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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 |
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9 | runnable under C<use strict>. The third biggest trap is not reading |
10 | the list of changes in this version of Perl; see L<perldelta>. |
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11 | |
12 | =head2 Awk Traps |
13 | |
14 | Accustomed B<awk> users should take special note of the following: |
15 | |
16 | =over 4 |
17 | |
18 | =item * |
19 | |
20 | The English module, loaded via |
21 | |
22 | use English; |
23 | |
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24 | allows you to refer to special variables (like C<$/>) with names (like |
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25 | $RS), as though they were in B<awk>; see L<perlvar> for details. |
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26 | |
27 | =item * |
28 | |
29 | Semicolons are required after all simple statements in Perl (except |
30 | at the end of a block). Newline is not a statement delimiter. |
31 | |
32 | =item * |
33 | |
34 | Curly brackets are required on C<if>s and C<while>s. |
35 | |
36 | =item * |
37 | |
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38 | Variables begin with "$", "@" or "%" in Perl. |
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39 | |
40 | =item * |
41 | |
42 | Arrays index from 0. Likewise string positions in substr() and |
43 | index(). |
44 | |
45 | =item * |
46 | |
47 | You have to decide whether your array has numeric or string indices. |
48 | |
49 | =item * |
50 | |
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51 | Hash values do not spring into existence upon mere reference. |
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52 | |
53 | =item * |
54 | |
55 | You have to decide whether you want to use string or numeric |
56 | comparisons. |
57 | |
58 | =item * |
59 | |
60 | Reading an input line does not split it for you. You get to split it |
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61 | to an array yourself. And the split() operator has different |
62 | arguments than B<awk>'s. |
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63 | |
64 | =item * |
65 | |
66 | The current input line is normally in $_, not $0. It generally does |
67 | not have the newline stripped. ($0 is the name of the program |
68 | executed.) See L<perlvar>. |
69 | |
70 | =item * |
71 | |
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72 | $E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched |
73 | by the last match pattern. |
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74 | |
75 | =item * |
76 | |
77 | The print() statement does not add field and record separators unless |
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78 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
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79 | the English module. |
80 | |
81 | =item * |
82 | |
83 | You must open your files before you print to them. |
84 | |
85 | =item * |
86 | |
87 | The range operator is "..", not comma. The comma operator works as in |
88 | C. |
89 | |
90 | =item * |
91 | |
92 | The match operator is "=~", not "~". ("~" is the one's complement |
93 | operator, as in C.) |
94 | |
95 | =item * |
96 | |
97 | The exponentiation operator is "**", not "^". "^" is the XOR |
98 | operator, as in C. (You know, one could get the feeling that B<awk> is |
99 | basically incompatible with C.) |
100 | |
101 | =item * |
102 | |
103 | The concatenation operator is ".", not the null string. (Using the |
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104 | null string would render C</pat/ /pat/> unparsable, because the third slash |
105 | would be interpreted as a division operator--the tokenizer is in fact |
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106 | slightly context sensitive for operators like "/", "?", and "E<gt>". |
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107 | And in fact, "." itself can be the beginning of a number.) |
108 | |
109 | =item * |
110 | |
111 | The C<next>, C<exit>, and C<continue> keywords work differently. |
112 | |
113 | =item * |
114 | |
115 | |
116 | The following variables work differently: |
117 | |
118 | Awk Perl |
119 | ARGC $#ARGV or scalar @ARGV |
120 | ARGV[0] $0 |
121 | FILENAME $ARGV |
122 | FNR $. - something |
123 | FS (whatever you like) |
124 | NF $#Fld, or some such |
125 | NR $. |
126 | OFMT $# |
127 | OFS $, |
128 | ORS $\ |
129 | RLENGTH length($&) |
130 | RS $/ |
131 | RSTART length($`) |
132 | SUBSEP $; |
133 | |
134 | =item * |
135 | |
136 | You cannot set $RS to a pattern, only a string. |
137 | |
138 | =item * |
139 | |
140 | When in doubt, run the B<awk> construct through B<a2p> and see what it |
141 | gives you. |
142 | |
143 | =back |
144 | |
145 | =head2 C Traps |
146 | |
147 | Cerebral C programmers should take note of the following: |
148 | |
149 | =over 4 |
150 | |
151 | =item * |
152 | |
153 | Curly brackets are required on C<if>'s and C<while>'s. |
154 | |
155 | =item * |
156 | |
157 | You must use C<elsif> rather than C<else if>. |
158 | |
159 | =item * |
160 | |
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161 | The C<break> and C<continue> keywords from C become in |
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162 | Perl C<last> and C<next>, respectively. |
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163 | Unlike in C, these do I<not> work within a C<do { } while> construct. |
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164 | |
165 | =item * |
166 | |
167 | There's no switch statement. (But it's easy to build one on the fly.) |
168 | |
169 | =item * |
170 | |
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171 | Variables begin with "$", "@" or "%" in Perl. |
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172 | |
173 | =item * |
174 | |
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175 | C<printf()> does not implement the "*" format for interpolating |
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176 | field widths, but it's trivial to use interpolation of double-quoted |
177 | strings to achieve the same effect. |
178 | |
179 | =item * |
180 | |
181 | Comments begin with "#", not "/*". |
182 | |
183 | =item * |
184 | |
185 | You can't take the address of anything, although a similar operator |
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186 | in Perl is the backslash, which creates a reference. |
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187 | |
188 | =item * |
189 | |
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190 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
191 | ends up in C<$0>. |
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192 | |
193 | =item * |
194 | |
195 | System calls such as link(), unlink(), rename(), etc. return nonzero for |
196 | success, not 0. |
197 | |
198 | =item * |
199 | |
200 | Signal handlers deal with signal names, not numbers. Use C<kill -l> |
201 | to find their names on your system. |
202 | |
203 | =back |
204 | |
205 | =head2 Sed Traps |
206 | |
207 | Seasoned B<sed> programmers should take note of the following: |
208 | |
209 | =over 4 |
210 | |
211 | =item * |
212 | |
213 | Backreferences in substitutions use "$" rather than "\". |
214 | |
215 | =item * |
216 | |
217 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes |
218 | in front. |
219 | |
220 | =item * |
221 | |
222 | The range operator is C<...>, rather than comma. |
223 | |
224 | =back |
225 | |
226 | =head2 Shell Traps |
227 | |
228 | Sharp shell programmers should take note of the following: |
229 | |
230 | =over 4 |
231 | |
232 | =item * |
233 | |
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234 | The backtick operator does variable interpolation without regard to |
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235 | the presence of single quotes in the command. |
236 | |
237 | =item * |
238 | |
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239 | The backtick operator does no translation of the return value, unlike B<csh>. |
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240 | |
241 | =item * |
242 | |
243 | Shells (especially B<csh>) do several levels of substitution on each |
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244 | command line. Perl does substitution in only certain constructs |
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245 | such as double quotes, backticks, angle brackets, and search patterns. |
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246 | |
247 | =item * |
248 | |
249 | Shells interpret scripts a little bit at a time. Perl compiles the |
250 | entire program before executing it (except for C<BEGIN> blocks, which |
251 | execute at compile time). |
252 | |
253 | =item * |
254 | |
255 | The arguments are available via @ARGV, not $1, $2, etc. |
256 | |
257 | =item * |
258 | |
259 | The environment is not automatically made available as separate scalar |
260 | variables. |
261 | |
262 | =back |
263 | |
264 | =head2 Perl Traps |
265 | |
266 | Practicing Perl Programmers should take note of the following: |
267 | |
268 | =over 4 |
269 | |
270 | =item * |
271 | |
272 | Remember that many operations behave differently in a list |
273 | context than they do in a scalar one. See L<perldata> for details. |
274 | |
275 | =item * |
276 | |
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277 | Avoid barewords if you can, especially all lowercase ones. |
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278 | You can't tell by just looking at it whether a bareword is |
279 | a function or a string. By using quotes on strings and |
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280 | parentheses on function calls, you won't ever get them confused. |
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281 | |
282 | =item * |
283 | |
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284 | You cannot discern from mere inspection which builtins |
285 | are unary operators (like chop() and chdir()) |
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286 | and which are list operators (like print() and unlink()). |
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287 | (User-defined subroutines can be B<only> list operators, never |
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288 | unary ones.) See L<perlop>. |
289 | |
290 | =item * |
291 | |
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292 | People have a hard time remembering that some functions |
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293 | default to $_, or @ARGV, or whatever, but that others which |
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294 | you might expect to do not. |
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295 | |
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296 | =item * |
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297 | |
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298 | The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline |
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299 | operation on that handle. The data read is assigned to $_ only if the |
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300 | file read is the sole condition in a while loop: |
301 | |
302 | while (<FH>) { } |
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303 | while (defined($_ = <FH>)) { }.. |
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304 | <FH>; # data discarded! |
305 | |
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306 | =item * |
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307 | |
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308 | Remember not to use C<=> when you need C<=~>; |
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309 | these two constructs are quite different: |
310 | |
311 | $x = /foo/; |
312 | $x =~ /foo/; |
313 | |
314 | =item * |
315 | |
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316 | The C<do {}> construct isn't a real loop that you can use |
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317 | loop control on. |
318 | |
319 | =item * |
320 | |
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321 | Use C<my()> for local variables whenever you can get away with |
322 | it (but see L<perlform> for where you can't). |
323 | Using C<local()> actually gives a local value to a global |
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324 | variable, which leaves you open to unforeseen side-effects |
325 | of dynamic scoping. |
326 | |
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327 | =item * |
328 | |
329 | If you localize an exported variable in a module, its exported value will |
330 | not change. The local name becomes an alias to a new value but the |
331 | external name is still an alias for the original. |
332 | |
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333 | =back |
334 | |
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335 | =head2 Perl4 to Perl5 Traps |
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336 | |
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337 | Practicing Perl4 Programmers should take note of the following |
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338 | Perl4-to-Perl5 specific traps. |
339 | |
340 | They're crudely ordered according to the following list: |
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341 | |
342 | =over 4 |
343 | |
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344 | =item Discontinuance, Deprecation, and BugFix traps |
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345 | |
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346 | Anything that's been fixed as a perl4 bug, removed as a perl4 feature |
347 | or deprecated as a perl4 feature with the intent to encourage usage of |
348 | some other perl5 feature. |
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349 | |
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350 | =item Parsing Traps |
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351 | |
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352 | Traps that appear to stem from the new parser. |
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353 | |
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354 | =item Numerical Traps |
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355 | |
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356 | Traps having to do with numerical or mathematical operators. |
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357 | |
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358 | =item General data type traps |
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359 | |
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360 | Traps involving perl standard data types. |
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361 | |
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362 | =item Context Traps - scalar, list contexts |
363 | |
364 | Traps related to context within lists, scalar statements/declarations. |
365 | |
366 | =item Precedence Traps |
367 | |
368 | Traps related to the precedence of parsing, evaluation, and execution of |
369 | code. |
370 | |
371 | =item General Regular Expression Traps using s///, etc. |
372 | |
373 | Traps related to the use of pattern matching. |
374 | |
375 | =item Subroutine, Signal, Sorting Traps |
376 | |
377 | Traps related to the use of signals and signal handlers, general subroutines, |
378 | and sorting, along with sorting subroutines. |
379 | |
380 | =item OS Traps |
381 | |
382 | OS-specific traps. |
383 | |
384 | =item DBM Traps |
385 | |
386 | Traps specific to the use of C<dbmopen()>, and specific dbm implementations. |
387 | |
388 | =item Unclassified Traps |
389 | |
390 | Everything else. |
391 | |
392 | =back |
393 | |
394 | If you find an example of a conversion trap that is not listed here, |
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395 | please submit it to Bill Middleton <F<wjm@best.com>> for inclusion. |
396 | Also note that at least some of these can be caught with B<-w>. |
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397 | |
398 | =head2 Discontinuance, Deprecation, and BugFix traps |
399 | |
400 | Anything that has been discontinued, deprecated, or fixed as |
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401 | a bug from perl4. |
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402 | |
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403 | =over 4 |
404 | |
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405 | =item * Discontinuance |
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406 | |
407 | Symbols starting with "_" are no longer forced into package main, except |
408 | for C<$_> itself (and C<@_>, etc.). |
409 | |
410 | package test; |
411 | $_legacy = 1; |
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412 | |
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413 | package main; |
414 | print "\$_legacy is ",$_legacy,"\n"; |
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415 | |
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416 | # perl4 prints: $_legacy is 1 |
417 | # perl5 prints: $_legacy is |
418 | |
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419 | =item * Deprecation |
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420 | |
421 | Double-colon is now a valid package separator in a variable name. Thus these |
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422 | behave differently in perl4 vs. perl5, because the packages don't exist. |
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423 | |
424 | $a=1;$b=2;$c=3;$var=4; |
425 | print "$a::$b::$c "; |
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426 | print "$var::abc::xyz\n"; |
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427 | |
428 | # perl4 prints: 1::2::3 4::abc::xyz |
429 | # perl5 prints: 3 |
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430 | |
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431 | Given that C<::> is now the preferred package delimiter, it is debatable |
432 | whether this should be classed as a bug or not. |
433 | (The older package delimiter, ' ,is used here) |
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434 | |
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435 | $x = 10 ; |
436 | print "x=${'x}\n" ; |
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437 | |
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438 | # perl4 prints: x=10 |
439 | # perl5 prints: Can't find string terminator "'" anywhere before EOF |
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440 | |
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441 | You can avoid this problem, and remain compatible with perl4, if you |
442 | always explicitly include the package name: |
443 | |
444 | $x = 10 ; |
445 | print "x=${main'x}\n" ; |
446 | |
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447 | Also see precedence traps, for parsing C<$:>. |
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448 | |
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449 | =item * BugFix |
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450 | |
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451 | The second and third arguments of C<splice()> are now evaluated in scalar |
452 | context (as the Camel says) rather than list context. |
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453 | |
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454 | sub sub1{return(0,2) } # return a 2-element list |
455 | sub sub2{ return(1,2,3)} # return a 3-element list |
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456 | @a1 = ("a","b","c","d","e"); |
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457 | @a2 = splice(@a1,&sub1,&sub2); |
458 | print join(' ',@a2),"\n"; |
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459 | |
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460 | # perl4 prints: a b |
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461 | # perl5 prints: c d e |
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462 | |
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463 | =item * Discontinuance |
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464 | |
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465 | You can't do a C<goto> into a block that is optimized away. Darn. |
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466 | |
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467 | goto marker1; |
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468 | |
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469 | for(1){ |
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470 | marker1: |
471 | print "Here I is!\n"; |
54310121 |
472 | } |
473 | |
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474 | # perl4 prints: Here I is! |
475 | # perl5 dumps core (SEGV) |
476 | |
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477 | =item * Discontinuance |
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478 | |
479 | It is no longer syntactically legal to use whitespace as the name |
480 | of a variable, or as a delimiter for any kind of quote construct. |
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481 | Double darn. |
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482 | |
483 | $a = ("foo bar"); |
484 | $b = q baz ; |
485 | print "a is $a, b is $b\n"; |
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486 | |
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487 | # perl4 prints: a is foo bar, b is baz |
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488 | # perl5 errors: Bareword found where operator expected |
5e378fdf |
489 | |
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490 | =item * Discontinuance |
491 | |
492 | The archaic while/if BLOCK BLOCK syntax is no longer supported. |
493 | |
494 | if { 1 } { |
495 | print "True!"; |
496 | } |
497 | else { |
498 | print "False!"; |
499 | } |
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500 | |
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501 | # perl4 prints: True! |
502 | # perl5 errors: syntax error at test.pl line 1, near "if {" |
503 | |
504 | =item * BugFix |
505 | |
506 | The C<**> operator now binds more tightly than unary minus. |
507 | It was documented to work this way before, but didn't. |
508 | |
509 | print -4**2,"\n"; |
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510 | |
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511 | # perl4 prints: 16 |
512 | # perl5 prints: -16 |
513 | |
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514 | =item * Discontinuance |
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515 | |
516 | The meaning of C<foreach{}> has changed slightly when it is iterating over a |
517 | list which is not an array. This used to assign the list to a |
518 | temporary array, but no longer does so (for efficiency). This means |
519 | that you'll now be iterating over the actual values, not over copies of |
520 | the values. Modifications to the loop variable can change the original |
521 | values. |
522 | |
523 | @list = ('ab','abc','bcd','def'); |
524 | foreach $var (grep(/ab/,@list)){ |
525 | $var = 1; |
526 | } |
527 | print (join(':',@list)); |
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528 | |
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529 | # perl4 prints: ab:abc:bcd:def |
530 | # perl5 prints: 1:1:bcd:def |
531 | |
532 | To retain Perl4 semantics you need to assign your list |
54310121 |
533 | explicitly to a temporary array and then iterate over that. For |
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534 | example, you might need to change |
535 | |
536 | foreach $var (grep(/ab/,@list)){ |
537 | |
538 | to |
539 | |
540 | foreach $var (@tmp = grep(/ab/,@list)){ |
541 | |
542 | Otherwise changing $var will clobber the values of @list. (This most often |
543 | happens when you use C<$_> for the loop variable, and call subroutines in |
544 | the loop that don't properly localize C<$_>.) |
545 | |
5e378fdf |
546 | =item * Discontinuance |
547 | |
548 | C<split> with no arguments now behaves like C<split ' '> (which doesn't |
549 | return an initial null field if $_ starts with whitespace), it used to |
550 | behave like C<split /\s+/> (which does). |
551 | |
552 | $_ = ' hi mom'; |
553 | print join(':', split); |
554 | |
555 | # perl4 prints: :hi:mom |
556 | # perl5 prints: hi:mom |
557 | |
55497cff |
558 | =item * BugFix |
559 | |
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560 | Perl 4 would ignore any text which was attached to an B<-e> switch, |
55497cff |
561 | always taking the code snippet from the following arg. Additionally, it |
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562 | would silently accept an B<-e> switch without a following arg. Both of |
55497cff |
563 | these behaviors have been fixed. |
564 | |
565 | perl -e'print "attached to -e"' 'print "separate arg"' |
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566 | |
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567 | # perl4 prints: separate arg |
568 | # perl5 prints: attached to -e |
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569 | |
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570 | perl -e |
571 | |
572 | # perl4 prints: |
573 | # perl5 dies: No code specified for -e. |
574 | |
575 | =item * Discontinuance |
576 | |
577 | In Perl 4 the return value of C<push> was undocumented, but it was |
578 | actually the last value being pushed onto the target list. In Perl 5 |
579 | the return value of C<push> is documented, but has changed, it is the |
580 | number of elements in the resulting list. |
581 | |
582 | @x = ('existing'); |
583 | print push(@x, 'first new', 'second new'); |
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584 | |
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585 | # perl4 prints: second new |
586 | # perl5 prints: 3 |
587 | |
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588 | =item * Deprecation |
589 | |
590 | Some error messages will be different. |
591 | |
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592 | =item * Discontinuance |
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593 | |
594 | Some bugs may have been inadvertently removed. :-) |
595 | |
596 | =back |
597 | |
598 | =head2 Parsing Traps |
599 | |
600 | Perl4-to-Perl5 traps from having to do with parsing. |
601 | |
602 | =over 4 |
603 | |
604 | =item * Parsing |
605 | |
606 | Note the space between . and = |
607 | |
608 | $string . = "more string"; |
609 | print $string; |
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610 | |
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611 | # perl4 prints: more string |
612 | # perl5 prints: syntax error at - line 1, near ". =" |
613 | |
614 | =item * Parsing |
615 | |
616 | Better parsing in perl 5 |
617 | |
618 | sub foo {} |
619 | &foo |
620 | print("hello, world\n"); |
54310121 |
621 | |
6dbacca0 |
622 | # perl4 prints: hello, world |
623 | # perl5 prints: syntax error |
624 | |
625 | =item * Parsing |
626 | |
627 | "if it looks like a function, it is a function" rule. |
628 | |
629 | print |
630 | ($foo == 1) ? "is one\n" : "is zero\n"; |
54310121 |
631 | |
6dbacca0 |
632 | # perl4 prints: is zero |
633 | # perl5 warns: "Useless use of a constant in void context" if using -w |
634 | |
c12982c8 |
635 | =item * Parsing |
636 | |
637 | String interpolation of the C<$#array> construct differs when braces |
638 | are to used around the name. |
639 | |
640 | @ = (1..3); |
641 | print "${#a}"; |
642 | |
643 | # perl4 prints: 2 |
644 | # perl5 fails with syntax error |
645 | |
646 | @ = (1..3); |
647 | print "$#{a}"; |
648 | |
649 | # perl4 prints: {a} |
650 | # perl5 prints: 2 |
651 | |
6dbacca0 |
652 | =back |
653 | |
654 | =head2 Numerical Traps |
655 | |
656 | Perl4-to-Perl5 traps having to do with numerical operators, |
657 | operands, or output from same. |
658 | |
659 | =over 5 |
660 | |
661 | =item * Numerical |
662 | |
663 | Formatted output and significant digits |
664 | |
665 | print 7.373504 - 0, "\n"; |
54310121 |
666 | printf "%20.18f\n", 7.373504 - 0; |
667 | |
6dbacca0 |
668 | # Perl4 prints: |
669 | 7.375039999999996141 |
670 | 7.37503999999999614 |
54310121 |
671 | |
6dbacca0 |
672 | # Perl5 prints: |
673 | 7.373504 |
674 | 7.37503999999999614 |
675 | |
676 | =item * Numerical |
677 | |
5f05dabc |
678 | This specific item has been deleted. It demonstrated how the auto-increment |
5e378fdf |
679 | operator would not catch when a number went over the signed int limit. Fixed |
a6006777 |
680 | in version 5.003_04. But always be wary when using large integers. |
681 | If in doubt: |
6dbacca0 |
682 | |
5e378fdf |
683 | use Math::BigInt; |
6dbacca0 |
684 | |
54310121 |
685 | =item * Numerical |
6dbacca0 |
686 | |
687 | Assignment of return values from numeric equality tests |
688 | does not work in perl5 when the test evaluates to false (0). |
689 | Logical tests now return an null, instead of 0 |
a6006777 |
690 | |
6dbacca0 |
691 | $p = ($test == 1); |
692 | print $p,"\n"; |
a6006777 |
693 | |
6dbacca0 |
694 | # perl4 prints: 0 |
695 | # perl5 prints: |
696 | |
dc848c6f |
697 | Also see L<"General Regular Expression Traps using s///, etc."> |
698 | for another example of this new feature... |
6dbacca0 |
699 | |
651ad3b1 |
700 | =item * Bitwise string ops |
701 | |
702 | When bitwise operators which can operate upon either numbers or |
703 | strings (C<& | ^ ~>) are given only strings as arguments, perl4 would |
704 | treat the operands as bitstrings so long as the program contained a call |
705 | to the C<vec()> function. perl5 treats the string operands as bitstrings. |
706 | (See L<perlop/Bitwise String Operators> for more details.) |
707 | |
708 | $fred = "10"; |
709 | $barney = "12"; |
710 | $betty = $fred & $barney; |
711 | print "$betty\n"; |
712 | # Uncomment the next line to change perl4's behavior |
713 | # ($dummy) = vec("dummy", 0, 0); |
714 | |
715 | # Perl4 prints: |
716 | 8 |
717 | |
718 | # Perl5 prints: |
719 | 10 |
720 | |
721 | # If vec() is used anywhere in the program, both print: |
722 | 10 |
723 | |
6dbacca0 |
724 | =back |
725 | |
726 | =head2 General data type traps |
727 | |
728 | Perl4-to-Perl5 traps involving most data-types, and their usage |
729 | within certain expressions and/or context. |
730 | |
731 | =over 5 |
732 | |
733 | =item * (Arrays) |
734 | |
735 | Negative array subscripts now count from the end of the array. |
736 | |
737 | @a = (1, 2, 3, 4, 5); |
738 | print "The third element of the array is $a[3] also expressed as $a[-2] \n"; |
54310121 |
739 | |
6dbacca0 |
740 | # perl4 prints: The third element of the array is 4 also expressed as |
741 | # perl5 prints: The third element of the array is 4 also expressed as 4 |
742 | |
743 | =item * (Arrays) |
744 | |
745 | Setting C<$#array> lower now discards array elements, and makes them |
746 | impossible to recover. |
747 | |
54310121 |
748 | @a = (a,b,c,d,e); |
6dbacca0 |
749 | print "Before: ",join('',@a); |
54310121 |
750 | $#a =1; |
6dbacca0 |
751 | print ", After: ",join('',@a); |
752 | $#a =3; |
753 | print ", Recovered: ",join('',@a),"\n"; |
54310121 |
754 | |
6dbacca0 |
755 | # perl4 prints: Before: abcde, After: ab, Recovered: abcd |
756 | # perl5 prints: Before: abcde, After: ab, Recovered: ab |
757 | |
758 | =item * (Hashes) |
759 | |
760 | Hashes get defined before use |
761 | |
54310121 |
762 | local($s,@a,%h); |
6dbacca0 |
763 | die "scalar \$s defined" if defined($s); |
764 | die "array \@a defined" if defined(@a); |
765 | die "hash \%h defined" if defined(%h); |
54310121 |
766 | |
6dbacca0 |
767 | # perl4 prints: |
768 | # perl5 dies: hash %h defined |
769 | |
475342a6 |
770 | Perl will now generate a warning when it sees defined(@a) and |
771 | defined(%h). |
772 | |
6dbacca0 |
773 | =item * (Globs) |
774 | |
775 | glob assignment from variable to variable will fail if the assigned |
776 | variable is localized subsequent to the assignment |
777 | |
778 | @a = ("This is Perl 4"); |
779 | *b = *a; |
780 | local(@a); |
781 | print @b,"\n"; |
54310121 |
782 | |
6dbacca0 |
783 | # perl4 prints: This is Perl 4 |
784 | # perl5 prints: |
54310121 |
785 | |
a3cb178b |
786 | =item * (Globs) |
54310121 |
787 | |
a3cb178b |
788 | Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4 |
789 | it undefines the associated scalar (but may have other side effects |
790 | including SEGVs). |
5e378fdf |
791 | |
6dbacca0 |
792 | =item * (Scalar String) |
793 | |
794 | Changes in unary negation (of strings) |
795 | This change effects both the return value and what it |
796 | does to auto(magic)increment. |
797 | |
798 | $x = "aaa"; |
799 | print ++$x," : "; |
800 | print -$x," : "; |
801 | print ++$x,"\n"; |
54310121 |
802 | |
6dbacca0 |
803 | # perl4 prints: aab : -0 : 1 |
804 | # perl5 prints: aab : -aab : aac |
805 | |
806 | =item * (Constants) |
807 | |
808 | perl 4 lets you modify constants: |
809 | |
810 | $foo = "x"; |
811 | &mod($foo); |
812 | for ($x = 0; $x < 3; $x++) { |
813 | &mod("a"); |
814 | } |
815 | sub mod { |
816 | print "before: $_[0]"; |
817 | $_[0] = "m"; |
818 | print " after: $_[0]\n"; |
819 | } |
54310121 |
820 | |
6dbacca0 |
821 | # perl4: |
822 | # before: x after: m |
823 | # before: a after: m |
824 | # before: m after: m |
825 | # before: m after: m |
54310121 |
826 | |
6dbacca0 |
827 | # Perl5: |
828 | # before: x after: m |
829 | # Modification of a read-only value attempted at foo.pl line 12. |
830 | # before: a |
831 | |
832 | =item * (Scalars) |
833 | |
834 | The behavior is slightly different for: |
835 | |
836 | print "$x", defined $x |
54310121 |
837 | |
6dbacca0 |
838 | # perl 4: 1 |
839 | # perl 5: <no output, $x is not called into existence> |
840 | |
841 | =item * (Variable Suicide) |
842 | |
843 | Variable suicide behavior is more consistent under Perl 5. |
aa689395 |
844 | Perl5 exhibits the same behavior for hashes and scalars, |
5f05dabc |
845 | that perl4 exhibits for only scalars. |
6dbacca0 |
846 | |
847 | $aGlobal{ "aKey" } = "global value"; |
848 | print "MAIN:", $aGlobal{"aKey"}, "\n"; |
849 | $GlobalLevel = 0; |
850 | &test( *aGlobal ); |
851 | |
852 | sub test { |
853 | local( *theArgument ) = @_; |
854 | local( %aNewLocal ); # perl 4 != 5.001l,m |
54310121 |
855 | $aNewLocal{"aKey"} = "this should never appear"; |
6dbacca0 |
856 | print "SUB: ", $theArgument{"aKey"}, "\n"; |
857 | $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print |
858 | $GlobalLevel++; |
859 | if( $GlobalLevel<4 ) { |
860 | &test( *aNewLocal ); |
861 | } |
862 | } |
54310121 |
863 | |
6dbacca0 |
864 | # Perl4: |
865 | # MAIN:global value |
866 | # SUB: global value |
867 | # SUB: level 0 |
868 | # SUB: level 1 |
869 | # SUB: level 2 |
54310121 |
870 | |
6dbacca0 |
871 | # Perl5: |
872 | # MAIN:global value |
873 | # SUB: global value |
874 | # SUB: this should never appear |
875 | # SUB: this should never appear |
876 | # SUB: this should never appear |
877 | |
84dc3c4d |
878 | =back |
6dbacca0 |
879 | |
880 | =head2 Context Traps - scalar, list contexts |
881 | |
882 | =over 5 |
883 | |
884 | =item * (list context) |
885 | |
886 | The elements of argument lists for formats are now evaluated in list |
887 | context. This means you can interpolate list values now. |
888 | |
889 | @fmt = ("foo","bar","baz"); |
890 | format STDOUT= |
891 | @<<<<< @||||| @>>>>> |
892 | @fmt; |
893 | . |
54310121 |
894 | write; |
895 | |
6dbacca0 |
896 | # perl4 errors: Please use commas to separate fields in file |
897 | # perl5 prints: foo bar baz |
898 | |
899 | =item * (scalar context) |
900 | |
54310121 |
901 | The C<caller()> function now returns a false value in a scalar context |
902 | if there is no caller. This lets library files determine if they're |
6dbacca0 |
903 | being required. |
904 | |
905 | caller() ? (print "You rang?\n") : (print "Got a 0\n"); |
54310121 |
906 | |
6dbacca0 |
907 | # perl4 errors: There is no caller |
908 | # perl5 prints: Got a 0 |
5e378fdf |
909 | |
6dbacca0 |
910 | =item * (scalar context) |
911 | |
912 | The comma operator in a scalar context is now guaranteed to give a |
913 | scalar context to its arguments. |
914 | |
915 | @y= ('a','b','c'); |
916 | $x = (1, 2, @y); |
917 | print "x = $x\n"; |
54310121 |
918 | |
6dbacca0 |
919 | # Perl4 prints: x = c # Thinks list context interpolates list |
920 | # Perl5 prints: x = 3 # Knows scalar uses length of list |
921 | |
922 | =item * (list, builtin) |
923 | |
924 | C<sprintf()> funkiness (array argument converted to scalar array count) |
925 | This test could be added to t/op/sprintf.t |
926 | |
927 | @z = ('%s%s', 'foo', 'bar'); |
928 | $x = sprintf(@z); |
929 | if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";} |
54310121 |
930 | |
6dbacca0 |
931 | # perl4 prints: ok 2 |
932 | # perl5 prints: not ok 2 |
933 | |
934 | C<printf()> works fine, though: |
935 | |
936 | printf STDOUT (@z); |
54310121 |
937 | print "\n"; |
938 | |
6dbacca0 |
939 | # perl4 prints: foobar |
940 | # perl5 prints: foobar |
941 | |
942 | Probably a bug. |
943 | |
944 | =back |
945 | |
946 | =head2 Precedence Traps |
947 | |
948 | Perl4-to-Perl5 traps involving precedence order. |
949 | |
f4b17341 |
950 | Perl 4 has almost the same precedence rules as Perl 5 for the operators |
951 | that they both have. Perl 4 however, seems to have had some |
952 | inconsistencies that made the behavior differ from what was documented. |
953 | |
84dc3c4d |
954 | =over 5 |
955 | |
5e378fdf |
956 | =item * Precedence |
957 | |
8dbef698 |
958 | LHS vs. RHS of any assignment operator. LHS is evaluated first |
959 | in perl4, second in perl5; this can affect the relationship |
960 | between side-effects in sub-expressions. |
5e378fdf |
961 | |
962 | @arr = ( 'left', 'right' ); |
963 | $a{shift @arr} = shift @arr; |
964 | print join( ' ', keys %a ); |
965 | |
966 | # perl4 prints: left |
967 | # perl5 prints: right |
968 | |
969 | =item * Precedence |
6dbacca0 |
970 | |
971 | These are now semantic errors because of precedence: |
972 | |
973 | @list = (1,2,3,4,5); |
974 | %map = ("a",1,"b",2,"c",3,"d",4); |
975 | $n = shift @list + 2; # first item in list plus 2 |
976 | print "n is $n, "; |
977 | $m = keys %map + 2; # number of items in hash plus 2 |
978 | print "m is $m\n"; |
54310121 |
979 | |
6dbacca0 |
980 | # perl4 prints: n is 3, m is 6 |
981 | # perl5 errors and fails to compile |
982 | |
983 | =item * Precedence |
a0d0e21e |
984 | |
4633a7c4 |
985 | The precedence of assignment operators is now the same as the precedence |
986 | of assignment. Perl 4 mistakenly gave them the precedence of the associated |
987 | operator. So you now must parenthesize them in expressions like |
988 | |
989 | /foo/ ? ($a += 2) : ($a -= 2); |
a6006777 |
990 | |
4633a7c4 |
991 | Otherwise |
992 | |
6dbacca0 |
993 | /foo/ ? $a += 2 : $a -= 2 |
4633a7c4 |
994 | |
995 | would be erroneously parsed as |
996 | |
997 | (/foo/ ? $a += 2 : $a) -= 2; |
998 | |
999 | On the other hand, |
1000 | |
54310121 |
1001 | $a += /foo/ ? 1 : 2; |
4633a7c4 |
1002 | |
1003 | now works as a C programmer would expect. |
1004 | |
6dbacca0 |
1005 | =item * Precedence |
4633a7c4 |
1006 | |
6dbacca0 |
1007 | open FOO || die; |
a0d0e21e |
1008 | |
5f05dabc |
1009 | is now incorrect. You need parentheses around the filehandle. |
1010 | Otherwise, perl5 leaves the statement as its default precedence: |
a0d0e21e |
1011 | |
6dbacca0 |
1012 | open(FOO || die); |
54310121 |
1013 | |
6dbacca0 |
1014 | # perl4 opens or dies |
1015 | # perl5 errors: Precedence problem: open FOO should be open(FOO) |
a0d0e21e |
1016 | |
6dbacca0 |
1017 | =item * Precedence |
a0d0e21e |
1018 | |
6dbacca0 |
1019 | perl4 gives the special variable, C<$:> precedence, where perl5 |
1020 | treats C<$::> as main C<package> |
a0d0e21e |
1021 | |
6dbacca0 |
1022 | $a = "x"; print "$::a"; |
54310121 |
1023 | |
6dbacca0 |
1024 | # perl 4 prints: -:a |
1025 | # perl 5 prints: x |
5e378fdf |
1026 | |
6dbacca0 |
1027 | =item * Precedence |
a0d0e21e |
1028 | |
f4b17341 |
1029 | perl4 had buggy precedence for the file test operators vis-a-vis |
1030 | the assignment operators. Thus, although the precedence table |
1031 | for perl4 leads one to believe C<-e $foo .= "q"> should parse as |
1032 | C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>. |
1033 | In perl5, the precedence is as documented. |
54310121 |
1034 | |
1035 | -e $foo .= "q" |
a0d0e21e |
1036 | |
6dbacca0 |
1037 | # perl4 prints: no output |
1038 | # perl5 prints: Can't modify -e in concatenation |
a0d0e21e |
1039 | |
f4b17341 |
1040 | =item * Precedence |
1041 | |
1042 | In perl4, keys(), each() and values() were special high-precedence operators |
1043 | that operated on a single hash, but in perl5, they are regular named unary |
1044 | operators. As documented, named unary operators have lower precedence |
1045 | than the arithmetic and concatenation operators C<+ - .>, but the perl4 |
1046 | variants of these operators actually bind tighter than C<+ - .>. |
1047 | Thus, for: |
1048 | |
1049 | %foo = 1..10; |
1050 | print keys %foo - 1 |
1051 | |
1052 | # perl4 prints: 4 |
1053 | # perl5 prints: Type of arg 1 to keys must be hash (not subtraction) |
1054 | |
1055 | The perl4 behavior was probably more useful, if less consistent. |
1056 | |
6dbacca0 |
1057 | =back |
1058 | |
1059 | =head2 General Regular Expression Traps using s///, etc. |
1060 | |
1061 | All types of RE traps. |
1062 | |
1063 | =over 5 |
1064 | |
1065 | =item * Regular Expression |
1066 | |
1067 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to |
19799a22 |
1068 | interpolate $lhs but not $rhs. (And still does not match a literal |
6dbacca0 |
1069 | '$' in string) |
1070 | |
1071 | $a=1;$b=2; |
1072 | $string = '1 2 $a $b'; |
1073 | $string =~ s'$a'$b'; |
1074 | print $string,"\n"; |
54310121 |
1075 | |
6dbacca0 |
1076 | # perl4 prints: $b 2 $a $b |
1077 | # perl5 prints: 1 2 $a $b |
1078 | |
1079 | =item * Regular Expression |
a0d0e21e |
1080 | |
1081 | C<m//g> now attaches its state to the searched string rather than the |
6dbacca0 |
1082 | regular expression. (Once the scope of a block is left for the sub, the |
1083 | state of the searched string is lost) |
1084 | |
1085 | $_ = "ababab"; |
1086 | while(m/ab/g){ |
1087 | &doit("blah"); |
1088 | } |
1089 | sub doit{local($_) = shift; print "Got $_ "} |
54310121 |
1090 | |
6dbacca0 |
1091 | # perl4 prints: blah blah blah |
1092 | # perl5 prints: infinite loop blah... |
1093 | |
1094 | =item * Regular Expression |
1095 | |
68dc0745 |
1096 | Currently, if you use the C<m//o> qualifier on a regular expression |
1097 | within an anonymous sub, I<all> closures generated from that anonymous |
1098 | sub will use the regular expression as it was compiled when it was used |
1099 | the very first time in any such closure. For instance, if you say |
1100 | |
1101 | sub build_match { |
1102 | my($left,$right) = @_; |
1103 | return sub { $_[0] =~ /$left stuff $right/o; }; |
1104 | } |
1105 | |
1106 | build_match() will always return a sub which matches the contents of |
19799a22 |
1107 | $left and $right as they were the I<first> time that build_match() |
68dc0745 |
1108 | was called, not as they are in the current call. |
1109 | |
1110 | This is probably a bug, and may change in future versions of Perl. |
1111 | |
1112 | =item * Regular Expression |
1113 | |
6dbacca0 |
1114 | If no parentheses are used in a match, Perl4 sets C<$+> to |
1115 | the whole match, just like C<$&>. Perl5 does not. |
1116 | |
1117 | "abcdef" =~ /b.*e/; |
1118 | print "\$+ = $+\n"; |
54310121 |
1119 | |
6dbacca0 |
1120 | # perl4 prints: bcde |
1121 | # perl5 prints: |
1122 | |
1123 | =item * Regular Expression |
1124 | |
1125 | substitution now returns the null string if it fails |
1126 | |
1127 | $string = "test"; |
1128 | $value = ($string =~ s/foo//); |
1129 | print $value, "\n"; |
54310121 |
1130 | |
6dbacca0 |
1131 | # perl4 prints: 0 |
1132 | # perl5 prints: |
1133 | |
1134 | Also see L<Numerical Traps> for another example of this new feature. |
1135 | |
1136 | =item * Regular Expression |
1137 | |
54310121 |
1138 | C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no |
1139 | backtick expansion |
6dbacca0 |
1140 | |
1141 | $string = ""; |
1142 | $string =~ s`^`hostname`; |
1143 | print $string, "\n"; |
54310121 |
1144 | |
6dbacca0 |
1145 | # perl4 prints: <the local hostname> |
1146 | # perl5 prints: hostname |
1147 | |
1148 | =item * Regular Expression |
1149 | |
1150 | Stricter parsing of variables used in regular expressions |
1151 | |
1152 | s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; |
54310121 |
1153 | |
6dbacca0 |
1154 | # perl4: compiles w/o error |
1155 | # perl5: with Scalar found where operator expected ..., near "$opt$plus" |
1156 | |
1157 | an added component of this example, apparently from the same script, is |
1158 | the actual value of the s'd string after the substitution. |
1159 | C<[$opt]> is a character class in perl4 and an array subscript in perl5 |
1160 | |
54310121 |
1161 | $grpc = 'a'; |
6dbacca0 |
1162 | $opt = 'r'; |
1163 | $_ = 'bar'; |
1164 | s/^([^$grpc]*$grpc[$opt]?)/foo/; |
1165 | print ; |
54310121 |
1166 | |
6dbacca0 |
1167 | # perl4 prints: foo |
1168 | # perl5 prints: foobar |
1169 | |
1170 | =item * Regular Expression |
1171 | |
1172 | Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched |
1173 | repeatedly, like C</x/> or C<m!x!>. |
1174 | |
1175 | $test = "once"; |
1176 | sub match { $test =~ m?once?; } |
1177 | &match(); |
1178 | if( &match() ) { |
1179 | # m?x? matches more then once |
1180 | print "perl4\n"; |
54310121 |
1181 | } else { |
6dbacca0 |
1182 | # m?x? matches only once |
54310121 |
1183 | print "perl5\n"; |
6dbacca0 |
1184 | } |
54310121 |
1185 | |
6dbacca0 |
1186 | # perl4 prints: perl4 |
1187 | # perl5 prints: perl5 |
a0d0e21e |
1188 | |
a0d0e21e |
1189 | |
6dbacca0 |
1190 | =back |
1191 | |
1192 | =head2 Subroutine, Signal, Sorting Traps |
a0d0e21e |
1193 | |
6dbacca0 |
1194 | The general group of Perl4-to-Perl5 traps having to do with |
1195 | Signals, Sorting, and their related subroutines, as well as |
1196 | general subroutine traps. Includes some OS-Specific traps. |
a0d0e21e |
1197 | |
6dbacca0 |
1198 | =over 5 |
a0d0e21e |
1199 | |
6dbacca0 |
1200 | =item * (Signals) |
a0d0e21e |
1201 | |
6dbacca0 |
1202 | Barewords that used to look like strings to Perl will now look like subroutine |
1203 | calls if a subroutine by that name is defined before the compiler sees them. |
a0d0e21e |
1204 | |
6dbacca0 |
1205 | sub SeeYa { warn"Hasta la vista, baby!" } |
1206 | $SIG{'TERM'} = SeeYa; |
1207 | print "SIGTERM is now $SIG{'TERM'}\n"; |
54310121 |
1208 | |
6dbacca0 |
1209 | # perl4 prints: SIGTERM is main'SeeYa |
1210 | # perl5 prints: SIGTERM is now main::1 |
a0d0e21e |
1211 | |
6dbacca0 |
1212 | Use B<-w> to catch this one |
a0d0e21e |
1213 | |
6dbacca0 |
1214 | =item * (Sort Subroutine) |
a0d0e21e |
1215 | |
6dbacca0 |
1216 | reverse is no longer allowed as the name of a sort subroutine. |
a0d0e21e |
1217 | |
6dbacca0 |
1218 | sub reverse{ print "yup "; $a <=> $b } |
54310121 |
1219 | print sort reverse a,b,c; |
1220 | |
6dbacca0 |
1221 | # perl4 prints: yup yup yup yup abc |
54310121 |
1222 | # perl5 prints: abc |
a0d0e21e |
1223 | |
b996531f |
1224 | =item * warn() won't let you specify a filehandle. |
1225 | |
1226 | Although it _always_ printed to STDERR, warn() would let you specify a |
1227 | filehandle in perl4. With perl5 it does not. |
5e378fdf |
1228 | |
1229 | warn STDERR "Foo!"; |
1230 | |
1231 | # perl4 prints: Foo! |
54310121 |
1232 | # perl5 prints: String found where operator expected |
5e378fdf |
1233 | |
6dbacca0 |
1234 | =back |
a0d0e21e |
1235 | |
6dbacca0 |
1236 | =head2 OS Traps |
1237 | |
1238 | =over 5 |
1239 | |
1240 | =item * (SysV) |
1241 | |
54310121 |
1242 | Under HPUX, and some other SysV OSes, one had to reset any signal handler, |
1243 | within the signal handler function, each time a signal was handled with |
1244 | perl4. With perl5, the reset is now done correctly. Any code relying |
6dbacca0 |
1245 | on the handler _not_ being reset will have to be reworked. |
1246 | |
a6006777 |
1247 | Since version 5.002, Perl uses sigaction() under SysV. |
6dbacca0 |
1248 | |
1249 | sub gotit { |
54310121 |
1250 | print "Got @_... "; |
1251 | } |
6dbacca0 |
1252 | $SIG{'INT'} = 'gotit'; |
54310121 |
1253 | |
6dbacca0 |
1254 | $| = 1; |
1255 | $pid = fork; |
1256 | if ($pid) { |
1257 | kill('INT', $pid); |
1258 | sleep(1); |
1259 | kill('INT', $pid); |
54310121 |
1260 | } else { |
6dbacca0 |
1261 | while (1) {sleep(10);} |
54310121 |
1262 | } |
1263 | |
6dbacca0 |
1264 | # perl4 (HPUX) prints: Got INT... |
1265 | # perl5 (HPUX) prints: Got INT... Got INT... |
1266 | |
1267 | =item * (SysV) |
1268 | |
54310121 |
1269 | Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does |
1270 | the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened |
6dbacca0 |
1271 | for append, it is impossible to overwrite information already in |
1272 | the file. |
1273 | |
1274 | open(TEST,">>seek.test"); |
54310121 |
1275 | $start = tell TEST ; |
6dbacca0 |
1276 | foreach(1 .. 9){ |
1277 | print TEST "$_ "; |
1278 | } |
1279 | $end = tell TEST ; |
1280 | seek(TEST,$start,0); |
1281 | print TEST "18 characters here"; |
54310121 |
1282 | |
6dbacca0 |
1283 | # perl4 (solaris) seek.test has: 18 characters here |
1284 | # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here |
a0d0e21e |
1285 | |
a0d0e21e |
1286 | |
a0d0e21e |
1287 | |
6dbacca0 |
1288 | =back |
a0d0e21e |
1289 | |
6dbacca0 |
1290 | =head2 Interpolation Traps |
a0d0e21e |
1291 | |
8b0a4b75 |
1292 | Perl4-to-Perl5 traps having to do with how things get interpolated |
1293 | within certain expressions, statements, contexts, or whatever. |
1294 | |
6dbacca0 |
1295 | =over 5 |
a0d0e21e |
1296 | |
6dbacca0 |
1297 | =item * Interpolation |
a0d0e21e |
1298 | |
6dbacca0 |
1299 | @ now always interpolates an array in double-quotish strings. |
1300 | |
54310121 |
1301 | print "To: someone@somewhere.com\n"; |
1302 | |
6dbacca0 |
1303 | # perl4 prints: To:someone@somewhere.com |
9607fc9c |
1304 | # perl5 errors : In string, @somewhere now must be written as \@somewhere |
6dbacca0 |
1305 | |
1306 | =item * Interpolation |
1307 | |
6dbacca0 |
1308 | Double-quoted strings may no longer end with an unescaped $ or @. |
1309 | |
1310 | $foo = "foo$"; |
1311 | $bar = "bar@"; |
1312 | print "foo is $foo, bar is $bar\n"; |
54310121 |
1313 | |
6dbacca0 |
1314 | # perl4 prints: foo is foo$, bar is bar@ |
1315 | # perl5 errors: Final $ should be \$ or $name |
1316 | |
1317 | Note: perl5 DOES NOT error on the terminating @ in $bar |
1318 | |
1319 | =item * Interpolation |
a0d0e21e |
1320 | |
8b0a4b75 |
1321 | Perl now sometimes evaluates arbitrary expressions inside braces that occur |
1322 | within double quotes (usually when the opening brace is preceded by C<$> |
1323 | or C<@>). |
1324 | |
1325 | @www = "buz"; |
1326 | $foo = "foo"; |
1327 | $bar = "bar"; |
1328 | sub foo { return "bar" }; |
1329 | print "|@{w.w.w}|${main'foo}|"; |
1330 | |
1331 | # perl4 prints: |@{w.w.w}|foo| |
1332 | # perl5 prints: |buz|bar| |
1333 | |
1334 | Note that you can C<use strict;> to ward off such trappiness under perl5. |
1335 | |
1336 | =item * Interpolation |
1337 | |
748a9306 |
1338 | The construct "this is $$x" used to interpolate the pid at that |
19799a22 |
1339 | point, but now apparently tries to dereference $x. C<$$> by itself still |
748a9306 |
1340 | works fine, however. |
1341 | |
6dbacca0 |
1342 | print "this is $$x\n"; |
748a9306 |
1343 | |
6dbacca0 |
1344 | # perl4 prints: this is XXXx (XXX is the current pid) |
1345 | # perl5 prints: this is |
1346 | |
1347 | =item * Interpolation |
1348 | |
54310121 |
1349 | Creation of hashes on the fly with C<eval "EXPR"> now requires either both |
1350 | C<$>'s to be protected in the specification of the hash name, or both curlies |
6dbacca0 |
1351 | to be protected. If both curlies are protected, the result will be compatible |
1352 | with perl4 and perl5. This is a very common practice, and should be changed |
1353 | to use the block form of C<eval{}> if possible. |
c07a80fd |
1354 | |
6dbacca0 |
1355 | $hashname = "foobar"; |
1356 | $key = "baz"; |
1357 | $value = 1234; |
1358 | eval "\$$hashname{'$key'} = q|$value|"; |
1359 | (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); |
1360 | |
1361 | # perl4 prints: Yup |
1362 | # perl5 prints: Nope |
1363 | |
1364 | Changing |
1365 | |
1366 | eval "\$$hashname{'$key'} = q|$value|"; |
c07a80fd |
1367 | |
1368 | to |
1369 | |
6dbacca0 |
1370 | eval "\$\$hashname{'$key'} = q|$value|"; |
c07a80fd |
1371 | |
6dbacca0 |
1372 | causes the following result: |
c07a80fd |
1373 | |
6dbacca0 |
1374 | # perl4 prints: Nope |
1375 | # perl5 prints: Yup |
c07a80fd |
1376 | |
6dbacca0 |
1377 | or, changing to |
a0d0e21e |
1378 | |
6dbacca0 |
1379 | eval "\$$hashname\{'$key'\} = q|$value|"; |
1380 | |
1381 | causes the following result: |
1382 | |
1383 | # perl4 prints: Yup |
1384 | # perl5 prints: Yup |
1385 | # and is compatible for both versions |
1386 | |
1387 | |
1388 | =item * Interpolation |
1389 | |
1390 | perl4 programs which unconsciously rely on the bugs in earlier perl versions. |
1391 | |
1392 | perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' |
54310121 |
1393 | |
6dbacca0 |
1394 | # perl4 prints: This is not perl5 |
1395 | # perl5 prints: This is perl5 |
1396 | |
1397 | =item * Interpolation |
1398 | |
54310121 |
1399 | You also have to be careful about array references. |
6dbacca0 |
1400 | |
1401 | print "$foo{" |
1402 | |
1403 | perl 4 prints: { |
1404 | perl 5 prints: syntax error |
1405 | |
1406 | =item * Interpolation |
1407 | |
1408 | Similarly, watch out for: |
1409 | |
1410 | $foo = "array"; |
1411 | print "\$$foo{bar}\n"; |
54310121 |
1412 | |
6dbacca0 |
1413 | # perl4 prints: $array{bar} |
1414 | # perl5 prints: $ |
1415 | |
1416 | Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is |
1417 | happy just to expand $foo to "array" by itself. Watch out for this |
1418 | especially in C<eval>'s. |
1419 | |
1420 | =item * Interpolation |
1421 | |
1422 | C<qq()> string passed to C<eval> |
1423 | |
1424 | eval qq( |
1425 | foreach \$y (keys %\$x\) { |
1426 | \$count++; |
1427 | } |
1428 | ); |
54310121 |
1429 | |
6dbacca0 |
1430 | # perl4 runs this ok |
54310121 |
1431 | # perl5 prints: Can't find string terminator ")" |
a0d0e21e |
1432 | |
6dbacca0 |
1433 | =back |
1434 | |
1435 | =head2 DBM Traps |
1436 | |
1437 | General DBM traps. |
1438 | |
1439 | =over 5 |
1440 | |
1441 | =item * DBM |
1442 | |
1443 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1444 | may cause the same script, run under perl5, to fail. The build of perl5 |
1445 | must have been linked with the same dbm/ndbm as the default for C<dbmopen()> |
1446 | to function properly without C<tie>'ing to an extension dbm implementation. |
1447 | |
1448 | dbmopen (%dbm, "file", undef); |
1449 | print "ok\n"; |
1450 | |
1451 | # perl4 prints: ok |
1452 | # perl5 prints: ok (IFF linked with -ldbm or -lndbm) |
1453 | |
1454 | |
1455 | =item * DBM |
1456 | |
1457 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1458 | may cause the same script, run under perl5, to fail. The error generated |
1459 | when exceeding the limit on the key/value size will cause perl5 to exit |
1460 | immediately. |
1461 | |
1462 | dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; |
1463 | $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm |
1464 | print "YUP\n"; |
1465 | |
1466 | # perl4 prints: |
1467 | dbm store returned -1, errno 28, key "trap" at - line 3. |
1468 | YUP |
1469 | |
1470 | # perl5 prints: |
1471 | dbm store returned -1, errno 28, key "trap" at - line 3. |
a0d0e21e |
1472 | |
1473 | =back |
6dbacca0 |
1474 | |
1475 | =head2 Unclassified Traps |
1476 | |
1477 | Everything else. |
1478 | |
84dc3c4d |
1479 | =over 5 |
1480 | |
5db417f7 |
1481 | =item * C<require>/C<do> trap using returned value |
6dbacca0 |
1482 | |
1483 | If the file doit.pl has: |
1484 | |
1485 | sub foo { |
1486 | $rc = do "./do.pl"; |
1487 | return 8; |
54310121 |
1488 | } |
6dbacca0 |
1489 | print &foo, "\n"; |
1490 | |
1491 | And the do.pl file has the following single line: |
1492 | |
1493 | return 3; |
1494 | |
1495 | Running doit.pl gives the following: |
1496 | |
1497 | # perl 4 prints: 3 (aborts the subroutine early) |
54310121 |
1498 | # perl 5 prints: 8 |
6dbacca0 |
1499 | |
1500 | Same behavior if you replace C<do> with C<require>. |
1501 | |
5db417f7 |
1502 | =item * C<split> on empty string with LIMIT specified |
1503 | |
1504 | $string = ''; |
1505 | @list = split(/foo/, $string, 2) |
1506 | |
1507 | Perl4 returns a one element list containing the empty string but Perl5 |
1508 | returns an empty list. |
1509 | |
6dbacca0 |
1510 | =back |
1511 | |
54310121 |
1512 | As always, if any of these are ever officially declared as bugs, |
6dbacca0 |
1513 | they'll be fixed and removed. |
1514 | |