8 A Perl program consists of a sequence of declarations and statements
9 which run from the top to the bottom. Loops, subroutines and other
10 control structures allow you to jump around within the code.
12 Perl is a B<free-form> language, you can format and indent it however
13 you like. Whitespace mostly serves to separate tokens, unlike
14 languages like Python where it is an important part of the syntax.
16 Many of Perl's syntactic elements are B<optional>. Rather than
17 requiring you to put parentheses around every function call and
18 declare every variable, you can often leave such explicit elements off
19 and Perl will figure out what you meant. This is known as B<Do What I
20 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
21 code in a style with which they are comfortable.
23 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
24 Bourne Shell, Smalltalk, Lisp and even English. Other
25 languages have borrowed syntax from Perl, particularly its regular
26 expression extensions. So if you have programmed in another language
27 you will see familiar pieces in Perl. They often work the same, but
28 see L<perltrap> for information about how they differ.
31 X<declaration> X<undef> X<undefined> X<uninitialized>
33 The only things you need to declare in Perl are report formats and
34 subroutines (and sometimes not even subroutines). A variable holds
35 the undefined value (C<undef>) until it has been assigned a defined
36 value, which is anything other than C<undef>. When used as a number,
37 C<undef> is treated as C<0>; when used as a string, it is treated as
38 the empty string, C<"">; and when used as a reference that isn't being
39 assigned to, it is treated as an error. If you enable warnings,
40 you'll be notified of an uninitialized value whenever you treat
41 C<undef> as a string or a number. Well, usually. Boolean contexts,
47 are exempt from warnings (because they care about truth rather than
48 definedness). Operators such as C<++>, C<-->, C<+=>,
49 C<-=>, and C<.=>, that operate on undefined left values such as:
54 are also always exempt from such warnings.
56 A declaration can be put anywhere a statement can, but has no effect on
57 the execution of the primary sequence of statements--declarations all
58 take effect at compile time. Typically all the declarations are put at
59 the beginning or the end of the script. However, if you're using
60 lexically-scoped private variables created with C<my()>, you'll
62 your format or subroutine definition is within the same block scope
63 as the my if you expect to be able to access those private variables.
65 Declaring a subroutine allows a subroutine name to be used as if it were a
66 list operator from that point forward in the program. You can declare a
67 subroutine without defining it by saying C<sub name>, thus:
68 X<subroutine, declaration>
71 $me = myname $0 or die "can't get myname";
73 Note that myname() functions as a list operator, not as a unary operator;
74 so be careful to use C<or> instead of C<||> in this case. However, if
75 you were to declare the subroutine as C<sub myname ($)>, then
76 C<myname> would function as a unary operator, so either C<or> or
79 Subroutines declarations can also be loaded up with the C<require> statement
80 or both loaded and imported into your namespace with a C<use> statement.
81 See L<perlmod> for details on this.
83 A statement sequence may contain declarations of lexically-scoped
84 variables, but apart from declaring a variable name, the declaration acts
85 like an ordinary statement, and is elaborated within the sequence of
86 statements as if it were an ordinary statement. That means it actually
87 has both compile-time and run-time effects.
92 Text from a C<"#"> character until the end of the line is a comment,
93 and is ignored. Exceptions include C<"#"> inside a string or regular
96 =head2 Simple Statements
97 X<statement> X<semicolon> X<expression> X<;>
99 The only kind of simple statement is an expression evaluated for its
100 side effects. Every simple statement must be terminated with a
101 semicolon, unless it is the final statement in a block, in which case
102 the semicolon is optional. (A semicolon is still encouraged if the
103 block takes up more than one line, because you may eventually add
104 another line.) Note that there are some operators like C<eval {}> and
105 C<do {}> that look like compound statements, but aren't (they're just
106 TERMs in an expression), and thus need an explicit termination if used
107 as the last item in a statement.
109 =head2 Truth and Falsehood
110 X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
112 The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
113 C<undef> are all false in a boolean context. All other values are true.
114 Negation of a true value by C<!> or C<not> returns a special false value.
115 When evaluated as a string it is treated as C<''>, but as a number, it
118 =head2 Statement Modifiers
119 X<statement modifier> X<modifier> X<if> X<unless> X<while>
120 X<until> X<foreach> X<for>
122 Any simple statement may optionally be followed by a I<SINGLE> modifier,
123 just before the terminating semicolon (or block ending). The possible
132 The C<EXPR> following the modifier is referred to as the "condition".
133 Its truth or falsehood determines how the modifier will behave.
135 C<if> executes the statement once I<if> and only if the condition is
136 true. C<unless> is the opposite, it executes the statement I<unless>
137 the condition is true (i.e., if the condition is false).
139 print "Basset hounds got long ears" if length $ear >= 10;
140 go_outside() and play() unless $is_raining;
142 The C<foreach> modifier is an iterator: it executes the statement once
143 for each item in the LIST (with C<$_> aliased to each item in turn).
145 print "Hello $_!\n" foreach qw(world Dolly nurse);
147 C<while> repeats the statement I<while> the condition is true.
148 C<until> does the opposite, it repeats the statement I<until> the
149 condition is true (or while the condition is false):
151 # Both of these count from 0 to 10.
152 print $i++ while $i <= 10;
153 print $j++ until $j > 10;
155 The C<while> and C<until> modifiers have the usual "C<while> loop"
156 semantics (conditional evaluated first), except when applied to a
157 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
158 which case the block executes once before the conditional is
159 evaluated. This is so that you can write loops like:
164 } until $line eq ".\n";
166 See L<perlfunc/do>. Note also that the loop control statements described
167 later will I<NOT> work in this construct, because modifiers don't take
168 loop labels. Sorry. You can always put another block inside of it
169 (for C<next>) or around it (for C<last>) to do that sort of thing.
170 For C<next>, just double the braces:
171 X<next> X<last> X<redo>
178 For C<last>, you have to be more elaborate:
188 B<NOTE:> The behaviour of a C<my> statement modified with a statement
189 modifier conditional or loop construct (e.g. C<my $x if ...>) is
190 B<undefined>. The value of the C<my> variable may be C<undef>, any
191 previously assigned value, or possibly anything else. Don't rely on
192 it. Future versions of perl might do something different from the
193 version of perl you try it out on. Here be dragons.
196 =head2 Compound Statements
197 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
198 X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>
200 In Perl, a sequence of statements that defines a scope is called a block.
201 Sometimes a block is delimited by the file containing it (in the case
202 of a required file, or the program as a whole), and sometimes a block
203 is delimited by the extent of a string (in the case of an eval).
205 But generally, a block is delimited by curly brackets, also known as braces.
206 We will call this syntactic construct a BLOCK.
208 The following compound statements may be used to control flow:
211 if (EXPR) BLOCK else BLOCK
212 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
213 LABEL while (EXPR) BLOCK
214 LABEL while (EXPR) BLOCK continue BLOCK
215 LABEL until (EXPR) BLOCK
216 LABEL until (EXPR) BLOCK continue BLOCK
217 LABEL for (EXPR; EXPR; EXPR) BLOCK
218 LABEL foreach VAR (LIST) BLOCK
219 LABEL foreach VAR (LIST) BLOCK continue BLOCK
220 LABEL BLOCK continue BLOCK
222 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
223 not statements. This means that the curly brackets are I<required>--no
224 dangling statements allowed. If you want to write conditionals without
225 curly brackets there are several other ways to do it. The following
226 all do the same thing:
228 if (!open(FOO)) { die "Can't open $FOO: $!"; }
229 die "Can't open $FOO: $!" unless open(FOO);
230 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
231 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
232 # a bit exotic, that last one
234 The C<if> statement is straightforward. Because BLOCKs are always
235 bounded by curly brackets, there is never any ambiguity about which
236 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
237 the sense of the test is reversed.
239 The C<while> statement executes the block as long as the expression is
240 true (does not evaluate to the null string C<""> or C<0> or C<"0">).
241 The C<until> statement executes the block as long as the expression is
243 The LABEL is optional, and if present, consists of an identifier followed
244 by a colon. The LABEL identifies the loop for the loop control
245 statements C<next>, C<last>, and C<redo>.
246 If the LABEL is omitted, the loop control statement
247 refers to the innermost enclosing loop. This may include dynamically
248 looking back your call-stack at run time to find the LABEL. Such
249 desperate behavior triggers a warning if you use the C<use warnings>
250 pragma or the B<-w> flag.
252 If there is a C<continue> BLOCK, it is always executed just before the
253 conditional is about to be evaluated again. Thus it can be used to
254 increment a loop variable, even when the loop has been continued via
255 the C<next> statement.
258 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
260 The C<next> command starts the next iteration of the loop:
262 LINE: while (<STDIN>) {
263 next LINE if /^#/; # discard comments
267 The C<last> command immediately exits the loop in question. The
268 C<continue> block, if any, is not executed:
270 LINE: while (<STDIN>) {
271 last LINE if /^$/; # exit when done with header
275 The C<redo> command restarts the loop block without evaluating the
276 conditional again. The C<continue> block, if any, is I<not> executed.
277 This command is normally used by programs that want to lie to themselves
278 about what was just input.
280 For example, when processing a file like F</etc/termcap>.
281 If your input lines might end in backslashes to indicate continuation, you
282 want to skip ahead and get the next record.
293 which is Perl short-hand for the more explicitly written version:
295 LINE: while (defined($line = <ARGV>)) {
297 if ($line =~ s/\\$//) {
299 redo LINE unless eof(); # not eof(ARGV)!
304 Note that if there were a C<continue> block on the above code, it would
305 get executed only on lines discarded by the regex (since redo skips the
306 continue block). A continue block is often used to reset line counters
307 or C<?pat?> one-time matches:
309 # inspired by :1,$g/fred/s//WILMA/
311 ?(fred)? && s//WILMA $1 WILMA/;
312 ?(barney)? && s//BETTY $1 BETTY/;
313 ?(homer)? && s//MARGE $1 MARGE/;
315 print "$ARGV $.: $_";
316 close ARGV if eof(); # reset $.
317 reset if eof(); # reset ?pat?
320 If the word C<while> is replaced by the word C<until>, the sense of the
321 test is reversed, but the conditional is still tested before the first
324 The loop control statements don't work in an C<if> or C<unless>, since
325 they aren't loops. You can double the braces to make them such, though.
329 next if /barney/; # same effect as "last", but doesn't document as well
333 This is caused by the fact that a block by itself acts as a loop that
334 executes once, see L<"Basic BLOCKs">.
336 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
337 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
342 Perl's C-style C<for> loop works like the corresponding C<while> loop;
343 that means that this:
345 for ($i = 1; $i < 10; $i++) {
358 There is one minor difference: if variables are declared with C<my>
359 in the initialization section of the C<for>, the lexical scope of
360 those variables is exactly the C<for> loop (the body of the loop
361 and the control sections).
364 Besides the normal array index looping, C<for> can lend itself
365 to many other interesting applications. Here's one that avoids the
366 problem you get into if you explicitly test for end-of-file on
367 an interactive file descriptor causing your program to appear to
369 X<eof> X<end-of-file> X<end of file>
371 $on_a_tty = -t STDIN && -t STDOUT;
372 sub prompt { print "yes? " if $on_a_tty }
373 for ( prompt(); <STDIN>; prompt() ) {
377 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
378 conditional of a C<for> loop is shorthand for the following. This
379 behaviour is the same as a C<while> loop conditional.
380 X<readline> X<< <> >>
382 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
389 The C<foreach> loop iterates over a normal list value and sets the
390 variable VAR to be each element of the list in turn. If the variable
391 is preceded with the keyword C<my>, then it is lexically scoped, and
392 is therefore visible only within the loop. Otherwise, the variable is
393 implicitly local to the loop and regains its former value upon exiting
394 the loop. If the variable was previously declared with C<my>, it uses
395 that variable instead of the global one, but it's still localized to
396 the loop. This implicit localisation occurs I<only> in a C<foreach>
400 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
401 you can use C<foreach> for readability or C<for> for brevity. (Or because
402 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
403 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
406 If any element of LIST is an lvalue, you can modify it by modifying
407 VAR inside the loop. Conversely, if any element of LIST is NOT an
408 lvalue, any attempt to modify that element will fail. In other words,
409 the C<foreach> loop index variable is an implicit alias for each item
410 in the list that you're looping over.
413 If any part of LIST is an array, C<foreach> will get very confused if
414 you add or remove elements within the loop body, for example with
415 C<splice>. So don't do that.
418 C<foreach> probably won't do what you expect if VAR is a tied or other
419 special variable. Don't do that either.
423 for (@ary) { s/foo/bar/ }
425 for my $elem (@elements) {
429 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
430 print $count, "\n"; sleep(1);
433 for (1..15) { print "Merry Christmas\n"; }
435 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
436 print "Item: $item\n";
439 Here's how a C programmer might code up a particular algorithm in Perl:
441 for (my $i = 0; $i < @ary1; $i++) {
442 for (my $j = 0; $j < @ary2; $j++) {
443 if ($ary1[$i] > $ary2[$j]) {
444 last; # can't go to outer :-(
446 $ary1[$i] += $ary2[$j];
448 # this is where that last takes me
451 Whereas here's how a Perl programmer more comfortable with the idiom might
454 OUTER: for my $wid (@ary1) {
455 INNER: for my $jet (@ary2) {
456 next OUTER if $wid > $jet;
461 See how much easier this is? It's cleaner, safer, and faster. It's
462 cleaner because it's less noisy. It's safer because if code gets added
463 between the inner and outer loops later on, the new code won't be
464 accidentally executed. The C<next> explicitly iterates the other loop
465 rather than merely terminating the inner one. And it's faster because
466 Perl executes a C<foreach> statement more rapidly than it would the
467 equivalent C<for> loop.
472 A BLOCK by itself (labeled or not) is semantically equivalent to a
473 loop that executes once. Thus you can use any of the loop control
474 statements in it to leave or restart the block. (Note that this is
475 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
476 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
479 The BLOCK construct can be used to emulate case structures.
482 if (/^abc/) { $abc = 1; last SWITCH; }
483 if (/^def/) { $def = 1; last SWITCH; }
484 if (/^xyz/) { $xyz = 1; last SWITCH; }
488 Such constructs are quite frequently used, because older versions
489 of Perl had no official C<switch> statement.
491 =head2 Switch statements
492 X<switch> X<case> X<given> X<when> X<default>
494 Starting from Perl 5.10, you can say
496 use feature "switch";
498 which enables a switch feature that is closely based on the
501 The keywords C<given> and C<when> are analogous
502 to C<switch> and C<case> in other languages, so the code
503 above could be written as
506 when (/^abc/) { $abc = 1; }
507 when (/^def/) { $def = 1; }
508 when (/^xyz/) { $xyz = 1; }
509 default { $nothing = 1; }
512 This construct is very flexible and powerful. For example:
518 Most of its power comes from the implicit smart matching:
522 is exactly equivalent to
526 (though you need to enable the "~~" feature before you
527 can use the C<~~> operator directly). In fact C<when(EXPR)>
528 is treated as an implicit smart match most of the time. The
529 exceptions are that when EXPR is:
535 a subroutine or method call
539 a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
540 or a negated regular expression match C<$foo !~ /REGEX/>.
544 a comparison (such as C<$_ E<lt> 10> or C<$x gt "abc">
548 C<defined(...)>, C<exists(...)>, or C<eof(...)>
552 A negated expression C<!(...)> or C<not (...)>, or a logical
553 exclusive-or C<(...) xor (...)>.
557 then the value of EXPR is used directly as a boolean.
564 If EXPR is C<... && ...> or C<... and ...>, the test
565 is applied recursively to both arguments. If I<both>
566 arguments pass the test, then the argument is treated
571 If EXPR is C<... || ...> or C<... or ...>, the test
572 is applied recursively to the first argument.
576 These rules look complicated, but usually they will do what
577 you want. For example you could write:
579 when (/^\d$/ && $_ < 75) { ... }
581 C<default> behaves exactly like C<when(1 == 1)>, which is
582 to say that it always matches.
584 See L</"Smart matching in detail"> for more information
589 You can use the C<continue> keyword to fall through from one
593 when (/x/) { print "\$foo contains an 'x'\n"; continue }
594 when (/y/) { print "\$foo contains a 'y'\n" }
595 default { print "\$foo contains neither an 'x' nor a 'y' }
598 =head3 Switching in a loop
600 Instead of using C<given()>, you can use a C<foreach()> loop.
601 For example, here's one way to count how many times a particular
602 string occurs in an array:
606 when ("foo") { ++$count }
608 print "\@array contains $count copies of 'foo'\n";
610 On exit from the C<when> block, there is an implicit C<next>.
611 You can override that with an explicit C<last> if you're only
612 interested in the first match.
614 This doesn't work if you explicitly specify a loop variable,
615 as in C<for $item (@array)>. You have to use the default
616 variable C<$_>. (You can use C<for my $_ (@array)>.)
618 =head3 Smart matching in detail
622 =head3 Custom matching via overloading
624 You can change the way that an object is matched by overloading
625 the C<'~~'> operator. This trumps the usual smart match semantics.
630 Although not for the faint of heart, Perl does support a C<goto>
631 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
632 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
633 a C<goto>; it's just the name of the loop.
635 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
636 execution there. It may not be used to go into any construct that
637 requires initialization, such as a subroutine or a C<foreach> loop. It
638 also can't be used to go into a construct that is optimized away. It
639 can be used to go almost anywhere else within the dynamic scope,
640 including out of subroutines, but it's usually better to use some other
641 construct such as C<last> or C<die>. The author of Perl has never felt the
642 need to use this form of C<goto> (in Perl, that is--C is another matter).
644 The C<goto>-EXPR form expects a label name, whose scope will be resolved
645 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
646 necessarily recommended if you're optimizing for maintainability:
648 goto(("FOO", "BAR", "GLARCH")[$i]);
650 The C<goto>-&NAME form is highly magical, and substitutes a call to the
651 named subroutine for the currently running subroutine. This is used by
652 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
653 pretend that the other subroutine had been called in the first place
654 (except that any modifications to C<@_> in the current subroutine are
655 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
656 will be able to tell that this routine was called first.
658 In almost all cases like this, it's usually a far, far better idea to use the
659 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
660 resorting to a C<goto>. For certain applications, the catch and throw pair of
661 C<eval{}> and die() for exception processing can also be a prudent approach.
663 =head2 PODs: Embedded Documentation
664 X<POD> X<documentation>
666 Perl has a mechanism for intermixing documentation with source code.
667 While it's expecting the beginning of a new statement, if the compiler
668 encounters a line that begins with an equal sign and a word, like this
670 =head1 Here There Be Pods!
672 Then that text and all remaining text up through and including a line
673 beginning with C<=cut> will be ignored. The format of the intervening
674 text is described in L<perlpod>.
676 This allows you to intermix your source code
677 and your documentation text freely, as in
681 The snazzle() function will behave in the most spectacular
682 form that you can possibly imagine, not even excepting
683 cybernetic pyrotechnics.
685 =cut back to the compiler, nuff of this pod stuff!
692 Note that pod translators should look at only paragraphs beginning
693 with a pod directive (it makes parsing easier), whereas the compiler
694 actually knows to look for pod escapes even in the middle of a
695 paragraph. This means that the following secret stuff will be
696 ignored by both the compiler and the translators.
700 warn "Neither POD nor CODE!?"
704 You probably shouldn't rely upon the C<warn()> being podded out forever.
705 Not all pod translators are well-behaved in this regard, and perhaps
706 the compiler will become pickier.
708 One may also use pod directives to quickly comment out a section
711 =head2 Plain Old Comments (Not!)
712 X<comment> X<line> X<#> X<preprocessor> X<eval>
714 Perl can process line directives, much like the C preprocessor. Using
715 this, one can control Perl's idea of filenames and line numbers in
716 error or warning messages (especially for strings that are processed
717 with C<eval()>). The syntax for this mechanism is the same as for most
718 C preprocessors: it matches the regular expression
720 # example: '# line 42 "new_filename.plx"'
723 (?:\s("?)([^"]+)\2)? \s*
726 with C<$1> being the line number for the next line, and C<$3> being
727 the optional filename (specified with or without quotes).
729 There is a fairly obvious gotcha included with the line directive:
730 Debuggers and profilers will only show the last source line to appear
731 at a particular line number in a given file. Care should be taken not
732 to cause line number collisions in code you'd like to debug later.
734 Here are some examples that you should be able to type into your command
739 # the `#' on the previous line must be the first char on line
742 foo at bzzzt line 201.
746 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
751 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
753 foo at foo bar line 200.
757 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
760 foo at goop line 345.