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1 | =head1 NAME |
2 | |
3 | perltrap - Perl traps for the unwary |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | The biggest trap of all is forgetting to use the B<-w> switch; see |
8 | L<perlrun>. The second biggest trap is not making your entire program |
9 | runnable under C<use strict>. |
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10 | |
11 | =head2 Awk Traps |
12 | |
13 | Accustomed B<awk> users should take special note of the following: |
14 | |
15 | =over 4 |
16 | |
17 | =item * |
18 | |
19 | The English module, loaded via |
20 | |
21 | use English; |
22 | |
23 | allows you to refer to special variables (like $RS) as |
24 | though they were in B<awk>; see L<perlvar> for details. |
25 | |
26 | =item * |
27 | |
28 | Semicolons are required after all simple statements in Perl (except |
29 | at the end of a block). Newline is not a statement delimiter. |
30 | |
31 | =item * |
32 | |
33 | Curly brackets are required on C<if>s and C<while>s. |
34 | |
35 | =item * |
36 | |
37 | Variables begin with "$" or "@" in Perl. |
38 | |
39 | =item * |
40 | |
41 | Arrays index from 0. Likewise string positions in substr() and |
42 | index(). |
43 | |
44 | =item * |
45 | |
46 | You have to decide whether your array has numeric or string indices. |
47 | |
48 | =item * |
49 | |
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50 | Hash values do not spring into existence upon mere reference. |
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51 | |
52 | =item * |
53 | |
54 | You have to decide whether you want to use string or numeric |
55 | comparisons. |
56 | |
57 | =item * |
58 | |
59 | Reading an input line does not split it for you. You get to split it |
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60 | yourself to an array. And the split() operator has different |
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61 | arguments. |
62 | |
63 | =item * |
64 | |
65 | The current input line is normally in $_, not $0. It generally does |
66 | not have the newline stripped. ($0 is the name of the program |
67 | executed.) See L<perlvar>. |
68 | |
69 | =item * |
70 | |
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71 | $E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched |
72 | by the last match pattern. |
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73 | |
74 | =item * |
75 | |
76 | The print() statement does not add field and record separators unless |
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77 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
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78 | the English module. |
79 | |
80 | =item * |
81 | |
82 | You must open your files before you print to them. |
83 | |
84 | =item * |
85 | |
86 | The range operator is "..", not comma. The comma operator works as in |
87 | C. |
88 | |
89 | =item * |
90 | |
91 | The match operator is "=~", not "~". ("~" is the one's complement |
92 | operator, as in C.) |
93 | |
94 | =item * |
95 | |
96 | The exponentiation operator is "**", not "^". "^" is the XOR |
97 | operator, as in C. (You know, one could get the feeling that B<awk> is |
98 | basically incompatible with C.) |
99 | |
100 | =item * |
101 | |
102 | The concatenation operator is ".", not the null string. (Using the |
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103 | null string would render C</pat/ /pat/> unparsable, because the third slash |
104 | would be interpreted as a division operator--the tokenizer is in fact |
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105 | slightly context sensitive for operators like "/", "?", and "E<gt>". |
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106 | And in fact, "." itself can be the beginning of a number.) |
107 | |
108 | =item * |
109 | |
110 | The C<next>, C<exit>, and C<continue> keywords work differently. |
111 | |
112 | =item * |
113 | |
114 | |
115 | The following variables work differently: |
116 | |
117 | Awk Perl |
118 | ARGC $#ARGV or scalar @ARGV |
119 | ARGV[0] $0 |
120 | FILENAME $ARGV |
121 | FNR $. - something |
122 | FS (whatever you like) |
123 | NF $#Fld, or some such |
124 | NR $. |
125 | OFMT $# |
126 | OFS $, |
127 | ORS $\ |
128 | RLENGTH length($&) |
129 | RS $/ |
130 | RSTART length($`) |
131 | SUBSEP $; |
132 | |
133 | =item * |
134 | |
135 | You cannot set $RS to a pattern, only a string. |
136 | |
137 | =item * |
138 | |
139 | When in doubt, run the B<awk> construct through B<a2p> and see what it |
140 | gives you. |
141 | |
142 | =back |
143 | |
144 | =head2 C Traps |
145 | |
146 | Cerebral C programmers should take note of the following: |
147 | |
148 | =over 4 |
149 | |
150 | =item * |
151 | |
152 | Curly brackets are required on C<if>'s and C<while>'s. |
153 | |
154 | =item * |
155 | |
156 | You must use C<elsif> rather than C<else if>. |
157 | |
158 | =item * |
159 | |
160 | The C<break> and C<continue> keywords from C become in |
161 | Perl C<last> and C<next>, respectively. |
162 | Unlike in C, these do I<NOT> work within a C<do { } while> construct. |
163 | |
164 | =item * |
165 | |
166 | There's no switch statement. (But it's easy to build one on the fly.) |
167 | |
168 | =item * |
169 | |
170 | Variables begin with "$" or "@" in Perl. |
171 | |
172 | =item * |
173 | |
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174 | C<printf()> does not implement the "*" format for interpolating |
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175 | field widths, but it's trivial to use interpolation of double-quoted |
176 | strings to achieve the same effect. |
177 | |
178 | =item * |
179 | |
180 | Comments begin with "#", not "/*". |
181 | |
182 | =item * |
183 | |
184 | You can't take the address of anything, although a similar operator |
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185 | in Perl is the backslash, which creates a reference. |
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186 | |
187 | =item * |
188 | |
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189 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
190 | ends up in C<$0>. |
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191 | |
192 | =item * |
193 | |
194 | System calls such as link(), unlink(), rename(), etc. return nonzero for |
195 | success, not 0. |
196 | |
197 | =item * |
198 | |
199 | Signal handlers deal with signal names, not numbers. Use C<kill -l> |
200 | to find their names on your system. |
201 | |
202 | =back |
203 | |
204 | =head2 Sed Traps |
205 | |
206 | Seasoned B<sed> programmers should take note of the following: |
207 | |
208 | =over 4 |
209 | |
210 | =item * |
211 | |
212 | Backreferences in substitutions use "$" rather than "\". |
213 | |
214 | =item * |
215 | |
216 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes |
217 | in front. |
218 | |
219 | =item * |
220 | |
221 | The range operator is C<...>, rather than comma. |
222 | |
223 | =back |
224 | |
225 | =head2 Shell Traps |
226 | |
227 | Sharp shell programmers should take note of the following: |
228 | |
229 | =over 4 |
230 | |
231 | =item * |
232 | |
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233 | The back-tick operator does variable interpolation without regard to |
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234 | the presence of single quotes in the command. |
235 | |
236 | =item * |
237 | |
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238 | The back-tick operator does no translation of the return value, unlike B<csh>. |
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239 | |
240 | =item * |
241 | |
242 | Shells (especially B<csh>) do several levels of substitution on each |
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243 | command line. Perl does substitution in only certain constructs |
244 | such as double quotes, back-ticks, angle brackets, and search patterns. |
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245 | |
246 | =item * |
247 | |
248 | Shells interpret scripts a little bit at a time. Perl compiles the |
249 | entire program before executing it (except for C<BEGIN> blocks, which |
250 | execute at compile time). |
251 | |
252 | =item * |
253 | |
254 | The arguments are available via @ARGV, not $1, $2, etc. |
255 | |
256 | =item * |
257 | |
258 | The environment is not automatically made available as separate scalar |
259 | variables. |
260 | |
261 | =back |
262 | |
263 | =head2 Perl Traps |
264 | |
265 | Practicing Perl Programmers should take note of the following: |
266 | |
267 | =over 4 |
268 | |
269 | =item * |
270 | |
271 | Remember that many operations behave differently in a list |
272 | context than they do in a scalar one. See L<perldata> for details. |
273 | |
274 | =item * |
275 | |
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276 | Avoid barewords if you can, especially all lowercase ones. |
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277 | You can't tell by just looking at it whether a bareword is |
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278 | a function or a string. By using quotes on strings and |
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279 | parentheses on function calls, you won't ever get them confused. |
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280 | |
281 | =item * |
282 | |
283 | You cannot discern from mere inspection which built-ins |
284 | are unary operators (like chop() and chdir()) |
285 | and which are list operators (like print() and unlink()). |
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286 | (User-defined subroutines can be B<only> list operators, never |
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287 | unary ones.) See L<perlop>. |
288 | |
289 | =item * |
290 | |
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291 | People have a hard time remembering that some functions |
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292 | default to $_, or @ARGV, or whatever, but that others which |
293 | you might expect to do not. |
294 | |
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295 | =item * |
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296 | |
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297 | The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline |
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298 | operation on that handle. The data read is assigned to $_ only if the |
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299 | file read is the sole condition in a while loop: |
300 | |
301 | while (<FH>) { } |
302 | while ($_ = <FH>) { }.. |
303 | <FH>; # data discarded! |
304 | |
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305 | =item * |
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306 | |
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307 | Remember not to use "C<=>" when you need "C<=~>"; |
308 | these two constructs are quite different: |
309 | |
310 | $x = /foo/; |
311 | $x =~ /foo/; |
312 | |
313 | =item * |
314 | |
315 | The C<do {}> construct isn't a real loop that you can use |
316 | loop control on. |
317 | |
318 | =item * |
319 | |
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320 | Use C<my()> for local variables whenever you can get away with |
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321 | it (but see L<perlform> for where you can't). |
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322 | Using C<local()> actually gives a local value to a global |
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323 | variable, which leaves you open to unforeseen side-effects |
324 | of dynamic scoping. |
325 | |
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326 | =item * |
327 | |
328 | If you localize an exported variable in a module, its exported value will |
329 | not change. The local name becomes an alias to a new value but the |
330 | external name is still an alias for the original. |
331 | |
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332 | =back |
333 | |
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334 | =head2 Perl4 to Perl5 Traps |
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335 | |
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336 | Practicing Perl4 Programmers should take note of the following |
337 | Perl4-to-Perl5 specific traps. |
338 | |
339 | They're crudely ordered according to the following list: |
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340 | |
341 | =over 4 |
342 | |
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343 | =item Discontinuance, Deprecation, and BugFix traps |
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344 | |
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345 | Anything that's been fixed as a perl4 bug, removed as a perl4 feature |
346 | or deprecated as a perl4 feature with the intent to encourage usage of |
347 | some other perl5 feature. |
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348 | |
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349 | =item Parsing Traps |
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350 | |
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351 | Traps that appear to stem from the new parser. |
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352 | |
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353 | =item Numerical Traps |
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354 | |
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355 | Traps having to do with numerical or mathematical operators. |
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356 | |
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357 | =item General data type traps |
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358 | |
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359 | Traps involving perl standard data types. |
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360 | |
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361 | =item Context Traps - scalar, list contexts |
362 | |
363 | Traps related to context within lists, scalar statements/declarations. |
364 | |
365 | =item Precedence Traps |
366 | |
367 | Traps related to the precedence of parsing, evaluation, and execution of |
368 | code. |
369 | |
370 | =item General Regular Expression Traps using s///, etc. |
371 | |
372 | Traps related to the use of pattern matching. |
373 | |
374 | =item Subroutine, Signal, Sorting Traps |
375 | |
376 | Traps related to the use of signals and signal handlers, general subroutines, |
377 | and sorting, along with sorting subroutines. |
378 | |
379 | =item OS Traps |
380 | |
381 | OS-specific traps. |
382 | |
383 | =item DBM Traps |
384 | |
385 | Traps specific to the use of C<dbmopen()>, and specific dbm implementations. |
386 | |
387 | =item Unclassified Traps |
388 | |
389 | Everything else. |
390 | |
391 | =back |
392 | |
393 | If you find an example of a conversion trap that is not listed here, |
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394 | please submit it to Bill Middleton <F<wjm@best.com>> for inclusion. |
395 | Also note that at least some of these can be caught with B<-w>. |
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396 | |
397 | =head2 Discontinuance, Deprecation, and BugFix traps |
398 | |
399 | Anything that has been discontinued, deprecated, or fixed as |
400 | a bug from perl4. |
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401 | |
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402 | =over 4 |
403 | |
404 | =item * Discontinuance |
405 | |
406 | Symbols starting with "_" are no longer forced into package main, except |
407 | for C<$_> itself (and C<@_>, etc.). |
408 | |
409 | package test; |
410 | $_legacy = 1; |
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411 | |
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412 | package main; |
413 | print "\$_legacy is ",$_legacy,"\n"; |
414 | |
415 | # perl4 prints: $_legacy is 1 |
416 | # perl5 prints: $_legacy is |
417 | |
418 | =item * Deprecation |
419 | |
420 | Double-colon is now a valid package separator in a variable name. Thus these |
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421 | behave differently in perl4 vs. perl5, because the packages don't exist. |
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422 | |
423 | $a=1;$b=2;$c=3;$var=4; |
424 | print "$a::$b::$c "; |
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425 | print "$var::abc::xyz\n"; |
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426 | |
427 | # perl4 prints: 1::2::3 4::abc::xyz |
428 | # perl5 prints: 3 |
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429 | |
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430 | Given that C<::> is now the preferred package delimiter, it is debatable |
431 | whether this should be classed as a bug or not. |
432 | (The older package delimiter, ' ,is used here) |
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433 | |
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434 | $x = 10 ; |
435 | print "x=${'x}\n" ; |
436 | |
437 | # perl4 prints: x=10 |
438 | # perl5 prints: Can't find string terminator "'" anywhere before EOF |
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439 | |
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440 | Also see precedence traps, for parsing C<$:>. |
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441 | |
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442 | =item * BugFix |
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443 | |
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444 | The second and third arguments of C<splice()> are now evaluated in scalar |
445 | context (as the Camel says) rather than list context. |
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446 | |
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447 | sub sub1{return(0,2) } # return a 2-elem array |
448 | sub sub2{ return(1,2,3)} # return a 3-elem array |
449 | @a1 = ("a","b","c","d","e"); |
450 | @a2 = splice(@a1,&sub1,&sub2); |
451 | print join(' ',@a2),"\n"; |
452 | |
453 | # perl4 prints: a b |
454 | # perl5 prints: c d e |
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455 | |
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456 | =item * Discontinuance |
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457 | |
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458 | You can't do a C<goto> into a block that is optimized away. Darn. |
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459 | |
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460 | goto marker1; |
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461 | |
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462 | for(1){ |
463 | marker1: |
464 | print "Here I is!\n"; |
465 | } |
466 | |
467 | # perl4 prints: Here I is! |
468 | # perl5 dumps core (SEGV) |
469 | |
470 | =item * Discontinuance |
471 | |
472 | It is no longer syntactically legal to use whitespace as the name |
473 | of a variable, or as a delimiter for any kind of quote construct. |
474 | Double darn. |
475 | |
476 | $a = ("foo bar"); |
477 | $b = q baz ; |
478 | print "a is $a, b is $b\n"; |
479 | |
480 | # perl4 prints: a is foo bar, b is baz |
481 | # perl5 errors: Bare word found where operator expected |
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482 | |
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483 | =item * Discontinuance |
484 | |
485 | The archaic while/if BLOCK BLOCK syntax is no longer supported. |
486 | |
487 | if { 1 } { |
488 | print "True!"; |
489 | } |
490 | else { |
491 | print "False!"; |
492 | } |
493 | |
494 | # perl4 prints: True! |
495 | # perl5 errors: syntax error at test.pl line 1, near "if {" |
496 | |
497 | =item * BugFix |
498 | |
499 | The C<**> operator now binds more tightly than unary minus. |
500 | It was documented to work this way before, but didn't. |
501 | |
502 | print -4**2,"\n"; |
503 | |
504 | # perl4 prints: 16 |
505 | # perl5 prints: -16 |
506 | |
507 | =item * Discontinuance |
508 | |
509 | The meaning of C<foreach{}> has changed slightly when it is iterating over a |
510 | list which is not an array. This used to assign the list to a |
511 | temporary array, but no longer does so (for efficiency). This means |
512 | that you'll now be iterating over the actual values, not over copies of |
513 | the values. Modifications to the loop variable can change the original |
514 | values. |
515 | |
516 | @list = ('ab','abc','bcd','def'); |
517 | foreach $var (grep(/ab/,@list)){ |
518 | $var = 1; |
519 | } |
520 | print (join(':',@list)); |
521 | |
522 | # perl4 prints: ab:abc:bcd:def |
523 | # perl5 prints: 1:1:bcd:def |
524 | |
525 | To retain Perl4 semantics you need to assign your list |
526 | explicitly to a temporary array and then iterate over that. For |
527 | example, you might need to change |
528 | |
529 | foreach $var (grep(/ab/,@list)){ |
530 | |
531 | to |
532 | |
533 | foreach $var (@tmp = grep(/ab/,@list)){ |
534 | |
535 | Otherwise changing $var will clobber the values of @list. (This most often |
536 | happens when you use C<$_> for the loop variable, and call subroutines in |
537 | the loop that don't properly localize C<$_>.) |
538 | |
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539 | =item * Discontinuance |
540 | |
541 | C<split> with no arguments now behaves like C<split ' '> (which doesn't |
542 | return an initial null field if $_ starts with whitespace), it used to |
543 | behave like C<split /\s+/> (which does). |
544 | |
545 | $_ = ' hi mom'; |
546 | print join(':', split); |
547 | |
548 | # perl4 prints: :hi:mom |
549 | # perl5 prints: hi:mom |
550 | |
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551 | =item * BugFix |
552 | |
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553 | Perl 4 would ignore any text which was attached to an B<-e> switch, |
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554 | always taking the code snippet from the following arg. Additionally, it |
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555 | would silently accept an B<-e> switch without a following arg. Both of |
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556 | these behaviors have been fixed. |
557 | |
558 | perl -e'print "attached to -e"' 'print "separate arg"' |
559 | |
560 | # perl4 prints: separate arg |
561 | # perl5 prints: attached to -e |
562 | |
563 | perl -e |
564 | |
565 | # perl4 prints: |
566 | # perl5 dies: No code specified for -e. |
567 | |
568 | =item * Discontinuance |
569 | |
570 | In Perl 4 the return value of C<push> was undocumented, but it was |
571 | actually the last value being pushed onto the target list. In Perl 5 |
572 | the return value of C<push> is documented, but has changed, it is the |
573 | number of elements in the resulting list. |
574 | |
575 | @x = ('existing'); |
576 | print push(@x, 'first new', 'second new'); |
577 | |
578 | # perl4 prints: second new |
579 | # perl5 prints: 3 |
580 | |
68dc0745 |
581 | =item * Discontinuance |
582 | |
583 | In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in |
584 | Perl code were silently allowed, although they could cause (mysterious!) |
585 | failures in certain constructs, particularly here documents. Now, |
586 | C<'\r'> characters cause an immediate fatal error. (Note: In this |
587 | example, the notation B<\015> represents the incorrect line |
588 | ending. Depending upon your text viewer, it will look different.) |
589 | |
590 | print "foo";\015 |
591 | print "bar"; |
592 | |
593 | # perl4 prints: foobar |
594 | # perl5.003 prints: foobar |
595 | # perl5.004 dies: Illegal character \015 (carriage return) |
596 | |
597 | See L<perldiag> for full details. |
598 | |
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599 | =item * Deprecation |
600 | |
601 | Some error messages will be different. |
602 | |
603 | =item * Discontinuance |
604 | |
605 | Some bugs may have been inadvertently removed. :-) |
606 | |
607 | =back |
608 | |
609 | =head2 Parsing Traps |
610 | |
611 | Perl4-to-Perl5 traps from having to do with parsing. |
612 | |
613 | =over 4 |
614 | |
615 | =item * Parsing |
616 | |
617 | Note the space between . and = |
618 | |
619 | $string . = "more string"; |
620 | print $string; |
621 | |
622 | # perl4 prints: more string |
623 | # perl5 prints: syntax error at - line 1, near ". =" |
624 | |
625 | =item * Parsing |
626 | |
627 | Better parsing in perl 5 |
628 | |
629 | sub foo {} |
630 | &foo |
631 | print("hello, world\n"); |
632 | |
633 | # perl4 prints: hello, world |
634 | # perl5 prints: syntax error |
635 | |
636 | =item * Parsing |
637 | |
638 | "if it looks like a function, it is a function" rule. |
639 | |
640 | print |
641 | ($foo == 1) ? "is one\n" : "is zero\n"; |
642 | |
643 | # perl4 prints: is zero |
644 | # perl5 warns: "Useless use of a constant in void context" if using -w |
645 | |
646 | =back |
647 | |
648 | =head2 Numerical Traps |
649 | |
650 | Perl4-to-Perl5 traps having to do with numerical operators, |
651 | operands, or output from same. |
652 | |
653 | =over 5 |
654 | |
655 | =item * Numerical |
656 | |
657 | Formatted output and significant digits |
658 | |
659 | print 7.373504 - 0, "\n"; |
660 | printf "%20.18f\n", 7.373504 - 0; |
661 | |
662 | # Perl4 prints: |
663 | 7.375039999999996141 |
664 | 7.37503999999999614 |
665 | |
666 | # Perl5 prints: |
667 | 7.373504 |
668 | 7.37503999999999614 |
669 | |
670 | =item * Numerical |
671 | |
5f05dabc |
672 | This specific item has been deleted. It demonstrated how the auto-increment |
5e378fdf |
673 | operator would not catch when a number went over the signed int limit. Fixed |
a6006777 |
674 | in version 5.003_04. But always be wary when using large integers. |
675 | If in doubt: |
6dbacca0 |
676 | |
5e378fdf |
677 | use Math::BigInt; |
6dbacca0 |
678 | |
679 | =item * Numerical |
680 | |
681 | Assignment of return values from numeric equality tests |
682 | does not work in perl5 when the test evaluates to false (0). |
683 | Logical tests now return an null, instead of 0 |
a6006777 |
684 | |
6dbacca0 |
685 | $p = ($test == 1); |
686 | print $p,"\n"; |
a6006777 |
687 | |
6dbacca0 |
688 | # perl4 prints: 0 |
689 | # perl5 prints: |
690 | |
55497cff |
691 | Also see the L<General Regular Expression Traps using s///, etc.> |
692 | tests for another example of this new feature... |
6dbacca0 |
693 | |
694 | =back |
695 | |
696 | =head2 General data type traps |
697 | |
698 | Perl4-to-Perl5 traps involving most data-types, and their usage |
699 | within certain expressions and/or context. |
700 | |
701 | =over 5 |
702 | |
703 | =item * (Arrays) |
704 | |
705 | Negative array subscripts now count from the end of the array. |
706 | |
707 | @a = (1, 2, 3, 4, 5); |
708 | print "The third element of the array is $a[3] also expressed as $a[-2] \n"; |
709 | |
710 | # perl4 prints: The third element of the array is 4 also expressed as |
711 | # perl5 prints: The third element of the array is 4 also expressed as 4 |
712 | |
713 | =item * (Arrays) |
714 | |
715 | Setting C<$#array> lower now discards array elements, and makes them |
716 | impossible to recover. |
717 | |
718 | @a = (a,b,c,d,e); |
719 | print "Before: ",join('',@a); |
720 | $#a =1; |
721 | print ", After: ",join('',@a); |
722 | $#a =3; |
723 | print ", Recovered: ",join('',@a),"\n"; |
724 | |
725 | # perl4 prints: Before: abcde, After: ab, Recovered: abcd |
726 | # perl5 prints: Before: abcde, After: ab, Recovered: ab |
727 | |
728 | =item * (Hashes) |
729 | |
730 | Hashes get defined before use |
731 | |
732 | local($s,@a,%h); |
733 | die "scalar \$s defined" if defined($s); |
734 | die "array \@a defined" if defined(@a); |
735 | die "hash \%h defined" if defined(%h); |
736 | |
737 | # perl4 prints: |
738 | # perl5 dies: hash %h defined |
739 | |
740 | =item * (Globs) |
741 | |
742 | glob assignment from variable to variable will fail if the assigned |
743 | variable is localized subsequent to the assignment |
744 | |
745 | @a = ("This is Perl 4"); |
746 | *b = *a; |
747 | local(@a); |
748 | print @b,"\n"; |
749 | |
750 | # perl4 prints: This is Perl 4 |
751 | # perl5 prints: |
752 | |
753 | # Another example |
754 | |
755 | *fred = *barney; # fred is aliased to barney |
756 | @barney = (1, 2, 4); |
757 | # @fred; |
758 | print "@fred"; # should print "1, 2, 4" |
759 | |
760 | # perl4 prints: 1 2 4 |
9607fc9c |
761 | # perl5 prints: In string, @fred now must be written as \@fred |
5e378fdf |
762 | |
6dbacca0 |
763 | =item * (Scalar String) |
764 | |
765 | Changes in unary negation (of strings) |
766 | This change effects both the return value and what it |
767 | does to auto(magic)increment. |
768 | |
769 | $x = "aaa"; |
770 | print ++$x," : "; |
771 | print -$x," : "; |
772 | print ++$x,"\n"; |
773 | |
774 | # perl4 prints: aab : -0 : 1 |
775 | # perl5 prints: aab : -aab : aac |
776 | |
777 | =item * (Constants) |
778 | |
779 | perl 4 lets you modify constants: |
780 | |
781 | $foo = "x"; |
782 | &mod($foo); |
783 | for ($x = 0; $x < 3; $x++) { |
784 | &mod("a"); |
785 | } |
786 | sub mod { |
787 | print "before: $_[0]"; |
788 | $_[0] = "m"; |
789 | print " after: $_[0]\n"; |
790 | } |
791 | |
792 | # perl4: |
793 | # before: x after: m |
794 | # before: a after: m |
795 | # before: m after: m |
796 | # before: m after: m |
797 | |
798 | # Perl5: |
799 | # before: x after: m |
800 | # Modification of a read-only value attempted at foo.pl line 12. |
801 | # before: a |
802 | |
803 | =item * (Scalars) |
804 | |
805 | The behavior is slightly different for: |
806 | |
807 | print "$x", defined $x |
808 | |
809 | # perl 4: 1 |
810 | # perl 5: <no output, $x is not called into existence> |
811 | |
812 | =item * (Variable Suicide) |
813 | |
814 | Variable suicide behavior is more consistent under Perl 5. |
aa689395 |
815 | Perl5 exhibits the same behavior for hashes and scalars, |
5f05dabc |
816 | that perl4 exhibits for only scalars. |
6dbacca0 |
817 | |
818 | $aGlobal{ "aKey" } = "global value"; |
819 | print "MAIN:", $aGlobal{"aKey"}, "\n"; |
820 | $GlobalLevel = 0; |
821 | &test( *aGlobal ); |
822 | |
823 | sub test { |
824 | local( *theArgument ) = @_; |
825 | local( %aNewLocal ); # perl 4 != 5.001l,m |
826 | $aNewLocal{"aKey"} = "this should never appear"; |
827 | print "SUB: ", $theArgument{"aKey"}, "\n"; |
828 | $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print |
829 | $GlobalLevel++; |
830 | if( $GlobalLevel<4 ) { |
831 | &test( *aNewLocal ); |
832 | } |
833 | } |
834 | |
835 | # Perl4: |
836 | # MAIN:global value |
837 | # SUB: global value |
838 | # SUB: level 0 |
839 | # SUB: level 1 |
840 | # SUB: level 2 |
841 | |
842 | # Perl5: |
843 | # MAIN:global value |
844 | # SUB: global value |
845 | # SUB: this should never appear |
846 | # SUB: this should never appear |
847 | # SUB: this should never appear |
848 | |
84dc3c4d |
849 | =back |
6dbacca0 |
850 | |
851 | =head2 Context Traps - scalar, list contexts |
852 | |
853 | =over 5 |
854 | |
855 | =item * (list context) |
856 | |
857 | The elements of argument lists for formats are now evaluated in list |
858 | context. This means you can interpolate list values now. |
859 | |
860 | @fmt = ("foo","bar","baz"); |
861 | format STDOUT= |
862 | @<<<<< @||||| @>>>>> |
863 | @fmt; |
864 | . |
865 | write; |
866 | |
867 | # perl4 errors: Please use commas to separate fields in file |
868 | # perl5 prints: foo bar baz |
869 | |
870 | =item * (scalar context) |
871 | |
872 | The C<caller()> function now returns a false value in a scalar context |
873 | if there is no caller. This lets library files determine if they're |
874 | being required. |
875 | |
876 | caller() ? (print "You rang?\n") : (print "Got a 0\n"); |
877 | |
878 | # perl4 errors: There is no caller |
879 | # perl5 prints: Got a 0 |
5e378fdf |
880 | |
6dbacca0 |
881 | =item * (scalar context) |
882 | |
883 | The comma operator in a scalar context is now guaranteed to give a |
884 | scalar context to its arguments. |
885 | |
886 | @y= ('a','b','c'); |
887 | $x = (1, 2, @y); |
888 | print "x = $x\n"; |
889 | |
890 | # Perl4 prints: x = c # Thinks list context interpolates list |
891 | # Perl5 prints: x = 3 # Knows scalar uses length of list |
892 | |
893 | =item * (list, builtin) |
894 | |
895 | C<sprintf()> funkiness (array argument converted to scalar array count) |
896 | This test could be added to t/op/sprintf.t |
897 | |
898 | @z = ('%s%s', 'foo', 'bar'); |
899 | $x = sprintf(@z); |
900 | if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";} |
901 | |
902 | # perl4 prints: ok 2 |
903 | # perl5 prints: not ok 2 |
904 | |
905 | C<printf()> works fine, though: |
906 | |
907 | printf STDOUT (@z); |
908 | print "\n"; |
909 | |
910 | # perl4 prints: foobar |
911 | # perl5 prints: foobar |
912 | |
913 | Probably a bug. |
914 | |
915 | =back |
916 | |
917 | =head2 Precedence Traps |
918 | |
919 | Perl4-to-Perl5 traps involving precedence order. |
920 | |
84dc3c4d |
921 | =over 5 |
922 | |
5e378fdf |
923 | =item * Precedence |
924 | |
925 | LHS vs. RHS when both sides are getting an op. |
926 | |
927 | @arr = ( 'left', 'right' ); |
928 | $a{shift @arr} = shift @arr; |
929 | print join( ' ', keys %a ); |
930 | |
931 | # perl4 prints: left |
932 | # perl5 prints: right |
933 | |
934 | =item * Precedence |
6dbacca0 |
935 | |
936 | These are now semantic errors because of precedence: |
937 | |
938 | @list = (1,2,3,4,5); |
939 | %map = ("a",1,"b",2,"c",3,"d",4); |
940 | $n = shift @list + 2; # first item in list plus 2 |
941 | print "n is $n, "; |
942 | $m = keys %map + 2; # number of items in hash plus 2 |
943 | print "m is $m\n"; |
944 | |
945 | # perl4 prints: n is 3, m is 6 |
946 | # perl5 errors and fails to compile |
947 | |
948 | =item * Precedence |
a0d0e21e |
949 | |
4633a7c4 |
950 | The precedence of assignment operators is now the same as the precedence |
951 | of assignment. Perl 4 mistakenly gave them the precedence of the associated |
952 | operator. So you now must parenthesize them in expressions like |
953 | |
954 | /foo/ ? ($a += 2) : ($a -= 2); |
a6006777 |
955 | |
4633a7c4 |
956 | Otherwise |
957 | |
6dbacca0 |
958 | /foo/ ? $a += 2 : $a -= 2 |
4633a7c4 |
959 | |
960 | would be erroneously parsed as |
961 | |
962 | (/foo/ ? $a += 2 : $a) -= 2; |
963 | |
964 | On the other hand, |
965 | |
6dbacca0 |
966 | $a += /foo/ ? 1 : 2; |
4633a7c4 |
967 | |
968 | now works as a C programmer would expect. |
969 | |
6dbacca0 |
970 | =item * Precedence |
4633a7c4 |
971 | |
6dbacca0 |
972 | open FOO || die; |
a0d0e21e |
973 | |
5f05dabc |
974 | is now incorrect. You need parentheses around the filehandle. |
975 | Otherwise, perl5 leaves the statement as its default precedence: |
a0d0e21e |
976 | |
6dbacca0 |
977 | open(FOO || die); |
978 | |
979 | # perl4 opens or dies |
980 | # perl5 errors: Precedence problem: open FOO should be open(FOO) |
a0d0e21e |
981 | |
6dbacca0 |
982 | =item * Precedence |
a0d0e21e |
983 | |
6dbacca0 |
984 | perl4 gives the special variable, C<$:> precedence, where perl5 |
985 | treats C<$::> as main C<package> |
a0d0e21e |
986 | |
6dbacca0 |
987 | $a = "x"; print "$::a"; |
988 | |
989 | # perl 4 prints: -:a |
990 | # perl 5 prints: x |
5e378fdf |
991 | |
6dbacca0 |
992 | =item * Precedence |
a0d0e21e |
993 | |
6dbacca0 |
994 | concatenation precedence over filetest operator? |
a0d0e21e |
995 | |
6dbacca0 |
996 | -e $foo .= "q" |
997 | |
998 | # perl4 prints: no output |
999 | # perl5 prints: Can't modify -e in concatenation |
a0d0e21e |
1000 | |
6dbacca0 |
1001 | =item * Precedence |
a0d0e21e |
1002 | |
6dbacca0 |
1003 | Assignment to value takes precedence over assignment to key in |
1004 | perl5 when using the shift operator on both sides. |
1005 | |
1006 | @arr = ( 'left', 'right' ); |
1007 | $a{shift @arr} = shift @arr; |
1008 | print join( ' ', keys %a ); |
1009 | |
1010 | # perl4 prints: left |
1011 | # perl5 prints: right |
1012 | |
1013 | =back |
1014 | |
1015 | =head2 General Regular Expression Traps using s///, etc. |
1016 | |
1017 | All types of RE traps. |
1018 | |
1019 | =over 5 |
1020 | |
1021 | =item * Regular Expression |
1022 | |
1023 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to |
1024 | interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal |
1025 | '$' in string) |
1026 | |
1027 | $a=1;$b=2; |
1028 | $string = '1 2 $a $b'; |
1029 | $string =~ s'$a'$b'; |
1030 | print $string,"\n"; |
1031 | |
1032 | # perl4 prints: $b 2 $a $b |
1033 | # perl5 prints: 1 2 $a $b |
1034 | |
1035 | =item * Regular Expression |
a0d0e21e |
1036 | |
1037 | C<m//g> now attaches its state to the searched string rather than the |
6dbacca0 |
1038 | regular expression. (Once the scope of a block is left for the sub, the |
1039 | state of the searched string is lost) |
1040 | |
1041 | $_ = "ababab"; |
1042 | while(m/ab/g){ |
1043 | &doit("blah"); |
1044 | } |
1045 | sub doit{local($_) = shift; print "Got $_ "} |
1046 | |
1047 | # perl4 prints: blah blah blah |
1048 | # perl5 prints: infinite loop blah... |
1049 | |
1050 | =item * Regular Expression |
1051 | |
68dc0745 |
1052 | Currently, if you use the C<m//o> qualifier on a regular expression |
1053 | within an anonymous sub, I<all> closures generated from that anonymous |
1054 | sub will use the regular expression as it was compiled when it was used |
1055 | the very first time in any such closure. For instance, if you say |
1056 | |
1057 | sub build_match { |
1058 | my($left,$right) = @_; |
1059 | return sub { $_[0] =~ /$left stuff $right/o; }; |
1060 | } |
1061 | |
1062 | build_match() will always return a sub which matches the contents of |
1063 | C<$left> and C<$right> as they were the I<first> time that build_match() |
1064 | was called, not as they are in the current call. |
1065 | |
1066 | This is probably a bug, and may change in future versions of Perl. |
1067 | |
1068 | =item * Regular Expression |
1069 | |
6dbacca0 |
1070 | If no parentheses are used in a match, Perl4 sets C<$+> to |
1071 | the whole match, just like C<$&>. Perl5 does not. |
1072 | |
1073 | "abcdef" =~ /b.*e/; |
1074 | print "\$+ = $+\n"; |
1075 | |
1076 | # perl4 prints: bcde |
1077 | # perl5 prints: |
1078 | |
1079 | =item * Regular Expression |
1080 | |
1081 | substitution now returns the null string if it fails |
1082 | |
1083 | $string = "test"; |
1084 | $value = ($string =~ s/foo//); |
1085 | print $value, "\n"; |
1086 | |
1087 | # perl4 prints: 0 |
1088 | # perl5 prints: |
1089 | |
1090 | Also see L<Numerical Traps> for another example of this new feature. |
1091 | |
1092 | =item * Regular Expression |
1093 | |
5f05dabc |
1094 | C<s`lhs`rhs`> (using back-ticks) is now a normal substitution, with no |
1095 | back-tick expansion |
6dbacca0 |
1096 | |
1097 | $string = ""; |
1098 | $string =~ s`^`hostname`; |
1099 | print $string, "\n"; |
1100 | |
1101 | # perl4 prints: <the local hostname> |
1102 | # perl5 prints: hostname |
1103 | |
1104 | =item * Regular Expression |
1105 | |
1106 | Stricter parsing of variables used in regular expressions |
1107 | |
1108 | s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; |
1109 | |
1110 | # perl4: compiles w/o error |
1111 | # perl5: with Scalar found where operator expected ..., near "$opt$plus" |
1112 | |
1113 | an added component of this example, apparently from the same script, is |
1114 | the actual value of the s'd string after the substitution. |
1115 | C<[$opt]> is a character class in perl4 and an array subscript in perl5 |
1116 | |
1117 | $grpc = 'a'; |
1118 | $opt = 'r'; |
1119 | $_ = 'bar'; |
1120 | s/^([^$grpc]*$grpc[$opt]?)/foo/; |
1121 | print ; |
1122 | |
1123 | # perl4 prints: foo |
1124 | # perl5 prints: foobar |
1125 | |
1126 | =item * Regular Expression |
1127 | |
1128 | Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched |
1129 | repeatedly, like C</x/> or C<m!x!>. |
1130 | |
1131 | $test = "once"; |
1132 | sub match { $test =~ m?once?; } |
1133 | &match(); |
1134 | if( &match() ) { |
1135 | # m?x? matches more then once |
1136 | print "perl4\n"; |
1137 | } else { |
1138 | # m?x? matches only once |
1139 | print "perl5\n"; |
1140 | } |
1141 | |
1142 | # perl4 prints: perl4 |
1143 | # perl5 prints: perl5 |
a0d0e21e |
1144 | |
a0d0e21e |
1145 | |
44a8e56a |
1146 | =item * Regular Expression |
1147 | |
1148 | Under perl4 and upto version 5.003, a failed C<m//g> match used to |
1149 | reset the internal iterator, so that subsequent C<m//g> match attempts |
1150 | began from the beginning of the string. In perl version 5.004 and later, |
1151 | failed C<m//g> matches do not reset the iterator position (which can be |
1152 | found using the C<pos()> function--see L<perlfunc/pos>). |
1153 | |
1154 | $test = "foop"; |
1155 | for (1..3) { |
1156 | print $1 while ($test =~ /(o)/g); |
1157 | # pos $test = 0; # to get old behavior |
1158 | } |
1159 | |
1160 | # perl4 prints: oooooo |
1161 | # perl5.004 prints: oo |
1162 | |
1163 | You may always reset the iterator yourself as shown in the commented line |
1164 | to get the old behavior. |
1165 | |
6dbacca0 |
1166 | =back |
1167 | |
1168 | =head2 Subroutine, Signal, Sorting Traps |
a0d0e21e |
1169 | |
6dbacca0 |
1170 | The general group of Perl4-to-Perl5 traps having to do with |
1171 | Signals, Sorting, and their related subroutines, as well as |
1172 | general subroutine traps. Includes some OS-Specific traps. |
a0d0e21e |
1173 | |
6dbacca0 |
1174 | =over 5 |
a0d0e21e |
1175 | |
6dbacca0 |
1176 | =item * (Signals) |
a0d0e21e |
1177 | |
6dbacca0 |
1178 | Barewords that used to look like strings to Perl will now look like subroutine |
1179 | calls if a subroutine by that name is defined before the compiler sees them. |
a0d0e21e |
1180 | |
6dbacca0 |
1181 | sub SeeYa { warn"Hasta la vista, baby!" } |
1182 | $SIG{'TERM'} = SeeYa; |
1183 | print "SIGTERM is now $SIG{'TERM'}\n"; |
1184 | |
1185 | # perl4 prints: SIGTERM is main'SeeYa |
1186 | # perl5 prints: SIGTERM is now main::1 |
a0d0e21e |
1187 | |
6dbacca0 |
1188 | Use B<-w> to catch this one |
a0d0e21e |
1189 | |
6dbacca0 |
1190 | =item * (Sort Subroutine) |
a0d0e21e |
1191 | |
6dbacca0 |
1192 | reverse is no longer allowed as the name of a sort subroutine. |
a0d0e21e |
1193 | |
6dbacca0 |
1194 | sub reverse{ print "yup "; $a <=> $b } |
1195 | print sort reverse a,b,c; |
1196 | |
1197 | # perl4 prints: yup yup yup yup abc |
1198 | # perl5 prints: abc |
a0d0e21e |
1199 | |
b996531f |
1200 | =item * warn() won't let you specify a filehandle. |
1201 | |
1202 | Although it _always_ printed to STDERR, warn() would let you specify a |
1203 | filehandle in perl4. With perl5 it does not. |
5e378fdf |
1204 | |
1205 | warn STDERR "Foo!"; |
1206 | |
1207 | # perl4 prints: Foo! |
1208 | # perl5 prints: String found where operator expected |
1209 | |
6dbacca0 |
1210 | =back |
a0d0e21e |
1211 | |
6dbacca0 |
1212 | =head2 OS Traps |
1213 | |
1214 | =over 5 |
1215 | |
1216 | =item * (SysV) |
1217 | |
1218 | Under HPUX, and some other SysV OS's, one had to reset any signal handler, |
1219 | within the signal handler function, each time a signal was handled with |
1220 | perl4. With perl5, the reset is now done correctly. Any code relying |
1221 | on the handler _not_ being reset will have to be reworked. |
1222 | |
a6006777 |
1223 | Since version 5.002, Perl uses sigaction() under SysV. |
6dbacca0 |
1224 | |
1225 | sub gotit { |
1226 | print "Got @_... "; |
1227 | } |
1228 | $SIG{'INT'} = 'gotit'; |
1229 | |
1230 | $| = 1; |
1231 | $pid = fork; |
1232 | if ($pid) { |
1233 | kill('INT', $pid); |
1234 | sleep(1); |
1235 | kill('INT', $pid); |
1236 | } else { |
1237 | while (1) {sleep(10);} |
1238 | } |
1239 | |
1240 | # perl4 (HPUX) prints: Got INT... |
1241 | # perl5 (HPUX) prints: Got INT... Got INT... |
1242 | |
1243 | =item * (SysV) |
1244 | |
1245 | Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does |
5f05dabc |
1246 | the right thing w.r.t. the fopen() man page. e.g., - When a file is opened |
6dbacca0 |
1247 | for append, it is impossible to overwrite information already in |
1248 | the file. |
1249 | |
1250 | open(TEST,">>seek.test"); |
1251 | $start = tell TEST ; |
1252 | foreach(1 .. 9){ |
1253 | print TEST "$_ "; |
1254 | } |
1255 | $end = tell TEST ; |
1256 | seek(TEST,$start,0); |
1257 | print TEST "18 characters here"; |
1258 | |
1259 | # perl4 (solaris) seek.test has: 18 characters here |
1260 | # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here |
a0d0e21e |
1261 | |
a0d0e21e |
1262 | |
a0d0e21e |
1263 | |
6dbacca0 |
1264 | =back |
a0d0e21e |
1265 | |
6dbacca0 |
1266 | =head2 Interpolation Traps |
a0d0e21e |
1267 | |
8b0a4b75 |
1268 | Perl4-to-Perl5 traps having to do with how things get interpolated |
1269 | within certain expressions, statements, contexts, or whatever. |
1270 | |
6dbacca0 |
1271 | =over 5 |
a0d0e21e |
1272 | |
6dbacca0 |
1273 | =item * Interpolation |
a0d0e21e |
1274 | |
6dbacca0 |
1275 | @ now always interpolates an array in double-quotish strings. |
1276 | |
1277 | print "To: someone@somewhere.com\n"; |
1278 | |
1279 | # perl4 prints: To:someone@somewhere.com |
9607fc9c |
1280 | # perl5 errors : In string, @somewhere now must be written as \@somewhere |
6dbacca0 |
1281 | |
1282 | =item * Interpolation |
1283 | |
6dbacca0 |
1284 | Double-quoted strings may no longer end with an unescaped $ or @. |
1285 | |
1286 | $foo = "foo$"; |
1287 | $bar = "bar@"; |
1288 | print "foo is $foo, bar is $bar\n"; |
1289 | |
1290 | # perl4 prints: foo is foo$, bar is bar@ |
1291 | # perl5 errors: Final $ should be \$ or $name |
1292 | |
1293 | Note: perl5 DOES NOT error on the terminating @ in $bar |
1294 | |
1295 | =item * Interpolation |
a0d0e21e |
1296 | |
8b0a4b75 |
1297 | Perl now sometimes evaluates arbitrary expressions inside braces that occur |
1298 | within double quotes (usually when the opening brace is preceded by C<$> |
1299 | or C<@>). |
1300 | |
1301 | @www = "buz"; |
1302 | $foo = "foo"; |
1303 | $bar = "bar"; |
1304 | sub foo { return "bar" }; |
1305 | print "|@{w.w.w}|${main'foo}|"; |
1306 | |
1307 | # perl4 prints: |@{w.w.w}|foo| |
1308 | # perl5 prints: |buz|bar| |
1309 | |
1310 | Note that you can C<use strict;> to ward off such trappiness under perl5. |
1311 | |
1312 | =item * Interpolation |
1313 | |
748a9306 |
1314 | The construct "this is $$x" used to interpolate the pid at that |
6dbacca0 |
1315 | point, but now apparently tries to dereference C<$x>. C<$$> by itself still |
748a9306 |
1316 | works fine, however. |
1317 | |
6dbacca0 |
1318 | print "this is $$x\n"; |
748a9306 |
1319 | |
6dbacca0 |
1320 | # perl4 prints: this is XXXx (XXX is the current pid) |
1321 | # perl5 prints: this is |
1322 | |
1323 | =item * Interpolation |
1324 | |
1325 | Creation of hashes on the fly with C<eval "EXPR"> now requires either both |
1326 | C<$>'s to be protected in the specification of the hash name, or both curlies |
1327 | to be protected. If both curlies are protected, the result will be compatible |
1328 | with perl4 and perl5. This is a very common practice, and should be changed |
1329 | to use the block form of C<eval{}> if possible. |
c07a80fd |
1330 | |
6dbacca0 |
1331 | $hashname = "foobar"; |
1332 | $key = "baz"; |
1333 | $value = 1234; |
1334 | eval "\$$hashname{'$key'} = q|$value|"; |
1335 | (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); |
1336 | |
1337 | # perl4 prints: Yup |
1338 | # perl5 prints: Nope |
1339 | |
1340 | Changing |
1341 | |
1342 | eval "\$$hashname{'$key'} = q|$value|"; |
c07a80fd |
1343 | |
1344 | to |
1345 | |
6dbacca0 |
1346 | eval "\$\$hashname{'$key'} = q|$value|"; |
c07a80fd |
1347 | |
6dbacca0 |
1348 | causes the following result: |
c07a80fd |
1349 | |
6dbacca0 |
1350 | # perl4 prints: Nope |
1351 | # perl5 prints: Yup |
c07a80fd |
1352 | |
6dbacca0 |
1353 | or, changing to |
a0d0e21e |
1354 | |
6dbacca0 |
1355 | eval "\$$hashname\{'$key'\} = q|$value|"; |
1356 | |
1357 | causes the following result: |
1358 | |
1359 | # perl4 prints: Yup |
1360 | # perl5 prints: Yup |
1361 | # and is compatible for both versions |
1362 | |
1363 | |
1364 | =item * Interpolation |
1365 | |
1366 | perl4 programs which unconsciously rely on the bugs in earlier perl versions. |
1367 | |
1368 | perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' |
1369 | |
1370 | # perl4 prints: This is not perl5 |
1371 | # perl5 prints: This is perl5 |
1372 | |
1373 | =item * Interpolation |
1374 | |
1375 | You also have to be careful about array references. |
1376 | |
1377 | print "$foo{" |
1378 | |
1379 | perl 4 prints: { |
1380 | perl 5 prints: syntax error |
1381 | |
1382 | =item * Interpolation |
1383 | |
1384 | Similarly, watch out for: |
1385 | |
1386 | $foo = "array"; |
1387 | print "\$$foo{bar}\n"; |
1388 | |
1389 | # perl4 prints: $array{bar} |
1390 | # perl5 prints: $ |
1391 | |
1392 | Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is |
1393 | happy just to expand $foo to "array" by itself. Watch out for this |
1394 | especially in C<eval>'s. |
1395 | |
1396 | =item * Interpolation |
1397 | |
1398 | C<qq()> string passed to C<eval> |
1399 | |
1400 | eval qq( |
1401 | foreach \$y (keys %\$x\) { |
1402 | \$count++; |
1403 | } |
1404 | ); |
1405 | |
1406 | # perl4 runs this ok |
1407 | # perl5 prints: Can't find string terminator ")" |
a0d0e21e |
1408 | |
6dbacca0 |
1409 | =back |
1410 | |
1411 | =head2 DBM Traps |
1412 | |
1413 | General DBM traps. |
1414 | |
1415 | =over 5 |
1416 | |
1417 | =item * DBM |
1418 | |
1419 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1420 | may cause the same script, run under perl5, to fail. The build of perl5 |
1421 | must have been linked with the same dbm/ndbm as the default for C<dbmopen()> |
1422 | to function properly without C<tie>'ing to an extension dbm implementation. |
1423 | |
1424 | dbmopen (%dbm, "file", undef); |
1425 | print "ok\n"; |
1426 | |
1427 | # perl4 prints: ok |
1428 | # perl5 prints: ok (IFF linked with -ldbm or -lndbm) |
1429 | |
1430 | |
1431 | =item * DBM |
1432 | |
1433 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1434 | may cause the same script, run under perl5, to fail. The error generated |
1435 | when exceeding the limit on the key/value size will cause perl5 to exit |
1436 | immediately. |
1437 | |
1438 | dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; |
1439 | $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm |
1440 | print "YUP\n"; |
1441 | |
1442 | # perl4 prints: |
1443 | dbm store returned -1, errno 28, key "trap" at - line 3. |
1444 | YUP |
1445 | |
1446 | # perl5 prints: |
1447 | dbm store returned -1, errno 28, key "trap" at - line 3. |
a0d0e21e |
1448 | |
1449 | =back |
6dbacca0 |
1450 | |
1451 | =head2 Unclassified Traps |
1452 | |
1453 | Everything else. |
1454 | |
84dc3c4d |
1455 | =over 5 |
1456 | |
6dbacca0 |
1457 | =item * Unclassified |
1458 | |
1459 | C<require>/C<do> trap using returned value |
1460 | |
1461 | If the file doit.pl has: |
1462 | |
1463 | sub foo { |
1464 | $rc = do "./do.pl"; |
1465 | return 8; |
1466 | } |
1467 | print &foo, "\n"; |
1468 | |
1469 | And the do.pl file has the following single line: |
1470 | |
1471 | return 3; |
1472 | |
1473 | Running doit.pl gives the following: |
1474 | |
1475 | # perl 4 prints: 3 (aborts the subroutine early) |
1476 | # perl 5 prints: 8 |
1477 | |
1478 | Same behavior if you replace C<do> with C<require>. |
1479 | |
1480 | =back |
1481 | |
1482 | As always, if any of these are ever officially declared as bugs, |
1483 | they'll be fixed and removed. |
1484 | |