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 an
107 explicit C<undef> are all false in a boolean context. Every other value
110 Note that while 0, 0.0 and C<'0'> are false, C<'0.0'> and C<'0e0'> are
111 true, but evaluate to 0 in a numeric context.
113 =head2 Statement Modifiers
115 Any simple statement may optionally be followed by a I<SINGLE> modifier,
116 just before the terminating semicolon (or block ending). The possible
125 The C<EXPR> following the modifier is referred to as the "condition".
126 Its truth or falsehood determines how the modifier will behave.
128 C<if> executes the statement once I<if> and only if the condition is
129 true. C<unless> is the opposite, it executes the statement I<unless>
130 the condition is true (i.e., if the condition is false).
132 print "Basset hounds got long ears" if length $ear >= 10;
133 go_outside() and play() unless $is_raining;
135 The C<foreach> modifier is an iterator: it executes the statement once
136 for each item in the LIST (with C<$_> aliased to each item in turn).
138 print "Hello $_!\n" foreach qw(world Dolly nurse);
140 C<while> repeats the statement I<while> the condition is true.
141 C<until> does the opposite, it repeats the statement I<until> the
142 condition is true (or while the condition is false):
144 # Both of these count from 0 to 10.
145 print $i++ while $i <= 10;
146 print $j++ until $j > 10;
148 The C<while> and C<until> modifiers have the usual "C<while> loop"
149 semantics (conditional evaluated first), except when applied to a
150 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
151 which case the block executes once before the conditional is
152 evaluated. This is so that you can write loops like:
157 } until $line eq ".\n";
159 See L<perlfunc/do>. Note also that the loop control statements described
160 later will I<NOT> work in this construct, because modifiers don't take
161 loop labels. Sorry. You can always put another block inside of it
162 (for C<next>) or around it (for C<last>) to do that sort of thing.
163 For C<next>, just double the braces:
170 For C<last>, you have to be more elaborate:
179 B<NOTE:> The behaviour of a C<my> statement modified with a statement
180 modifier conditional or loop construct (e.g. C<my $x if ...>) is
181 B<undefined>. The value of the C<my> variable may be C<undef>, any
182 previously assigned value, or possibly anything else. Don't rely on
183 it. Future versions of perl might do something different from the
184 version of perl you try it out on. Here be dragons.
186 =head2 Compound Statements
188 In Perl, a sequence of statements that defines a scope is called a block.
189 Sometimes a block is delimited by the file containing it (in the case
190 of a required file, or the program as a whole), and sometimes a block
191 is delimited by the extent of a string (in the case of an eval).
193 But generally, a block is delimited by curly brackets, also known as braces.
194 We will call this syntactic construct a BLOCK.
196 The following compound statements may be used to control flow:
199 if (EXPR) BLOCK else BLOCK
200 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
201 LABEL while (EXPR) BLOCK
202 LABEL while (EXPR) BLOCK continue BLOCK
203 LABEL for (EXPR; EXPR; EXPR) BLOCK
204 LABEL foreach VAR (LIST) BLOCK
205 LABEL foreach VAR (LIST) BLOCK continue BLOCK
206 LABEL BLOCK continue BLOCK
208 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
209 not statements. This means that the curly brackets are I<required>--no
210 dangling statements allowed. If you want to write conditionals without
211 curly brackets there are several other ways to do it. The following
212 all do the same thing:
214 if (!open(FOO)) { die "Can't open $FOO: $!"; }
215 die "Can't open $FOO: $!" unless open(FOO);
216 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
217 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
218 # a bit exotic, that last one
220 The C<if> statement is straightforward. Because BLOCKs are always
221 bounded by curly brackets, there is never any ambiguity about which
222 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
223 the sense of the test is reversed.
225 The C<while> statement executes the block as long as the expression is
226 true (does not evaluate to the null string C<""> or C<0> or C<"0">).
227 The LABEL is optional, and if present, consists of an identifier followed
228 by a colon. The LABEL identifies the loop for the loop control
229 statements C<next>, C<last>, and C<redo>.
230 If the LABEL is omitted, the loop control statement
231 refers to the innermost enclosing loop. This may include dynamically
232 looking back your call-stack at run time to find the LABEL. Such
233 desperate behavior triggers a warning if you use the C<use warnings>
234 pragma or the B<-w> flag.
236 If there is a C<continue> BLOCK, it is always executed just before the
237 conditional is about to be evaluated again. Thus it can be used to
238 increment a loop variable, even when the loop has been continued via
239 the C<next> statement.
243 The C<next> command starts the next iteration of the loop:
245 LINE: while (<STDIN>) {
246 next LINE if /^#/; # discard comments
250 The C<last> command immediately exits the loop in question. The
251 C<continue> block, if any, is not executed:
253 LINE: while (<STDIN>) {
254 last LINE if /^$/; # exit when done with header
258 The C<redo> command restarts the loop block without evaluating the
259 conditional again. The C<continue> block, if any, is I<not> executed.
260 This command is normally used by programs that want to lie to themselves
261 about what was just input.
263 For example, when processing a file like F</etc/termcap>.
264 If your input lines might end in backslashes to indicate continuation, you
265 want to skip ahead and get the next record.
276 which is Perl short-hand for the more explicitly written version:
278 LINE: while (defined($line = <ARGV>)) {
280 if ($line =~ s/\\$//) {
282 redo LINE unless eof(); # not eof(ARGV)!
287 Note that if there were a C<continue> block on the above code, it would
288 get executed only on lines discarded by the regex (since redo skips the
289 continue block). A continue block is often used to reset line counters
290 or C<?pat?> one-time matches:
292 # inspired by :1,$g/fred/s//WILMA/
294 ?(fred)? && s//WILMA $1 WILMA/;
295 ?(barney)? && s//BETTY $1 BETTY/;
296 ?(homer)? && s//MARGE $1 MARGE/;
298 print "$ARGV $.: $_";
299 close ARGV if eof(); # reset $.
300 reset if eof(); # reset ?pat?
303 If the word C<while> is replaced by the word C<until>, the sense of the
304 test is reversed, but the conditional is still tested before the first
307 The loop control statements don't work in an C<if> or C<unless>, since
308 they aren't loops. You can double the braces to make them such, though.
312 next if /barney/; # same effect as "last", but doesn't document as well
316 This is caused by the fact that a block by itself acts as a loop that
317 executes once, see L<"Basic BLOCKs and Switch Statements">.
319 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
320 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
324 Perl's C-style C<for> loop works like the corresponding C<while> loop;
325 that means that this:
327 for ($i = 1; $i < 10; $i++) {
340 There is one minor difference: if variables are declared with C<my>
341 in the initialization section of the C<for>, the lexical scope of
342 those variables is exactly the C<for> loop (the body of the loop
343 and the control sections).
345 Besides the normal array index looping, C<for> can lend itself
346 to many other interesting applications. Here's one that avoids the
347 problem you get into if you explicitly test for end-of-file on
348 an interactive file descriptor causing your program to appear to
351 $on_a_tty = -t STDIN && -t STDOUT;
352 sub prompt { print "yes? " if $on_a_tty }
353 for ( prompt(); <STDIN>; prompt() ) {
357 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
358 conditional of a C<for> loop is shorthand for the following. This
359 behaviour is the same as a C<while> loop conditional.
361 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
367 The C<foreach> loop iterates over a normal list value and sets the
368 variable VAR to be each element of the list in turn. If the variable
369 is preceded with the keyword C<my>, then it is lexically scoped, and
370 is therefore visible only within the loop. Otherwise, the variable is
371 implicitly local to the loop and regains its former value upon exiting
372 the loop. If the variable was previously declared with C<my>, it uses
373 that variable instead of the global one, but it's still localized to
374 the loop. This implicit localisation occurs I<only> in a C<foreach>
377 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
378 you can use C<foreach> for readability or C<for> for brevity. (Or because
379 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
380 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
382 If any element of LIST is an lvalue, you can modify it by modifying
383 VAR inside the loop. Conversely, if any element of LIST is NOT an
384 lvalue, any attempt to modify that element will fail. In other words,
385 the C<foreach> loop index variable is an implicit alias for each item
386 in the list that you're looping over.
388 If any part of LIST is an array, C<foreach> will get very confused if
389 you add or remove elements within the loop body, for example with
390 C<splice>. So don't do that.
392 C<foreach> probably won't do what you expect if VAR is a tied or other
393 special variable. Don't do that either.
397 for (@ary) { s/foo/bar/ }
399 for my $elem (@elements) {
403 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
404 print $count, "\n"; sleep(1);
407 for (1..15) { print "Merry Christmas\n"; }
409 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
410 print "Item: $item\n";
413 Here's how a C programmer might code up a particular algorithm in Perl:
415 for (my $i = 0; $i < @ary1; $i++) {
416 for (my $j = 0; $j < @ary2; $j++) {
417 if ($ary1[$i] > $ary2[$j]) {
418 last; # can't go to outer :-(
420 $ary1[$i] += $ary2[$j];
422 # this is where that last takes me
425 Whereas here's how a Perl programmer more comfortable with the idiom might
428 OUTER: for my $wid (@ary1) {
429 INNER: for my $jet (@ary2) {
430 next OUTER if $wid > $jet;
435 See how much easier this is? It's cleaner, safer, and faster. It's
436 cleaner because it's less noisy. It's safer because if code gets added
437 between the inner and outer loops later on, the new code won't be
438 accidentally executed. The C<next> explicitly iterates the other loop
439 rather than merely terminating the inner one. And it's faster because
440 Perl executes a C<foreach> statement more rapidly than it would the
441 equivalent C<for> loop.
443 =head2 Basic BLOCKs and Switch Statements
445 A BLOCK by itself (labeled or not) is semantically equivalent to a
446 loop that executes once. Thus you can use any of the loop control
447 statements in it to leave or restart the block. (Note that this is
448 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
449 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
452 The BLOCK construct is particularly nice for doing case
456 if (/^abc/) { $abc = 1; last SWITCH; }
457 if (/^def/) { $def = 1; last SWITCH; }
458 if (/^xyz/) { $xyz = 1; last SWITCH; }
462 There is no official C<switch> statement in Perl, because there are
463 already several ways to write the equivalent.
465 However, starting from Perl 5.8 to get switch and case one can use
466 the Switch extension and say:
470 after which one has switch and case. It is not as fast as it could be
471 because it's not really part of the language (it's done using source
472 filters) but it is available, and it's very flexible.
474 In addition to the above BLOCK construct, you could write
477 $abc = 1, last SWITCH if /^abc/;
478 $def = 1, last SWITCH if /^def/;
479 $xyz = 1, last SWITCH if /^xyz/;
483 (That's actually not as strange as it looks once you realize that you can
484 use loop control "operators" within an expression. That's just the binary
485 comma operator in scalar context. See L<perlop/"Comma Operator">.)
490 /^abc/ && do { $abc = 1; last SWITCH; };
491 /^def/ && do { $def = 1; last SWITCH; };
492 /^xyz/ && do { $xyz = 1; last SWITCH; };
496 or formatted so it stands out more as a "proper" C<switch> statement:
519 /^abc/ and $abc = 1, last SWITCH;
520 /^def/ and $def = 1, last SWITCH;
521 /^xyz/ and $xyz = 1, last SWITCH;
536 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
537 a temporary assignment to C<$_> for convenient matching:
539 SWITCH: for ($where) {
540 /In Card Names/ && do { push @flags, '-e'; last; };
541 /Anywhere/ && do { push @flags, '-h'; last; };
542 /In Rulings/ && do { last; };
543 die "unknown value for form variable where: `$where'";
546 Another interesting approach to a switch statement is arrange
547 for a C<do> block to return the proper value:
550 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
551 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
552 elsif ($flag & O_RDWR) {
553 if ($flag & O_CREAT) { "w+" }
554 else { ($flag & O_APPEND) ? "a+" : "r+" }
561 ($flags & O_WRONLY) ? "write-only" :
562 ($flags & O_RDWR) ? "read-write" :
566 Or if you are certain that all the C<&&> clauses are true, you can use
567 something like this, which "switches" on the value of the
568 C<HTTP_USER_AGENT> environment variable.
571 # pick out jargon file page based on browser
572 $dir = 'http://www.wins.uva.nl/~mes/jargon';
573 for ($ENV{HTTP_USER_AGENT}) {
574 $page = /Mac/ && 'm/Macintrash.html'
575 || /Win(dows )?NT/ && 'e/evilandrude.html'
576 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
577 || /Linux/ && 'l/Linux.html'
578 || /HP-UX/ && 'h/HP-SUX.html'
579 || /SunOS/ && 's/ScumOS.html'
580 || 'a/AppendixB.html';
582 print "Location: $dir/$page\015\012\015\012";
584 That kind of switch statement only works when you know the C<&&> clauses
585 will be true. If you don't, the previous C<?:> example should be used.
587 You might also consider writing a hash of subroutine references
588 instead of synthesizing a C<switch> statement.
592 Although not for the faint of heart, Perl does support a C<goto>
593 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
594 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
595 a C<goto>; it's just the name of the loop.
597 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
598 execution there. It may not be used to go into any construct that
599 requires initialization, such as a subroutine or a C<foreach> loop. It
600 also can't be used to go into a construct that is optimized away. It
601 can be used to go almost anywhere else within the dynamic scope,
602 including out of subroutines, but it's usually better to use some other
603 construct such as C<last> or C<die>. The author of Perl has never felt the
604 need to use this form of C<goto> (in Perl, that is--C is another matter).
606 The C<goto>-EXPR form expects a label name, whose scope will be resolved
607 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
608 necessarily recommended if you're optimizing for maintainability:
610 goto(("FOO", "BAR", "GLARCH")[$i]);
612 The C<goto>-&NAME form is highly magical, and substitutes a call to the
613 named subroutine for the currently running subroutine. This is used by
614 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
615 pretend that the other subroutine had been called in the first place
616 (except that any modifications to C<@_> in the current subroutine are
617 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
618 will be able to tell that this routine was called first.
620 In almost all cases like this, it's usually a far, far better idea to use the
621 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
622 resorting to a C<goto>. For certain applications, the catch and throw pair of
623 C<eval{}> and die() for exception processing can also be a prudent approach.
625 =head2 PODs: Embedded Documentation
627 Perl has a mechanism for intermixing documentation with source code.
628 While it's expecting the beginning of a new statement, if the compiler
629 encounters a line that begins with an equal sign and a word, like this
631 =head1 Here There Be Pods!
633 Then that text and all remaining text up through and including a line
634 beginning with C<=cut> will be ignored. The format of the intervening
635 text is described in L<perlpod>.
637 This allows you to intermix your source code
638 and your documentation text freely, as in
642 The snazzle() function will behave in the most spectacular
643 form that you can possibly imagine, not even excepting
644 cybernetic pyrotechnics.
646 =cut back to the compiler, nuff of this pod stuff!
653 Note that pod translators should look at only paragraphs beginning
654 with a pod directive (it makes parsing easier), whereas the compiler
655 actually knows to look for pod escapes even in the middle of a
656 paragraph. This means that the following secret stuff will be
657 ignored by both the compiler and the translators.
661 warn "Neither POD nor CODE!?"
665 You probably shouldn't rely upon the C<warn()> being podded out forever.
666 Not all pod translators are well-behaved in this regard, and perhaps
667 the compiler will become pickier.
669 One may also use pod directives to quickly comment out a section
672 =head2 Plain Old Comments (Not!)
674 Perl can process line directives, much like the C preprocessor. Using
675 this, one can control Perl's idea of filenames and line numbers in
676 error or warning messages (especially for strings that are processed
677 with C<eval()>). The syntax for this mechanism is the same as for most
678 C preprocessors: it matches the regular expression
680 # example: '# line 42 "new_filename.plx"'
683 (?:\s("?)([^"]+)\2)? \s*
686 with C<$1> being the line number for the next line, and C<$3> being
687 the optional filename (specified with or without quotes).
689 There is a fairly obvious gotcha included with the line directive:
690 Debuggers and profilers will only show the last source line to appear
691 at a particular line number in a given file. Care should be taken not
692 to cause line number collisions in code you'd like to debug later.
694 Here are some examples that you should be able to type into your command
699 # the `#' on the previous line must be the first char on line
702 foo at bzzzt line 201.
706 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
711 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
713 foo at foo bar line 200.
717 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
720 foo at goop line 345.