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 robust
63 it is recommened to start every program with the following lines:
69 The two additional lines request from perl to catch various common
70 problems in your code. They check different things so you need both. A
71 potential problem caught by C<use strict;> will cause your code to stop
72 immediately when it is encountered, while C<use warnings;> will merely
73 give a warning (like the command-line switch B<-w>) and let your code run.
74 To read more about them check their respective manual pages at L<strict>
77 =head2 Basic syntax overview
79 A Perl script or program consists of one or more statements. These
80 statements are simply written in the script in a straightforward
81 fashion. There is no need to have a C<main()> function or anything of
84 Perl statements end in a semi-colon:
88 Comments start with a hash symbol and run to the end of the line
92 Whitespace is irrelevant:
98 ... except inside quoted strings:
100 # this would print with a linebreak in the middle
104 Double quotes or single quotes may be used around literal strings:
106 print "Hello, world";
107 print 'Hello, world';
109 However, only double quotes "interpolate" variables and special
110 characters such as newlines (C<\n>):
112 print "Hello, $name\n"; # works fine
113 print 'Hello, $name\n'; # prints $name\n literally
115 Numbers don't need quotes around them:
119 You can use parentheses for functions' arguments or omit them
120 according to your personal taste. They are only required
121 occasionally to clarify issues of precedence.
123 print("Hello, world\n");
124 print "Hello, world\n";
126 More detailed information about Perl syntax can be found in L<perlsyn>.
128 =head2 Perl variable types
130 Perl has three main variable types: scalars, arrays, and hashes.
136 A scalar represents a single value:
138 my $animal = "camel";
141 Scalar values can be strings, integers or floating point numbers, and Perl
142 will automatically convert between them as required. There is no need
143 to pre-declare your variable types, but you have to declare them using
144 the C<my> keyword the first time you use them. (This is one of the
145 requirements of C<use strict;>.)
147 Scalar values can be used in various ways:
150 print "The animal is $animal\n";
151 print "The square of $answer is ", $answer * $answer, "\n";
153 There are a number of "magic" scalars with names that look like
154 punctuation or line noise. These special variables are used for all
155 kinds of purposes, and are documented in L<perlvar>. The only one you
156 need to know about for now is C<$_> which is the "default variable".
157 It's used as the default argument to a number of functions in Perl, and
158 it's set implicitly by certain looping constructs.
160 print; # prints contents of $_ by default
164 An array represents a list of values:
166 my @animals = ("camel", "llama", "owl");
167 my @numbers = (23, 42, 69);
168 my @mixed = ("camel", 42, 1.23);
170 Arrays are zero-indexed. Here's how you get at elements in an array:
172 print $animals[0]; # prints "camel"
173 print $animals[1]; # prints "llama"
175 The special variable C<$#array> tells you the index of the last element
178 print $mixed[$#mixed]; # last element, prints 1.23
180 You might be tempted to use C<$#array + 1> to tell you how many items there
181 are in an array. Don't bother. As it happens, using C<@array> where Perl
182 expects to find a scalar value ("in scalar context") will give you the number
183 of elements in the array:
185 if (@animals < 5) { ... }
187 The elements we're getting from the array start with a C<$> because
188 we're getting just a single value out of the array -- you ask for a scalar,
191 To get multiple values from an array:
193 @animals[0,1]; # gives ("camel", "llama");
194 @animals[0..2]; # gives ("camel", "llama", "owl");
195 @animals[1..$#animals]; # gives all except the first element
197 This is called an "array slice".
199 You can do various useful things to lists:
201 my @sorted = sort @animals;
202 my @backwards = reverse @numbers;
204 There are a couple of special arrays too, such as C<@ARGV> (the command
205 line arguments to your script) and C<@_> (the arguments passed to a
206 subroutine). These are documented in L<perlvar>.
210 A hash represents a set of key/value pairs:
212 my %fruit_color = ("apple", "red", "banana", "yellow");
214 You can use whitespace and the C<< => >> operator to lay them out more
222 To get at hash elements:
224 $fruit_color{"apple"}; # gives "red"
226 You can get at lists of keys and values with C<keys()> and
229 my @fruits = keys %fruit_colors;
230 my @colors = values %fruit_colors;
232 Hashes have no particular internal order, though you can sort the keys
233 and loop through them.
235 Just like special scalars and arrays, there are also special hashes.
236 The most well known of these is C<%ENV> which contains environment
237 variables. Read all about it (and other special variables) in
242 Scalars, arrays and hashes are documented more fully in L<perldata>.
244 More complex data types can be constructed using references, which allow
245 you to build lists and hashes within lists and hashes.
247 A reference is a scalar value and can refer to any other Perl data
248 type. So by storing a reference as the value of an array or hash
249 element, you can easily create lists and hashes within lists and
250 hashes. The following example shows a 2 level hash of hash
251 structure using anonymous hash references.
255 description => "single item",
259 description => "ordered list of items",
263 description => "key/value pairs",
268 print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
270 Exhaustive information on the topic of references can be found in
271 L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.
273 =head2 Variable scoping
275 Throughout the previous section all the examples have used the syntax:
279 The C<my> is actually not required; you could just use:
283 However, the above usage will create global variables throughout your
284 program, which is bad programming practice. C<my> creates lexically
285 scoped variables instead. The variables are scoped to the block
286 (i.e. a bunch of statements surrounded by curly-braces) in which they
290 my $some_condition = 1;
291 if ($some_condition) {
293 print $x; # prints "foo"
294 print $y; # prints "bar"
296 print $x; # prints "foo"
297 print $y; # prints nothing; $y has fallen out of scope
299 Using C<my> in combination with a C<use strict;> at the top of
300 your Perl scripts means that the interpreter will pick up certain common
301 programming errors. For instance, in the example above, the final
302 C<print $b> would cause a compile-time error and prevent you from
303 running the program. Using C<strict> is highly recommended.
305 =head2 Conditional and looping constructs
307 Perl has most of the usual conditional and looping constructs except for
308 case/switch (but if you really want it, there is a Switch module in Perl
309 5.8 and newer, and on CPAN. See the section on modules, below, for more
310 information about modules and CPAN).
312 The conditions can be any Perl expression. See the list of operators in
313 the next section for information on comparison and boolean logic operators,
314 which are commonly used in conditional statements.
322 } elsif ( other condition ) {
328 There's also a negated version of it:
330 unless ( condition ) {
334 This is provided as a more readable version of C<if (!I<condition>)>.
336 Note that the braces are required in Perl, even if you've only got one
337 line in the block. However, there is a clever way of making your one-line
338 conditional blocks more English like:
340 # the traditional way
345 # the Perlish post-condition way
346 print "Yow!" if $zippy;
347 print "We have no bananas" unless $bananas;
351 while ( condition ) {
355 There's also a negated version, for the same reason we have C<unless>:
357 until ( condition ) {
361 You can also use C<while> in a post-condition:
363 print "LA LA LA\n" while 1; # loops forever
369 for ($i=0; $i <= $max; $i++) {
373 The C style for loop is rarely needed in Perl since Perl provides
374 the more friendly list scanning C<foreach> loop.
379 print "This element is $_\n";
382 print $list[$_] foreach 0 .. $max;
384 # you don't have to use the default $_ either...
385 foreach my $key (keys %hash) {
386 print "The value of $key is $hash{$key}\n";
391 For more detail on looping constructs (and some that weren't mentioned in
392 this overview) see L<perlsyn>.
394 =head2 Builtin operators and functions
396 Perl comes with a wide selection of builtin functions. Some of the ones
397 we've already seen include C<print>, C<sort> and C<reverse>. A list of
398 them is given at the start of L<perlfunc> and you can easily read
399 about any given function by using C<perldoc -f I<functionname>>.
401 Perl operators are documented in full in L<perlop>, but here are a few
402 of the most common ones:
413 =item Numeric comparison
419 <= less than or equal
420 >= greater than or equal
422 =item String comparison
428 le less than or equal
429 ge greater than or equal
431 (Why do we have separate numeric and string comparisons? Because we don't
432 have special variable types, and Perl needs to know whether to sort
433 numerically (where 99 is less than 100) or alphabetically (where 100 comes
442 (C<and>, C<or> and C<not> aren't just in the above table as descriptions
443 of the operators -- they're also supported as operators in their own
444 right. They're more readable than the C-style operators, but have
445 different precedence to C<&&> and friends. Check L<perlop> for more
451 . string concatenation
452 x string multiplication
453 .. range operator (creates a list of numbers)
457 Many operators can be combined with a C<=> as follows:
459 $a += 1; # same as $a = $a + 1
460 $a -= 1; # same as $a = $a - 1
461 $a .= "\n"; # same as $a = $a . "\n";
465 You can open a file for input or output using the C<open()> function.
466 It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
469 open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";
470 open(my $out, ">", "output.txt") or die "Can't open output.txt: $!";
471 open(my $log, ">>", "my.log") or die "Can't open my.log: $!";
473 You can read from an open filehandle using the C<< <> >> operator. In
474 scalar context it reads a single line from the filehandle, and in list
475 context it reads the whole file in, assigning each line to an element of
481 Reading in the whole file at one time is called slurping. It can
482 be useful but it may be a memory hog. Most text file processing
483 can be done a line at a time with Perl's looping constructs.
485 The C<< <> >> operator is most often seen in a C<while> loop:
487 while (<$in>) { # assigns each line in turn to $_
488 print "Just read in this line: $_";
491 We've already seen how to print to standard output using C<print()>.
492 However, C<print()> can also take an optional first argument specifying
493 which filehandle to print to:
495 print STDERR "This is your final warning.\n";
497 print $log $logmessage;
499 When you're done with your filehandles, you should C<close()> them
500 (though to be honest, Perl will clean up after you if you forget):
502 close $in or die "$in: $!";
504 =head2 Regular expressions
506 Perl's regular expression support is both broad and deep, and is the
507 subject of lengthy documentation in L<perlrequick>, L<perlretut>, and
508 elsewhere. However, in short:
512 =item Simple matching
514 if (/foo/) { ... } # true if $_ contains "foo"
515 if ($a =~ /foo/) { ... } # true if $a contains "foo"
517 The C<//> matching operator is documented in L<perlop>. It operates on
518 C<$_> by default, or can be bound to another variable using the C<=~>
519 binding operator (also documented in L<perlop>).
521 =item Simple substitution
523 s/foo/bar/; # replaces foo with bar in $_
524 $a =~ s/foo/bar/; # replaces foo with bar in $a
525 $a =~ s/foo/bar/g; # replaces ALL INSTANCES of foo with bar in $a
527 The C<s///> substitution operator is documented in L<perlop>.
529 =item More complex regular expressions
531 You don't just have to match on fixed strings. In fact, you can match
532 on just about anything you could dream of by using more complex regular
533 expressions. These are documented at great length in L<perlre>, but for
534 the meantime, here's a quick cheat sheet:
537 \s a whitespace character (space, tab, newline)
538 \S non-whitespace character
541 \w a word character (a-z, A-Z, 0-9, _)
542 \W a non-word character
543 [aeiou] matches a single character in the given set
544 [^aeiou] matches a single character outside the given set
545 (foo|bar|baz) matches any of the alternatives specified
550 Quantifiers can be used to specify how many of the previous thing you
551 want to match on, where "thing" means either a literal character, one
552 of the metacharacters listed above, or a group of characters or
553 metacharacters in parentheses.
555 * zero or more of the previous thing
556 + one or more of the previous thing
557 ? zero or one of the previous thing
558 {3} matches exactly 3 of the previous thing
559 {3,6} matches between 3 and 6 of the previous thing
560 {3,} matches 3 or more of the previous thing
564 /^\d+/ string starts with one or more digits
565 /^$/ nothing in the string (start and end are adjacent)
566 /(\d\s){3}/ a three digits, each followed by a whitespace
567 character (eg "3 4 5 ")
568 /(a.)+/ matches a string in which every odd-numbered letter
571 # This loop reads from STDIN, and prints non-blank lines:
577 =item Parentheses for capturing
579 As well as grouping, parentheses serve a second purpose. They can be
580 used to capture the results of parts of the regexp match for later use.
581 The results end up in C<$1>, C<$2> and so on.
583 # a cheap and nasty way to break an email address up into parts
585 if ($email =~ /([^@]+)@(.+)/) {
586 print "Username is $1\n";
587 print "Hostname is $2\n";
590 =item Other regexp features
592 Perl regexps also support backreferences, lookaheads, and all kinds of
593 other complex details. Read all about them in L<perlrequick>,
594 L<perlretut>, and L<perlre>.
598 =head2 Writing subroutines
600 Writing subroutines is easy:
603 my $logmessage = shift;
604 open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
605 print $logfile $logmessage;
608 Now we can use the subroutine just as any other built-in function:
610 logger("We have a logger subroutine!");
612 What's that C<shift>? Well, the arguments to a subroutine are available
613 to us as a special array called C<@_> (see L<perlvar> for more on that).
614 The default argument to the C<shift> function just happens to be C<@_>.
615 So C<my $logmessage = shift;> shifts the first item off the list of
616 arguments and assigns it to C<$logmessage>.
618 We can manipulate C<@_> in other ways too:
620 my ($logmessage, $priority) = @_; # common
621 my $logmessage = $_[0]; # uncommon, and ugly
623 Subroutines can also return values:
627 my $result = $num * $num;
635 For more information on writing subroutines, see L<perlsub>.
639 OO Perl is relatively simple and is implemented using references which
640 know what sort of object they are based on Perl's concept of packages.
641 However, OO Perl is largely beyond the scope of this document.
642 Read L<perlboot>, L<perltoot>, L<perltooc> and L<perlobj>.
644 As a beginning Perl programmer, your most common use of OO Perl will be
645 in using third-party modules, which are documented below.
647 =head2 Using Perl modules
649 Perl modules provide a range of features to help you avoid reinventing
650 the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ). A
651 number of popular modules are included with the Perl distribution
654 Categories of modules range from text manipulation to network protocols
655 to database integration to graphics. A categorized list of modules is
656 also available from CPAN.
658 To learn how to install modules you download from CPAN, read
661 To learn how to use a particular module, use C<perldoc I<Module::Name>>.
662 Typically you will want to C<use I<Module::Name>>, which will then give
663 you access to exported functions or an OO interface to the module.
665 L<perlfaq> contains questions and answers related to many common
666 tasks, and often provides suggestions for good CPAN modules to use.
668 L<perlmod> describes Perl modules in general. L<perlmodlib> lists the
669 modules which came with your Perl installation.
671 If you feel the urge to write Perl modules, L<perlnewmod> will give you
676 Kirrily "Skud" Robert <skud@cpan.org>