7 A Perl script consists of a sequence of declarations and statements.
8 The sequence of statements is executed just once, unlike in B<sed>
9 and B<awk> scripts, where the sequence of statements is executed
10 for each input line. While this means that you must explicitly
11 loop over the lines of your input file (or files), it also means
12 you have much more control over which files and which lines you look at.
13 (Actually, I'm lying--it is possible to do an implicit loop with
14 either the B<-n> or B<-p> switch. It's just not the mandatory
15 default like it is in B<sed> and B<awk>.)
17 Perl is, for the most part, a free-form language. (The only exception
18 to this is format declarations, for obvious reasons.) Text from a
19 C<"#"> character until the end of the line is a comment, and is
20 ignored. If you attempt to use C</* */> C-style comments, it will be
21 interpreted either as division or pattern matching, depending on the
22 context, and C++ C<//> comments just look like a null regular
23 expression, so don't do that.
27 The only things you need to declare in Perl are report formats
28 and subroutines--and even undefined subroutines can be handled
29 through AUTOLOAD. A variable holds the undefined value (C<undef>)
30 until it has been assigned a defined value, which is anything
31 other than C<undef>. When used as a number, C<undef> is treated
32 as C<0>; when used as a string, it is treated the empty string,
33 C<"">; and when used as a reference that isn't being assigned
34 to, it is treated as an error. If you enable warnings, you'll
35 be notified of an uninitialized value whenever you treat C<undef>
36 as a string or a number. Well, usually. Boolean contexts, such as:
41 are exempt from warnings (because they care about truth rather than
42 definedness). Operators such as C<++>, C<-->, C<+=>,
43 C<-=>, and C<.=>, that operate on undefined left values such as:
48 are also always exempt from such warnings.
50 A declaration can be put anywhere a statement can, but has no effect on
51 the execution of the primary sequence of statements--declarations all
52 take effect at compile time. Typically all the declarations are put at
53 the beginning or the end of the script. However, if you're using
54 lexically-scoped private variables created with C<my()>, you'll
56 your format or subroutine definition is within the same block scope
57 as the my if you expect to be able to access those private variables.
59 Declaring a subroutine allows a subroutine name to be used as if it were a
60 list operator from that point forward in the program. You can declare a
61 subroutine without defining it by saying C<sub name>, thus:
64 $me = myname $0 or die "can't get myname";
66 Note that myname() functions as a list operator, not as a unary operator;
67 so be careful to use C<or> instead of C<||> in this case. However, if
68 you were to declare the subroutine as C<sub myname ($)>, then
69 C<myname> would function as a unary operator, so either C<or> or
72 Subroutines declarations can also be loaded up with the C<require> statement
73 or both loaded and imported into your namespace with a C<use> statement.
74 See L<perlmod> for details on this.
76 A statement sequence may contain declarations of lexically-scoped
77 variables, but apart from declaring a variable name, the declaration acts
78 like an ordinary statement, and is elaborated within the sequence of
79 statements as if it were an ordinary statement. That means it actually
80 has both compile-time and run-time effects.
82 =head2 Simple statements
84 The only kind of simple statement is an expression evaluated for its
85 side effects. Every simple statement must be terminated with a
86 semicolon, unless it is the final statement in a block, in which case
87 the semicolon is optional. (A semicolon is still encouraged there if the
88 block takes up more than one line, because you may eventually add another line.)
89 Note that there are some operators like C<eval {}> and C<do {}> that look
90 like compound statements, but aren't (they're just TERMs in an expression),
91 and thus need an explicit termination if used as the last item in a statement.
93 Any simple statement may optionally be followed by a I<SINGLE> modifier,
94 just before the terminating semicolon (or block ending). The possible
103 The C<if> and C<unless> modifiers have the expected semantics,
104 presuming you're a speaker of English. The C<foreach> modifier is an
105 iterator: For each value in EXPR, it aliases C<$_> to the value and
106 executes the statement. The C<while> and C<until> modifiers have the
107 usual "C<while> loop" semantics (conditional evaluated first), except
108 when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE
109 statement), in which case the block executes once before the
110 conditional is evaluated. This is so that you can write loops like:
115 } until $line eq ".\n";
117 See L<perlfunc/do>. Note also that the loop control statements described
118 later will I<NOT> work in this construct, because modifiers don't take
119 loop labels. Sorry. You can always put another block inside of it
120 (for C<next>) or around it (for C<last>) to do that sort of thing.
121 For C<next>, just double the braces:
128 For C<last>, you have to be more elaborate:
137 B<NOTE:> The behaviour of a C<my> statement modified with a statement
138 modifier conditional or loop construct (e.g. C<my $x if ...>) is
139 B<undefined>. The value of the C<my> variable may be C<undef>, any
140 previously assigned value, or possibly anything else. Don't rely on
141 it. Future versions of perl might do something different from the
142 version of perl you try it out on. Here be dragons.
144 =head2 Compound statements
146 In Perl, a sequence of statements that defines a scope is called a block.
147 Sometimes a block is delimited by the file containing it (in the case
148 of a required file, or the program as a whole), and sometimes a block
149 is delimited by the extent of a string (in the case of an eval).
151 But generally, a block is delimited by curly brackets, also known as braces.
152 We will call this syntactic construct a BLOCK.
154 The following compound statements may be used to control flow:
157 if (EXPR) BLOCK else BLOCK
158 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
159 LABEL while (EXPR) BLOCK
160 LABEL while (EXPR) BLOCK continue BLOCK
161 LABEL for (EXPR; EXPR; EXPR) BLOCK
162 LABEL foreach VAR (LIST) BLOCK
163 LABEL foreach VAR (LIST) BLOCK continue BLOCK
164 LABEL BLOCK continue BLOCK
166 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
167 not statements. This means that the curly brackets are I<required>--no
168 dangling statements allowed. If you want to write conditionals without
169 curly brackets there are several other ways to do it. The following
170 all do the same thing:
172 if (!open(FOO)) { die "Can't open $FOO: $!"; }
173 die "Can't open $FOO: $!" unless open(FOO);
174 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
175 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
176 # a bit exotic, that last one
178 The C<if> statement is straightforward. Because BLOCKs are always
179 bounded by curly brackets, there is never any ambiguity about which
180 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
181 the sense of the test is reversed.
183 The C<while> statement executes the block as long as the expression is
184 true (does not evaluate to the null string C<""> or C<0> or C<"0">).
185 The LABEL is optional, and if present, consists of an identifier followed
186 by a colon. The LABEL identifies the loop for the loop control
187 statements C<next>, C<last>, and C<redo>.
188 If the LABEL is omitted, the loop control statement
189 refers to the innermost enclosing loop. This may include dynamically
190 looking back your call-stack at run time to find the LABEL. Such
191 desperate behavior triggers a warning if you use the C<use warnings>
192 pragma or the B<-w> flag.
194 If there is a C<continue> BLOCK, it is always executed just before the
195 conditional is about to be evaluated again, just like the third part of a
196 C<for> loop in C. Thus it can be used to increment a loop variable, even
197 when the loop has been continued via the C<next> statement (which is
198 similar to the C C<continue> statement).
202 The C<next> command is like the C<continue> statement in C; it starts
203 the next iteration of the loop:
205 LINE: while (<STDIN>) {
206 next LINE if /^#/; # discard comments
210 The C<last> command is like the C<break> statement in C (as used in
211 loops); it immediately exits the loop in question. The
212 C<continue> block, if any, is not executed:
214 LINE: while (<STDIN>) {
215 last LINE if /^$/; # exit when done with header
219 The C<redo> command restarts the loop block without evaluating the
220 conditional again. The C<continue> block, if any, is I<not> executed.
221 This command is normally used by programs that want to lie to themselves
222 about what was just input.
224 For example, when processing a file like F</etc/termcap>.
225 If your input lines might end in backslashes to indicate continuation, you
226 want to skip ahead and get the next record.
237 which is Perl short-hand for the more explicitly written version:
239 LINE: while (defined($line = <ARGV>)) {
241 if ($line =~ s/\\$//) {
243 redo LINE unless eof(); # not eof(ARGV)!
248 Note that if there were a C<continue> block on the above code, it would
249 get executed only on lines discarded by the regex (since redo skips the
250 continue block). A continue block is often used to reset line counters
251 or C<?pat?> one-time matches:
253 # inspired by :1,$g/fred/s//WILMA/
255 ?(fred)? && s//WILMA $1 WILMA/;
256 ?(barney)? && s//BETTY $1 BETTY/;
257 ?(homer)? && s//MARGE $1 MARGE/;
259 print "$ARGV $.: $_";
260 close ARGV if eof(); # reset $.
261 reset if eof(); # reset ?pat?
264 If the word C<while> is replaced by the word C<until>, the sense of the
265 test is reversed, but the conditional is still tested before the first
268 The loop control statements don't work in an C<if> or C<unless>, since
269 they aren't loops. You can double the braces to make them such, though.
273 next if /barney/; # same effect as "last", but doesn't document as well
277 This is caused by the fact that a block by itself acts as a loop that
278 executes once, see L<"Basic BLOCKs and Switch Statements">.
280 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
281 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
285 Perl's C-style C<for> loop works like the corresponding C<while> loop;
286 that means that this:
288 for ($i = 1; $i < 10; $i++) {
301 There is one minor difference: if variables are declared with C<my>
302 in the initialization section of the C<for>, the lexical scope of
303 those variables is exactly the C<for> loop (the body of the loop
304 and the control sections).
306 Besides the normal array index looping, C<for> can lend itself
307 to many other interesting applications. Here's one that avoids the
308 problem you get into if you explicitly test for end-of-file on
309 an interactive file descriptor causing your program to appear to
312 $on_a_tty = -t STDIN && -t STDOUT;
313 sub prompt { print "yes? " if $on_a_tty }
314 for ( prompt(); <STDIN>; prompt() ) {
318 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
319 conditional of a C<for> loop is shorthand for the following. This
320 behaviour is the same as a C<while> loop conditional.
322 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
328 The C<foreach> loop iterates over a normal list value and sets the
329 variable VAR to be each element of the list in turn. If the variable
330 is preceded with the keyword C<my>, then it is lexically scoped, and
331 is therefore visible only within the loop. Otherwise, the variable is
332 implicitly local to the loop and regains its former value upon exiting
333 the loop. If the variable was previously declared with C<my>, it uses
334 that variable instead of the global one, but it's still localized to
335 the loop. This implicit localisation occurs I<only> in a C<foreach>
338 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
339 you can use C<foreach> for readability or C<for> for brevity. (Or because
340 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
341 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
343 If any element of LIST is an lvalue, you can modify it by modifying
344 VAR inside the loop. Conversely, if any element of LIST is NOT an
345 lvalue, any attempt to modify that element will fail. In other words,
346 the C<foreach> loop index variable is an implicit alias for each item
347 in the list that you're looping over.
349 If any part of LIST is an array, C<foreach> will get very confused if
350 you add or remove elements within the loop body, for example with
351 C<splice>. So don't do that.
353 C<foreach> probably won't do what you expect if VAR is a tied or other
354 special variable. Don't do that either.
358 for (@ary) { s/foo/bar/ }
360 for my $elem (@elements) {
364 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
365 print $count, "\n"; sleep(1);
368 for (1..15) { print "Merry Christmas\n"; }
370 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
371 print "Item: $item\n";
374 Here's how a C programmer might code up a particular algorithm in Perl:
376 for (my $i = 0; $i < @ary1; $i++) {
377 for (my $j = 0; $j < @ary2; $j++) {
378 if ($ary1[$i] > $ary2[$j]) {
379 last; # can't go to outer :-(
381 $ary1[$i] += $ary2[$j];
383 # this is where that last takes me
386 Whereas here's how a Perl programmer more comfortable with the idiom might
389 OUTER: for my $wid (@ary1) {
390 INNER: for my $jet (@ary2) {
391 next OUTER if $wid > $jet;
396 See how much easier this is? It's cleaner, safer, and faster. It's
397 cleaner because it's less noisy. It's safer because if code gets added
398 between the inner and outer loops later on, the new code won't be
399 accidentally executed. The C<next> explicitly iterates the other loop
400 rather than merely terminating the inner one. And it's faster because
401 Perl executes a C<foreach> statement more rapidly than it would the
402 equivalent C<for> loop.
404 =head2 Basic BLOCKs and Switch Statements
406 A BLOCK by itself (labeled or not) is semantically equivalent to a
407 loop that executes once. Thus you can use any of the loop control
408 statements in it to leave or restart the block. (Note that this is
409 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
410 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
413 The BLOCK construct is particularly nice for doing case
417 if (/^abc/) { $abc = 1; last SWITCH; }
418 if (/^def/) { $def = 1; last SWITCH; }
419 if (/^xyz/) { $xyz = 1; last SWITCH; }
423 There is no official C<switch> statement in Perl, because there are
424 already several ways to write the equivalent.
426 However, starting from Perl 5.8 to get switch and case one can use
427 the Switch extension and say:
431 after which one has switch and case. It is not as fast as it could be
432 because it's not really part of the language (it's done using source
433 filters) but it is available, and it's very flexible.
435 In addition to the above BLOCK construct, you could write
438 $abc = 1, last SWITCH if /^abc/;
439 $def = 1, last SWITCH if /^def/;
440 $xyz = 1, last SWITCH if /^xyz/;
444 (That's actually not as strange as it looks once you realize that you can
445 use loop control "operators" within an expression, That's just the normal
451 /^abc/ && do { $abc = 1; last SWITCH; };
452 /^def/ && do { $def = 1; last SWITCH; };
453 /^xyz/ && do { $xyz = 1; last SWITCH; };
457 or formatted so it stands out more as a "proper" C<switch> statement:
480 /^abc/ and $abc = 1, last SWITCH;
481 /^def/ and $def = 1, last SWITCH;
482 /^xyz/ and $xyz = 1, last SWITCH;
497 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
498 a temporary assignment to C<$_> for convenient matching:
500 SWITCH: for ($where) {
501 /In Card Names/ && do { push @flags, '-e'; last; };
502 /Anywhere/ && do { push @flags, '-h'; last; };
503 /In Rulings/ && do { last; };
504 die "unknown value for form variable where: `$where'";
507 Another interesting approach to a switch statement is arrange
508 for a C<do> block to return the proper value:
511 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
512 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
513 elsif ($flag & O_RDWR) {
514 if ($flag & O_CREAT) { "w+" }
515 else { ($flag & O_APPEND) ? "a+" : "r+" }
522 ($flags & O_WRONLY) ? "write-only" :
523 ($flags & O_RDWR) ? "read-write" :
527 Or if you are certain that all the C<&&> clauses are true, you can use
528 something like this, which "switches" on the value of the
529 C<HTTP_USER_AGENT> environment variable.
532 # pick out jargon file page based on browser
533 $dir = 'http://www.wins.uva.nl/~mes/jargon';
534 for ($ENV{HTTP_USER_AGENT}) {
535 $page = /Mac/ && 'm/Macintrash.html'
536 || /Win(dows )?NT/ && 'e/evilandrude.html'
537 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
538 || /Linux/ && 'l/Linux.html'
539 || /HP-UX/ && 'h/HP-SUX.html'
540 || /SunOS/ && 's/ScumOS.html'
541 || 'a/AppendixB.html';
543 print "Location: $dir/$page\015\012\015\012";
545 That kind of switch statement only works when you know the C<&&> clauses
546 will be true. If you don't, the previous C<?:> example should be used.
548 You might also consider writing a hash of subroutine references
549 instead of synthesizing a C<switch> statement.
553 Although not for the faint of heart, Perl does support a C<goto>
554 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
555 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
556 a C<goto>; it's just the name of the loop.
558 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
559 execution there. It may not be used to go into any construct that
560 requires initialization, such as a subroutine or a C<foreach> loop. It
561 also can't be used to go into a construct that is optimized away. It
562 can be used to go almost anywhere else within the dynamic scope,
563 including out of subroutines, but it's usually better to use some other
564 construct such as C<last> or C<die>. The author of Perl has never felt the
565 need to use this form of C<goto> (in Perl, that is--C is another matter).
567 The C<goto>-EXPR form expects a label name, whose scope will be resolved
568 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
569 necessarily recommended if you're optimizing for maintainability:
571 goto(("FOO", "BAR", "GLARCH")[$i]);
573 The C<goto>-&NAME form is highly magical, and substitutes a call to the
574 named subroutine for the currently running subroutine. This is used by
575 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
576 pretend that the other subroutine had been called in the first place
577 (except that any modifications to C<@_> in the current subroutine are
578 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
579 will be able to tell that this routine was called first.
581 In almost all cases like this, it's usually a far, far better idea to use the
582 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
583 resorting to a C<goto>. For certain applications, the catch and throw pair of
584 C<eval{}> and die() for exception processing can also be a prudent approach.
586 =head2 PODs: Embedded Documentation
588 Perl has a mechanism for intermixing documentation with source code.
589 While it's expecting the beginning of a new statement, if the compiler
590 encounters a line that begins with an equal sign and a word, like this
592 =head1 Here There Be Pods!
594 Then that text and all remaining text up through and including a line
595 beginning with C<=cut> will be ignored. The format of the intervening
596 text is described in L<perlpod>.
598 This allows you to intermix your source code
599 and your documentation text freely, as in
603 The snazzle() function will behave in the most spectacular
604 form that you can possibly imagine, not even excepting
605 cybernetic pyrotechnics.
607 =cut back to the compiler, nuff of this pod stuff!
614 Note that pod translators should look at only paragraphs beginning
615 with a pod directive (it makes parsing easier), whereas the compiler
616 actually knows to look for pod escapes even in the middle of a
617 paragraph. This means that the following secret stuff will be
618 ignored by both the compiler and the translators.
622 warn "Neither POD nor CODE!?"
626 You probably shouldn't rely upon the C<warn()> being podded out forever.
627 Not all pod translators are well-behaved in this regard, and perhaps
628 the compiler will become pickier.
630 One may also use pod directives to quickly comment out a section
633 =head2 Plain Old Comments (Not!)
635 Much like the C preprocessor, Perl can process line directives. Using
636 this, one can control Perl's idea of filenames and line numbers in
637 error or warning messages (especially for strings that are processed
638 with C<eval()>). The syntax for this mechanism is the same as for most
639 C preprocessors: it matches the regular expression
640 C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line
641 number for the next line, and C<$2> being the optional filename
642 (specified within quotes).
644 There is a fairly obvious gotcha included with the line directive:
645 Debuggers and profilers will only show the last source line to appear
646 at a particular line number in a given file. Care should be taken not
647 to cause line number collisions in code you'd like to debug later.
649 Here are some examples that you should be able to type into your command
654 # the `#' on the previous line must be the first char on line
657 foo at bzzzt line 201.
661 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
666 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
668 foo at foo bar line 200.
672 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
675 foo at goop line 345.