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<when> 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
134 The C<EXPR> following the modifier is referred to as the "condition".
135 Its truth or falsehood determines how the modifier will behave.
137 C<if> executes the statement once I<if> and only if the condition is
138 true. C<unless> is the opposite, it executes the statement I<unless>
139 the condition is true (i.e., if the condition is false).
141 print "Basset hounds got long ears" if length $ear >= 10;
142 go_outside() and play() unless $is_raining;
144 C<when> executes the statement I<when> C<$_> smart matches C<EXPR>, and
145 then either C<break>s out if it's enclosed in a C<given> scope or skips
146 to the C<next> element when it lies directly inside a C<for> loop.
147 See also L</"Switch statements">.
150 $abc = 1 when /^abc/;
151 $just_a = 1 when /^a/;
156 admin($_) when [ qw/Alice Bob/ ];
157 regular($_) when [ qw/Chris David Ellen/ ];
160 The C<foreach> modifier is an iterator: it executes the statement once
161 for each item in the LIST (with C<$_> aliased to each item in turn).
163 print "Hello $_!\n" foreach qw(world Dolly nurse);
165 C<while> repeats the statement I<while> the condition is true.
166 C<until> does the opposite, it repeats the statement I<until> the
167 condition is true (or while the condition is false):
169 # Both of these count from 0 to 10.
170 print $i++ while $i <= 10;
171 print $j++ until $j > 10;
173 The C<while> and C<until> modifiers have the usual "C<while> loop"
174 semantics (conditional evaluated first), except when applied to a
175 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
176 which case the block executes once before the conditional is
177 evaluated. This is so that you can write loops like:
182 } until $line eq ".\n";
184 See L<perlfunc/do>. Note also that the loop control statements described
185 later will I<NOT> work in this construct, because modifiers don't take
186 loop labels. Sorry. You can always put another block inside of it
187 (for C<next>) or around it (for C<last>) to do that sort of thing.
188 For C<next>, just double the braces:
189 X<next> X<last> X<redo>
196 For C<last>, you have to be more elaborate:
206 B<NOTE:> The behaviour of a C<my> statement modified with a statement
207 modifier conditional or loop construct (e.g. C<my $x if ...>) is
208 B<undefined>. The value of the C<my> variable may be C<undef>, any
209 previously assigned value, or possibly anything else. Don't rely on
210 it. Future versions of perl might do something different from the
211 version of perl you try it out on. Here be dragons.
214 =head2 Compound Statements
215 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
216 X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>
218 In Perl, a sequence of statements that defines a scope is called a block.
219 Sometimes a block is delimited by the file containing it (in the case
220 of a required file, or the program as a whole), and sometimes a block
221 is delimited by the extent of a string (in the case of an eval).
223 But generally, a block is delimited by curly brackets, also known as braces.
224 We will call this syntactic construct a BLOCK.
226 The following compound statements may be used to control flow:
229 if (EXPR) BLOCK else BLOCK
230 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
232 unless (EXPR) BLOCK else BLOCK
233 unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
234 LABEL while (EXPR) BLOCK
235 LABEL while (EXPR) BLOCK continue BLOCK
236 LABEL until (EXPR) BLOCK
237 LABEL until (EXPR) BLOCK continue BLOCK
238 LABEL for (EXPR; EXPR; EXPR) BLOCK
239 LABEL for VAR (LIST) BLOCK
240 LABEL for VAR (LIST) BLOCK continue BLOCK
241 LABEL foreach (EXPR; EXPR; EXPR) BLOCK
242 LABEL foreach VAR (LIST) BLOCK
243 LABEL foreach VAR (LIST) BLOCK continue BLOCK
244 LABEL BLOCK continue BLOCK
247 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
248 not statements. This means that the curly brackets are I<required>--no
249 dangling statements allowed. If you want to write conditionals without
250 curly brackets there are several other ways to do it. The following
251 all do the same thing:
253 if (!open(FOO)) { die "Can't open $FOO: $!"; }
254 die "Can't open $FOO: $!" unless open(FOO);
255 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
256 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
257 # a bit exotic, that last one
259 The C<if> statement is straightforward. Because BLOCKs are always
260 bounded by curly brackets, there is never any ambiguity about which
261 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
262 the sense of the test is reversed. Like C<if>, C<unless> can be followed
263 by C<else>. C<unless> can even be followed by one or more C<elsif>
264 statements, though you may want to think twice before using that particular
265 language construct, as everyone reading your code will have to think at least
266 twice before they can understand what's going on.
268 The C<while> statement executes the block as long as the expression is
269 L<true|/"Truth and Falsehood">.
270 The C<until> statement executes the block as long as the expression is
272 The LABEL is optional, and if present, consists of an identifier followed
273 by a colon. The LABEL identifies the loop for the loop control
274 statements C<next>, C<last>, and C<redo>.
275 If the LABEL is omitted, the loop control statement
276 refers to the innermost enclosing loop. This may include dynamically
277 looking back your call-stack at run time to find the LABEL. Such
278 desperate behavior triggers a warning if you use the C<use warnings>
279 pragma or the B<-w> flag.
281 If there is a C<continue> BLOCK, it is always executed just before the
282 conditional is about to be evaluated again. Thus it can be used to
283 increment a loop variable, even when the loop has been continued via
284 the C<next> statement.
286 When a block is preceding by a compilation phase keyword such as C<BEGIN>,
287 C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only
288 during the corresponding phase of execution. See L<perlmod> for more details.
290 Extension modules can also hook into the Perl parser to define new
291 kinds of compound statement. These are introduced by a keyword which
292 the extension recognizes, and the syntax following the keyword is
293 defined entirely by the extension. If you are an implementor, see
294 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
295 a module, see the module's documentation for details of the syntax that
299 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
301 The C<next> command starts the next iteration of the loop:
303 LINE: while (<STDIN>) {
304 next LINE if /^#/; # discard comments
308 The C<last> command immediately exits the loop in question. The
309 C<continue> block, if any, is not executed:
311 LINE: while (<STDIN>) {
312 last LINE if /^$/; # exit when done with header
316 The C<redo> command restarts the loop block without evaluating the
317 conditional again. The C<continue> block, if any, is I<not> executed.
318 This command is normally used by programs that want to lie to themselves
319 about what was just input.
321 For example, when processing a file like F</etc/termcap>.
322 If your input lines might end in backslashes to indicate continuation, you
323 want to skip ahead and get the next record.
334 which is Perl short-hand for the more explicitly written version:
336 LINE: while (defined($line = <ARGV>)) {
338 if ($line =~ s/\\$//) {
340 redo LINE unless eof(); # not eof(ARGV)!
345 Note that if there were a C<continue> block on the above code, it would
346 get executed only on lines discarded by the regex (since redo skips the
347 continue block). A continue block is often used to reset line counters
348 or C<m?pat?> one-time matches:
350 # inspired by :1,$g/fred/s//WILMA/
352 m?(fred)? && s//WILMA $1 WILMA/;
353 m?(barney)? && s//BETTY $1 BETTY/;
354 m?(homer)? && s//MARGE $1 MARGE/;
356 print "$ARGV $.: $_";
357 close ARGV if eof; # reset $.
358 reset if eof; # reset ?pat?
361 If the word C<while> is replaced by the word C<until>, the sense of the
362 test is reversed, but the conditional is still tested before the first
365 The loop control statements don't work in an C<if> or C<unless>, since
366 they aren't loops. You can double the braces to make them such, though.
370 next if /barney/; # same effect as "last", but doesn't document as well
374 This is caused by the fact that a block by itself acts as a loop that
375 executes once, see L<"Basic BLOCKs">.
377 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
378 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
383 Perl's C-style C<for> loop works like the corresponding C<while> loop;
384 that means that this:
386 for ($i = 1; $i < 10; $i++) {
399 There is one minor difference: if variables are declared with C<my>
400 in the initialization section of the C<for>, the lexical scope of
401 those variables is exactly the C<for> loop (the body of the loop
402 and the control sections).
405 Besides the normal array index looping, C<for> can lend itself
406 to many other interesting applications. Here's one that avoids the
407 problem you get into if you explicitly test for end-of-file on
408 an interactive file descriptor causing your program to appear to
410 X<eof> X<end-of-file> X<end of file>
412 $on_a_tty = -t STDIN && -t STDOUT;
413 sub prompt { print "yes? " if $on_a_tty }
414 for ( prompt(); <STDIN>; prompt() ) {
418 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
419 conditional of a C<for> loop is shorthand for the following. This
420 behaviour is the same as a C<while> loop conditional.
421 X<readline> X<< <> >>
423 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
430 The C<foreach> loop iterates over a normal list value and sets the
431 variable VAR to be each element of the list in turn. If the variable
432 is preceded with the keyword C<my>, then it is lexically scoped, and
433 is therefore visible only within the loop. Otherwise, the variable is
434 implicitly local to the loop and regains its former value upon exiting
435 the loop. If the variable was previously declared with C<my>, it uses
436 that variable instead of the global one, but it's still localized to
437 the loop. This implicit localization occurs I<only> in a C<foreach>
441 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
442 you can use C<foreach> for readability or C<for> for brevity. (Or because
443 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
444 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
447 If any element of LIST is an lvalue, you can modify it by modifying
448 VAR inside the loop. Conversely, if any element of LIST is NOT an
449 lvalue, any attempt to modify that element will fail. In other words,
450 the C<foreach> loop index variable is an implicit alias for each item
451 in the list that you're looping over.
454 If any part of LIST is an array, C<foreach> will get very confused if
455 you add or remove elements within the loop body, for example with
456 C<splice>. So don't do that.
459 C<foreach> probably won't do what you expect if VAR is a tied or other
460 special variable. Don't do that either.
464 for (@ary) { s/foo/bar/ }
466 for my $elem (@elements) {
470 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
471 print $count, "\n"; sleep(1);
474 for (1..15) { print "Merry Christmas\n"; }
476 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
477 print "Item: $item\n";
480 Here's how a C programmer might code up a particular algorithm in Perl:
482 for (my $i = 0; $i < @ary1; $i++) {
483 for (my $j = 0; $j < @ary2; $j++) {
484 if ($ary1[$i] > $ary2[$j]) {
485 last; # can't go to outer :-(
487 $ary1[$i] += $ary2[$j];
489 # this is where that last takes me
492 Whereas here's how a Perl programmer more comfortable with the idiom might
495 OUTER: for my $wid (@ary1) {
496 INNER: for my $jet (@ary2) {
497 next OUTER if $wid > $jet;
502 See how much easier this is? It's cleaner, safer, and faster. It's
503 cleaner because it's less noisy. It's safer because if code gets added
504 between the inner and outer loops later on, the new code won't be
505 accidentally executed. The C<next> explicitly iterates the other loop
506 rather than merely terminating the inner one. And it's faster because
507 Perl executes a C<foreach> statement more rapidly than it would the
508 equivalent C<for> loop.
513 A BLOCK by itself (labeled or not) is semantically equivalent to a
514 loop that executes once. Thus you can use any of the loop control
515 statements in it to leave or restart the block. (Note that this is
516 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
517 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
520 The BLOCK construct can be used to emulate case structures.
523 if (/^abc/) { $abc = 1; last SWITCH; }
524 if (/^def/) { $def = 1; last SWITCH; }
525 if (/^xyz/) { $xyz = 1; last SWITCH; }
529 Such constructs are quite frequently used, because older versions
530 of Perl had no official C<switch> statement.
532 =head2 Switch statements
534 X<switch> X<case> X<given> X<when> X<default>
536 Starting from Perl 5.10, you can say
538 use feature "switch";
540 which enables a switch feature that is closely based on the
541 Perl 6 proposal. Starting from Perl 5.16, one can prefix the switch
542 keywords with C<CORE::> to access the feature without a C<use feature>
545 The keywords C<given> and C<when> are analogous
546 to C<switch> and C<case> in other languages, so the code
547 above could be written as
550 when (/^abc/) { $abc = 1; }
551 when (/^def/) { $def = 1; }
552 when (/^xyz/) { $xyz = 1; }
553 default { $nothing = 1; }
556 This construct is very flexible and powerful. For example:
561 say '$foo is undefined';
564 say '$foo is the string "foo"';
567 say '$foo is an odd digit';
568 continue; # Fall through
571 say '$foo is numerically less than 100';
573 when (\&complicated_check) {
574 say 'a complicated check for $foo is true';
577 die q(I don't know what to do with $foo);
581 C<given(EXPR)> will assign the value of EXPR to C<$_>
582 within the lexical scope of the block, so it's similar to
584 do { my $_ = EXPR; ... }
586 except that the block is automatically broken out of by a
587 successful C<when> or an explicit C<break>.
589 Most of the power comes from implicit smart matching:
593 is exactly equivalent to
597 Most of the time, C<when(EXPR)> is treated as an implicit smart match of
598 C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more
599 information on smart matching.) But when EXPR is one of the below
600 exceptional cases, it is used directly as a boolean:
606 a subroutine or method call
610 a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
611 or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>).
615 a comparison such as C<$_ E<lt> 10> or C<$x eq "abc">
616 (or of course C<$_ ~~ $c>)
620 C<defined(...)>, C<exists(...)>, or C<eof(...)>
624 a negated expression C<!(...)> or C<not (...)>, or a logical
625 exclusive-or C<(...) xor (...)>.
629 a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>,
630 that return numerical values, not boolean ones.
634 the C<..> and C<...> flip-flop operators.
638 In those cases the value of EXPR is used directly as a boolean.
640 Furthermore, Perl inspects the operands of the binary boolean operators to
641 decide whether to use smart matching for each one by applying the above test to
648 If EXPR is C<... && ...> or C<... and ...>, the test
649 is applied recursively to both operands. If I<both>
650 operands pass the test, then the expression is treated
651 as boolean; otherwise, smart matching is used.
655 If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test
656 is applied recursively to the first operand (which may be a
657 higher-precedence AND operator, for example). If the first operand
658 is to use smart matching, then both operands will do so; if it is
659 not, then the second argument will not be either.
663 These rules look complicated, but usually they will do what
664 you want. For example:
666 when (/^\d+$/ && $_ < 75) { ... }
668 will be treated as a boolean match because the rules say both a regex match and
669 an explicit test on $_ will be treated as boolean.
673 when ([qw(foo bar)] && /baz/) { ... }
675 will use smart matching because only I<one> of the operands is a boolean; the
676 other uses smart matching, and that wins.
680 when ([qw(foo bar)] || /^baz/) { ... }
682 will use smart matching (only the first operand is considered), whereas
684 when (/^baz/ || [qw(foo bar)]) { ... }
686 will test only the regex, which causes both operands to be treated as boolean.
687 Watch out for this one, then, because an arrayref is always a true value, which
688 makes it effectively redundant.
690 Tautologous boolean operators are still going to be optimized away. Don't be
693 when ('foo' or 'bar') { ... }
695 This will optimize down to C<'foo'>, so C<'bar'> will never be considered (even
696 though the rules say to use a smart match on C<'foo'>). For an alternation like
697 this, an array ref will work, because this will instigate smart matching:
699 when ([qw(foo bar)] { ... }
701 This is somewhat equivalent to the C-style switch statement's fallthrough
702 functionality (not to be confused with I<Perl's> fallthrough functionality - see
703 below), wherein the same block is used for several C<case> statements.
705 Another useful shortcut is that, if you use a literal array
706 or hash as the argument to C<given>, it is turned into a
707 reference. So C<given(@foo)> is the same as C<given(\@foo)>,
710 C<default> behaves exactly like C<when(1 == 1)>, which is
711 to say that it always matches.
715 You can use the C<break> keyword to break out of the enclosing
716 C<given> block. Every C<when> block is implicitly ended with
721 You can use the C<continue> keyword to fall through from one
725 when (/x/) { say '$foo contains an x'; continue }
726 when (/y/) { say '$foo contains a y' }
727 default { say '$foo does not contain a y' }
732 When a C<given> statement is also a valid expression (e.g.
733 when it's the last statement of a block), it evaluates to :
739 an empty list as soon as an explicit C<break> is encountered.
743 the value of the last evaluated expression of the successful
744 C<when>/C<default> clause, if there's one.
748 the value of the last evaluated expression of the C<given> block if no
753 In both last cases, the last expression is evaluated in the context that
754 was applied to the C<given> block.
756 Note that, unlike C<if> and C<unless>, failed C<when> statements always
757 evaluate to an empty list.
759 my $price = do { given ($item) {
760 when ([ 'pear', 'apple' ]) { 1 }
761 break when 'vote'; # My vote cannot be bought
762 1e10 when /Mona Lisa/;
766 Currently, C<given> blocks can't always be used as proper expressions. This
767 may be addressed in a future version of perl.
769 =head3 Switching in a loop
771 Instead of using C<given()>, you can use a C<foreach()> loop.
772 For example, here's one way to count how many times a particular
773 string occurs in an array:
777 when ("foo") { ++$count }
779 print "\@array contains $count copies of 'foo'\n";
781 At the end of all C<when> blocks, there is an implicit C<next>.
782 You can override that with an explicit C<last> if you're only
783 interested in the first match.
785 This doesn't work if you explicitly specify a loop variable,
786 as in C<for $item (@array)>. You have to use the default
787 variable C<$_>. (You can use C<for my $_ (@array)>.)
789 =head3 Smart matching in detail
791 The behaviour of a smart match depends on what type of thing its arguments
792 are. The behaviour is determined by the following table: the first row
793 that applies determines the match behaviour (which is thus mostly
794 determined by the type of the right operand). Note that the smart match
795 implicitly dereferences any non-blessed hash or array ref, so the "Hash"
796 and "Array" entries apply in those cases. (For blessed references, the
797 "Object" entries apply.)
799 Note that the "Matching Code" column is not always an exact rendition. For
800 example, the smart match operator short-circuits whenever possible, but
803 $a $b Type of Match Implied Matching Code
804 ====== ===== ===================== =============
805 Any undef undefined !defined $a
807 Any Object invokes ~~ overloading on $object, or dies
809 Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
810 Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
811 Any CodeRef scalar sub truth $b->($a)
813 Hash Hash hash keys identical (every key is found in both hashes)
814 Array Hash hash keys intersection grep { exists $b->{$_} } @$a
815 Regex Hash hash key grep grep /$a/, keys %$b
816 undef Hash always false (undef can't be a key)
817 Any Hash hash entry existence exists $b->{$a}
819 Hash Array hash keys intersection grep { exists $a->{$_} } @$b
820 Array Array arrays are comparable[2]
821 Regex Array array grep grep /$a/, @$b
822 undef Array array contains undef grep !defined, @$b
823 Any Array match against an array element[3]
826 Hash Regex hash key grep grep /$b/, keys %$a
827 Array Regex array grep grep /$b/, @$a
828 Any Regex pattern match $a =~ /$b/
830 Object Any invokes ~~ overloading on $object, or falls back:
831 undef Any undefined !defined($b)
832 Any Num numeric equality $a == $b
833 Num numish[4] numeric equality $a == $b
834 Any Any string equality $a eq $b
836 1 - empty hashes or arrays will match.
837 2 - that is, each element smart-matches the element of same index in the
839 3 - If a circular reference is found, we fall back to referential equality.
840 4 - either a real number, or a string that looks like a number
842 =head3 Custom matching via overloading
844 You can change the way that an object is matched by overloading
845 the C<~~> operator. This may alter the usual smart match semantics.
847 It should be noted that C<~~> will refuse to work on objects that
848 don't overload it (in order to avoid relying on the object's
849 underlying structure).
851 Note also that smart match's matching rules take precedence over
852 overloading, so if C<$obj> has smart match overloading, then
856 will not automatically invoke the overload method with X as an argument;
857 instead the table above is consulted as normal, and based in the type of X,
858 overloading may or may not be invoked.
862 =head3 Differences from Perl 6
864 The Perl 5 smart match and C<given>/C<when> constructs are not
865 absolutely identical to their Perl 6 analogues. The most visible
866 difference is that, in Perl 5, parentheses are required around
867 the argument to C<given()> and C<when()> (except when this last
868 one is used as a statement modifier). Parentheses in Perl 6
869 are always optional in a control construct such as C<if()>,
870 C<while()>, or C<when()>; they can't be made optional in Perl
871 5 without a great deal of potential confusion, because Perl 5
872 would parse the expression
878 as though the argument to C<given> were an element of the hash
879 C<%foo>, interpreting the braces as hash-element syntax.
881 The table of smart matches is not identical to that proposed by the
882 Perl 6 specification, mainly due to the differences between Perl 6's
883 and Perl 5's data models.
885 In Perl 6, C<when()> will always do an implicit smart match
886 with its argument, whilst it is convenient in Perl 5 to
887 suppress this implicit smart match in certain situations,
888 as documented above. (The difference is largely because Perl 5
889 does not, even internally, have a boolean type.)
894 Although not for the faint of heart, Perl does support a C<goto>
895 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
896 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
897 a C<goto>; it's just the name of the loop.
899 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
900 execution there. It may not be used to go into any construct that
901 requires initialization, such as a subroutine or a C<foreach> loop. It
902 also can't be used to go into a construct that is optimized away. It
903 can be used to go almost anywhere else within the dynamic scope,
904 including out of subroutines, but it's usually better to use some other
905 construct such as C<last> or C<die>. The author of Perl has never felt the
906 need to use this form of C<goto> (in Perl, that is--C is another matter).
908 The C<goto>-EXPR form expects a label name, whose scope will be resolved
909 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
910 necessarily recommended if you're optimizing for maintainability:
912 goto(("FOO", "BAR", "GLARCH")[$i]);
914 The C<goto>-&NAME form is highly magical, and substitutes a call to the
915 named subroutine for the currently running subroutine. This is used by
916 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
917 pretend that the other subroutine had been called in the first place
918 (except that any modifications to C<@_> in the current subroutine are
919 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
920 will be able to tell that this routine was called first.
922 In almost all cases like this, it's usually a far, far better idea to use the
923 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
924 resorting to a C<goto>. For certain applications, the catch and throw pair of
925 C<eval{}> and die() for exception processing can also be a prudent approach.
927 =head2 PODs: Embedded Documentation
928 X<POD> X<documentation>
930 Perl has a mechanism for intermixing documentation with source code.
931 While it's expecting the beginning of a new statement, if the compiler
932 encounters a line that begins with an equal sign and a word, like this
934 =head1 Here There Be Pods!
936 Then that text and all remaining text up through and including a line
937 beginning with C<=cut> will be ignored. The format of the intervening
938 text is described in L<perlpod>.
940 This allows you to intermix your source code
941 and your documentation text freely, as in
945 The snazzle() function will behave in the most spectacular
946 form that you can possibly imagine, not even excepting
947 cybernetic pyrotechnics.
949 =cut back to the compiler, nuff of this pod stuff!
956 Note that pod translators should look at only paragraphs beginning
957 with a pod directive (it makes parsing easier), whereas the compiler
958 actually knows to look for pod escapes even in the middle of a
959 paragraph. This means that the following secret stuff will be
960 ignored by both the compiler and the translators.
964 warn "Neither POD nor CODE!?"
968 You probably shouldn't rely upon the C<warn()> being podded out forever.
969 Not all pod translators are well-behaved in this regard, and perhaps
970 the compiler will become pickier.
972 One may also use pod directives to quickly comment out a section
975 =head2 Plain Old Comments (Not!)
976 X<comment> X<line> X<#> X<preprocessor> X<eval>
978 Perl can process line directives, much like the C preprocessor. Using
979 this, one can control Perl's idea of filenames and line numbers in
980 error or warning messages (especially for strings that are processed
981 with C<eval()>). The syntax for this mechanism is almost the same as for
982 most C preprocessors: it matches the regular expression
984 # example: '# line 42 "new_filename.plx"'
987 (?:\s("?)([^"]+)\g2)? \s*
990 with C<$1> being the line number for the next line, and C<$3> being
991 the optional filename (specified with or without quotes). Note that
992 no whitespace may precede the C<< # >>, unlike modern C preprocessors.
994 There is a fairly obvious gotcha included with the line directive:
995 Debuggers and profilers will only show the last source line to appear
996 at a particular line number in a given file. Care should be taken not
997 to cause line number collisions in code you'd like to debug later.
999 Here are some examples that you should be able to type into your command
1004 # the '#' on the previous line must be the first char on line
1007 foo at bzzzt line 201.
1011 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
1016 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
1018 foo at foo bar line 200.
1022 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
1025 foo at goop line 345.