7 A Perl script consists of a sequence of declarations and statements.
8 The only things that need to be declared in Perl are report formats
9 and subroutines. See the sections below for more information on those
10 declarations. All uninitialized user-created objects are assumed to
11 start with a null or 0 value until they are defined by some explicit
12 operation such as assignment. (Though you can get warnings about the
13 use of undefined values if you like.) The sequence of statements is
14 executed just once, unlike in B<sed> and B<awk> scripts, where the
15 sequence of statements is executed for each input line. While this means
16 that you must explicitly loop over the lines of your input file (or
17 files), it also means you have much more control over which files and
18 which lines you look at. (Actually, I'm lying--it is possible to do an
19 implicit loop with either the B<-n> or B<-p> switch. It's just not the
20 mandatory default like it is in B<sed> and B<awk>.)
24 Perl is, for the most part, a free-form language. (The only
25 exception to this is format declarations, for obvious reasons.) Comments
26 are indicated by the "#" character, and extend to the end of the line. If
27 you attempt to use C</* */> C-style comments, it will be interpreted
28 either as division or pattern matching, depending on the context, and C++
29 C<//> comments just look like a null regular expression, so don't do
32 A declaration can be put anywhere a statement can, but has no effect on
33 the execution of the primary sequence of statements--declarations all
34 take effect at compile time. Typically all the declarations are put at
35 the beginning or the end of the script. However, if you're using
36 lexically-scoped private variables created with my(), you'll have to make sure
37 your format or subroutine definition is within the same block scope
38 as the my if you expect to be able to access those private variables.
40 Declaring a subroutine allows a subroutine name to be used as if it were a
41 list operator from that point forward in the program. You can declare a
42 subroutine without defining it by saying C<sub name>, thus:
45 $me = myname $0 or die "can't get myname";
47 Note that it functions as a list operator, not as a unary operator; so
48 be careful to use C<or> instead of C<||> in this case. However, if
49 you were to declare the subroutine as C<sub myname ($)>, then
50 C<myname> would functonion as a unary operator, so either C<or> or
53 Subroutines declarations can also be loaded up with the C<require> statement
54 or both loaded and imported into your namespace with a C<use> statement.
55 See L<perlmod> for details on this.
57 A statement sequence may contain declarations of lexically-scoped
58 variables, but apart from declaring a variable name, the declaration acts
59 like an ordinary statement, and is elaborated within the sequence of
60 statements as if it were an ordinary statement. That means it actually
61 has both compile-time and run-time effects.
63 =head2 Simple statements
65 The only kind of simple statement is an expression evaluated for its
66 side effects. Every simple statement must be terminated with a
67 semicolon, unless it is the final statement in a block, in which case
68 the semicolon is optional. (A semicolon is still encouraged there if the
69 block takes up more than one line, because you may eventually add another line.)
70 Note that there are some operators like C<eval {}> and C<do {}> that look
71 like compound statements, but aren't (they're just TERMs in an expression),
72 and thus need an explicit termination if used as the last item in a statement.
74 Any simple statement may optionally be followed by a I<SINGLE> modifier,
75 just before the terminating semicolon (or block ending). The possible
83 The C<if> and C<unless> modifiers have the expected semantics,
84 presuming you're a speaker of English. The C<while> and C<until>
85 modifiers also have the usual "while loop" semantics (conditional
86 evaluated first), except when applied to a do-BLOCK (or to the
87 now-deprecated do-SUBROUTINE statement), in which case the block
88 executes once before the conditional is evaluated. This is so that you
94 } until $line eq ".\n";
96 See L<perlfunc/do>. Note also that the loop control
97 statements described later will I<NOT> work in this construct, because
98 modifiers don't take loop labels. Sorry. You can always wrap
99 another block around it to do that sort of thing.
101 =head2 Compound statements
103 In Perl, a sequence of statements that defines a scope is called a block.
104 Sometimes a block is delimited by the file containing it (in the case
105 of a required file, or the program as a whole), and sometimes a block
106 is delimited by the extent of a string (in the case of an eval).
108 But generally, a block is delimited by curly brackets, also known as braces.
109 We will call this syntactic construct a BLOCK.
111 The following compound statements may be used to control flow:
114 if (EXPR) BLOCK else BLOCK
115 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
116 LABEL while (EXPR) BLOCK
117 LABEL while (EXPR) BLOCK continue BLOCK
118 LABEL for (EXPR; EXPR; EXPR) BLOCK
119 LABEL foreach VAR (LIST) BLOCK
120 LABEL BLOCK continue BLOCK
122 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
123 not statements. This means that the curly brackets are I<required>--no
124 dangling statements allowed. If you want to write conditionals without
125 curly brackets there are several other ways to do it. The following
126 all do the same thing:
128 if (!open(FOO)) { die "Can't open $FOO: $!"; }
129 die "Can't open $FOO: $!" unless open(FOO);
130 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
131 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
132 # a bit exotic, that last one
134 The C<if> statement is straightforward. Because BLOCKs are always
135 bounded by curly brackets, there is never any ambiguity about which
136 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
137 the sense of the test is reversed.
139 The C<while> statement executes the block as long as the expression is
140 true (does not evaluate to the null string or 0 or "0"). The LABEL is
141 optional, and if present, consists of an identifier followed by a colon.
142 The LABEL identifies the loop for the loop control statements C<next>,
143 C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
144 refers to the innermost enclosing loop. This may include dynamically
145 looking back your call-stack at run time to find the LABEL. Such
146 desperate behavior triggers a warning if you use the B<-w> flag.
148 If there is a C<continue> BLOCK, it is always executed just before the
149 conditional is about to be evaluated again, just like the third part of a
150 C<for> loop in C. Thus it can be used to increment a loop variable, even
151 when the loop has been continued via the C<next> statement (which is
152 similar to the C C<continue> statement).
156 The C<next> command is like the C<continue> statement in C; it starts
157 the next iteration of the loop:
159 LINE: while (<STDIN>) {
160 next LINE if /^#/; # discard comments
164 The C<last> command is like the C<break> statement in C (as used in
165 loops); it immediately exits the loop in question. The
166 C<continue> block, if any, is not executed:
168 LINE: while (<STDIN>) {
169 last LINE if /^$/; # exit when done with header
173 The C<redo> command restarts the loop block without evaluating the
174 conditional again. The C<continue> block, if any, is I<not> executed.
175 This command is normally used by programs that want to lie to themselves
176 about what was just input.
178 For example, when processing a file like F</etc/termcap>.
179 If your input lines might end in backslashes to indicate continuation, you
180 want to skip ahead and get the next record.
191 which is Perl short-hand for the more explicitly written version:
193 LINE: while (defined($line = <ARGV>)) {
195 if ($line =~ s/\\$//) {
197 redo LINE unless eof(); # not eof(ARGV)!
202 Or here's a simpleminded Pascal comment stripper (warning: assumes no
205 LINE: while (<STDIN>) {
206 while (s|({.*}.*){.*}|$1 |) {}
211 if (/}/) { # end of comment?
220 Note that if there were a C<continue> block on the above code, it would get
221 executed even on discarded lines.
223 If the word C<while> is replaced by the word C<until>, the sense of the
224 test is reversed, but the conditional is still tested before the first
227 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
228 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
232 Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
233 that means that this:
235 for ($i = 1; $i < 10; $i++) {
248 (There is one minor difference: The first form implies a lexical scope
249 for variables declared with C<my> in the initialization expression.)
251 Besides the normal array index looping, C<for> can lend itself
252 to many other interesting applications. Here's one that avoids the
253 problem you get into if you explicitly test for end-of-file on
254 an interactive file descriptor causing your program to appear to
257 $on_a_tty = -t STDIN && -t STDOUT;
258 sub prompt { print "yes? " if $on_a_tty }
259 for ( prompt(); <STDIN>; prompt() ) {
265 The C<foreach> loop iterates over a normal list value and sets the
266 variable VAR to be each element of the list in turn. If the variable
267 is preceded with the keyword C<my>, then it is lexically scoped, and
268 is therefore visible only within the loop. Otherwise, the variable is
269 implicitly local to the loop and regains its former value upon exiting
270 the loop. If the variable was previously declared with C<my>, it uses
271 that variable instead of the global one, but it's still localized to
272 the loop. (Note that a lexically scoped variable can cause problems
273 with you have subroutine or format declarations.)
275 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
276 you can use C<foreach> for readability or C<for> for brevity. If VAR is
277 omitted, $_ is set to each value. If LIST is an actual array (as opposed
278 to an expression returning a list value), you can modify each element of
279 the array by modifying VAR inside the loop. That's because the C<foreach>
280 loop index variable is an implicit alias for each item in the list that
285 for (@ary) { s/foo/bar/ }
287 foreach my $elem (@elements) {
291 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
292 print $count, "\n"; sleep(1);
295 for (1..15) { print "Merry Christmas\n"; }
297 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
298 print "Item: $item\n";
301 Here's how a C programmer might code up a particular algorithm in Perl:
303 for (my $i = 0; $i < @ary1; $i++) {
304 for (my $j = 0; $j < @ary2; $j++) {
305 if ($ary1[$i] > $ary2[$j]) {
306 last; # can't go to outer :-(
308 $ary1[$i] += $ary2[$j];
310 # this is where that last takes me
313 Whereas here's how a Perl programmer more comfortable with the idiom might
316 OUTER: foreach my $wid (@ary1) {
317 INNER: foreach my $jet (@ary2) {
318 next OUTER if $wid > $jet;
323 See how much easier this is? It's cleaner, safer, and faster. It's
324 cleaner because it's less noisy. It's safer because if code gets added
325 between the inner and outer loops later on, the new code won't be
326 accidentally executed. The C<next> explicitly iterates the other loop
327 rather than merely terminating the inner one. And it's faster because
328 Perl executes a C<foreach> statement more rapidly than it would the
329 equivalent C<for> loop.
331 =head2 Basic BLOCKs and Switch Statements
333 A BLOCK by itself (labeled or not) is semantically equivalent to a
334 loop that executes once. Thus you can use any of the loop control
335 statements in it to leave or restart the block. (Note that this is
336 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
337 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
340 The BLOCK construct is particularly nice for doing case
344 if (/^abc/) { $abc = 1; last SWITCH; }
345 if (/^def/) { $def = 1; last SWITCH; }
346 if (/^xyz/) { $xyz = 1; last SWITCH; }
350 There is no official switch statement in Perl, because there are
351 already several ways to write the equivalent. In addition to the
352 above, you could write
355 $abc = 1, last SWITCH if /^abc/;
356 $def = 1, last SWITCH if /^def/;
357 $xyz = 1, last SWITCH if /^xyz/;
361 (That's actually not as strange as it looks once you realize that you can
362 use loop control "operators" within an expression, That's just the normal
368 /^abc/ && do { $abc = 1; last SWITCH; };
369 /^def/ && do { $def = 1; last SWITCH; };
370 /^xyz/ && do { $xyz = 1; last SWITCH; };
374 or formatted so it stands out more as a "proper" switch statement:
397 /^abc/ and $abc = 1, last SWITCH;
398 /^def/ and $def = 1, last SWITCH;
399 /^xyz/ and $xyz = 1, last SWITCH;
415 A common idiom for a switch statement is to use C<foreach>'s aliasing to make
416 a temporary assignment to $_ for convenient matching:
418 SWITCH: for ($where) {
419 /In Card Names/ && do { push @flags, '-e'; last; };
420 /Anywhere/ && do { push @flags, '-h'; last; };
421 /In Rulings/ && do { last; };
422 die "unknown value for form variable where: `$where'";
425 Another interesting approach to a switch statement is arrange
426 for a C<do> block to return the proper value:
429 if ($flag & O_RDONLY) { "r" }
430 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
431 elsif ($flag & O_RDWR) {
432 if ($flag & O_CREAT) { "w+" }
433 else { ($flag & O_APPEND) ? "a+" : "r+" }
439 Although not for the faint of heart, Perl does support a C<goto> statement.
440 A loop's LABEL is not actually a valid target for a C<goto>;
441 it's just the name of the loop. There are three forms: goto-LABEL,
442 goto-EXPR, and goto-&NAME.
444 The goto-LABEL form finds the statement labeled with LABEL and resumes
445 execution there. It may not be used to go into any construct that
446 requires initialization, such as a subroutine or a foreach loop. It
447 also can't be used to go into a construct that is optimized away. It
448 can be used to go almost anywhere else within the dynamic scope,
449 including out of subroutines, but it's usually better to use some other
450 construct such as last or die. The author of Perl has never felt the
451 need to use this form of goto (in Perl, that is--C is another matter).
453 The goto-EXPR form expects a label name, whose scope will be resolved
454 dynamically. This allows for computed gotos per FORTRAN, but isn't
455 necessarily recommended if you're optimizing for maintainability:
457 goto ("FOO", "BAR", "GLARCH")[$i];
459 The goto-&NAME form is highly magical, and substitutes a call to the
460 named subroutine for the currently running subroutine. This is used by
461 AUTOLOAD() subroutines that wish to load another subroutine and then
462 pretend that the other subroutine had been called in the first place
463 (except that any modifications to @_ in the current subroutine are
464 propagated to the other subroutine.) After the C<goto>, not even caller()
465 will be able to tell that this routine was called first.
467 In almost all cases like this, it's usually a far, far better idea to use the
468 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
469 resorting to a C<goto>. For certain applications, the catch and throw pair of
470 C<eval{}> and die() for exception processing can also be a prudent approach.
472 =head2 PODs: Embedded Documentation
474 Perl has a mechanism for intermixing documentation with source code.
475 While it's expecting the beginning of a new statement, if the compiler
476 encounters a line that begins with an equal sign and a word, like this
478 =head1 Here There Be Pods!
480 Then that text and all remaining text up through and including a line
481 beginning with C<=cut> will be ignored. The format of the intervening
482 text is described in L<perlpod>.
484 This allows you to intermix your source code
485 and your documentation text freely, as in
489 The snazzle() function will behave in the most spectacular
490 form that you can possibly imagine, not even excepting
491 cybernetic pyrotechnics.
493 =cut back to the compiler, nuff of this pod stuff!
500 Note that pod translators should look at only paragraphs beginning
501 with a pod directive (it makes parsing easier), whereas the compiler
502 actually knows to look for pod escapes even in the middle of a
503 paragraph. This means that the following secret stuff will be
504 ignored by both the compiler and the translators.
508 warn "Neither POD nor CODE!?"
512 You probably shouldn't rely upon the warn() being podded out forever.
513 Not all pod translators are well-behaved in this regard, and perhaps
514 the compiler will become pickier.
516 One may also use pod directives to quickly comment out a section
519 =head2 Plain Old Comments (Not!)
521 Much like the C preprocessor, perl can process line directives. Using
522 this, one can control perl's idea of filenames and line numbers in
523 error or warning messages (especially for strings that are processed
524 with eval()). The syntax for this mechanism is the same as for most
525 C preprocessors: it matches the regular expression
526 C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
527 number for the next line, and C<$2> being the optional filename
528 (specified within quotes).
530 Here are some examples that you should be able to type into your command
535 # the `#' on the previous line must be the first char on line
538 foo at bzzzt line 201.
542 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
547 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
549 foo at foo bar line 200.
553 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
556 foo at goop line 345.