 Scalar values can be used in various ways:
153 print "The animal is $animal\n";
154 print "The square of $answer is ", $answer * $answer, "\n";
156 There are a number of "magic" scalars with names that look like
157 punctuation or line noise. These special variables are used for all
158 kinds of purposes, and are documented in L<perlvar>. The only one you
159 need to know about for now is C<$_> which is the "default variable".
160 It's used as the default argument to a number of functions in Perl, and
161 it's set implicitly by certain looping constructs.
163 print; # prints contents of $_ by default
167 An array represents a list of values:
169 my @animals = ("camel", "llama", "owl");
170 my @numbers = (23, 42, 69);
171 my @mixed = ("camel", 42, 1.23);
173 Arrays are zero-indexed. Here's how you get at elements in an array:
175 print $animals[0]; # prints "camel"
176 print $animals[1]; # prints "llama"
178 The special variable C<$#array> tells you the index of the last element
181 print $mixed[$#mixed]; # last element, prints 1.23
183 You might be tempted to use C<$#array + 1> to tell you how many items there
184 are in an array. Don't bother. As it happens, using C<@array> where Perl
185 expects to find a scalar value ("in scalar context") will give you the number
186 of elements in the array:
188 if (@animals < 5) { ... }
190 The elements we're getting from the array start with a C<$> because
191 we're getting just a single value out of the array -- you ask for a scalar,
194 To get multiple values from an array:
196 @animals[0,1]; # gives ("camel", "llama");
197 @animals[0..2]; # gives ("camel", "llama", "owl");
198 @animals[1..$#animals]; # gives all except the first element
200 This is called an "array slice".
202 You can do various useful things to lists:
204 my @sorted = sort @animals;
205 my @backwards = reverse @numbers;
207 There are a couple of special arrays too, such as C<@ARGV> (the command
208 line arguments to your script) and C<@_> (the arguments passed to a
209 subroutine). These are documented in L<perlvar>.
213 A hash represents a set of key/value pairs:
215 my %fruit_color = ("apple", "red", "banana", "yellow");
217 You can use whitespace and the C<< => >> operator to lay them out more
225 To get at hash elements:
227 $fruit_color{"apple"}; # gives "red"
229 You can get at lists of keys and values with C<keys()> and
232 my @fruits = keys %fruit_colors;
233 my @colors = values %fruit_colors;
235 Hashes have no particular internal order, though you can sort the keys
236 and loop through them.
238 Just like special scalars and arrays, there are also special hashes.
239 The most well known of these is C<%ENV> which contains environment
240 variables. Read all about it (and other special variables) in
245 Scalars, arrays and hashes are documented more fully in L<perldata>.
247 More complex data types can be constructed using references, which allow
248 you to build lists and hashes within lists and hashes.
250 A reference is a scalar value and can refer to any other Perl data
251 type. So by storing a reference as the value of an array or hash
252 element, you can easily create lists and hashes within lists and
253 hashes. The following example shows a 2 level hash of hash
254 structure using anonymous hash references.
258 description => "single item",
262 description => "ordered list of items",
266 description => "key/value pairs",
271 print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
273 Exhaustive information on the topic of references can be found in
274 L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.
276 =head2 Variable scoping
278 Throughout the previous section all the examples have used the syntax:
282 The C<my> is actually not required; you could just use:
286 However, the above usage will create global variables throughout your
287 program, which is bad programming practice. C<my> creates lexically
288 scoped variables instead. The variables are scoped to the block
289 (i.e. a bunch of statements surrounded by curly-braces) in which they
293 my $some_condition = 1;
294 if ($some_condition) {
296 print $x; # prints "foo"
297 print $y; # prints "bar"
299 print $x; # prints "foo"
300 print $y; # prints nothing; $y has fallen out of scope
302 Using C<my> in combination with a C<use strict;> at the top of
303 your Perl scripts means that the interpreter will pick up certain common
304 programming errors. For instance, in the example above, the final
305 C<print $y> would cause a compile-time error and prevent you from
306 running the program. Using C<strict> is highly recommended.
308 =head2 Conditional and looping constructs
310 Perl has most of the usual conditional and looping constructs except for
311 case/switch (but if you really want it, there is a Switch module in Perl
312 5.8 and newer, and on CPAN. See the section on modules, below, for more
313 information about modules and CPAN).
315 The conditions can be any Perl expression. See the list of operators in
316 the next section for information on comparison and boolean logic operators,
317 which are commonly used in conditional statements.
325 } elsif ( other condition ) {
331 There's also a negated version of it:
333 unless ( condition ) {
337 This is provided as a more readable version of C<if (!I<condition>)>.
339 Note that the braces are required in Perl, even if you've only got one
340 line in the block. However, there is a clever way of making your one-line
341 conditional blocks more English like:
343 # the traditional way
348 # the Perlish post-condition way
349 print "Yow!" if $zippy;
350 print "We have no bananas" unless $bananas;
354 while ( condition ) {
358 There's also a negated version, for the same reason we have C<unless>:
360 until ( condition ) {
364 You can also use C<while> in a post-condition:
366 print "LA LA LA\n" while 1; # loops forever
372 for ($i = 0; $i <= $max; $i++) {
376 The C style for loop is rarely needed in Perl since Perl provides
377 the more friendly list scanning C<foreach> loop.
382 print "This element is $_\n";
385 print $list[$_] foreach 0 .. $max;
387 # you don't have to use the default $_ either...
388 foreach my $key (keys %hash) {
389 print "The value of $key is $hash{$key}\n";
394 For more detail on looping constructs (and some that weren't mentioned in
395 this overview) see L<perlsyn>.
397 =head2 Builtin operators and functions
399 Perl comes with a wide selection of builtin functions. Some of the ones
400 we've already seen include C<print>, C<sort> and C<reverse>. A list of
401 them is given at the start of L<perlfunc> and you can easily read
402 about any given function by using C<perldoc -f I<functionname>>.
404 Perl operators are documented in full in L<perlop>, but here are a few
405 of the most common ones:
416 =item Numeric comparison
422 <= less than or equal
423 >= greater than or equal
425 =item String comparison
431 le less than or equal
432 ge greater than or equal
434 (Why do we have separate numeric and string comparisons? Because we don't
435 have special variable types, and Perl needs to know whether to sort
436 numerically (where 99 is less than 100) or alphabetically (where 100 comes
445 (C<and>, C<or> and C<not> aren't just in the above table as descriptions
446 of the operators -- they're also supported as operators in their own
447 right. They're more readable than the C-style operators, but have
448 different precedence to C<&&> and friends. Check L<perlop> for more
454 . string concatenation
455 x string multiplication
456 .. range operator (creates a list of numbers)
460 Many operators can be combined with a C<=> as follows:
462 $a += 1; # same as $a = $a + 1
463 $a -= 1; # same as $a = $a - 1
464 $a .= "\n"; # same as $a = $a . "\n";
468 You can open a file for input or output using the C<open()> function.
469 It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
472 open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
473 open(my $out, ">", "output.txt") or die "Can't open output.txt: $!";
474 open(my $log, ">>", "my.log") or die "Can't open my.log: $!";
476 You can read from an open filehandle using the C<< <> >> operator. In
477 scalar context it reads a single line from the filehandle, and in list
478 context it reads the whole file in, assigning each line to an element of
484 Reading in the whole file at one time is called slurping. It can
485 be useful but it may be a memory hog. Most text file processing
486 can be done a line at a time with Perl's looping constructs.
488 The C<< <> >> operator is most often seen in a C<while> loop:
490 while (<$in>) { # assigns each line in turn to $_
491 print "Just read in this line: $_";
494 We've already seen how to print to standard output using C<print()>.
495 However, C<print()> can also take an optional first argument specifying
496 which filehandle to print to:
498 print STDERR "This is your final warning.\n";
500 print $log $logmessage;
502 When you're done with your filehandles, you should C<close()> them
503 (though to be honest, Perl will clean up after you if you forget):
505 close $in or die "$in: $!";
507 =head2 Regular expressions
509 Perl's regular expression support is both broad and deep, and is the
510 subject of lengthy documentation in L<perlrequick>, L<perlretut>, and
511 elsewhere. However, in short:
515 =item Simple matching
517 if (/foo/) { ... } # true if $_ contains "foo"
518 if ($a =~ /foo/) { ... } # true if $a contains "foo"
520 The C<//> matching operator is documented in L<perlop>. It operates on
521 C<$_> by default, or can be bound to another variable using the C<=~>
522 binding operator (also documented in L<perlop>).
524 =item Simple substitution
526 s/foo/bar/; # replaces foo with bar in $_
527 $a =~ s/foo/bar/; # replaces foo with bar in $a
528 $a =~ s/foo/bar/g; # replaces ALL INSTANCES of foo with bar in $a
530 The C<s///> substitution operator is documented in L<perlop>.
532 =item More complex regular expressions
534 You don't just have to match on fixed strings. In fact, you can match
535 on just about anything you could dream of by using more complex regular
536 expressions. These are documented at great length in L<perlre>, but for
537 the meantime, here's a quick cheat sheet:
540 \s a whitespace character (space, tab, newline, ...)
541 \S non-whitespace character
544 \w a word character (a-z, A-Z, 0-9, _)
545 \W a non-word character
546 [aeiou] matches a single character in the given set
547 [^aeiou] matches a single character outside the given set
548 (foo|bar|baz) matches any of the alternatives specified
553 Quantifiers can be used to specify how many of the previous thing you
554 want to match on, where "thing" means either a literal character, one
555 of the metacharacters listed above, or a group of characters or
556 metacharacters in parentheses.
558 * zero or more of the previous thing
559 + one or more of the previous thing
560 ? zero or one of the previous thing
561 {3} matches exactly 3 of the previous thing
562 {3,6} matches between 3 and 6 of the previous thing
563 {3,} matches 3 or more of the previous thing
567 /^\d+/ string starts with one or more digits
568 /^$/ nothing in the string (start and end are adjacent)
569 /(\d\s){3}/ a three digits, each followed by a whitespace
570 character (eg "3 4 5 ")
571 /(a.)+/ matches a string in which every odd-numbered letter
574 # This loop reads from STDIN, and prints non-blank lines:
580 =item Parentheses for capturing
582 As well as grouping, parentheses serve a second purpose. They can be
583 used to capture the results of parts of the regexp match for later use.
584 The results end up in C<$1>, C<$2> and so on.
586 # a cheap and nasty way to break an email address up into parts
588 if ($email =~ /([^@]+)@(.+)/) {
589 print "Username is $1\n";
590 print "Hostname is $2\n";
593 =item Other regexp features
595 Perl regexps also support backreferences, lookaheads, and all kinds of
596 other complex details. Read all about them in L<perlrequick>,
597 L<perlretut>, and L<perlre>.
601 =head2 Writing subroutines
603 Writing subroutines is easy:
606 my $logmessage = shift;
607 open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
608 print $logfile $logmessage;
611 Now we can use the subroutine just as any other built-in function:
613 logger("We have a logger subroutine!");
615 What's that C<shift>? Well, the arguments to a subroutine are available
616 to us as a special array called C<@_> (see L<perlvar> for more on that).
617 The default argument to the C<shift> function just happens to be C<@_>.
618 So C<my $logmessage = shift;> shifts the first item off the list of
619 arguments and assigns it to C<$logmessage>.
621 We can manipulate C<@_> in other ways too:
623 my ($logmessage, $priority) = @_; # common
624 my $logmessage = $_[0]; # uncommon, and ugly
626 Subroutines can also return values:
630 my $result = $num * $num;
638 For more information on writing subroutines, see L<perlsub>.
642 OO Perl is relatively simple and is implemented using references which
643 know what sort of object they are based on Perl's concept of packages.
644 However, OO Perl is largely beyond the scope of this document.
645 Read L<perlboot>, L<perltoot>, L<perltooc> and L<perlobj>.
647 As a beginning Perl programmer, your most common use of OO Perl will be
648 in using third-party modules, which are documented below.
650 =head2 Using Perl modules
652 Perl modules provide a range of features to help you avoid reinventing
653 the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ). A
654 number of popular modules are included with the Perl distribution
657 Categories of modules range from text manipulation to network protocols
658 to database integration to graphics. A categorized list of modules is
659 also available from CPAN.
661 To learn how to install modules you download from CPAN, read
664 To learn how to use a particular module, use C<perldoc I<Module::Name>>.
665 Typically you will want to C<use I<Module::Name>>, which will then give
666 you access to exported functions or an OO interface to the module.
668 L<perlfaq> contains questions and answers related to many common
669 tasks, and often provides suggestions for good CPAN modules to use.
671 L<perlmod> describes Perl modules in general. L<perlmodlib> lists the
672 modules which came with your Perl installation.
674 If you feel the urge to write Perl modules, L<perlnewmod> will give you
679 Kirrily "Skud" Robert <skud@cpan.org>