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1=head1 NAME
2
3perlsyn - Perl syntax
4
5=head1 DESCRIPTION
6
7A Perl script consists of a sequence of declarations and statements.
8The only things that need to be declared in Perl are report formats
9and subroutines. See the sections below for more information on those
10declarations. All uninitialized user-created objects are assumed to
11start with a null or 0 value until they are defined by some explicit
12operation such as assignment. (Though you can get warnings about the
13use of undefined values if you like.) The sequence of statements is
14executed just once, unlike in B<sed> and B<awk> scripts, where the
15sequence of statements is executed for each input line. While this means
16that you must explicitly loop over the lines of your input file (or
17files), it also means you have much more control over which files and
18which lines you look at. (Actually, I'm lying--it is possible to do an
19implicit loop with either the B<-n> or B<-p> switch. It's just not the
20mandatory default like it is in B<sed> and B<awk>.)
21
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22=head2 Declarations
23
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24Perl is, for the most part, a free-form language. (The only
25exception to this is format declarations, for obvious reasons.) Comments
26are indicated by the "#" character, and extend to the end of the line. If
27you attempt to use C</* */> C-style comments, it will be interpreted
28either as division or pattern matching, depending on the context, and C++
4633a7c4 29C<//> comments just look like a null regular expression, so don't do
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30that.
31
32A declaration can be put anywhere a statement can, but has no effect on
33the execution of the primary sequence of statements--declarations all
34take effect at compile time. Typically all the declarations are put at
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35the beginning or the end of the script. However, if you're using
36lexically-scoped private variables created with my(), you'll have to make sure
37your format or subroutine definition is within the same block scope
38as the my if you expect to to be able to access those private variables.
a0d0e21e 39
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40Declaring a subroutine allows a subroutine name to be used as if it were a
41list operator from that point forward in the program. You can declare a
c07a80fd 42subroutine (prototyped to take one scalar parameter) without defining it by saying just:
a0d0e21e 43
c07a80fd 44 sub myname ($);
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45 $me = myname $0 or die "can't get myname";
46
4633a7c4 47Note that it functions as a list operator though, not as a unary
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48operator, so be careful to use C<or> instead of C<||> there.
49
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50Subroutines declarations can also be loaded up with the C<require> statement
51or both loaded and imported into your namespace with a C<use> statement.
52See L<perlmod> for details on this.
a0d0e21e 53
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54A statement sequence may contain declarations of lexically-scoped
55variables, but apart from declaring a variable name, the declaration acts
56like an ordinary statement, and is elaborated within the sequence of
57statements as if it were an ordinary statement. That means it actually
58has both compile-time and run-time effects.
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59
60=head2 Simple statements
61
62The only kind of simple statement is an expression evaluated for its
63side effects. Every simple statement must be terminated with a
64semicolon, unless it is the final statement in a block, in which case
65the semicolon is optional. (A semicolon is still encouraged there if the
748a9306 66block takes up more than one line, since you may eventually add another line.)
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67Note that there are some operators like C<eval {}> and C<do {}> that look
68like compound statements, but aren't (they're just TERMs in an expression),
4633a7c4 69and thus need an explicit termination if used as the last item in a statement.
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70
71Any simple statement may optionally be followed by a I<SINGLE> modifier,
72just before the terminating semicolon (or block ending). The possible
73modifiers are:
74
75 if EXPR
76 unless EXPR
77 while EXPR
78 until EXPR
79
80The C<if> and C<unless> modifiers have the expected semantics,
81presuming you're a speaker of English. The C<while> and C<until>
82modifiers also have the usual "while loop" semantics (conditional
83evaluated first), except when applied to a do-BLOCK (or to the
84now-deprecated do-SUBROUTINE statement), in which case the block
85executes once before the conditional is evaluated. This is so that you
86can write loops like:
87
88 do {
4633a7c4 89 $line = <STDIN>;
a0d0e21e 90 ...
4633a7c4 91 } until $line eq ".\n";
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92
93See L<perlfunc/do>. Note also that the loop control
94statements described later will I<NOT> work in this construct, since
95modifiers don't take loop labels. Sorry. You can always wrap
4633a7c4 96another block around it to do that sort of thing.
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97
98=head2 Compound statements
99
100In Perl, a sequence of statements that defines a scope is called a block.
101Sometimes a block is delimited by the file containing it (in the case
102of a required file, or the program as a whole), and sometimes a block
103is delimited by the extent of a string (in the case of an eval).
104
105But generally, a block is delimited by curly brackets, also known as braces.
106We will call this syntactic construct a BLOCK.
107
108The following compound statements may be used to control flow:
109
110 if (EXPR) BLOCK
111 if (EXPR) BLOCK else BLOCK
112 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
113 LABEL while (EXPR) BLOCK
114 LABEL while (EXPR) BLOCK continue BLOCK
115 LABEL for (EXPR; EXPR; EXPR) BLOCK
748a9306 116 LABEL foreach VAR (LIST) BLOCK
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117 LABEL BLOCK continue BLOCK
118
119Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
120not statements. This means that the curly brackets are I<required>--no
121dangling statements allowed. If you want to write conditionals without
122curly brackets there are several other ways to do it. The following
123all do the same thing:
124
125 if (!open(FOO)) { die "Can't open $FOO: $!"; }
126 die "Can't open $FOO: $!" unless open(FOO);
127 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
128 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
129 # a bit exotic, that last one
130
131The C<if> statement is straightforward. Since BLOCKs are always
132bounded by curly brackets, there is never any ambiguity about which
133C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
134the sense of the test is reversed.
135
136The C<while> statement executes the block as long as the expression is
137true (does not evaluate to the null string or 0 or "0"). The LABEL is
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138optional, and if present, consists of an identifier followed by a colon.
139The LABEL identifies the loop for the loop control statements C<next>,
140C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
141refers to the innermost enclosing loop. This may include dynamically
142looking back your call-stack at run time to find the LABEL. Such
143desperate behavior triggers a warning if you use the B<-w> flag.
144
145If there is a C<continue> BLOCK, it is always executed just before the
146conditional is about to be evaluated again, just like the third part of a
147C<for> loop in C. Thus it can be used to increment a loop variable, even
148when the loop has been continued via the C<next> statement (which is
149similar to the C C<continue> statement).
150
151=head2 Loop Control
152
153The C<next> command is like the C<continue> statement in C; it starts
154the next iteration of the loop:
155
156 LINE: while (<STDIN>) {
157 next LINE if /^#/; # discard comments
158 ...
159 }
160
161The C<last> command is like the C<break> statement in C (as used in
162loops); it immediately exits the loop in question. The
163C<continue> block, if any, is not executed:
164
165 LINE: while (<STDIN>) {
166 last LINE if /^$/; # exit when done with header
167 ...
168 }
169
170The C<redo> command restarts the loop block without evaluating the
171conditional again. The C<continue> block, if any, is I<not> executed.
172This command is normally used by programs that want to lie to themselves
173about what was just input.
174
175For example, when processing a file like F</etc/termcap>.
176If your input lines might end in backslashes to indicate continuation, you
177want to skip ahead and get the next record.
178
179 while (<>) {
180 chomp;
181 if (s/\\$//) {
182 $_ .= <>;
183 redo unless eof();
184 }
185 # now process $_
186 }
187
188which is Perl short-hand for the more explicitly written version:
189
190 LINE: while ($line = <ARGV>) {
191 chomp($line);
192 if ($line =~ s/\\$//) {
193 $line .= <ARGV>;
194 redo LINE unless eof(); # not eof(ARGV)!
195 }
196 # now process $line
197 }
198
184e9718 199Or here's a simpleminded Pascal comment stripper (warning: assumes no { or } in strings).
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200
201 LINE: while (<STDIN>) {
202 while (s|({.*}.*){.*}|$1 |) {}
203 s|{.*}| |;
204 if (s|{.*| |) {
205 $front = $_;
206 while (<STDIN>) {
207 if (/}/) { # end of comment?
208 s|^|$front{|;
209 redo LINE;
210 }
211 }
212 }
213 print;
214 }
215
216Note that if there were a C<continue> block on the above code, it would get
217executed even on discarded lines.
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218
219If the word C<while> is replaced by the word C<until>, the sense of the
220test is reversed, but the conditional is still tested before the first
221iteration.
222
223In either the C<if> or the C<while> statement, you may replace "(EXPR)"
224with a BLOCK, and the conditional is true if the value of the last
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225statement in that block is true. While this "feature" continues to work in
226version 5, it has been deprecated, so please change any occurrences of "if BLOCK" to
227"if (do BLOCK)".
228
cb1a09d0 229=head2 For Loops
a0d0e21e 230
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231Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
232that means that this:
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233
234 for ($i = 1; $i < 10; $i++) {
235 ...
236 }
237
cb1a09d0 238is the same as this:
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239
240 $i = 1;
241 while ($i < 10) {
242 ...
243 } continue {
244 $i++;
245 }
246
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247Besides the normal array index looping, C<for> can lend itself
248to many other interesting applications. Here's one that avoids the
249problem you get into if you explicitly test for end-of-file on
250an interactive file descriptor causing your program to appear to
251hang.
252
253 $on_a_tty = -t STDIN && -t STDOUT;
254 sub prompt { print "yes? " if $on_a_tty }
255 for ( prompt(); <STDIN>; prompt() ) {
256 # do something
257 }
258
259=head2 Foreach Loops
260
4633a7c4 261The C<foreach> loop iterates over a normal list value and sets the
a0d0e21e 262variable VAR to be each element of the list in turn. The variable is
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263implicitly local to the loop and regains its former value upon exiting the
264loop. If the variable was previously declared with C<my>, it uses that
265variable instead of the global one, but it's still localized to the loop.
266This can cause problems if you have subroutine or format declarations
267within that block's scope.
268
269The C<foreach> keyword is actually a synonym for the C<for> keyword, so
270you can use C<foreach> for readability or C<for> for brevity. If VAR is
271omitted, $_ is set to each value. If LIST is an actual array (as opposed
272to an expression returning a list value), you can modify each element of
273the array by modifying VAR inside the loop. That's because the C<foreach>
274loop index variable is an implicit alias for each item in the list that
275you're looping over.
276
748a9306 277Examples:
a0d0e21e 278
4633a7c4 279 for (@ary) { s/foo/bar/ }
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280
281 foreach $elem (@elements) {
282 $elem *= 2;
283 }
284
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285 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
286 print $count, "\n"; sleep(1);
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287 }
288
289 for (1..15) { print "Merry Christmas\n"; }
290
4633a7c4 291 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
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292 print "Item: $item\n";
293 }
294
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295Here's how a C programmer might code up a particular algorithm in Perl:
296
297 for ($i = 0; $i < @ary1; $i++) {
298 for ($j = 0; $j < @ary2; $j++) {
299 if ($ary1[$i] > $ary2[$j]) {
300 last; # can't go to outer :-(
301 }
302 $ary1[$i] += $ary2[$j];
303 }
cb1a09d0 304 # this is where that last takes me
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305 }
306
184e9718 307Whereas here's how a Perl programmer more comfortable with the idiom might
cb1a09d0 308do it:
4633a7c4 309
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310 OUTER: foreach $wid (@ary1) {
311 INNER: foreach $jet (@ary2) {
312 next OUTER if $wid > $jet;
313 $wid += $jet;
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314 }
315 }
316
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317See how much easier this is? It's cleaner, safer, and faster. It's
318cleaner because it's less noisy. It's safer because if code gets added
c07a80fd 319between the inner and outer loops later on, the new code won't be
9b1b54d0 320accidentally executed, the C<next> explicitly iterates the other loop
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321rather than merely terminating the inner one. And it's faster because
322Perl executes a C<foreach> statement more rapidly than it would the
323equivalent C<for> loop.
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324
325=head2 Basic BLOCKs and Switch Statements
326
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327A BLOCK by itself (labeled or not) is semantically equivalent to a loop
328that executes once. Thus you can use any of the loop control
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329statements in it to leave or restart the block. (Note that this
330is I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief C<do{}> blocks,
331which do I<NOT> count as loops.) The C<continue> block
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332is optional.
333
334The BLOCK construct is particularly nice for doing case
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335structures.
336
337 SWITCH: {
338 if (/^abc/) { $abc = 1; last SWITCH; }
339 if (/^def/) { $def = 1; last SWITCH; }
340 if (/^xyz/) { $xyz = 1; last SWITCH; }
341 $nothing = 1;
342 }
343
344There is no official switch statement in Perl, because there are
345already several ways to write the equivalent. In addition to the
346above, you could write
347
348 SWITCH: {
349 $abc = 1, last SWITCH if /^abc/;
350 $def = 1, last SWITCH if /^def/;
351 $xyz = 1, last SWITCH if /^xyz/;
352 $nothing = 1;
353 }
354
cb1a09d0 355(That's actually not as strange as it looks once you realize that you can
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356use loop control "operators" within an expression, That's just the normal
357C comma operator.)
358
359or
360
361 SWITCH: {
362 /^abc/ && do { $abc = 1; last SWITCH; };
363 /^def/ && do { $def = 1; last SWITCH; };
364 /^xyz/ && do { $xyz = 1; last SWITCH; };
365 $nothing = 1;
366 }
367
368or formatted so it stands out more as a "proper" switch statement:
369
370 SWITCH: {
371 /^abc/ && do {
372 $abc = 1;
373 last SWITCH;
374 };
375
376 /^def/ && do {
377 $def = 1;
378 last SWITCH;
379 };
380
381 /^xyz/ && do {
382 $xyz = 1;
383 last SWITCH;
384 };
385 $nothing = 1;
386 }
387
388or
389
390 SWITCH: {
391 /^abc/ and $abc = 1, last SWITCH;
392 /^def/ and $def = 1, last SWITCH;
393 /^xyz/ and $xyz = 1, last SWITCH;
394 $nothing = 1;
395 }
396
397or even, horrors,
398
399 if (/^abc/)
400 { $abc = 1 }
401 elsif (/^def/)
402 { $def = 1 }
403 elsif (/^xyz/)
404 { $xyz = 1 }
405 else
406 { $nothing = 1 }
407
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408
409A common idiom for a switch statement is to use C<foreach>'s aliasing to make
410a temporary assignment to $_ for convenient matching:
411
412 SWITCH: for ($where) {
413 /In Card Names/ && do { push @flags, '-e'; last; };
414 /Anywhere/ && do { push @flags, '-h'; last; };
415 /In Rulings/ && do { last; };
416 die "unknown value for form variable where: `$where'";
417 }
418
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419Another interesting approach to a switch statement is arrange
420for a C<do> block to return the proper value:
421
422 $amode = do {
423 if ($flag & O_RDONLY) { "r" }
c07a80fd 424 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
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425 elsif ($flag & O_RDWR) {
426 if ($flag & O_CREAT) { "w+" }
c07a80fd 427 else { ($flag & O_APPEND) ? "a+" : "r+" }
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428 }
429 };
430
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431=head2 Goto
432
433Although not for the faint of heart, Perl does support a C<goto> statement.
434A loop's LABEL is not actually a valid target for a C<goto>;
435it's just the name of the loop. There are three forms: goto-LABEL,
436goto-EXPR, and goto-&NAME.
437
438The goto-LABEL form finds the statement labeled with LABEL and resumes
439execution there. It may not be used to go into any construct that
440requires initialization, such as a subroutine or a foreach loop. It
441also can't be used to go into a construct that is optimized away. It
442can be used to go almost anywhere else within the dynamic scope,
443including out of subroutines, but it's usually better to use some other
444construct such as last or die. The author of Perl has never felt the
445need to use this form of goto (in Perl, that is--C is another matter).
446
447The goto-EXPR form expects a label name, whose scope will be resolved
448dynamically. This allows for computed gotos per FORTRAN, but isn't
449necessarily recommended if you're optimizing for maintainability:
450
451 goto ("FOO", "BAR", "GLARCH")[$i];
452
453The goto-&NAME form is highly magical, and substitutes a call to the
454named subroutine for the currently running subroutine. This is used by
455AUTOLOAD() subroutines that wish to load another subroutine and then
456pretend that the other subroutine had been called in the first place
457(except that any modifications to @_ in the current subroutine are
458propagated to the other subroutine.) After the C<goto>, not even caller()
459will be able to tell that this routine was called first.
460
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461In almost all cases like this, it's usually a far, far better idea to use the
462structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
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463resorting to a C<goto>. For certain applications, the catch and throw pair of
464C<eval{}> and die() for exception processing can also be a prudent approach.
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465
466=head2 PODs: Embedded Documentation
467
468Perl has a mechanism for intermixing documentation with source code.
c07a80fd 469While it's expecting the beginning of a new statement, if the compiler
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470encounters a line that begins with an equal sign and a word, like this
471
472 =head1 Here There Be Pods!
473
474Then that text and all remaining text up through and including a line
475beginning with C<=cut> will be ignored. The format of the intervening
476text is described in L<perlpod>.
477
478This allows you to intermix your source code
479and your documentation text freely, as in
480
481 =item snazzle($)
482
483 The snazzle() function will behave in the most spectacular
484 form that you can possibly imagine, not even excepting
485 cybernetic pyrotechnics.
486
487 =cut back to the compiler, nuff of this pod stuff!
488
489 sub snazzle($) {
490 my $thingie = shift;
491 .........
492 }
493
494Note that pod translators should only look at paragraphs beginning
184e9718 495with a pod directive (it makes parsing easier), whereas the compiler
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496actually knows to look for pod escapes even in the middle of a
497paragraph. This means that the following secret stuff will be
498ignored by both the compiler and the translators.
499
500 $a=3;
501 =secret stuff
502 warn "Neither POD nor CODE!?"
503 =cut back
504 print "got $a\n";
505
506You probably shouldn't rely upon the warn() being podded out forever.
507Not all pod translators are well-behaved in this regard, and perhaps
508the compiler will become pickier.