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 =head2 Compound statements
139 In Perl, a sequence of statements that defines a scope is called a block.
140 Sometimes a block is delimited by the file containing it (in the case
141 of a required file, or the program as a whole), and sometimes a block
142 is delimited by the extent of a string (in the case of an eval).
144 But generally, a block is delimited by curly brackets, also known as braces.
145 We will call this syntactic construct a BLOCK.
147 The following compound statements may be used to control flow:
150 if (EXPR) BLOCK else BLOCK
151 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
152 LABEL while (EXPR) BLOCK
153 LABEL while (EXPR) BLOCK continue BLOCK
154 LABEL for (EXPR; EXPR; EXPR) BLOCK
155 LABEL foreach VAR (LIST) BLOCK
156 LABEL foreach VAR (LIST) BLOCK continue BLOCK
157 LABEL BLOCK continue BLOCK
159 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
160 not statements. This means that the curly brackets are I<required>--no
161 dangling statements allowed. If you want to write conditionals without
162 curly brackets there are several other ways to do it. The following
163 all do the same thing:
165 if (!open(FOO)) { die "Can't open $FOO: $!"; }
166 die "Can't open $FOO: $!" unless open(FOO);
167 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
168 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
169 # a bit exotic, that last one
171 The C<if> statement is straightforward. Because BLOCKs are always
172 bounded by curly brackets, there is never any ambiguity about which
173 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
174 the sense of the test is reversed.
176 The C<while> statement executes the block as long as the expression is
177 true (does not evaluate to the null string C<""> or C<0> or C<"0">).
178 The LABEL is optional, and if present, consists of an identifier followed
179 by a colon. The LABEL identifies the loop for the loop control
180 statements C<next>, C<last>, and C<redo>.
181 If the LABEL is omitted, the loop control statement
182 refers to the innermost enclosing loop. This may include dynamically
183 looking back your call-stack at run time to find the LABEL. Such
184 desperate behavior triggers a warning if you use the C<use warnings>
185 pragma or the B<-w> flag.
187 If there is a C<continue> BLOCK, it is always executed just before the
188 conditional is about to be evaluated again, just like the third part of a
189 C<for> loop in C. Thus it can be used to increment a loop variable, even
190 when the loop has been continued via the C<next> statement (which is
191 similar to the C C<continue> statement).
195 The C<next> command is like the C<continue> statement in C; it starts
196 the next iteration of the loop:
198 LINE: while (<STDIN>) {
199 next LINE if /^#/; # discard comments
203 The C<last> command is like the C<break> statement in C (as used in
204 loops); it immediately exits the loop in question. The
205 C<continue> block, if any, is not executed:
207 LINE: while (<STDIN>) {
208 last LINE if /^$/; # exit when done with header
212 The C<redo> command restarts the loop block without evaluating the
213 conditional again. The C<continue> block, if any, is I<not> executed.
214 This command is normally used by programs that want to lie to themselves
215 about what was just input.
217 For example, when processing a file like F</etc/termcap>.
218 If your input lines might end in backslashes to indicate continuation, you
219 want to skip ahead and get the next record.
230 which is Perl short-hand for the more explicitly written version:
232 LINE: while (defined($line = <ARGV>)) {
234 if ($line =~ s/\\$//) {
236 redo LINE unless eof(); # not eof(ARGV)!
241 Note that if there were a C<continue> block on the above code, it would
242 get executed only on lines discarded by the regex (since redo skips the
243 continue block). A continue block is often used to reset line counters
244 or C<?pat?> one-time matches:
246 # inspired by :1,$g/fred/s//WILMA/
248 ?(fred)? && s//WILMA $1 WILMA/;
249 ?(barney)? && s//BETTY $1 BETTY/;
250 ?(homer)? && s//MARGE $1 MARGE/;
252 print "$ARGV $.: $_";
253 close ARGV if eof(); # reset $.
254 reset if eof(); # reset ?pat?
257 If the word C<while> is replaced by the word C<until>, the sense of the
258 test is reversed, but the conditional is still tested before the first
261 The loop control statements don't work in an C<if> or C<unless>, since
262 they aren't loops. You can double the braces to make them such, though.
266 next if /barney/; # same effect as "last", but doesn't document as well
270 This is caused by the fact that a block by itself acts as a loop that
271 executes once, see L<"Basic BLOCKs and Switch Statements">.
273 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
274 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
278 Perl's C-style C<for> loop works like the corresponding C<while> loop;
279 that means that this:
281 for ($i = 1; $i < 10; $i++) {
294 There is one minor difference: if variables are declared with C<my>
295 in the initialization section of the C<for>, the lexical scope of
296 those variables is exactly the C<for> loop (the body of the loop
297 and the control sections).
299 Besides the normal array index looping, C<for> can lend itself
300 to many other interesting applications. Here's one that avoids the
301 problem you get into if you explicitly test for end-of-file on
302 an interactive file descriptor causing your program to appear to
305 $on_a_tty = -t STDIN && -t STDOUT;
306 sub prompt { print "yes? " if $on_a_tty }
307 for ( prompt(); <STDIN>; prompt() ) {
313 The C<foreach> loop iterates over a normal list value and sets the
314 variable VAR to be each element of the list in turn. If the variable
315 is preceded with the keyword C<my>, then it is lexically scoped, and
316 is therefore visible only within the loop. Otherwise, the variable is
317 implicitly local to the loop and regains its former value upon exiting
318 the loop. If the variable was previously declared with C<my>, it uses
319 that variable instead of the global one, but it's still localized to
320 the loop. This implicit localisation occurs I<only> in a C<foreach>
323 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
324 you can use C<foreach> for readability or C<for> for brevity. (Or because
325 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
326 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
328 If any element of LIST is an lvalue, you can modify it by modifying
329 VAR inside the loop. Conversely, if any element of LIST is NOT an
330 lvalue, any attempt to modify that element will fail. In other words,
331 the C<foreach> loop index variable is an implicit alias for each item
332 in the list that you're looping over.
334 If any part of LIST is an array, C<foreach> will get very confused if
335 you add or remove elements within the loop body, for example with
336 C<splice>. So don't do that.
338 C<foreach> probably won't do what you expect if VAR is a tied or other
339 special variable. Don't do that either.
343 for (@ary) { s/foo/bar/ }
345 for my $elem (@elements) {
349 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
350 print $count, "\n"; sleep(1);
353 for (1..15) { print "Merry Christmas\n"; }
355 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
356 print "Item: $item\n";
359 Here's how a C programmer might code up a particular algorithm in Perl:
361 for (my $i = 0; $i < @ary1; $i++) {
362 for (my $j = 0; $j < @ary2; $j++) {
363 if ($ary1[$i] > $ary2[$j]) {
364 last; # can't go to outer :-(
366 $ary1[$i] += $ary2[$j];
368 # this is where that last takes me
371 Whereas here's how a Perl programmer more comfortable with the idiom might
374 OUTER: for my $wid (@ary1) {
375 INNER: for my $jet (@ary2) {
376 next OUTER if $wid > $jet;
381 See how much easier this is? It's cleaner, safer, and faster. It's
382 cleaner because it's less noisy. It's safer because if code gets added
383 between the inner and outer loops later on, the new code won't be
384 accidentally executed. The C<next> explicitly iterates the other loop
385 rather than merely terminating the inner one. And it's faster because
386 Perl executes a C<foreach> statement more rapidly than it would the
387 equivalent C<for> loop.
389 =head2 Basic BLOCKs and Switch Statements
391 A BLOCK by itself (labeled or not) is semantically equivalent to a
392 loop that executes once. Thus you can use any of the loop control
393 statements in it to leave or restart the block. (Note that this is
394 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
395 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
398 The BLOCK construct is particularly nice for doing case
402 if (/^abc/) { $abc = 1; last SWITCH; }
403 if (/^def/) { $def = 1; last SWITCH; }
404 if (/^xyz/) { $xyz = 1; last SWITCH; }
408 There is no official C<switch> statement in Perl, because there are
409 already several ways to write the equivalent.
411 However, starting from Perl 5.8 to get switch and case one can use
412 the Switch extension and say:
416 after which one has switch and case. It is not as fast as it could be
417 because it's not really part of the language (it's done using source
418 filters) but it is available, and it's very flexible.
420 In addition to the above BLOCK construct, you could write
423 $abc = 1, last SWITCH if /^abc/;
424 $def = 1, last SWITCH if /^def/;
425 $xyz = 1, last SWITCH if /^xyz/;
429 (That's actually not as strange as it looks once you realize that you can
430 use loop control "operators" within an expression, That's just the normal
436 /^abc/ && do { $abc = 1; last SWITCH; };
437 /^def/ && do { $def = 1; last SWITCH; };
438 /^xyz/ && do { $xyz = 1; last SWITCH; };
442 or formatted so it stands out more as a "proper" C<switch> statement:
465 /^abc/ and $abc = 1, last SWITCH;
466 /^def/ and $def = 1, last SWITCH;
467 /^xyz/ and $xyz = 1, last SWITCH;
482 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
483 a temporary assignment to C<$_> for convenient matching:
485 SWITCH: for ($where) {
486 /In Card Names/ && do { push @flags, '-e'; last; };
487 /Anywhere/ && do { push @flags, '-h'; last; };
488 /In Rulings/ && do { last; };
489 die "unknown value for form variable where: `$where'";
492 Another interesting approach to a switch statement is arrange
493 for a C<do> block to return the proper value:
496 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
497 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
498 elsif ($flag & O_RDWR) {
499 if ($flag & O_CREAT) { "w+" }
500 else { ($flag & O_APPEND) ? "a+" : "r+" }
507 ($flags & O_WRONLY) ? "write-only" :
508 ($flags & O_RDWR) ? "read-write" :
512 Or if you are certain that all the C<&&> clauses are true, you can use
513 something like this, which "switches" on the value of the
514 C<HTTP_USER_AGENT> environment variable.
517 # pick out jargon file page based on browser
518 $dir = 'http://www.wins.uva.nl/~mes/jargon';
519 for ($ENV{HTTP_USER_AGENT}) {
520 $page = /Mac/ && 'm/Macintrash.html'
521 || /Win(dows )?NT/ && 'e/evilandrude.html'
522 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
523 || /Linux/ && 'l/Linux.html'
524 || /HP-UX/ && 'h/HP-SUX.html'
525 || /SunOS/ && 's/ScumOS.html'
526 || 'a/AppendixB.html';
528 print "Location: $dir/$page\015\012\015\012";
530 That kind of switch statement only works when you know the C<&&> clauses
531 will be true. If you don't, the previous C<?:> example should be used.
533 You might also consider writing a hash of subroutine references
534 instead of synthesizing a C<switch> statement.
538 Although not for the faint of heart, Perl does support a C<goto>
539 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
540 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
541 a C<goto>; it's just the name of the loop.
543 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
544 execution there. It may not be used to go into any construct that
545 requires initialization, such as a subroutine or a C<foreach> loop. It
546 also can't be used to go into a construct that is optimized away. It
547 can be used to go almost anywhere else within the dynamic scope,
548 including out of subroutines, but it's usually better to use some other
549 construct such as C<last> or C<die>. The author of Perl has never felt the
550 need to use this form of C<goto> (in Perl, that is--C is another matter).
552 The C<goto>-EXPR form expects a label name, whose scope will be resolved
553 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
554 necessarily recommended if you're optimizing for maintainability:
556 goto(("FOO", "BAR", "GLARCH")[$i]);
558 The C<goto>-&NAME form is highly magical, and substitutes a call to the
559 named subroutine for the currently running subroutine. This is used by
560 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
561 pretend that the other subroutine had been called in the first place
562 (except that any modifications to C<@_> in the current subroutine are
563 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
564 will be able to tell that this routine was called first.
566 In almost all cases like this, it's usually a far, far better idea to use the
567 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
568 resorting to a C<goto>. For certain applications, the catch and throw pair of
569 C<eval{}> and die() for exception processing can also be a prudent approach.
571 =head2 PODs: Embedded Documentation
573 Perl has a mechanism for intermixing documentation with source code.
574 While it's expecting the beginning of a new statement, if the compiler
575 encounters a line that begins with an equal sign and a word, like this
577 =head1 Here There Be Pods!
579 Then that text and all remaining text up through and including a line
580 beginning with C<=cut> will be ignored. The format of the intervening
581 text is described in L<perlpod>.
583 This allows you to intermix your source code
584 and your documentation text freely, as in
588 The snazzle() function will behave in the most spectacular
589 form that you can possibly imagine, not even excepting
590 cybernetic pyrotechnics.
592 =cut back to the compiler, nuff of this pod stuff!
599 Note that pod translators should look at only paragraphs beginning
600 with a pod directive (it makes parsing easier), whereas the compiler
601 actually knows to look for pod escapes even in the middle of a
602 paragraph. This means that the following secret stuff will be
603 ignored by both the compiler and the translators.
607 warn "Neither POD nor CODE!?"
611 You probably shouldn't rely upon the C<warn()> being podded out forever.
612 Not all pod translators are well-behaved in this regard, and perhaps
613 the compiler will become pickier.
615 One may also use pod directives to quickly comment out a section
618 =head2 Plain Old Comments (Not!)
620 Much like the C preprocessor, Perl can process line directives. Using
621 this, one can control Perl's idea of filenames and line numbers in
622 error or warning messages (especially for strings that are processed
623 with C<eval()>). The syntax for this mechanism is the same as for most
624 C preprocessors: it matches the regular expression
625 C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line
626 number for the next line, and C<$2> being the optional filename
627 (specified within quotes).
629 There is a fairly obvious gotcha included with the line directive:
630 Debuggers and profilers will only show the last source line to appear
631 at a particular line number in a given file. Care should be taken not
632 to cause line number collisions in code you'd like to debug later.
634 Here are some examples that you should be able to type into your command
639 # the `#' on the previous line must be the first char on line
642 foo at bzzzt line 201.
646 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
651 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
653 foo at foo bar line 200.
657 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
660 foo at goop line 345.