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 $.
317 reset if eof(); # reset ?pat?
320 If the word C<while> is replaced by the word C<until>, the sense of the
321 test is reversed, but the conditional is still tested before the first
324 The loop control statements don't work in an C<if> or C<unless>, since
325 they aren't loops. You can double the braces to make them such, though.
329 next if /barney/; # same effect as "last", but doesn't document as well
333 This is caused by the fact that a block by itself acts as a loop that
334 executes once, see L<"Basic BLOCKs and Switch Statements">.
336 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
337 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
342 Perl's C-style C<for> loop works like the corresponding C<while> loop;
343 that means that this:
345 for ($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.
469 =head2 Basic BLOCKs and Switch Statements
470 X<switch> X<block> X<case>
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 is particularly nice for doing case
483 if (/^abc/) { $abc = 1; last SWITCH; }
484 if (/^def/) { $def = 1; last SWITCH; }
485 if (/^xyz/) { $xyz = 1; last SWITCH; }
489 There is no official C<switch> statement in Perl, because there are
490 already several ways to write the equivalent.
492 However, starting from Perl 5.8 to get switch and case one can use
493 the Switch extension and say:
497 after which one has switch and case. It is not as fast as it could be
498 because it's not really part of the language (it's done using source
499 filters) but it is available, and it's very flexible.
501 In addition to the above BLOCK construct, you could write
504 $abc = 1, last SWITCH if /^abc/;
505 $def = 1, last SWITCH if /^def/;
506 $xyz = 1, last SWITCH if /^xyz/;
510 (That's actually not as strange as it looks once you realize that you can
511 use loop control "operators" within an expression. That's just the binary
512 comma operator in scalar context. See L<perlop/"Comma Operator">.)
517 /^abc/ && do { $abc = 1; last SWITCH; };
518 /^def/ && do { $def = 1; last SWITCH; };
519 /^xyz/ && do { $xyz = 1; last SWITCH; };
523 or formatted so it stands out more as a "proper" C<switch> statement:
546 /^abc/ and $abc = 1, last SWITCH;
547 /^def/ and $def = 1, last SWITCH;
548 /^xyz/ and $xyz = 1, last SWITCH;
563 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
564 a temporary assignment to C<$_> for convenient matching:
566 SWITCH: for ($where) {
567 /In Card Names/ && do { push @flags, '-e'; last; };
568 /Anywhere/ && do { push @flags, '-h'; last; };
569 /In Rulings/ && do { last; };
570 die "unknown value for form variable where: `$where'";
573 Another interesting approach to a switch statement is arrange
574 for a C<do> block to return the proper value:
577 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
578 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
579 elsif ($flag & O_RDWR) {
580 if ($flag & O_CREAT) { "w+" }
581 else { ($flag & O_APPEND) ? "a+" : "r+" }
588 ($flags & O_WRONLY) ? "write-only" :
589 ($flags & O_RDWR) ? "read-write" :
593 Or if you are certain that all the C<&&> clauses are true, you can use
594 something like this, which "switches" on the value of the
595 C<HTTP_USER_AGENT> environment variable.
598 # pick out jargon file page based on browser
599 $dir = 'http://www.wins.uva.nl/~mes/jargon';
600 for ($ENV{HTTP_USER_AGENT}) {
601 $page = /Mac/ && 'm/Macintrash.html'
602 || /Win(dows )?NT/ && 'e/evilandrude.html'
603 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
604 || /Linux/ && 'l/Linux.html'
605 || /HP-UX/ && 'h/HP-SUX.html'
606 || /SunOS/ && 's/ScumOS.html'
607 || 'a/AppendixB.html';
609 print "Location: $dir/$page\015\012\015\012";
611 That kind of switch statement only works when you know the C<&&> clauses
612 will be true. If you don't, the previous C<?:> example should be used.
614 You might also consider writing a hash of subroutine references
615 instead of synthesizing a C<switch> statement.
620 Although not for the faint of heart, Perl does support a C<goto>
621 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
622 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
623 a C<goto>; it's just the name of the loop.
625 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
626 execution there. It may not be used to go into any construct that
627 requires initialization, such as a subroutine or a C<foreach> loop. It
628 also can't be used to go into a construct that is optimized away. It
629 can be used to go almost anywhere else within the dynamic scope,
630 including out of subroutines, but it's usually better to use some other
631 construct such as C<last> or C<die>. The author of Perl has never felt the
632 need to use this form of C<goto> (in Perl, that is--C is another matter).
634 The C<goto>-EXPR form expects a label name, whose scope will be resolved
635 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
636 necessarily recommended if you're optimizing for maintainability:
638 goto(("FOO", "BAR", "GLARCH")[$i]);
640 The C<goto>-&NAME form is highly magical, and substitutes a call to the
641 named subroutine for the currently running subroutine. This is used by
642 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
643 pretend that the other subroutine had been called in the first place
644 (except that any modifications to C<@_> in the current subroutine are
645 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
646 will be able to tell that this routine was called first.
648 In almost all cases like this, it's usually a far, far better idea to use the
649 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
650 resorting to a C<goto>. For certain applications, the catch and throw pair of
651 C<eval{}> and die() for exception processing can also be a prudent approach.
653 =head2 PODs: Embedded Documentation
654 X<POD> X<documentation>
656 Perl has a mechanism for intermixing documentation with source code.
657 While it's expecting the beginning of a new statement, if the compiler
658 encounters a line that begins with an equal sign and a word, like this
660 =head1 Here There Be Pods!
662 Then that text and all remaining text up through and including a line
663 beginning with C<=cut> will be ignored. The format of the intervening
664 text is described in L<perlpod>.
666 This allows you to intermix your source code
667 and your documentation text freely, as in
671 The snazzle() function will behave in the most spectacular
672 form that you can possibly imagine, not even excepting
673 cybernetic pyrotechnics.
675 =cut back to the compiler, nuff of this pod stuff!
682 Note that pod translators should look at only paragraphs beginning
683 with a pod directive (it makes parsing easier), whereas the compiler
684 actually knows to look for pod escapes even in the middle of a
685 paragraph. This means that the following secret stuff will be
686 ignored by both the compiler and the translators.
690 warn "Neither POD nor CODE!?"
694 You probably shouldn't rely upon the C<warn()> being podded out forever.
695 Not all pod translators are well-behaved in this regard, and perhaps
696 the compiler will become pickier.
698 One may also use pod directives to quickly comment out a section
701 =head2 Plain Old Comments (Not!)
702 X<comment> X<line> X<#> X<preprocessor> X<eval>
704 Perl can process line directives, much like the C preprocessor. Using
705 this, one can control Perl's idea of filenames and line numbers in
706 error or warning messages (especially for strings that are processed
707 with C<eval()>). The syntax for this mechanism is the same as for most
708 C preprocessors: it matches the regular expression
710 # example: '# line 42 "new_filename.plx"'
713 (?:\s("?)([^"]+)\2)? \s*
716 with C<$1> being the line number for the next line, and C<$3> being
717 the optional filename (specified with or without quotes).
719 There is a fairly obvious gotcha included with the line directive:
720 Debuggers and profilers will only show the last source line to appear
721 at a particular line number in a given file. Care should be taken not
722 to cause line number collisions in code you'd like to debug later.
724 Here are some examples that you should be able to type into your command
729 # the `#' on the previous line must be the first char on line
732 foo at bzzzt line 201.
736 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
741 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
743 foo at foo bar line 200.
747 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
750 foo at goop line 345.