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1 | =head1 NAME |
2 | |
3 | perlsyn - Perl syntax |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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>.) |
21 | |
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22 | =head2 Declarations |
23 | |
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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++ |
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29 | C<//> comments just look like a null regular expression, so don't do |
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30 | that. |
31 | |
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 |
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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 to be able to access those private variables. |
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39 | |
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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 just |
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43 | |
44 | sub myname; |
45 | $me = myname $0 or die "can't get myname"; |
46 | |
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47 | Note that it functions as a list operator though, not as a unary |
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48 | operator, so be careful to use C<or> instead of C<||> there. |
49 | |
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50 | Subroutines declarations can also be loaded up with the C<require> statement |
51 | or both loaded and imported into your namespace with a C<use> statement. |
52 | See L<perlmod> for details on this. |
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53 | |
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54 | A statement sequence may contain declarations of lexically-scoped |
55 | variables, but apart from declaring a variable name, the declaration acts |
56 | like an ordinary statement, and is elaborated within the sequence of |
57 | statements as if it were an ordinary statement. That means it actually |
58 | has both compile-time and run-time effects. |
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59 | |
60 | =head2 Simple statements |
61 | |
62 | The only kind of simple statement is an expression evaluated for its |
63 | side effects. Every simple statement must be terminated with a |
64 | semicolon, unless it is the final statement in a block, in which case |
65 | the semicolon is optional. (A semicolon is still encouraged there if the |
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66 | block takes up more than one line, since you may eventually add another line.) |
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67 | Note that there are some operators like C<eval {}> and C<do {}> that look |
68 | like compound statements, but aren't (they're just TERMs in an expression), |
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69 | and thus need an explicit termination if used as the last item in a statement. |
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70 | |
71 | Any simple statement may optionally be followed by a I<SINGLE> modifier, |
72 | just before the terminating semicolon (or block ending). The possible |
73 | modifiers are: |
74 | |
75 | if EXPR |
76 | unless EXPR |
77 | while EXPR |
78 | until EXPR |
79 | |
80 | The C<if> and C<unless> modifiers have the expected semantics, |
81 | presuming you're a speaker of English. The C<while> and C<until> |
82 | modifiers also have the usual "while loop" semantics (conditional |
83 | evaluated first), except when applied to a do-BLOCK (or to the |
84 | now-deprecated do-SUBROUTINE statement), in which case the block |
85 | executes once before the conditional is evaluated. This is so that you |
86 | can write loops like: |
87 | |
88 | do { |
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89 | $line = <STDIN>; |
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90 | ... |
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91 | } until $line eq ".\n"; |
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92 | |
93 | See L<perlfunc/do>. Note also that the loop control |
94 | statements described later will I<NOT> work in this construct, since |
95 | modifiers don't take loop labels. Sorry. You can always wrap |
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96 | another block around it to do that sort of thing. |
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97 | |
98 | =head2 Compound statements |
99 | |
100 | In Perl, a sequence of statements that defines a scope is called a block. |
101 | Sometimes a block is delimited by the file containing it (in the case |
102 | of a required file, or the program as a whole), and sometimes a block |
103 | is delimited by the extent of a string (in the case of an eval). |
104 | |
105 | But generally, a block is delimited by curly brackets, also known as braces. |
106 | We will call this syntactic construct a BLOCK. |
107 | |
108 | The 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 |
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116 | LABEL foreach VAR (LIST) BLOCK |
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117 | LABEL BLOCK continue BLOCK |
118 | |
119 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
120 | not statements. This means that the curly brackets are I<required>--no |
121 | dangling statements allowed. If you want to write conditionals without |
122 | curly brackets there are several other ways to do it. The following |
123 | all 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: $!"; |
129 | # a bit exotic, that last one |
130 | |
131 | The C<if> statement is straightforward. Since BLOCKs are always |
132 | bounded by curly brackets, there is never any ambiguity about which |
133 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
134 | the sense of the test is reversed. |
135 | |
136 | The C<while> statement executes the block as long as the expression is |
137 | true (does not evaluate to the null string or 0 or "0"). The LABEL is |
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138 | optional, and if present, consists of an identifier followed by a colon. |
139 | The LABEL identifies the loop for the loop control statements C<next>, |
140 | C<last>, and C<redo>. If the LABEL is omitted, the loop control statement |
141 | refers to the innermost enclosing loop. This may include dynamically |
142 | looking back your call-stack at run time to find the LABEL. Such |
143 | desperate behavior triggers a warning if you use the B<-w> flag. |
144 | |
145 | If there is a C<continue> BLOCK, it is always executed just before the |
146 | conditional is about to be evaluated again, just like the third part of a |
147 | C<for> loop in C. Thus it can be used to increment a loop variable, even |
148 | when the loop has been continued via the C<next> statement (which is |
149 | similar to the C C<continue> statement). |
150 | |
151 | =head2 Loop Control |
152 | |
153 | The C<next> command is like the C<continue> statement in C; it starts |
154 | the next iteration of the loop: |
155 | |
156 | LINE: while (<STDIN>) { |
157 | next LINE if /^#/; # discard comments |
158 | ... |
159 | } |
160 | |
161 | The C<last> command is like the C<break> statement in C (as used in |
162 | loops); it immediately exits the loop in question. The |
163 | C<continue> block, if any, is not executed: |
164 | |
165 | LINE: while (<STDIN>) { |
166 | last LINE if /^$/; # exit when done with header |
167 | ... |
168 | } |
169 | |
170 | The C<redo> command restarts the loop block without evaluating the |
171 | conditional again. The C<continue> block, if any, is I<not> executed. |
172 | This command is normally used by programs that want to lie to themselves |
173 | about what was just input. |
174 | |
175 | For example, when processing a file like F</etc/termcap>. |
176 | If your input lines might end in backslashes to indicate continuation, you |
177 | want 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 $_ |
186 | } |
187 | |
188 | which is Perl short-hand for the more explicitly written version: |
189 | |
190 | LINE: while ($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 | |
199 | Or here's a a simpleminded Pascal comment stripper (warning: assumes no { or } in strings) |
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 | |
216 | Note that if there were a C<continue> block on the above code, it would get |
217 | executed even on discarded lines. |
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218 | |
219 | If the word C<while> is replaced by the word C<until>, the sense of the |
220 | test is reversed, but the conditional is still tested before the first |
221 | iteration. |
222 | |
223 | In either the C<if> or the C<while> statement, you may replace "(EXPR)" |
224 | with a BLOCK, and the conditional is true if the value of the last |
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225 | statement in that block is true. While this "feature" continues to work in |
226 | version 5, it has been deprecated, so please change any occurrences of "if BLOCK" to |
227 | "if (do BLOCK)". |
228 | |
229 | =head2 For and Foreach |
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230 | |
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231 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop: |
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232 | |
233 | for ($i = 1; $i < 10; $i++) { |
234 | ... |
235 | } |
236 | |
237 | is the same as |
238 | |
239 | $i = 1; |
240 | while ($i < 10) { |
241 | ... |
242 | } continue { |
243 | $i++; |
244 | } |
245 | |
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246 | The C<foreach> loop iterates over a normal list value and sets the |
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247 | variable VAR to be each element of the list in turn. The variable is |
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248 | implicitly local to the loop and regains its former value upon exiting the |
249 | loop. If the variable was previously declared with C<my>, it uses that |
250 | variable instead of the global one, but it's still localized to the loop. |
251 | This can cause problems if you have subroutine or format declarations |
252 | within that block's scope. |
253 | |
254 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
255 | you can use C<foreach> for readability or C<for> for brevity. If VAR is |
256 | omitted, $_ is set to each value. If LIST is an actual array (as opposed |
257 | to an expression returning a list value), you can modify each element of |
258 | the array by modifying VAR inside the loop. That's because the C<foreach> |
259 | loop index variable is an implicit alias for each item in the list that |
260 | you're looping over. |
261 | |
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262 | Examples: |
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263 | |
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264 | for (@ary) { s/foo/bar/ } |
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265 | |
266 | foreach $elem (@elements) { |
267 | $elem *= 2; |
268 | } |
269 | |
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270 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
271 | print $count, "\n"; sleep(1); |
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272 | } |
273 | |
274 | for (1..15) { print "Merry Christmas\n"; } |
275 | |
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276 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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277 | print "Item: $item\n"; |
278 | } |
279 | |
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280 | Here's how a C programmer might code up a particular algorithm in Perl: |
281 | |
282 | for ($i = 0; $i < @ary1; $i++) { |
283 | for ($j = 0; $j < @ary2; $j++) { |
284 | if ($ary1[$i] > $ary2[$j]) { |
285 | last; # can't go to outer :-( |
286 | } |
287 | $ary1[$i] += $ary2[$j]; |
288 | } |
289 | } |
290 | |
291 | Whereas here's how a Perl programmer more confortable with the idiom might |
292 | do it this way: |
293 | |
294 | OUTER: foreach $i (@ary1) { |
295 | INNER: foreach $j (@ary2) { |
296 | next OUTER if $i > $j; |
297 | $i += $j; |
298 | } |
299 | } |
300 | |
301 | See how much easier this is? It's cleaner, safer, and faster. |
302 | It's cleaner because it's less noisy. |
303 | It's safer because if code gets added |
304 | between the inner and outer loops later, you won't accidentally excecute |
305 | it because you've explicitly asked to iterate the other loop rather than |
306 | merely terminating the inner one. |
307 | And it's faster because Perl exececute C<foreach> statement more |
308 | rapidly than it would the equivalent C<for> loop. |
309 | |
310 | =head2 Basic BLOCKs and Switch Statements |
311 | |
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312 | A BLOCK by itself (labeled or not) is semantically equivalent to a loop |
313 | that executes once. Thus you can use any of the loop control |
314 | statements in it to leave or restart the block. The C<continue> block |
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315 | is optional. |
316 | |
317 | The BLOCK construct is particularly nice for doing case |
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318 | structures. |
319 | |
320 | SWITCH: { |
321 | if (/^abc/) { $abc = 1; last SWITCH; } |
322 | if (/^def/) { $def = 1; last SWITCH; } |
323 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
324 | $nothing = 1; |
325 | } |
326 | |
327 | There is no official switch statement in Perl, because there are |
328 | already several ways to write the equivalent. In addition to the |
329 | above, you could write |
330 | |
331 | SWITCH: { |
332 | $abc = 1, last SWITCH if /^abc/; |
333 | $def = 1, last SWITCH if /^def/; |
334 | $xyz = 1, last SWITCH if /^xyz/; |
335 | $nothing = 1; |
336 | } |
337 | |
338 | (That's actually not as strange as it looks one you realize that you can |
339 | use loop control "operators" within an expression, That's just the normal |
340 | C comma operator.) |
341 | |
342 | or |
343 | |
344 | SWITCH: { |
345 | /^abc/ && do { $abc = 1; last SWITCH; }; |
346 | /^def/ && do { $def = 1; last SWITCH; }; |
347 | /^xyz/ && do { $xyz = 1; last SWITCH; }; |
348 | $nothing = 1; |
349 | } |
350 | |
351 | or formatted so it stands out more as a "proper" switch statement: |
352 | |
353 | SWITCH: { |
354 | /^abc/ && do { |
355 | $abc = 1; |
356 | last SWITCH; |
357 | }; |
358 | |
359 | /^def/ && do { |
360 | $def = 1; |
361 | last SWITCH; |
362 | }; |
363 | |
364 | /^xyz/ && do { |
365 | $xyz = 1; |
366 | last SWITCH; |
367 | }; |
368 | $nothing = 1; |
369 | } |
370 | |
371 | or |
372 | |
373 | SWITCH: { |
374 | /^abc/ and $abc = 1, last SWITCH; |
375 | /^def/ and $def = 1, last SWITCH; |
376 | /^xyz/ and $xyz = 1, last SWITCH; |
377 | $nothing = 1; |
378 | } |
379 | |
380 | or even, horrors, |
381 | |
382 | if (/^abc/) |
383 | { $abc = 1 } |
384 | elsif (/^def/) |
385 | { $def = 1 } |
386 | elsif (/^xyz/) |
387 | { $xyz = 1 } |
388 | else |
389 | { $nothing = 1 } |
390 | |
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391 | |
392 | A common idiom for a switch statement is to use C<foreach>'s aliasing to make |
393 | a temporary assignment to $_ for convenient matching: |
394 | |
395 | SWITCH: for ($where) { |
396 | /In Card Names/ && do { push @flags, '-e'; last; }; |
397 | /Anywhere/ && do { push @flags, '-h'; last; }; |
398 | /In Rulings/ && do { last; }; |
399 | die "unknown value for form variable where: `$where'"; |
400 | } |
401 | |
402 | =head2 Goto |
403 | |
404 | Although not for the faint of heart, Perl does support a C<goto> statement. |
405 | A loop's LABEL is not actually a valid target for a C<goto>; |
406 | it's just the name of the loop. There are three forms: goto-LABEL, |
407 | goto-EXPR, and goto-&NAME. |
408 | |
409 | The goto-LABEL form finds the statement labeled with LABEL and resumes |
410 | execution there. It may not be used to go into any construct that |
411 | requires initialization, such as a subroutine or a foreach loop. It |
412 | also can't be used to go into a construct that is optimized away. It |
413 | can be used to go almost anywhere else within the dynamic scope, |
414 | including out of subroutines, but it's usually better to use some other |
415 | construct such as last or die. The author of Perl has never felt the |
416 | need to use this form of goto (in Perl, that is--C is another matter). |
417 | |
418 | The goto-EXPR form expects a label name, whose scope will be resolved |
419 | dynamically. This allows for computed gotos per FORTRAN, but isn't |
420 | necessarily recommended if you're optimizing for maintainability: |
421 | |
422 | goto ("FOO", "BAR", "GLARCH")[$i]; |
423 | |
424 | The goto-&NAME form is highly magical, and substitutes a call to the |
425 | named subroutine for the currently running subroutine. This is used by |
426 | AUTOLOAD() subroutines that wish to load another subroutine and then |
427 | pretend that the other subroutine had been called in the first place |
428 | (except that any modifications to @_ in the current subroutine are |
429 | propagated to the other subroutine.) After the C<goto>, not even caller() |
430 | will be able to tell that this routine was called first. |
431 | |
432 | In almost cases like this, it's usually a far, far better idea to use the |
433 | structured control flow mechanisms of C<next>, C<last>, or C<redo> insetad |
434 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
435 | C<eval{}> and die() for exception processing can also be a prudent approach. |