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 For more information, including instructions for other platforms such as
58 Windows and Mac OS, read L<perlrun>.
62 Perl by default is very forgiving. In order to make it more roboust
63 it is recommened to start every program with the following lines:
69 The C<use strict;> line imposes some restrictions that will mainly stop
70 you from introducing bugs in your code. The C<use warnings;> is more or
71 less equivalent to the command line switch B<-w> (see L<perlrun>). This will
72 catch various problems in your code and give warnings.
74 =head2 Basic syntax overview
76 A Perl script or program consists of one or more statements. These
77 statements are simply written in the script in a straightforward
78 fashion. There is no need to have a C<main()> function or anything of
81 Perl statements end in a semi-colon:
85 Comments start with a hash symbol and run to the end of the line
89 Whitespace is irrelevant:
95 ... except inside quoted strings:
97 # this would print with a linebreak in the middle
101 Double quotes or single quotes may be used around literal strings:
103 print "Hello, world";
104 print 'Hello, world';
106 However, only double quotes "interpolate" variables and special
107 characters such as newlines (C<\n>):
109 print "Hello, $name\n"; # works fine
110 print 'Hello, $name\n'; # prints $name\n literally
112 Numbers don't need quotes around them:
116 You can use parentheses for functions' arguments or omit them
117 according to your personal taste. They are only required
118 occasionally to clarify issues of precedence.
120 print("Hello, world\n");
121 print "Hello, world\n";
123 More detailed information about Perl syntax can be found in L<perlsyn>.
125 =head2 Perl variable types
127 Perl has three main variable types: scalars, arrays, and hashes.
133 A scalar represents a single value:
135 my $animal = "camel";
138 Scalar values can be strings, integers or floating point numbers, and Perl
139 will automatically convert between them as required. There is no need
140 to pre-declare your variable types, but you have to declare them using
141 the C<my> keyword the first time you use them. (This is one of the
142 requirements of C<use strict;>.)
144 Scalar values can be used in various ways:
147 print "The animal is $animal\n";
148 print "The square of $answer is ", $answer * $answer, "\n";
150 There are a number of "magic" scalars with names that look like
151 punctuation or line noise. These special variables are used for all
152 kinds of purposes, and are documented in L<perlvar>. The only one you
153 need to know about for now is C<$_> which is the "default variable".
154 It's used as the default argument to a number of functions in Perl, and
155 it's set implicitly by certain looping constructs.
157 print; # prints contents of $_ by default
161 An array represents a list of values:
163 my @animals = ("camel", "llama", "owl");
164 my @numbers = (23, 42, 69);
165 my @mixed = ("camel", 42, 1.23);
167 Arrays are zero-indexed. Here's how you get at elements in an array:
169 print $animals[0]; # prints "camel"
170 print $animals[1]; # prints "llama"
172 The special variable C<$#array> tells you the index of the last element
175 print $mixed[$#mixed]; # last element, prints 1.23
177 You might be tempted to use C<$#array + 1> to tell you how many items there
178 are in an array. Don't bother. As it happens, using C<@array> where Perl
179 expects to find a scalar value ("in scalar context") will give you the number
180 of elements in the array:
182 if (@animals < 5) { ... }
184 The elements we're getting from the array start with a C<$> because
185 we're getting just a single value out of the array -- you ask for a scalar,
188 To get multiple values from an array:
190 @animals[0,1]; # gives ("camel", "llama");
191 @animals[0..2]; # gives ("camel", "llama", "owl");
192 @animals[1..$#animals]; # gives all except the first element
194 This is called an "array slice".
196 You can do various useful things to lists:
198 my @sorted = sort @animals;
199 my @backwards = reverse @numbers;
201 There are a couple of special arrays too, such as C<@ARGV> (the command
202 line arguments to your script) and C<@_> (the arguments passed to a
203 subroutine). These are documented in L<perlvar>.
207 A hash represents a set of key/value pairs:
209 my %fruit_color = ("apple", "red", "banana", "yellow");
211 You can use whitespace and the C<< => >> operator to lay them out more
219 To get at hash elements:
221 $fruit_color{"apple"}; # gives "red"
223 You can get at lists of keys and values with C<keys()> and
226 my @fruits = keys %fruit_colors;
227 my @colors = values %fruit_colors;
229 Hashes have no particular internal order, though you can sort the keys
230 and loop through them.
232 Just like special scalars and arrays, there are also special hashes.
233 The most well known of these is C<%ENV> which contains environment
234 variables. Read all about it (and other special variables) in
239 Scalars, arrays and hashes are documented more fully in L<perldata>.
241 More complex data types can be constructed using references, which allow
242 you to build lists and hashes within lists and hashes.
244 A reference is a scalar value and can refer to any other Perl data
245 type. So by storing a reference as the value of an array or hash
246 element, you can easily create lists and hashes within lists and
247 hashes. The following example shows a 2 level hash of hash
248 structure using anonymous hash references.
252 description => "single item",
256 description => "ordered list of items",
260 description => "key/value pairs",
265 print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
267 Exhaustive information on the topic of references can be found in
268 L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.
270 =head2 Variable scoping
272 Throughout the previous section all the examples have used the syntax:
276 The C<my> is actually not required; you could just use:
280 However, the above usage will create global variables throughout your
281 program, which is bad programming practice. C<my> creates lexically
282 scoped variables instead. The variables are scoped to the block
283 (i.e. a bunch of statements surrounded by curly-braces) in which they
287 my $some_condition = 1;
288 if ($some_condition) {
290 print $x; # prints "foo"
291 print $y; # prints "bar"
293 print $x; # prints "foo"
294 print $y; # prints nothing; $y has fallen out of scope
296 Using C<my> in combination with a C<use strict;> at the top of
297 your Perl scripts means that the interpreter will pick up certain common
298 programming errors. For instance, in the example above, the final
299 C<print $b> would cause a compile-time error and prevent you from
300 running the program. Using C<strict> is highly recommended.
302 =head2 Conditional and looping constructs
304 Perl has most of the usual conditional and looping constructs except for
305 case/switch (but if you really want it, there is a Switch module in Perl
306 5.8 and newer, and on CPAN. See the section on modules, below, for more
307 information about modules and CPAN).
309 The conditions can be any Perl expression. See the list of operators in
310 the next section for information on comparison and boolean logic operators,
311 which are commonly used in conditional statements.
319 } elsif ( other condition ) {
325 There's also a negated version of it:
327 unless ( condition ) {
331 This is provided as a more readable version of C<if (!I<condition>)>.
333 Note that the braces are required in Perl, even if you've only got one
334 line in the block. However, there is a clever way of making your one-line
335 conditional blocks more English like:
337 # the traditional way
342 # the Perlish post-condition way
343 print "Yow!" if $zippy;
344 print "We have no bananas" unless $bananas;
348 while ( condition ) {
352 There's also a negated version, for the same reason we have C<unless>:
354 until ( condition ) {
358 You can also use C<while> in a post-condition:
360 print "LA LA LA\n" while 1; # loops forever
366 for ($i=0; $i <= $max; $i++) {
370 The C style for loop is rarely needed in Perl since Perl provides
371 the more friendly list scanning C<foreach> loop.
376 print "This element is $_\n";
379 print $list[$_] foreach 0 .. $max;
381 # you don't have to use the default $_ either...
382 foreach my $key (keys %hash) {
383 print "The value of $key is $hash{$key}\n";
388 For more detail on looping constructs (and some that weren't mentioned in
389 this overview) see L<perlsyn>.
391 =head2 Builtin operators and functions
393 Perl comes with a wide selection of builtin functions. Some of the ones
394 we've already seen include C<print>, C<sort> and C<reverse>. A list of
395 them is given at the start of L<perlfunc> and you can easily read
396 about any given function by using C<perldoc -f I<functionname>>.
398 Perl operators are documented in full in L<perlop>, but here are a few
399 of the most common ones:
410 =item Numeric comparison
416 <= less than or equal
417 >= greater than or equal
419 =item String comparison
425 le less than or equal
426 ge greater than or equal
428 (Why do we have separate numeric and string comparisons? Because we don't
429 have special variable types, and Perl needs to know whether to sort
430 numerically (where 99 is less than 100) or alphabetically (where 100 comes
439 (C<and>, C<or> and C<not> aren't just in the above table as descriptions
440 of the operators -- they're also supported as operators in their own
441 right. They're more readable than the C-style operators, but have
442 different precedence to C<&&> and friends. Check L<perlop> for more
448 . string concatenation
449 x string multiplication
450 .. range operator (creates a list of numbers)
454 Many operators can be combined with a C<=> as follows:
456 $a += 1; # same as $a = $a + 1
457 $a -= 1; # same as $a = $a - 1
458 $a .= "\n"; # same as $a = $a . "\n";
462 You can open a file for input or output using the C<open()> function.
463 It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
466 open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
467 open(my $out, ">", "output.txt") or die "Can't open output.txt: $!";
468 open(my $log, ">>", "my.log") or die "Can't open my.log: $!";
470 You can read from an open filehandle using the C<< <> >> operator. In
471 scalar context it reads a single line from the filehandle, and in list
472 context it reads the whole file in, assigning each line to an element of
478 Reading in the whole file at one time is called slurping. It can
479 be useful but it may be a memory hog. Most text file processing
480 can be done a line at a time with Perl's looping constructs.
482 The C<< <> >> operator is most often seen in a C<while> loop:
484 while (<$in>) { # assigns each line in turn to $_
485 print "Just read in this line: $_";
488 We've already seen how to print to standard output using C<print()>.
489 However, C<print()> can also take an optional first argument specifying
490 which filehandle to print to:
492 print STDERR "This is your final warning.\n";
494 print $log $logmessage;
496 When you're done with your filehandles, you should C<close()> them
497 (though to be honest, Perl will clean up after you if you forget):
499 close $in or die "$in: $!";
501 =head2 Regular expressions
503 Perl's regular expression support is both broad and deep, and is the
504 subject of lengthy documentation in L<perlrequick>, L<perlretut>, and
505 elsewhere. However, in short:
509 =item Simple matching
511 if (/foo/) { ... } # true if $_ contains "foo"
512 if ($a =~ /foo/) { ... } # true if $a contains "foo"
514 The C<//> matching operator is documented in L<perlop>. It operates on
515 C<$_> by default, or can be bound to another variable using the C<=~>
516 binding operator (also documented in L<perlop>).
518 =item Simple substitution
520 s/foo/bar/; # replaces foo with bar in $_
521 $a =~ s/foo/bar/; # replaces foo with bar in $a
522 $a =~ s/foo/bar/g; # replaces ALL INSTANCES of foo with bar in $a
524 The C<s///> substitution operator is documented in L<perlop>.
526 =item More complex regular expressions
528 You don't just have to match on fixed strings. In fact, you can match
529 on just about anything you could dream of by using more complex regular
530 expressions. These are documented at great length in L<perlre>, but for
531 the meantime, here's a quick cheat sheet:
534 \s a whitespace character (space, tab, newline)
535 \S non-whitespace character
538 \w a word character (a-z, A-Z, 0-9, _)
539 \W a non-word character
540 [aeiou] matches a single character in the given set
541 [^aeiou] matches a single character outside the given set
542 (foo|bar|baz) matches any of the alternatives specified
547 Quantifiers can be used to specify how many of the previous thing you
548 want to match on, where "thing" means either a literal character, one
549 of the metacharacters listed above, or a group of characters or
550 metacharacters in parentheses.
552 * zero or more of the previous thing
553 + one or more of the previous thing
554 ? zero or one of the previous thing
555 {3} matches exactly 3 of the previous thing
556 {3,6} matches between 3 and 6 of the previous thing
557 {3,} matches 3 or more of the previous thing
561 /^\d+/ string starts with one or more digits
562 /^$/ nothing in the string (start and end are adjacent)
563 /(\d\s){3}/ a three digits, each followed by a whitespace
564 character (eg "3 4 5 ")
565 /(a.)+/ matches a string in which every odd-numbered letter
568 # This loop reads from STDIN, and prints non-blank lines:
574 =item Parentheses for capturing
576 As well as grouping, parentheses serve a second purpose. They can be
577 used to capture the results of parts of the regexp match for later use.
578 The results end up in C<$1>, C<$2> and so on.
580 # a cheap and nasty way to break an email address up into parts
582 if ($email =~ /([^@]+)@(.+)/) {
583 print "Username is $1\n";
584 print "Hostname is $2\n";
587 =item Other regexp features
589 Perl regexps also support backreferences, lookaheads, and all kinds of
590 other complex details. Read all about them in L<perlrequick>,
591 L<perlretut>, and L<perlre>.
595 =head2 Writing subroutines
597 Writing subroutines is easy:
600 my $logmessage = shift;
601 open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
602 print $logfile $logmessage;
605 Now we can use the subroutine just as any other built-in function:
607 logger("We have a logger subroutine!");
609 What's that C<shift>? Well, the arguments to a subroutine are available
610 to us as a special array called C<@_> (see L<perlvar> for more on that).
611 The default argument to the C<shift> function just happens to be C<@_>.
612 So C<my $logmessage = shift;> shifts the first item off the list of
613 arguments and assigns it to C<$logmessage>.
615 We can manipulate C<@_> in other ways too:
617 my ($logmessage, $priority) = @_; # common
618 my $logmessage = $_[0]; # uncommon, and ugly
620 Subroutines can also return values:
624 my $result = $num * $num;
632 For more information on writing subroutines, see L<perlsub>.
636 OO Perl is relatively simple and is implemented using references which
637 know what sort of object they are based on Perl's concept of packages.
638 However, OO Perl is largely beyond the scope of this document.
639 Read L<perlboot>, L<perltoot>, L<perltooc> and L<perlobj>.
641 As a beginning Perl programmer, your most common use of OO Perl will be
642 in using third-party modules, which are documented below.
644 =head2 Using Perl modules
646 Perl modules provide a range of features to help you avoid reinventing
647 the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ). A
648 number of popular modules are included with the Perl distribution
651 Categories of modules range from text manipulation to network protocols
652 to database integration to graphics. A categorized list of modules is
653 also available from CPAN.
655 To learn how to install modules you download from CPAN, read
658 To learn how to use a particular module, use C<perldoc I<Module::Name>>.
659 Typically you will want to C<use I<Module::Name>>, which will then give
660 you access to exported functions or an OO interface to the module.
662 L<perlfaq> contains questions and answers related to many common
663 tasks, and often provides suggestions for good CPAN modules to use.
665 L<perlmod> describes Perl modules in general. L<perlmodlib> lists the
666 modules which came with your Perl installation.
668 If you feel the urge to write Perl modules, L<perlnewmod> will give you
673 Kirrily "Skud" Robert <skud@cpan.org>