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| 97 | . ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u' |
| 98 | . ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u' |
| 99 | . ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u' |
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| 103 | .ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V' |
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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLFAQ6 1" |
| 132 | .TH PERLFAQ6 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlfaq6 \- Regular Expressions ($Revision: 1.12 $, $Date: 2002/06/01 22:31:09 $) |
| 135 | .SH "DESCRIPTION" |
| 136 | .IX Header "DESCRIPTION" |
| 137 | This section is surprisingly small because the rest of the \s-1FAQ\s0 is |
| 138 | littered with answers involving regular expressions. For example, |
| 139 | decoding a \s-1URL\s0 and checking whether something is a number are handled |
| 140 | with regular expressions, but those answers are found elsewhere in |
| 141 | this document (in perlfaq9: ``How do I decode or create those %\-encodings |
| 142 | on the web'' and perlfaq4: ``How do I determine whether a scalar is |
| 143 | a number/whole/integer/float'', to be precise). |
| 144 | .Sh "How can I hope to use regular expressions without creating illegible and unmaintainable code?" |
| 145 | .IX Subsection "How can I hope to use regular expressions without creating illegible and unmaintainable code?" |
| 146 | Three techniques can make regular expressions maintainable and |
| 147 | understandable. |
| 148 | .IP "Comments Outside the Regex" 4 |
| 149 | .IX Item "Comments Outside the Regex" |
| 150 | Describe what you're doing and how you're doing it, using normal Perl |
| 151 | comments. |
| 152 | .Sp |
| 153 | .Vb 3 |
| 154 | \& # turn the line into the first word, a colon, and the |
| 155 | \& # number of characters on the rest of the line |
| 156 | \& s/^(\ew+)(.*)/ lc($1) . ":" . length($2) /meg; |
| 157 | .Ve |
| 158 | .IP "Comments Inside the Regex" 4 |
| 159 | .IX Item "Comments Inside the Regex" |
| 160 | The \f(CW\*(C`/x\*(C'\fR modifier causes whitespace to be ignored in a regex pattern |
| 161 | (except in a character class), and also allows you to use normal |
| 162 | comments there, too. As you can imagine, whitespace and comments help |
| 163 | a lot. |
| 164 | .Sp |
| 165 | \&\f(CW\*(C`/x\*(C'\fR lets you turn this: |
| 166 | .Sp |
| 167 | .Vb 1 |
| 168 | \& s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; |
| 169 | .Ve |
| 170 | .Sp |
| 171 | into this: |
| 172 | .Sp |
| 173 | .Vb 10 |
| 174 | \& s{ < # opening angle bracket |
| 175 | \& (?: # Non-backreffing grouping paren |
| 176 | \& [^>'"] * # 0 or more things that are neither > nor ' nor " |
| 177 | \& | # or else |
| 178 | \& ".*?" # a section between double quotes (stingy match) |
| 179 | \& | # or else |
| 180 | \& '.*?' # a section between single quotes (stingy match) |
| 181 | \& ) + # all occurring one or more times |
| 182 | \& > # closing angle bracket |
| 183 | \& }{}gsx; # replace with nothing, i.e. delete |
| 184 | .Ve |
| 185 | .Sp |
| 186 | It's still not quite so clear as prose, but it is very useful for |
| 187 | describing the meaning of each part of the pattern. |
| 188 | .IP "Different Delimiters" 4 |
| 189 | .IX Item "Different Delimiters" |
| 190 | While we normally think of patterns as being delimited with \f(CW\*(C`/\*(C'\fR |
| 191 | characters, they can be delimited by almost any character. perlre |
| 192 | describes this. For example, the \f(CW\*(C`s///\*(C'\fR above uses braces as |
| 193 | delimiters. Selecting another delimiter can avoid quoting the |
| 194 | delimiter within the pattern: |
| 195 | .Sp |
| 196 | .Vb 2 |
| 197 | \& s/\e/usr\e/local/\e/usr\e/share/g; # bad delimiter choice |
| 198 | \& s#/usr/local#/usr/share#g; # better |
| 199 | .Ve |
| 200 | .Sh "I'm having trouble matching over more than one line. What's wrong?" |
| 201 | .IX Subsection "I'm having trouble matching over more than one line. What's wrong?" |
| 202 | Either you don't have more than one line in the string you're looking |
| 203 | at (probably), or else you aren't using the correct modifier(s) on |
| 204 | your pattern (possibly). |
| 205 | .PP |
| 206 | There are many ways to get multiline data into a string. If you want |
| 207 | it to happen automatically while reading input, you'll want to set $/ |
| 208 | (probably to '' for paragraphs or \f(CW\*(C`undef\*(C'\fR for the whole file) to |
| 209 | allow you to read more than one line at a time. |
| 210 | .PP |
| 211 | Read perlre to help you decide which of \f(CW\*(C`/s\*(C'\fR and \f(CW\*(C`/m\*(C'\fR (or both) |
| 212 | you might want to use: \f(CW\*(C`/s\*(C'\fR allows dot to include newline, and \f(CW\*(C`/m\*(C'\fR |
| 213 | allows caret and dollar to match next to a newline, not just at the |
| 214 | end of the string. You do need to make sure that you've actually |
| 215 | got a multiline string in there. |
| 216 | .PP |
| 217 | For example, this program detects duplicate words, even when they span |
| 218 | line breaks (but not paragraph ones). For this example, we don't need |
| 219 | \&\f(CW\*(C`/s\*(C'\fR because we aren't using dot in a regular expression that we want |
| 220 | to cross line boundaries. Neither do we need \f(CW\*(C`/m\*(C'\fR because we aren't |
| 221 | wanting caret or dollar to match at any point inside the record next |
| 222 | to newlines. But it's imperative that $/ be set to something other |
| 223 | than the default, or else we won't actually ever have a multiline |
| 224 | record read in. |
| 225 | .PP |
| 226 | .Vb 6 |
| 227 | \& $/ = ''; # read in more whole paragraph, not just one line |
| 228 | \& while ( <> ) { |
| 229 | \& while ( /\eb([\ew'-]+)(\es+\e1)+\eb/gi ) { # word starts alpha |
| 230 | \& print "Duplicate $1 at paragraph $.\en"; |
| 231 | \& } |
| 232 | \& } |
| 233 | .Ve |
| 234 | .PP |
| 235 | Here's code that finds sentences that begin with \*(L"From \*(R" (which would |
| 236 | be mangled by many mailers): |
| 237 | .PP |
| 238 | .Vb 6 |
| 239 | \& $/ = ''; # read in more whole paragraph, not just one line |
| 240 | \& while ( <> ) { |
| 241 | \& while ( /^From /gm ) { # /m makes ^ match next to \en |
| 242 | \& print "leading from in paragraph $.\en"; |
| 243 | \& } |
| 244 | \& } |
| 245 | .Ve |
| 246 | .PP |
| 247 | Here's code that finds everything between \s-1START\s0 and \s-1END\s0 in a paragraph: |
| 248 | .PP |
| 249 | .Vb 6 |
| 250 | \& undef $/; # read in whole file, not just one line or paragraph |
| 251 | \& while ( <> ) { |
| 252 | \& while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries |
| 253 | \& print "$1\en"; |
| 254 | \& } |
| 255 | \& } |
| 256 | .Ve |
| 257 | .Sh "How can I pull out lines between two patterns that are themselves on different lines?" |
| 258 | .IX Subsection "How can I pull out lines between two patterns that are themselves on different lines?" |
| 259 | You can use Perl's somewhat exotic \f(CW\*(C`..\*(C'\fR operator (documented in |
| 260 | perlop): |
| 261 | .PP |
| 262 | .Vb 1 |
| 263 | \& perl -ne 'print if /START/ .. /END/' file1 file2 ... |
| 264 | .Ve |
| 265 | .PP |
| 266 | If you wanted text and not lines, you would use |
| 267 | .PP |
| 268 | .Vb 1 |
| 269 | \& perl -0777 -ne 'print "$1\en" while /START(.*?)END/gs' file1 file2 ... |
| 270 | .Ve |
| 271 | .PP |
| 272 | But if you want nested occurrences of \f(CW\*(C`START\*(C'\fR through \f(CW\*(C`END\*(C'\fR, you'll |
| 273 | run up against the problem described in the question in this section |
| 274 | on matching balanced text. |
| 275 | .PP |
| 276 | Here's another example of using \f(CW\*(C`..\*(C'\fR: |
| 277 | .PP |
| 278 | .Vb 7 |
| 279 | \& while (<>) { |
| 280 | \& $in_header = 1 .. /^$/; |
| 281 | \& $in_body = /^$/ .. eof(); |
| 282 | \& # now choose between them |
| 283 | \& } continue { |
| 284 | \& reset if eof(); # fix $. |
| 285 | \& } |
| 286 | .Ve |
| 287 | .Sh "I put a regular expression into $/ but it didn't work. What's wrong?" |
| 288 | .IX Subsection "I put a regular expression into $/ but it didn't work. What's wrong?" |
| 289 | $/ must be a string, not a regular expression. Awk has to be better |
| 290 | for something. :\-) |
| 291 | .PP |
| 292 | Actually, you could do this if you don't mind reading the whole file |
| 293 | into memory: |
| 294 | .PP |
| 295 | .Vb 2 |
| 296 | \& undef $/; |
| 297 | \& @records = split /your_pattern/, <FH>; |
| 298 | .Ve |
| 299 | .PP |
| 300 | The Net::Telnet module (available from \s-1CPAN\s0) has the capability to |
| 301 | wait for a pattern in the input stream, or timeout if it doesn't |
| 302 | appear within a certain time. |
| 303 | .PP |
| 304 | .Vb 4 |
| 305 | \& ## Create a file with three lines. |
| 306 | \& open FH, ">file"; |
| 307 | \& print FH "The first line\enThe second line\enThe third line\en"; |
| 308 | \& close FH; |
| 309 | .Ve |
| 310 | .PP |
| 311 | .Vb 2 |
| 312 | \& ## Get a read/write filehandle to it. |
| 313 | \& $fh = new IO::File "+<file"; |
| 314 | .Ve |
| 315 | .PP |
| 316 | .Vb 3 |
| 317 | \& ## Attach it to a "stream" object. |
| 318 | \& use Net::Telnet; |
| 319 | \& $file = new Net::Telnet (-fhopen => $fh); |
| 320 | .Ve |
| 321 | .PP |
| 322 | .Vb 3 |
| 323 | \& ## Search for the second line and print out the third. |
| 324 | \& $file->waitfor('/second line\en/'); |
| 325 | \& print $file->getline; |
| 326 | .Ve |
| 327 | .Sh "How do I substitute case insensitively on the \s-1LHS\s0 while preserving case on the \s-1RHS\s0?" |
| 328 | .IX Subsection "How do I substitute case insensitively on the LHS while preserving case on the RHS?" |
| 329 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
| 330 | properties of bitwise xor on \s-1ASCII\s0 strings. |
| 331 | .PP |
| 332 | .Vb 1 |
| 333 | \& $_= "this is a TEsT case"; |
| 334 | .Ve |
| 335 | .PP |
| 336 | .Vb 2 |
| 337 | \& $old = 'test'; |
| 338 | \& $new = 'success'; |
| 339 | .Ve |
| 340 | .PP |
| 341 | .Vb 5 |
| 342 | \& s{(\eQ$old\eE)} |
| 343 | \& { uc $new | (uc $1 ^ $1) . |
| 344 | \& (uc(substr $1, -1) ^ substr $1, -1) x |
| 345 | \& (length($new) - length $1) |
| 346 | \& }egi; |
| 347 | .Ve |
| 348 | .PP |
| 349 | .Vb 1 |
| 350 | \& print; |
| 351 | .Ve |
| 352 | .PP |
| 353 | And here it is as a subroutine, modeled after the above: |
| 354 | .PP |
| 355 | .Vb 3 |
| 356 | \& sub preserve_case($$) { |
| 357 | \& my ($old, $new) = @_; |
| 358 | \& my $mask = uc $old ^ $old; |
| 359 | .Ve |
| 360 | .PP |
| 361 | .Vb 3 |
| 362 | \& uc $new | $mask . |
| 363 | \& substr($mask, -1) x (length($new) - length($old)) |
| 364 | \& } |
| 365 | .Ve |
| 366 | .PP |
| 367 | .Vb 3 |
| 368 | \& $a = "this is a TEsT case"; |
| 369 | \& $a =~ s/(test)/preserve_case($1, "success")/egi; |
| 370 | \& print "$a\en"; |
| 371 | .Ve |
| 372 | .PP |
| 373 | This prints: |
| 374 | .PP |
| 375 | .Vb 1 |
| 376 | \& this is a SUcCESS case |
| 377 | .Ve |
| 378 | .PP |
| 379 | As an alternative, to keep the case of the replacement word if it is |
| 380 | longer than the original, you can use this code, by Jeff Pinyan: |
| 381 | .PP |
| 382 | .Vb 3 |
| 383 | \& sub preserve_case { |
| 384 | \& my ($from, $to) = @_; |
| 385 | \& my ($lf, $lt) = map length, @_; |
| 386 | .Ve |
| 387 | .PP |
| 388 | .Vb 2 |
| 389 | \& if ($lt < $lf) { $from = substr $from, 0, $lt } |
| 390 | \& else { $from .= substr $to, $lf } |
| 391 | .Ve |
| 392 | .PP |
| 393 | .Vb 2 |
| 394 | \& return uc $to | ($from ^ uc $from); |
| 395 | \& } |
| 396 | .Ve |
| 397 | .PP |
| 398 | This changes the sentence to \*(L"this is a SUcCess case.\*(R" |
| 399 | .PP |
| 400 | Just to show that C programmers can write C in any programming language, |
| 401 | if you prefer a more C\-like solution, the following script makes the |
| 402 | substitution have the same case, letter by letter, as the original. |
| 403 | (It also happens to run about 240% slower than the Perlish solution runs.) |
| 404 | If the substitution has more characters than the string being substituted, |
| 405 | the case of the last character is used for the rest of the substitution. |
| 406 | .PP |
| 407 | .Vb 8 |
| 408 | \& # Original by Nathan Torkington, massaged by Jeffrey Friedl |
| 409 | \& # |
| 410 | \& sub preserve_case($$) |
| 411 | \& { |
| 412 | \& my ($old, $new) = @_; |
| 413 | \& my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc |
| 414 | \& my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); |
| 415 | \& my ($len) = $oldlen < $newlen ? $oldlen : $newlen; |
| 416 | .Ve |
| 417 | .PP |
| 418 | .Vb 21 |
| 419 | \& for ($i = 0; $i < $len; $i++) { |
| 420 | \& if ($c = substr($old, $i, 1), $c =~ /[\eW\ed_]/) { |
| 421 | \& $state = 0; |
| 422 | \& } elsif (lc $c eq $c) { |
| 423 | \& substr($new, $i, 1) = lc(substr($new, $i, 1)); |
| 424 | \& $state = 1; |
| 425 | \& } else { |
| 426 | \& substr($new, $i, 1) = uc(substr($new, $i, 1)); |
| 427 | \& $state = 2; |
| 428 | \& } |
| 429 | \& } |
| 430 | \& # finish up with any remaining new (for when new is longer than old) |
| 431 | \& if ($newlen > $oldlen) { |
| 432 | \& if ($state == 1) { |
| 433 | \& substr($new, $oldlen) = lc(substr($new, $oldlen)); |
| 434 | \& } elsif ($state == 2) { |
| 435 | \& substr($new, $oldlen) = uc(substr($new, $oldlen)); |
| 436 | \& } |
| 437 | \& } |
| 438 | \& return $new; |
| 439 | \& } |
| 440 | .Ve |
| 441 | .ie n .Sh "How can I make ""\ew"" match national character sets?" |
| 442 | .el .Sh "How can I make \f(CW\ew\fP match national character sets?" |
| 443 | .IX Subsection "How can I make w match national character sets?" |
| 444 | See perllocale. |
| 445 | .ie n .Sh "How can I match a locale-smart version of ""/[a\-zA\-Z]/""?" |
| 446 | .el .Sh "How can I match a locale-smart version of \f(CW/[a\-zA\-Z]/\fP?" |
| 447 | .IX Subsection "How can I match a locale-smart version of /[a-zA-Z]/?" |
| 448 | One alphabetic character would be \f(CW\*(C`/[^\eW\ed_]/\*(C'\fR, no matter what locale |
| 449 | you're in. Non-alphabetics would be \f(CW\*(C`/[\eW\ed_]/\*(C'\fR (assuming you don't |
| 450 | consider an underscore a letter). |
| 451 | .Sh "How can I quote a variable to use in a regex?" |
| 452 | .IX Subsection "How can I quote a variable to use in a regex?" |
| 453 | The Perl parser will expand \f(CW$variable\fR and \f(CW@variable\fR references in |
| 454 | regular expressions unless the delimiter is a single quote. Remember, |
| 455 | too, that the right-hand side of a \f(CW\*(C`s///\*(C'\fR substitution is considered |
| 456 | a double-quoted string (see perlop for more details). Remember |
| 457 | also that any regex special characters will be acted on unless you |
| 458 | precede the substitution with \eQ. Here's an example: |
| 459 | .PP |
| 460 | .Vb 3 |
| 461 | \& $string = "to die?"; |
| 462 | \& $lhs = "die?"; |
| 463 | \& $rhs = "sleep, no more"; |
| 464 | .Ve |
| 465 | .PP |
| 466 | .Vb 2 |
| 467 | \& $string =~ s/\eQ$lhs/$rhs/; |
| 468 | \& # $string is now "to sleep no more" |
| 469 | .Ve |
| 470 | .PP |
| 471 | Without the \eQ, the regex would also spuriously match \*(L"di\*(R". |
| 472 | .ie n .Sh "What is ""/o"" really for?" |
| 473 | .el .Sh "What is \f(CW/o\fP really for?" |
| 474 | .IX Subsection "What is /o really for?" |
| 475 | Using a variable in a regular expression match forces a re-evaluation |
| 476 | (and perhaps recompilation) each time the regular expression is |
| 477 | encountered. The \f(CW\*(C`/o\*(C'\fR modifier locks in the regex the first time |
| 478 | it's used. This always happens in a constant regular expression, and |
| 479 | in fact, the pattern was compiled into the internal format at the same |
| 480 | time your entire program was. |
| 481 | .PP |
| 482 | Use of \f(CW\*(C`/o\*(C'\fR is irrelevant unless variable interpolation is used in |
| 483 | the pattern, and if so, the regex engine will neither know nor care |
| 484 | whether the variables change after the pattern is evaluated the \fIvery |
| 485 | first\fR time. |
| 486 | .PP |
| 487 | \&\f(CW\*(C`/o\*(C'\fR is often used to gain an extra measure of efficiency by not |
| 488 | performing subsequent evaluations when you know it won't matter |
| 489 | (because you know the variables won't change), or more rarely, when |
| 490 | you don't want the regex to notice if they do. |
| 491 | .PP |
| 492 | For example, here's a \*(L"paragrep\*(R" program: |
| 493 | .PP |
| 494 | .Vb 5 |
| 495 | \& $/ = ''; # paragraph mode |
| 496 | \& $pat = shift; |
| 497 | \& while (<>) { |
| 498 | \& print if /$pat/o; |
| 499 | \& } |
| 500 | .Ve |
| 501 | .Sh "How do I use a regular expression to strip C style comments from a file?" |
| 502 | .IX Subsection "How do I use a regular expression to strip C style comments from a file?" |
| 503 | While this actually can be done, it's much harder than you'd think. |
| 504 | For example, this one-liner |
| 505 | .PP |
| 506 | .Vb 1 |
| 507 | \& perl -0777 -pe 's{/\e*.*?\e*/}{}gs' foo.c |
| 508 | .Ve |
| 509 | .PP |
| 510 | will work in many but not all cases. You see, it's too simple-minded for |
| 511 | certain kinds of C programs, in particular, those with what appear to be |
| 512 | comments in quoted strings. For that, you'd need something like this, |
| 513 | created by Jeffrey Friedl and later modified by Fred Curtis. |
| 514 | .PP |
| 515 | .Vb 4 |
| 516 | \& $/ = undef; |
| 517 | \& $_ = <>; |
| 518 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#$2#gs |
| 519 | \& print; |
| 520 | .Ve |
| 521 | .PP |
| 522 | This could, of course, be more legibly written with the \f(CW\*(C`/x\*(C'\fR modifier, adding |
| 523 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
| 524 | .PP |
| 525 | .Vb 8 |
| 526 | \& s{ |
| 527 | \& /\e* ## Start of /* ... */ comment |
| 528 | \& [^*]*\e*+ ## Non-* followed by 1-or-more *'s |
| 529 | \& ( |
| 530 | \& [^/*][^*]*\e*+ |
| 531 | \& )* ## 0-or-more things which don't start with / |
| 532 | \& ## but do end with '*' |
| 533 | \& / ## End of /* ... */ comment |
| 534 | .Ve |
| 535 | .PP |
| 536 | .Vb 1 |
| 537 | \& | ## OR various things which aren't comments: |
| 538 | .Ve |
| 539 | .PP |
| 540 | .Vb 8 |
| 541 | \& ( |
| 542 | \& " ## Start of " ... " string |
| 543 | \& ( |
| 544 | \& \e\e. ## Escaped char |
| 545 | \& | ## OR |
| 546 | \& [^"\e\e] ## Non "\e |
| 547 | \& )* |
| 548 | \& " ## End of " ... " string |
| 549 | .Ve |
| 550 | .PP |
| 551 | .Vb 1 |
| 552 | \& | ## OR |
| 553 | .Ve |
| 554 | .PP |
| 555 | .Vb 7 |
| 556 | \& ' ## Start of ' ... ' string |
| 557 | \& ( |
| 558 | \& \e\e. ## Escaped char |
| 559 | \& | ## OR |
| 560 | \& [^'\e\e] ## Non '\e |
| 561 | \& )* |
| 562 | \& ' ## End of ' ... ' string |
| 563 | .Ve |
| 564 | .PP |
| 565 | .Vb 1 |
| 566 | \& | ## OR |
| 567 | .Ve |
| 568 | .PP |
| 569 | .Vb 4 |
| 570 | \& . ## Anything other char |
| 571 | \& [^/"'\e\e]* ## Chars which doesn't start a comment, string or escape |
| 572 | \& ) |
| 573 | \& }{$2}gxs; |
| 574 | .Ve |
| 575 | .PP |
| 576 | A slight modification also removes \*(C+ comments: |
| 577 | .PP |
| 578 | .Vb 1 |
| 579 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|//[^\en]*|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#$2#gs; |
| 580 | .Ve |
| 581 | .Sh "Can I use Perl regular expressions to match balanced text?" |
| 582 | .IX Subsection "Can I use Perl regular expressions to match balanced text?" |
| 583 | Historically, Perl regular expressions were not capable of matching |
| 584 | balanced text. As of more recent versions of perl including 5.6.1 |
| 585 | experimental features have been added that make it possible to do this. |
| 586 | Look at the documentation for the (??{ }) construct in recent perlre manual |
| 587 | pages to see an example of matching balanced parentheses. Be sure to take |
| 588 | special notice of the warnings present in the manual before making use |
| 589 | of this feature. |
| 590 | .PP |
| 591 | \&\s-1CPAN\s0 contains many modules that can be useful for matching text |
| 592 | depending on the context. Damian Conway provides some useful |
| 593 | patterns in Regexp::Common. The module Text::Balanced provides a |
| 594 | general solution to this problem. |
| 595 | .PP |
| 596 | One of the common applications of balanced text matching is working |
| 597 | with \s-1XML\s0 and \s-1HTML\s0. There are many modules available that support |
| 598 | these needs. Two examples are HTML::Parser and XML::Parser. There |
| 599 | are many others. |
| 600 | .PP |
| 601 | An elaborate subroutine (for 7\-bit \s-1ASCII\s0 only) to pull out balanced |
| 602 | and possibly nested single chars, like \f(CW\*(C``\*(C'\fR and \f(CW\*(C`'\*(C'\fR, \f(CW\*(C`{\*(C'\fR and \f(CW\*(C`}\*(C'\fR, |
| 603 | or \f(CW\*(C`(\*(C'\fR and \f(CW\*(C`)\*(C'\fR can be found in |
| 604 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . |
| 605 | .PP |
| 606 | The C::Scan module from \s-1CPAN\s0 also contains such subs for internal use, |
| 607 | but they are undocumented. |
| 608 | .Sh "What does it mean that regexes are greedy? How can I get around it?" |
| 609 | .IX Subsection "What does it mean that regexes are greedy? How can I get around it?" |
| 610 | Most people mean that greedy regexes match as much as they can. |
| 611 | Technically speaking, it's actually the quantifiers (\f(CW\*(C`?\*(C'\fR, \f(CW\*(C`*\*(C'\fR, \f(CW\*(C`+\*(C'\fR, |
| 612 | \&\f(CW\*(C`{}\*(C'\fR) that are greedy rather than the whole pattern; Perl prefers local |
| 613 | greed and immediate gratification to overall greed. To get non-greedy |
| 614 | versions of the same quantifiers, use (\f(CW\*(C`??\*(C'\fR, \f(CW\*(C`*?\*(C'\fR, \f(CW\*(C`+?\*(C'\fR, \f(CW\*(C`{}?\*(C'\fR). |
| 615 | .PP |
| 616 | An example: |
| 617 | .PP |
| 618 | .Vb 3 |
| 619 | \& $s1 = $s2 = "I am very very cold"; |
| 620 | \& $s1 =~ s/ve.*y //; # I am cold |
| 621 | \& $s2 =~ s/ve.*?y //; # I am very cold |
| 622 | .Ve |
| 623 | .PP |
| 624 | Notice how the second substitution stopped matching as soon as it |
| 625 | encountered \*(L"y \*(R". The \f(CW\*(C`*?\*(C'\fR quantifier effectively tells the regular |
| 626 | expression engine to find a match as quickly as possible and pass |
| 627 | control on to whatever is next in line, like you would if you were |
| 628 | playing hot potato. |
| 629 | .Sh "How do I process each word on each line?" |
| 630 | .IX Subsection "How do I process each word on each line?" |
| 631 | Use the split function: |
| 632 | .PP |
| 633 | .Vb 5 |
| 634 | \& while (<>) { |
| 635 | \& foreach $word ( split ) { |
| 636 | \& # do something with $word here |
| 637 | \& } |
| 638 | \& } |
| 639 | .Ve |
| 640 | .PP |
| 641 | Note that this isn't really a word in the English sense; it's just |
| 642 | chunks of consecutive non-whitespace characters. |
| 643 | .PP |
| 644 | To work with only alphanumeric sequences (including underscores), you |
| 645 | might consider |
| 646 | .PP |
| 647 | .Vb 5 |
| 648 | \& while (<>) { |
| 649 | \& foreach $word (m/(\ew+)/g) { |
| 650 | \& # do something with $word here |
| 651 | \& } |
| 652 | \& } |
| 653 | .Ve |
| 654 | .Sh "How can I print out a word-frequency or line-frequency summary?" |
| 655 | .IX Subsection "How can I print out a word-frequency or line-frequency summary?" |
| 656 | To do this, you have to parse out each word in the input stream. We'll |
| 657 | pretend that by word you mean chunk of alphabetics, hyphens, or |
| 658 | apostrophes, rather than the non-whitespace chunk idea of a word given |
| 659 | in the previous question: |
| 660 | .PP |
| 661 | .Vb 8 |
| 662 | \& while (<>) { |
| 663 | \& while ( /(\eb[^\eW_\ed][\ew'-]+\eb)/g ) { # misses "`sheep'" |
| 664 | \& $seen{$1}++; |
| 665 | \& } |
| 666 | \& } |
| 667 | \& while ( ($word, $count) = each %seen ) { |
| 668 | \& print "$count $word\en"; |
| 669 | \& } |
| 670 | .Ve |
| 671 | .PP |
| 672 | If you wanted to do the same thing for lines, you wouldn't need a |
| 673 | regular expression: |
| 674 | .PP |
| 675 | .Vb 6 |
| 676 | \& while (<>) { |
| 677 | \& $seen{$_}++; |
| 678 | \& } |
| 679 | \& while ( ($line, $count) = each %seen ) { |
| 680 | \& print "$count $line"; |
| 681 | \& } |
| 682 | .Ve |
| 683 | .PP |
| 684 | If you want these output in a sorted order, see perlfaq4: ``How do I |
| 685 | sort a hash (optionally by value instead of key)?''. |
| 686 | .Sh "How can I do approximate matching?" |
| 687 | .IX Subsection "How can I do approximate matching?" |
| 688 | See the module String::Approx available from \s-1CPAN\s0. |
| 689 | .Sh "How do I efficiently match many regular expressions at once?" |
| 690 | .IX Subsection "How do I efficiently match many regular expressions at once?" |
| 691 | The following is extremely inefficient: |
| 692 | .PP |
| 693 | .Vb 10 |
| 694 | \& # slow but obvious way |
| 695 | \& @popstates = qw(CO ON MI WI MN); |
| 696 | \& while (defined($line = <>)) { |
| 697 | \& for $state (@popstates) { |
| 698 | \& if ($line =~ /\eb$state\eb/i) { |
| 699 | \& print $line; |
| 700 | \& last; |
| 701 | \& } |
| 702 | \& } |
| 703 | \& } |
| 704 | .Ve |
| 705 | .PP |
| 706 | That's because Perl has to recompile all those patterns for each of |
| 707 | the lines of the file. As of the 5.005 release, there's a much better |
| 708 | approach, one which makes use of the new \f(CW\*(C`qr//\*(C'\fR operator: |
| 709 | .PP |
| 710 | .Vb 9 |
| 711 | \& # use spiffy new qr// operator, with /i flag even |
| 712 | \& use 5.005; |
| 713 | \& @popstates = qw(CO ON MI WI MN); |
| 714 | \& @poppats = map { qr/\eb$_\eb/i } @popstates; |
| 715 | \& while (defined($line = <>)) { |
| 716 | \& for $patobj (@poppats) { |
| 717 | \& print $line if $line =~ /$patobj/; |
| 718 | \& } |
| 719 | \& } |
| 720 | .Ve |
| 721 | .ie n .Sh "Why don't word-boundary searches with ""\eb"" work for me?" |
| 722 | .el .Sh "Why don't word-boundary searches with \f(CW\eb\fP work for me?" |
| 723 | .IX Subsection "Why don't word-boundary searches with b work for me?" |
| 724 | Two common misconceptions are that \f(CW\*(C`\eb\*(C'\fR is a synonym for \f(CW\*(C`\es+\*(C'\fR and |
| 725 | that it's the edge between whitespace characters and non-whitespace |
| 726 | characters. Neither is correct. \f(CW\*(C`\eb\*(C'\fR is the place between a \f(CW\*(C`\ew\*(C'\fR |
| 727 | character and a \f(CW\*(C`\eW\*(C'\fR character (that is, \f(CW\*(C`\eb\*(C'\fR is the edge of a |
| 728 | \&\*(L"word\*(R"). It's a zero-width assertion, just like \f(CW\*(C`^\*(C'\fR, \f(CW\*(C`$\*(C'\fR, and all |
| 729 | the other anchors, so it doesn't consume any characters. perlre |
| 730 | describes the behavior of all the regex metacharacters. |
| 731 | .PP |
| 732 | Here are examples of the incorrect application of \f(CW\*(C`\eb\*(C'\fR, with fixes: |
| 733 | .PP |
| 734 | .Vb 2 |
| 735 | \& "two words" =~ /(\ew+)\eb(\ew+)/; # WRONG |
| 736 | \& "two words" =~ /(\ew+)\es+(\ew+)/; # right |
| 737 | .Ve |
| 738 | .PP |
| 739 | .Vb 2 |
| 740 | \& " =matchless= text" =~ /\eb=(\ew+)=\eb/; # WRONG |
| 741 | \& " =matchless= text" =~ /=(\ew+)=/; # right |
| 742 | .Ve |
| 743 | .PP |
| 744 | Although they may not do what you thought they did, \f(CW\*(C`\eb\*(C'\fR and \f(CW\*(C`\eB\*(C'\fR |
| 745 | can still be quite useful. For an example of the correct use of |
| 746 | \&\f(CW\*(C`\eb\*(C'\fR, see the example of matching duplicate words over multiple |
| 747 | lines. |
| 748 | .PP |
| 749 | An example of using \f(CW\*(C`\eB\*(C'\fR is the pattern \f(CW\*(C`\eBis\eB\*(C'\fR. This will find |
| 750 | occurrences of \*(L"is\*(R" on the insides of words only, as in \*(L"thistle\*(R", but |
| 751 | not \*(L"this\*(R" or \*(L"island\*(R". |
| 752 | .Sh "Why does using $&, $`, or $' slow my program down?" |
| 753 | .IX Subsection "Why does using $&, $`, or $' slow my program down?" |
| 754 | Once Perl sees that you need one of these variables anywhere in |
| 755 | the program, it provides them on each and every pattern match. |
| 756 | The same mechanism that handles these provides for the use of \f(CW$1\fR, \f(CW$2\fR, |
| 757 | etc., so you pay the same price for each regex that contains capturing |
| 758 | parentheses. If you never use $&, etc., in your script, then regexes |
| 759 | \&\fIwithout\fR capturing parentheses won't be penalized. So avoid $&, $', |
| 760 | and $` if you can, but if you can't, once you've used them at all, use |
| 761 | them at will because you've already paid the price. Remember that some |
| 762 | algorithms really appreciate them. As of the 5.005 release. the $& |
| 763 | variable is no longer \*(L"expensive\*(R" the way the other two are. |
| 764 | .ie n .Sh "What good is ""\eG"" in a regular expression?" |
| 765 | .el .Sh "What good is \f(CW\eG\fP in a regular expression?" |
| 766 | .IX Subsection "What good is G in a regular expression?" |
| 767 | The notation \f(CW\*(C`\eG\*(C'\fR is used in a match or substitution in conjunction with |
| 768 | the \f(CW\*(C`/g\*(C'\fR modifier to anchor the regular expression to the point just past |
| 769 | where the last match occurred, i.e. the \fIpos()\fR point. A failed match resets |
| 770 | the position of \f(CW\*(C`\eG\*(C'\fR unless the \f(CW\*(C`/c\*(C'\fR modifier is in effect. \f(CW\*(C`\eG\*(C'\fR can be |
| 771 | used in a match without the \f(CW\*(C`/g\*(C'\fR modifier; it acts the same (i.e. still |
| 772 | anchors at the \fIpos()\fR point) but of course only matches once and does not |
| 773 | update \fIpos()\fR, as non\-\f(CW\*(C`/g\*(C'\fR expressions never do. \f(CW\*(C`\eG\*(C'\fR in an expression |
| 774 | applied to a target string that has never been matched against a \f(CW\*(C`/g\*(C'\fR |
| 775 | expression before or has had its \fIpos()\fR reset is functionally equivalent to |
| 776 | \&\f(CW\*(C`\eA\*(C'\fR, which matches at the beginning of the string. |
| 777 | .PP |
| 778 | For example, suppose you had a line of text quoted in standard mail |
| 779 | and Usenet notation, (that is, with leading \f(CW\*(C`>\*(C'\fR characters), and |
| 780 | you want change each leading \f(CW\*(C`>\*(C'\fR into a corresponding \f(CW\*(C`:\*(C'\fR. You |
| 781 | could do so in this way: |
| 782 | .PP |
| 783 | .Vb 1 |
| 784 | \& s/^(>+)/':' x length($1)/gem; |
| 785 | .Ve |
| 786 | .PP |
| 787 | Or, using \f(CW\*(C`\eG\*(C'\fR, the much simpler (and faster): |
| 788 | .PP |
| 789 | .Vb 1 |
| 790 | \& s/\eG>/:/g; |
| 791 | .Ve |
| 792 | .PP |
| 793 | A more sophisticated use might involve a tokenizer. The following |
| 794 | lex-like example is courtesy of Jeffrey Friedl. It did not work in |
| 795 | 5.003 due to bugs in that release, but does work in 5.004 or better. |
| 796 | (Note the use of \f(CW\*(C`/c\*(C'\fR, which prevents a failed match with \f(CW\*(C`/g\*(C'\fR from |
| 797 | resetting the search position back to the beginning of the string.) |
| 798 | .PP |
| 799 | .Vb 9 |
| 800 | \& while (<>) { |
| 801 | \& chomp; |
| 802 | \& PARSER: { |
| 803 | \& m/ \eG( \ed+\eb )/gcx && do { print "number: $1\en"; redo; }; |
| 804 | \& m/ \eG( \ew+ )/gcx && do { print "word: $1\en"; redo; }; |
| 805 | \& m/ \eG( \es+ )/gcx && do { print "space: $1\en"; redo; }; |
| 806 | \& m/ \eG( [^\ew\ed]+ )/gcx && do { print "other: $1\en"; redo; }; |
| 807 | \& } |
| 808 | \& } |
| 809 | .Ve |
| 810 | .PP |
| 811 | Of course, that could have been written as |
| 812 | .PP |
| 813 | .Vb 21 |
| 814 | \& while (<>) { |
| 815 | \& chomp; |
| 816 | \& PARSER: { |
| 817 | \& if ( /\eG( \ed+\eb )/gcx { |
| 818 | \& print "number: $1\en"; |
| 819 | \& redo PARSER; |
| 820 | \& } |
| 821 | \& if ( /\eG( \ew+ )/gcx { |
| 822 | \& print "word: $1\en"; |
| 823 | \& redo PARSER; |
| 824 | \& } |
| 825 | \& if ( /\eG( \es+ )/gcx { |
| 826 | \& print "space: $1\en"; |
| 827 | \& redo PARSER; |
| 828 | \& } |
| 829 | \& if ( /\eG( [^\ew\ed]+ )/gcx { |
| 830 | \& print "other: $1\en"; |
| 831 | \& redo PARSER; |
| 832 | \& } |
| 833 | \& } |
| 834 | \& } |
| 835 | .Ve |
| 836 | .PP |
| 837 | but then you lose the vertical alignment of the regular expressions. |
| 838 | .Sh "Are Perl regexes DFAs or NFAs? Are they \s-1POSIX\s0 compliant?" |
| 839 | .IX Subsection "Are Perl regexes DFAs or NFAs? Are they POSIX compliant?" |
| 840 | While it's true that Perl's regular expressions resemble the DFAs |
| 841 | (deterministic finite automata) of the \fIegrep\fR\|(1) program, they are in |
| 842 | fact implemented as NFAs (non\-deterministic finite automata) to allow |
| 843 | backtracking and backreferencing. And they aren't POSIX-style either, |
| 844 | because those guarantee worst-case behavior for all cases. (It seems |
| 845 | that some people prefer guarantees of consistency, even when what's |
| 846 | guaranteed is slowness.) See the book \*(L"Mastering Regular Expressions\*(R" |
| 847 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever |
| 848 | hope to know on these matters (a full citation appears in |
| 849 | perlfaq2). |
| 850 | .Sh "What's wrong with using grep or map in a void context?" |
| 851 | .IX Subsection "What's wrong with using grep or map in a void context?" |
| 852 | The problem is that both grep and map build a return list, |
| 853 | regardless of the context. This means you're making Perl go |
| 854 | to the trouble of building a list that you then just throw away. |
| 855 | If the list is large, you waste both time and space. If your |
| 856 | intent is to iterate over the list then use a for loop for this |
| 857 | purpose. |
| 858 | .Sh "How can I match strings with multibyte characters?" |
| 859 | .IX Subsection "How can I match strings with multibyte characters?" |
| 860 | Starting from Perl 5.6 Perl has had some level of multibyte character |
| 861 | support. Perl 5.8 or later is recommended. Supported multibyte |
| 862 | character repertoires include Unicode, and legacy encodings |
| 863 | through the Encode module. See perluniintro, perlunicode, |
| 864 | and Encode. |
| 865 | .PP |
| 866 | If you are stuck with older Perls, you can do Unicode with the |
| 867 | \&\f(CW\*(C`Unicode::String\*(C'\fR module, and character conversions using the |
| 868 | \&\f(CW\*(C`Unicode::Map8\*(C'\fR and \f(CW\*(C`Unicode::Map\*(C'\fR modules. If you are using |
| 869 | Japanese encodings, you might try using the jperl 5.005_03. |
| 870 | .PP |
| 871 | Finally, the following set of approaches was offered by Jeffrey |
| 872 | Friedl, whose article in issue #5 of The Perl Journal talks about |
| 873 | this very matter. |
| 874 | .PP |
| 875 | Let's suppose you have some weird Martian encoding where pairs of |
| 876 | \&\s-1ASCII\s0 uppercase letters encode single Martian letters (i.e. the two |
| 877 | bytes \*(L"\s-1CV\s0\*(R" make a single Martian letter, as do the two bytes \*(L"\s-1SG\s0\*(R", |
| 878 | \&\*(L"\s-1VS\s0\*(R", \*(L"\s-1XX\s0\*(R", etc.). Other bytes represent single characters, just like |
| 879 | \&\s-1ASCII\s0. |
| 880 | .PP |
| 881 | So, the string of Martian \*(L"I am \s-1CVSGXX\s0!\*(R" uses 12 bytes to encode the |
| 882 | nine characters 'I', ' ', 'a', 'm', ' ', '\s-1CV\s0', '\s-1SG\s0', '\s-1XX\s0', '!'. |
| 883 | .PP |
| 884 | Now, say you want to search for the single character \f(CW\*(C`/GX/\*(C'\fR. Perl |
| 885 | doesn't know about Martian, so it'll find the two bytes \*(L"\s-1GX\s0\*(R" in the \*(L"I |
| 886 | am \s-1CVSGXX\s0!\*(R" string, even though that character isn't there: it just |
| 887 | looks like it is because \*(L"\s-1SG\s0\*(R" is next to \*(L"\s-1XX\s0\*(R", but there's no real |
| 888 | \&\*(L"\s-1GX\s0\*(R". This is a big problem. |
| 889 | .PP |
| 890 | Here are a few ways, all painful, to deal with it: |
| 891 | .PP |
| 892 | .Vb 3 |
| 893 | \& $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes |
| 894 | \& # are no longer adjacent. |
| 895 | \& print "found GX!\en" if $martian =~ /GX/; |
| 896 | .Ve |
| 897 | .PP |
| 898 | Or like this: |
| 899 | .PP |
| 900 | .Vb 6 |
| 901 | \& @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
| 902 | \& # above is conceptually similar to: @chars = $text =~ m/(.)/g; |
| 903 | \& # |
| 904 | \& foreach $char (@chars) { |
| 905 | \& print "found GX!\en", last if $char eq 'GX'; |
| 906 | \& } |
| 907 | .Ve |
| 908 | .PP |
| 909 | Or like this: |
| 910 | .PP |
| 911 | .Vb 3 |
| 912 | \& while ($martian =~ m/\eG([A-Z][A-Z]|.)/gs) { # \eG probably unneeded |
| 913 | \& print "found GX!\en", last if $1 eq 'GX'; |
| 914 | \& } |
| 915 | .Ve |
| 916 | .PP |
| 917 | Or like this: |
| 918 | .PP |
| 919 | .Vb 1 |
| 920 | \& die "sorry, Perl doesn't (yet) have Martian support )-:\en"; |
| 921 | .Ve |
| 922 | .PP |
| 923 | There are many double\- (and multi\-) byte encodings commonly used these |
| 924 | days. Some versions of these have 1\-, 2\-, 3\-, and 4\-byte characters, |
| 925 | all mixed. |
| 926 | .Sh "How do I match a pattern that is supplied by the user?" |
| 927 | .IX Subsection "How do I match a pattern that is supplied by the user?" |
| 928 | Well, if it's really a pattern, then just use |
| 929 | .PP |
| 930 | .Vb 2 |
| 931 | \& chomp($pattern = <STDIN>); |
| 932 | \& if ($line =~ /$pattern/) { } |
| 933 | .Ve |
| 934 | .PP |
| 935 | Alternatively, since you have no guarantee that your user entered |
| 936 | a valid regular expression, trap the exception this way: |
| 937 | .PP |
| 938 | .Vb 1 |
| 939 | \& if (eval { $line =~ /$pattern/ }) { } |
| 940 | .Ve |
| 941 | .PP |
| 942 | If all you really want to search for a string, not a pattern, |
| 943 | then you should either use the \fIindex()\fR function, which is made for |
| 944 | string searching, or if you can't be disabused of using a pattern |
| 945 | match on a non\-pattern, then be sure to use \f(CW\*(C`\eQ\*(C'\fR...\f(CW\*(C`\eE\*(C'\fR, documented |
| 946 | in perlre. |
| 947 | .PP |
| 948 | .Vb 1 |
| 949 | \& $pattern = <STDIN>; |
| 950 | .Ve |
| 951 | .PP |
| 952 | .Vb 5 |
| 953 | \& open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; |
| 954 | \& while (<FILE>) { |
| 955 | \& print if /\eQ$pattern\eE/; |
| 956 | \& } |
| 957 | \& close FILE; |
| 958 | .Ve |
| 959 | .SH "AUTHOR AND COPYRIGHT" |
| 960 | .IX Header "AUTHOR AND COPYRIGHT" |
| 961 | Copyright (c) 1997\-2002 Tom Christiansen and Nathan Torkington. |
| 962 | All rights reserved. |
| 963 | .PP |
| 964 | This documentation is free; you can redistribute it and/or modify it |
| 965 | under the same terms as Perl itself. |
| 966 | .PP |
| 967 | Irrespective of its distribution, all code examples in this file |
| 968 | are hereby placed into the public domain. You are permitted and |
| 969 | encouraged to use this code in your own programs for fun |
| 970 | or for profit as you see fit. A simple comment in the code giving |
| 971 | credit would be courteous but is not required. |