7 A Perl program consists of a sequence of declarations and statements
8 which run from the top to the bottom. Loops, subroutines and other
9 control structures allow you to jump around within the code.
11 Perl is a B<free-form> language, you can format and indent it however
12 you like. Whitespace mostly serves to separate tokens, unlike
13 languages like Python where it is an important part of the syntax.
15 Many of Perl's syntactic elements are B<optional>. Rather than
16 requiring you to put parentheses around every function call and
17 declare every variable, you can often leave such explicit elements off
18 and Perl will figure out what you meant. This is known as B<Do What I
19 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
20 code in a style with which they are comfortable.
22 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
23 Bourne Shell, Smalltalk, Lisp and even English. Other
24 languages have borrowed syntax from Perl, particularly its regular
25 expression extensions. So if you have programmed in another language
26 you will see familiar pieces in Perl. They often work the same, but
27 see L<perltrap> for information about how they differ.
31 The only things you need to declare in Perl are report formats and
32 subroutines (and sometimes not even subroutines). A variable holds
33 the undefined value (C<undef>) until it has been assigned a defined
34 value, which is anything other than C<undef>. When used as a number,
35 C<undef> is treated as C<0>; when used as a string, it is treated as
36 the empty string, C<"">; and when used as a reference that isn't being
37 assigned to, it is treated as an error. If you enable warnings,
38 you'll be notified of an uninitialized value whenever you treat
39 C<undef> as a string or a number. Well, usually. Boolean contexts,
45 are exempt from warnings (because they care about truth rather than
46 definedness). Operators such as C<++>, C<-->, C<+=>,
47 C<-=>, and C<.=>, that operate on undefined left values such as:
52 are also always exempt from such warnings.
54 A declaration can be put anywhere a statement can, but has no effect on
55 the execution of the primary sequence of statements--declarations all
56 take effect at compile time. Typically all the declarations are put at
57 the beginning or the end of the script. However, if you're using
58 lexically-scoped private variables created with C<my()>, you'll
60 your format or subroutine definition is within the same block scope
61 as the my if you expect to be able to access those private variables.
63 Declaring a subroutine allows a subroutine name to be used as if it were a
64 list operator from that point forward in the program. You can declare a
65 subroutine without defining it by saying C<sub name>, thus:
68 $me = myname $0 or die "can't get myname";
70 Note that myname() functions as a list operator, not as a unary operator;
71 so be careful to use C<or> instead of C<||> in this case. However, if
72 you were to declare the subroutine as C<sub myname ($)>, then
73 C<myname> would function as a unary operator, so either C<or> or
76 Subroutines declarations can also be loaded up with the C<require> statement
77 or both loaded and imported into your namespace with a C<use> statement.
78 See L<perlmod> for details on this.
80 A statement sequence may contain declarations of lexically-scoped
81 variables, but apart from declaring a variable name, the declaration acts
82 like an ordinary statement, and is elaborated within the sequence of
83 statements as if it were an ordinary statement. That means it actually
84 has both compile-time and run-time effects.
88 Text from a C<"#"> character until the end of the line is a comment,
89 and is ignored. Exceptions include C<"#"> inside a string or regular
92 =head2 Simple Statements
94 The only kind of simple statement is an expression evaluated for its
95 side effects. Every simple statement must be terminated with a
96 semicolon, unless it is the final statement in a block, in which case
97 the semicolon is optional. (A semicolon is still encouraged if the
98 block takes up more than one line, because you may eventually add
99 another line.) Note that there are some operators like C<eval {}> and
100 C<do {}> that look like compound statements, but aren't (they're just
101 TERMs in an expression), and thus need an explicit termination if used
102 as the last item in a statement.
104 =head2 Truth and Falsehood
106 The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
107 C<undef> are all false in a boolean context. All other values are true.
109 Note that while 0, 0.0 and C<'0'> are false, C<'0.0'> and C<'0e0'> are
110 true, but evaluate to 0 in a numeric context.
112 =head2 Statement Modifiers
114 Any simple statement may optionally be followed by a I<SINGLE> modifier,
115 just before the terminating semicolon (or block ending). The possible
124 The C<EXPR> following the modifier is referred to as the "condition".
125 Its truth or falsehood determines how the modifier will behave.
127 C<if> executes the statement once I<if> and only if the condition is
128 true. C<unless> is the opposite, it executes the statement I<unless>
129 the condition is true (i.e., if the condition is false).
131 print "Basset hounds got long ears" if length $ear >= 10;
132 go_outside() and play() unless $is_raining;
134 The C<foreach> modifier is an iterator: it executes the statement once
135 for each item in the LIST (with C<$_> aliased to each item in turn).
137 print "Hello $_!\n" foreach qw(world Dolly nurse);
139 C<while> repeats the statement I<while> the condition is true.
140 C<until> does the opposite, it repeats the statement I<until> the
141 condition is true (or while the condition is false):
143 # Both of these count from 0 to 10.
144 print $i++ while $i <= 10;
145 print $j++ until $j > 10;
147 The C<while> and C<until> modifiers have the usual "C<while> loop"
148 semantics (conditional evaluated first), except when applied to a
149 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
150 which case the block executes once before the conditional is
151 evaluated. This is so that you can write loops like:
156 } until $line eq ".\n";
158 See L<perlfunc/do>. Note also that the loop control statements described
159 later will I<NOT> work in this construct, because modifiers don't take
160 loop labels. Sorry. You can always put another block inside of it
161 (for C<next>) or around it (for C<last>) to do that sort of thing.
162 For C<next>, just double the braces:
169 For C<last>, you have to be more elaborate:
178 B<NOTE:> The behaviour of a C<my> statement modified with a statement
179 modifier conditional or loop construct (e.g. C<my $x if ...>) is
180 B<undefined>. The value of the C<my> variable may be C<undef>, any
181 previously assigned value, or possibly anything else. Don't rely on
182 it. Future versions of perl might do something different from the
183 version of perl you try it out on. Here be dragons.
185 =head2 Compound Statements
187 In Perl, a sequence of statements that defines a scope is called a block.
188 Sometimes a block is delimited by the file containing it (in the case
189 of a required file, or the program as a whole), and sometimes a block
190 is delimited by the extent of a string (in the case of an eval).
192 But generally, a block is delimited by curly brackets, also known as braces.
193 We will call this syntactic construct a BLOCK.
195 The following compound statements may be used to control flow:
198 if (EXPR) BLOCK else BLOCK
199 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
200 LABEL while (EXPR) BLOCK
201 LABEL while (EXPR) BLOCK continue BLOCK
202 LABEL for (EXPR; EXPR; EXPR) BLOCK
203 LABEL foreach VAR (LIST) BLOCK
204 LABEL foreach VAR (LIST) BLOCK continue BLOCK
205 LABEL BLOCK continue BLOCK
207 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
208 not statements. This means that the curly brackets are I<required>--no
209 dangling statements allowed. If you want to write conditionals without
210 curly brackets there are several other ways to do it. The following
211 all do the same thing:
213 if (!open(FOO)) { die "Can't open $FOO: $!"; }
214 die "Can't open $FOO: $!" unless open(FOO);
215 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
216 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
217 # a bit exotic, that last one
219 The C<if> statement is straightforward. Because BLOCKs are always
220 bounded by curly brackets, there is never any ambiguity about which
221 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
222 the sense of the test is reversed.
224 The C<while> statement executes the block as long as the expression is
225 true (does not evaluate to the null string C<""> or C<0> or C<"0">).
226 The LABEL is optional, and if present, consists of an identifier followed
227 by a colon. The LABEL identifies the loop for the loop control
228 statements C<next>, C<last>, and C<redo>.
229 If the LABEL is omitted, the loop control statement
230 refers to the innermost enclosing loop. This may include dynamically
231 looking back your call-stack at run time to find the LABEL. Such
232 desperate behavior triggers a warning if you use the C<use warnings>
233 pragma or the B<-w> flag.
235 If there is a C<continue> BLOCK, it is always executed just before the
236 conditional is about to be evaluated again. Thus it can be used to
237 increment a loop variable, even when the loop has been continued via
238 the C<next> statement.
242 The C<next> command starts the next iteration of the loop:
244 LINE: while (<STDIN>) {
245 next LINE if /^#/; # discard comments
249 The C<last> command immediately exits the loop in question. The
250 C<continue> block, if any, is not executed:
252 LINE: while (<STDIN>) {
253 last LINE if /^$/; # exit when done with header
257 The C<redo> command restarts the loop block without evaluating the
258 conditional again. The C<continue> block, if any, is I<not> executed.
259 This command is normally used by programs that want to lie to themselves
260 about what was just input.
262 For example, when processing a file like F</etc/termcap>.
263 If your input lines might end in backslashes to indicate continuation, you
264 want to skip ahead and get the next record.
275 which is Perl short-hand for the more explicitly written version:
277 LINE: while (defined($line = <ARGV>)) {
279 if ($line =~ s/\\$//) {
281 redo LINE unless eof(); # not eof(ARGV)!
286 Note that if there were a C<continue> block on the above code, it would
287 get executed only on lines discarded by the regex (since redo skips the
288 continue block). A continue block is often used to reset line counters
289 or C<?pat?> one-time matches:
291 # inspired by :1,$g/fred/s//WILMA/
293 ?(fred)? && s//WILMA $1 WILMA/;
294 ?(barney)? && s//BETTY $1 BETTY/;
295 ?(homer)? && s//MARGE $1 MARGE/;
297 print "$ARGV $.: $_";
298 close ARGV if eof(); # reset $.
299 reset if eof(); # reset ?pat?
302 If the word C<while> is replaced by the word C<until>, the sense of the
303 test is reversed, but the conditional is still tested before the first
306 The loop control statements don't work in an C<if> or C<unless>, since
307 they aren't loops. You can double the braces to make them such, though.
311 next if /barney/; # same effect as "last", but doesn't document as well
315 This is caused by the fact that a block by itself acts as a loop that
316 executes once, see L<"Basic BLOCKs and Switch Statements">.
318 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
319 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
323 Perl's C-style C<for> loop works like the corresponding C<while> loop;
324 that means that this:
326 for ($i = 1; $i < 10; $i++) {
339 There is one minor difference: if variables are declared with C<my>
340 in the initialization section of the C<for>, the lexical scope of
341 those variables is exactly the C<for> loop (the body of the loop
342 and the control sections).
344 Besides the normal array index looping, C<for> can lend itself
345 to many other interesting applications. Here's one that avoids the
346 problem you get into if you explicitly test for end-of-file on
347 an interactive file descriptor causing your program to appear to
350 $on_a_tty = -t STDIN && -t STDOUT;
351 sub prompt { print "yes? " if $on_a_tty }
352 for ( prompt(); <STDIN>; prompt() ) {
356 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
357 conditional of a C<for> loop is shorthand for the following. This
358 behaviour is the same as a C<while> loop conditional.
360 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
366 The C<foreach> loop iterates over a normal list value and sets the
367 variable VAR to be each element of the list in turn. If the variable
368 is preceded with the keyword C<my>, then it is lexically scoped, and
369 is therefore visible only within the loop. Otherwise, the variable is
370 implicitly local to the loop and regains its former value upon exiting
371 the loop. If the variable was previously declared with C<my>, it uses
372 that variable instead of the global one, but it's still localized to
373 the loop. This implicit localisation occurs I<only> in a C<foreach>
376 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
377 you can use C<foreach> for readability or C<for> for brevity. (Or because
378 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
379 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
381 If any element of LIST is an lvalue, you can modify it by modifying
382 VAR inside the loop. Conversely, if any element of LIST is NOT an
383 lvalue, any attempt to modify that element will fail. In other words,
384 the C<foreach> loop index variable is an implicit alias for each item
385 in the list that you're looping over.
387 If any part of LIST is an array, C<foreach> will get very confused if
388 you add or remove elements within the loop body, for example with
389 C<splice>. So don't do that.
391 C<foreach> probably won't do what you expect if VAR is a tied or other
392 special variable. Don't do that either.
396 for (@ary) { s/foo/bar/ }
398 for my $elem (@elements) {
402 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
403 print $count, "\n"; sleep(1);
406 for (1..15) { print "Merry Christmas\n"; }
408 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
409 print "Item: $item\n";
412 Here's how a C programmer might code up a particular algorithm in Perl:
414 for (my $i = 0; $i < @ary1; $i++) {
415 for (my $j = 0; $j < @ary2; $j++) {
416 if ($ary1[$i] > $ary2[$j]) {
417 last; # can't go to outer :-(
419 $ary1[$i] += $ary2[$j];
421 # this is where that last takes me
424 Whereas here's how a Perl programmer more comfortable with the idiom might
427 OUTER: for my $wid (@ary1) {
428 INNER: for my $jet (@ary2) {
429 next OUTER if $wid > $jet;
434 See how much easier this is? It's cleaner, safer, and faster. It's
435 cleaner because it's less noisy. It's safer because if code gets added
436 between the inner and outer loops later on, the new code won't be
437 accidentally executed. The C<next> explicitly iterates the other loop
438 rather than merely terminating the inner one. And it's faster because
439 Perl executes a C<foreach> statement more rapidly than it would the
440 equivalent C<for> loop.
442 =head2 Basic BLOCKs and Switch Statements
444 A BLOCK by itself (labeled or not) is semantically equivalent to a
445 loop that executes once. Thus you can use any of the loop control
446 statements in it to leave or restart the block. (Note that this is
447 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
448 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
451 The BLOCK construct is particularly nice for doing case
455 if (/^abc/) { $abc = 1; last SWITCH; }
456 if (/^def/) { $def = 1; last SWITCH; }
457 if (/^xyz/) { $xyz = 1; last SWITCH; }
461 There is no official C<switch> statement in Perl, because there are
462 already several ways to write the equivalent.
464 However, starting from Perl 5.8 to get switch and case one can use
465 the Switch extension and say:
469 after which one has switch and case. It is not as fast as it could be
470 because it's not really part of the language (it's done using source
471 filters) but it is available, and it's very flexible.
473 In addition to the above BLOCK construct, you could write
476 $abc = 1, last SWITCH if /^abc/;
477 $def = 1, last SWITCH if /^def/;
478 $xyz = 1, last SWITCH if /^xyz/;
482 (That's actually not as strange as it looks once you realize that you can
483 use loop control "operators" within an expression. That's just the binary
484 comma operator in scalar context. See L<perlop/"Comma Operator">.)
489 /^abc/ && do { $abc = 1; last SWITCH; };
490 /^def/ && do { $def = 1; last SWITCH; };
491 /^xyz/ && do { $xyz = 1; last SWITCH; };
495 or formatted so it stands out more as a "proper" C<switch> statement:
518 /^abc/ and $abc = 1, last SWITCH;
519 /^def/ and $def = 1, last SWITCH;
520 /^xyz/ and $xyz = 1, last SWITCH;
535 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
536 a temporary assignment to C<$_> for convenient matching:
538 SWITCH: for ($where) {
539 /In Card Names/ && do { push @flags, '-e'; last; };
540 /Anywhere/ && do { push @flags, '-h'; last; };
541 /In Rulings/ && do { last; };
542 die "unknown value for form variable where: `$where'";
545 Another interesting approach to a switch statement is arrange
546 for a C<do> block to return the proper value:
549 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
550 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
551 elsif ($flag & O_RDWR) {
552 if ($flag & O_CREAT) { "w+" }
553 else { ($flag & O_APPEND) ? "a+" : "r+" }
560 ($flags & O_WRONLY) ? "write-only" :
561 ($flags & O_RDWR) ? "read-write" :
565 Or if you are certain that all the C<&&> clauses are true, you can use
566 something like this, which "switches" on the value of the
567 C<HTTP_USER_AGENT> environment variable.
570 # pick out jargon file page based on browser
571 $dir = 'http://www.wins.uva.nl/~mes/jargon';
572 for ($ENV{HTTP_USER_AGENT}) {
573 $page = /Mac/ && 'm/Macintrash.html'
574 || /Win(dows )?NT/ && 'e/evilandrude.html'
575 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
576 || /Linux/ && 'l/Linux.html'
577 || /HP-UX/ && 'h/HP-SUX.html'
578 || /SunOS/ && 's/ScumOS.html'
579 || 'a/AppendixB.html';
581 print "Location: $dir/$page\015\012\015\012";
583 That kind of switch statement only works when you know the C<&&> clauses
584 will be true. If you don't, the previous C<?:> example should be used.
586 You might also consider writing a hash of subroutine references
587 instead of synthesizing a C<switch> statement.
591 Although not for the faint of heart, Perl does support a C<goto>
592 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
593 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
594 a C<goto>; it's just the name of the loop.
596 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
597 execution there. It may not be used to go into any construct that
598 requires initialization, such as a subroutine or a C<foreach> loop. It
599 also can't be used to go into a construct that is optimized away. It
600 can be used to go almost anywhere else within the dynamic scope,
601 including out of subroutines, but it's usually better to use some other
602 construct such as C<last> or C<die>. The author of Perl has never felt the
603 need to use this form of C<goto> (in Perl, that is--C is another matter).
605 The C<goto>-EXPR form expects a label name, whose scope will be resolved
606 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
607 necessarily recommended if you're optimizing for maintainability:
609 goto(("FOO", "BAR", "GLARCH")[$i]);
611 The C<goto>-&NAME form is highly magical, and substitutes a call to the
612 named subroutine for the currently running subroutine. This is used by
613 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
614 pretend that the other subroutine had been called in the first place
615 (except that any modifications to C<@_> in the current subroutine are
616 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
617 will be able to tell that this routine was called first.
619 In almost all cases like this, it's usually a far, far better idea to use the
620 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
621 resorting to a C<goto>. For certain applications, the catch and throw pair of
622 C<eval{}> and die() for exception processing can also be a prudent approach.
624 =head2 PODs: Embedded Documentation
626 Perl has a mechanism for intermixing documentation with source code.
627 While it's expecting the beginning of a new statement, if the compiler
628 encounters a line that begins with an equal sign and a word, like this
630 =head1 Here There Be Pods!
632 Then that text and all remaining text up through and including a line
633 beginning with C<=cut> will be ignored. The format of the intervening
634 text is described in L<perlpod>.
636 This allows you to intermix your source code
637 and your documentation text freely, as in
641 The snazzle() function will behave in the most spectacular
642 form that you can possibly imagine, not even excepting
643 cybernetic pyrotechnics.
645 =cut back to the compiler, nuff of this pod stuff!
652 Note that pod translators should look at only paragraphs beginning
653 with a pod directive (it makes parsing easier), whereas the compiler
654 actually knows to look for pod escapes even in the middle of a
655 paragraph. This means that the following secret stuff will be
656 ignored by both the compiler and the translators.
660 warn "Neither POD nor CODE!?"
664 You probably shouldn't rely upon the C<warn()> being podded out forever.
665 Not all pod translators are well-behaved in this regard, and perhaps
666 the compiler will become pickier.
668 One may also use pod directives to quickly comment out a section
671 =head2 Plain Old Comments (Not!)
673 Perl can process line directives, much like the C preprocessor. Using
674 this, one can control Perl's idea of filenames and line numbers in
675 error or warning messages (especially for strings that are processed
676 with C<eval()>). The syntax for this mechanism is the same as for most
677 C preprocessors: it matches the regular expression
679 # example: '# line 42 "new_filename.plx"'
682 (?:\s("?)([^"]+)\2)? \s*
685 with C<$1> being the line number for the next line, and C<$3> being
686 the optional filename (specified with or without quotes).
688 There is a fairly obvious gotcha included with the line directive:
689 Debuggers and profilers will only show the last source line to appear
690 at a particular line number in a given file. Care should be taken not
691 to cause line number collisions in code you'd like to debug later.
693 Here are some examples that you should be able to type into your command
698 # the `#' on the previous line must be the first char on line
701 foo at bzzzt line 201.
705 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
710 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
712 foo at foo bar line 200.
716 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
719 foo at goop line 345.