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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 "2006-01-07" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlfaq6 \- Regular Expressions ($Revision: 1.38 $, $Date: 2005/12/31 00:54:37 $) |
| 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: \*(L"How do I decode or create those %\-encodings |
| 142 | on the web\*(R" and perlfaq4: \*(L"How do I determine whether a scalar is |
| 143 | a number/whole/integer/float\*(R", to be precise). |
| 144 | .Sh "How can I hope to use regular expressions without creating illegible and unmaintainable code?" |
| 145 | .IX Xref "regex, legibility regexp, legibility regular expression, legibility x" |
| 146 | .IX Subsection "How can I hope to use regular expressions without creating illegible and unmaintainable code?" |
| 147 | Three techniques can make regular expressions maintainable and |
| 148 | understandable. |
| 149 | .IP "Comments Outside the Regex" 4 |
| 150 | .IX Item "Comments Outside the Regex" |
| 151 | Describe what you're doing and how you're doing it, using normal Perl |
| 152 | comments. |
| 153 | .Sp |
| 154 | .Vb 3 |
| 155 | \& # turn the line into the first word, a colon, and the |
| 156 | \& # number of characters on the rest of the line |
| 157 | \& s/^(\ew+)(.*)/ lc($1) . ":" . length($2) /meg; |
| 158 | .Ve |
| 159 | .IP "Comments Inside the Regex" 4 |
| 160 | .IX Item "Comments Inside the Regex" |
| 161 | The \f(CW\*(C`/x\*(C'\fR modifier causes whitespace to be ignored in a regex pattern |
| 162 | (except in a character class), and also allows you to use normal |
| 163 | comments there, too. As you can imagine, whitespace and comments help |
| 164 | a lot. |
| 165 | .Sp |
| 166 | \&\f(CW\*(C`/x\*(C'\fR lets you turn this: |
| 167 | .Sp |
| 168 | .Vb 1 |
| 169 | \& s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; |
| 170 | .Ve |
| 171 | .Sp |
| 172 | into this: |
| 173 | .Sp |
| 174 | .Vb 10 |
| 175 | \& s{ < # opening angle bracket |
| 176 | \& (?: # Non-backreffing grouping paren |
| 177 | \& [^>'"] * # 0 or more things that are neither > nor ' nor " |
| 178 | \& | # or else |
| 179 | \& ".*?" # a section between double quotes (stingy match) |
| 180 | \& | # or else |
| 181 | \& '.*?' # a section between single quotes (stingy match) |
| 182 | \& ) + # all occurring one or more times |
| 183 | \& > # closing angle bracket |
| 184 | \& }{}gsx; # replace with nothing, i.e. delete |
| 185 | .Ve |
| 186 | .Sp |
| 187 | It's still not quite so clear as prose, but it is very useful for |
| 188 | describing the meaning of each part of the pattern. |
| 189 | .IP "Different Delimiters" 4 |
| 190 | .IX Item "Different Delimiters" |
| 191 | While we normally think of patterns as being delimited with \f(CW\*(C`/\*(C'\fR |
| 192 | characters, they can be delimited by almost any character. perlre |
| 193 | describes this. For example, the \f(CW\*(C`s///\*(C'\fR above uses braces as |
| 194 | delimiters. Selecting another delimiter can avoid quoting the |
| 195 | delimiter within the pattern: |
| 196 | .Sp |
| 197 | .Vb 2 |
| 198 | \& s/\e/usr\e/local/\e/usr\e/share/g; # bad delimiter choice |
| 199 | \& s#/usr/local#/usr/share#g; # better |
| 200 | .Ve |
| 201 | .Sh "I'm having trouble matching over more than one line. What's wrong?" |
| 202 | .IX Xref "regex, multiline regexp, multiline regular expression, multiline" |
| 203 | .IX Subsection "I'm having trouble matching over more than one line. What's wrong?" |
| 204 | Either you don't have more than one line in the string you're looking |
| 205 | at (probably), or else you aren't using the correct modifier(s) on |
| 206 | your pattern (possibly). |
| 207 | .PP |
| 208 | There are many ways to get multiline data into a string. If you want |
| 209 | it to happen automatically while reading input, you'll want to set $/ |
| 210 | (probably to '' for paragraphs or \f(CW\*(C`undef\*(C'\fR for the whole file) to |
| 211 | allow you to read more than one line at a time. |
| 212 | .PP |
| 213 | Read perlre to help you decide which of \f(CW\*(C`/s\*(C'\fR and \f(CW\*(C`/m\*(C'\fR (or both) |
| 214 | you might want to use: \f(CW\*(C`/s\*(C'\fR allows dot to include newline, and \f(CW\*(C`/m\*(C'\fR |
| 215 | allows caret and dollar to match next to a newline, not just at the |
| 216 | end of the string. You do need to make sure that you've actually |
| 217 | got a multiline string in there. |
| 218 | .PP |
| 219 | For example, this program detects duplicate words, even when they span |
| 220 | line breaks (but not paragraph ones). For this example, we don't need |
| 221 | \&\f(CW\*(C`/s\*(C'\fR because we aren't using dot in a regular expression that we want |
| 222 | to cross line boundaries. Neither do we need \f(CW\*(C`/m\*(C'\fR because we aren't |
| 223 | wanting caret or dollar to match at any point inside the record next |
| 224 | to newlines. But it's imperative that $/ be set to something other |
| 225 | than the default, or else we won't actually ever have a multiline |
| 226 | record read in. |
| 227 | .PP |
| 228 | .Vb 6 |
| 229 | \& $/ = ''; # read in more whole paragraph, not just one line |
| 230 | \& while ( <> ) { |
| 231 | \& while ( /\eb([\ew'-]+)(\es+\e1)+\eb/gi ) { # word starts alpha |
| 232 | \& print "Duplicate $1 at paragraph $.\en"; |
| 233 | \& } |
| 234 | \& } |
| 235 | .Ve |
| 236 | .PP |
| 237 | Here's code that finds sentences that begin with \*(L"From \*(R" (which would |
| 238 | be mangled by many mailers): |
| 239 | .PP |
| 240 | .Vb 6 |
| 241 | \& $/ = ''; # read in more whole paragraph, not just one line |
| 242 | \& while ( <> ) { |
| 243 | \& while ( /^From /gm ) { # /m makes ^ match next to \en |
| 244 | \& print "leading from in paragraph $.\en"; |
| 245 | \& } |
| 246 | \& } |
| 247 | .Ve |
| 248 | .PP |
| 249 | Here's code that finds everything between \s-1START\s0 and \s-1END\s0 in a paragraph: |
| 250 | .PP |
| 251 | .Vb 6 |
| 252 | \& undef $/; # read in whole file, not just one line or paragraph |
| 253 | \& while ( <> ) { |
| 254 | \& while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries |
| 255 | \& print "$1\en"; |
| 256 | \& } |
| 257 | \& } |
| 258 | .Ve |
| 259 | .Sh "How can I pull out lines between two patterns that are themselves on different lines?" |
| 260 | .IX Xref ".." |
| 261 | .IX Subsection "How can I pull out lines between two patterns that are themselves on different lines?" |
| 262 | You can use Perl's somewhat exotic \f(CW\*(C`..\*(C'\fR operator (documented in |
| 263 | perlop): |
| 264 | .PP |
| 265 | .Vb 1 |
| 266 | \& perl -ne 'print if /START/ .. /END/' file1 file2 ... |
| 267 | .Ve |
| 268 | .PP |
| 269 | If you wanted text and not lines, you would use |
| 270 | .PP |
| 271 | .Vb 1 |
| 272 | \& perl -0777 -ne 'print "$1\en" while /START(.*?)END/gs' file1 file2 ... |
| 273 | .Ve |
| 274 | .PP |
| 275 | But if you want nested occurrences of \f(CW\*(C`START\*(C'\fR through \f(CW\*(C`END\*(C'\fR, you'll |
| 276 | run up against the problem described in the question in this section |
| 277 | on matching balanced text. |
| 278 | .PP |
| 279 | Here's another example of using \f(CW\*(C`..\*(C'\fR: |
| 280 | .PP |
| 281 | .Vb 7 |
| 282 | \& while (<>) { |
| 283 | \& $in_header = 1 .. /^$/; |
| 284 | \& $in_body = /^$/ .. eof(); |
| 285 | \& # now choose between them |
| 286 | \& } continue { |
| 287 | \& reset if eof(); # fix $. |
| 288 | \& } |
| 289 | .Ve |
| 290 | .Sh "I put a regular expression into $/ but it didn't work. What's wrong?" |
| 291 | .IX Xref "$ , regexes in $INPUT_RECORD_SEPARATOR, regexes in $RS, regexes in" |
| 292 | .IX Subsection "I put a regular expression into $/ but it didn't work. What's wrong?" |
| 293 | Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, |
| 294 | but don't get your hopes up. Until then, you can use these examples |
| 295 | if you really need to do this. |
| 296 | .PP |
| 297 | If you have File::Stream, this is easy. |
| 298 | .PP |
| 299 | .Vb 5 |
| 300 | \& use File::Stream; |
| 301 | \& my $stream = File::Stream->new( |
| 302 | \& $filehandle, |
| 303 | \& separator => qr/\es*,\es*/, |
| 304 | \& ); |
| 305 | .Ve |
| 306 | .PP |
| 307 | .Vb 1 |
| 308 | \& print "$_\en" while <$stream>; |
| 309 | .Ve |
| 310 | .PP |
| 311 | If you don't have File::Stream, you have to do a little more work. |
| 312 | .PP |
| 313 | You can use the four argument form of sysread to continually add to |
| 314 | a buffer. After you add to the buffer, you check if you have a |
| 315 | complete line (using your regular expression). |
| 316 | .PP |
| 317 | .Vb 7 |
| 318 | \& local $_ = ""; |
| 319 | \& while( sysread FH, $_, 8192, length ) { |
| 320 | \& while( s/^((?s).*?)your_pattern/ ) { |
| 321 | \& my $record = $1; |
| 322 | \& # do stuff here. |
| 323 | \& } |
| 324 | \& } |
| 325 | .Ve |
| 326 | .PP |
| 327 | .Vb 3 |
| 328 | \& You can do the same thing with foreach and a match using the |
| 329 | \& c flag and the \eG anchor, if you do not mind your entire file |
| 330 | \& being in memory at the end. |
| 331 | .Ve |
| 332 | .PP |
| 333 | .Vb 7 |
| 334 | \& local $_ = ""; |
| 335 | \& while( sysread FH, $_, 8192, length ) { |
| 336 | \& foreach my $record ( m/\eG((?s).*?)your_pattern/gc ) { |
| 337 | \& # do stuff here. |
| 338 | \& } |
| 339 | \& substr( $_, 0, pos ) = "" if pos; |
| 340 | \& } |
| 341 | .Ve |
| 342 | .Sh "How do I substitute case insensitively on the \s-1LHS\s0 while preserving case on the \s-1RHS\s0?" |
| 343 | .IX Xref "replace, case preserving substitute, case preserving substitution, case preserving s, case preserving" |
| 344 | .IX Subsection "How do I substitute case insensitively on the LHS while preserving case on the RHS?" |
| 345 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
| 346 | properties of bitwise xor on \s-1ASCII\s0 strings. |
| 347 | .PP |
| 348 | .Vb 1 |
| 349 | \& $_= "this is a TEsT case"; |
| 350 | .Ve |
| 351 | .PP |
| 352 | .Vb 2 |
| 353 | \& $old = 'test'; |
| 354 | \& $new = 'success'; |
| 355 | .Ve |
| 356 | .PP |
| 357 | .Vb 5 |
| 358 | \& s{(\eQ$old\eE)} |
| 359 | \& { uc $new | (uc $1 ^ $1) . |
| 360 | \& (uc(substr $1, -1) ^ substr $1, -1) x |
| 361 | \& (length($new) - length $1) |
| 362 | \& }egi; |
| 363 | .Ve |
| 364 | .PP |
| 365 | .Vb 1 |
| 366 | \& print; |
| 367 | .Ve |
| 368 | .PP |
| 369 | And here it is as a subroutine, modeled after the above: |
| 370 | .PP |
| 371 | .Vb 3 |
| 372 | \& sub preserve_case($$) { |
| 373 | \& my ($old, $new) = @_; |
| 374 | \& my $mask = uc $old ^ $old; |
| 375 | .Ve |
| 376 | .PP |
| 377 | .Vb 3 |
| 378 | \& uc $new | $mask . |
| 379 | \& substr($mask, -1) x (length($new) - length($old)) |
| 380 | \& } |
| 381 | .Ve |
| 382 | .PP |
| 383 | .Vb 3 |
| 384 | \& $a = "this is a TEsT case"; |
| 385 | \& $a =~ s/(test)/preserve_case($1, "success")/egi; |
| 386 | \& print "$a\en"; |
| 387 | .Ve |
| 388 | .PP |
| 389 | This prints: |
| 390 | .PP |
| 391 | .Vb 1 |
| 392 | \& this is a SUcCESS case |
| 393 | .Ve |
| 394 | .PP |
| 395 | As an alternative, to keep the case of the replacement word if it is |
| 396 | longer than the original, you can use this code, by Jeff Pinyan: |
| 397 | .PP |
| 398 | .Vb 3 |
| 399 | \& sub preserve_case { |
| 400 | \& my ($from, $to) = @_; |
| 401 | \& my ($lf, $lt) = map length, @_; |
| 402 | .Ve |
| 403 | .PP |
| 404 | .Vb 2 |
| 405 | \& if ($lt < $lf) { $from = substr $from, 0, $lt } |
| 406 | \& else { $from .= substr $to, $lf } |
| 407 | .Ve |
| 408 | .PP |
| 409 | .Vb 2 |
| 410 | \& return uc $to | ($from ^ uc $from); |
| 411 | \& } |
| 412 | .Ve |
| 413 | .PP |
| 414 | This changes the sentence to \*(L"this is a SUcCess case.\*(R" |
| 415 | .PP |
| 416 | Just to show that C programmers can write C in any programming language, |
| 417 | if you prefer a more C\-like solution, the following script makes the |
| 418 | substitution have the same case, letter by letter, as the original. |
| 419 | (It also happens to run about 240% slower than the Perlish solution runs.) |
| 420 | If the substitution has more characters than the string being substituted, |
| 421 | the case of the last character is used for the rest of the substitution. |
| 422 | .PP |
| 423 | .Vb 8 |
| 424 | \& # Original by Nathan Torkington, massaged by Jeffrey Friedl |
| 425 | \& # |
| 426 | \& sub preserve_case($$) |
| 427 | \& { |
| 428 | \& my ($old, $new) = @_; |
| 429 | \& my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc |
| 430 | \& my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); |
| 431 | \& my ($len) = $oldlen < $newlen ? $oldlen : $newlen; |
| 432 | .Ve |
| 433 | .PP |
| 434 | .Vb 21 |
| 435 | \& for ($i = 0; $i < $len; $i++) { |
| 436 | \& if ($c = substr($old, $i, 1), $c =~ /[\eW\ed_]/) { |
| 437 | \& $state = 0; |
| 438 | \& } elsif (lc $c eq $c) { |
| 439 | \& substr($new, $i, 1) = lc(substr($new, $i, 1)); |
| 440 | \& $state = 1; |
| 441 | \& } else { |
| 442 | \& substr($new, $i, 1) = uc(substr($new, $i, 1)); |
| 443 | \& $state = 2; |
| 444 | \& } |
| 445 | \& } |
| 446 | \& # finish up with any remaining new (for when new is longer than old) |
| 447 | \& if ($newlen > $oldlen) { |
| 448 | \& if ($state == 1) { |
| 449 | \& substr($new, $oldlen) = lc(substr($new, $oldlen)); |
| 450 | \& } elsif ($state == 2) { |
| 451 | \& substr($new, $oldlen) = uc(substr($new, $oldlen)); |
| 452 | \& } |
| 453 | \& } |
| 454 | \& return $new; |
| 455 | \& } |
| 456 | .Ve |
| 457 | .ie n .Sh "How can I make ""\ew"" match national character sets?" |
| 458 | .el .Sh "How can I make \f(CW\ew\fP match national character sets?" |
| 459 | .IX Xref "\w" |
| 460 | .IX Subsection "How can I make w match national character sets?" |
| 461 | Put \f(CW\*(C`use locale;\*(C'\fR in your script. The \ew character class is taken |
| 462 | from the current locale. |
| 463 | .PP |
| 464 | See perllocale for details. |
| 465 | .ie n .Sh "How can I match a locale-smart version of ""/[a\-zA\-Z]/""?" |
| 466 | .el .Sh "How can I match a locale-smart version of \f(CW/[a\-zA\-Z]/\fP?" |
| 467 | .IX Xref "alpha" |
| 468 | .IX Subsection "How can I match a locale-smart version of /[a-zA-Z]/?" |
| 469 | You can use the \s-1POSIX\s0 character class syntax \f(CW\*(C`/[[:alpha:]]/\*(C'\fR |
| 470 | documented in perlre. |
| 471 | .PP |
| 472 | No matter which locale you are in, the alphabetic characters are |
| 473 | the characters in \ew without the digits and the underscore. |
| 474 | As a regex, that looks like \f(CW\*(C`/[^\eW\ed_]/\*(C'\fR. Its complement, |
| 475 | the non\-alphabetics, is then everything in \eW along with |
| 476 | the digits and the underscore, or \f(CW\*(C`/[\eW\ed_]/\*(C'\fR. |
| 477 | .Sh "How can I quote a variable to use in a regex?" |
| 478 | .IX Xref "regex, escaping regexp, escaping regular expression, escaping" |
| 479 | .IX Subsection "How can I quote a variable to use in a regex?" |
| 480 | The Perl parser will expand \f(CW$variable\fR and \f(CW@variable\fR references in |
| 481 | regular expressions unless the delimiter is a single quote. Remember, |
| 482 | too, that the right-hand side of a \f(CW\*(C`s///\*(C'\fR substitution is considered |
| 483 | a double-quoted string (see perlop for more details). Remember |
| 484 | also that any regex special characters will be acted on unless you |
| 485 | precede the substitution with \eQ. Here's an example: |
| 486 | .PP |
| 487 | .Vb 2 |
| 488 | \& $string = "Placido P. Octopus"; |
| 489 | \& $regex = "P."; |
| 490 | .Ve |
| 491 | .PP |
| 492 | .Vb 2 |
| 493 | \& $string =~ s/$regex/Polyp/; |
| 494 | \& # $string is now "Polypacido P. Octopus" |
| 495 | .Ve |
| 496 | .PP |
| 497 | Because \f(CW\*(C`.\*(C'\fR is special in regular expressions, and can match any |
| 498 | single character, the regex \f(CW\*(C`P.\*(C'\fR here has matched the <Pl> in the |
| 499 | original string. |
| 500 | .PP |
| 501 | To escape the special meaning of \f(CW\*(C`.\*(C'\fR, we use \f(CW\*(C`\eQ\*(C'\fR: |
| 502 | .PP |
| 503 | .Vb 2 |
| 504 | \& $string = "Placido P. Octopus"; |
| 505 | \& $regex = "P."; |
| 506 | .Ve |
| 507 | .PP |
| 508 | .Vb 2 |
| 509 | \& $string =~ s/\eQ$regex/Polyp/; |
| 510 | \& # $string is now "Placido Polyp Octopus" |
| 511 | .Ve |
| 512 | .PP |
| 513 | The use of \f(CW\*(C`\eQ\*(C'\fR causes the <.> in the regex to be treated as a |
| 514 | regular character, so that \f(CW\*(C`P.\*(C'\fR matches a \f(CW\*(C`P\*(C'\fR followed by a dot. |
| 515 | .ie n .Sh "What is ""/o"" really for?" |
| 516 | .el .Sh "What is \f(CW/o\fP really for?" |
| 517 | .IX Xref " o" |
| 518 | .IX Subsection "What is /o really for?" |
| 519 | Using a variable in a regular expression match forces a re-evaluation |
| 520 | (and perhaps recompilation) each time the regular expression is |
| 521 | encountered. The \f(CW\*(C`/o\*(C'\fR modifier locks in the regex the first time |
| 522 | it's used. This always happens in a constant regular expression, and |
| 523 | in fact, the pattern was compiled into the internal format at the same |
| 524 | time your entire program was. |
| 525 | .PP |
| 526 | Use of \f(CW\*(C`/o\*(C'\fR is irrelevant unless variable interpolation is used in |
| 527 | the pattern, and if so, the regex engine will neither know nor care |
| 528 | whether the variables change after the pattern is evaluated the \fIvery |
| 529 | first\fR time. |
| 530 | .PP |
| 531 | \&\f(CW\*(C`/o\*(C'\fR is often used to gain an extra measure of efficiency by not |
| 532 | performing subsequent evaluations when you know it won't matter |
| 533 | (because you know the variables won't change), or more rarely, when |
| 534 | you don't want the regex to notice if they do. |
| 535 | .PP |
| 536 | For example, here's a \*(L"paragrep\*(R" program: |
| 537 | .PP |
| 538 | .Vb 5 |
| 539 | \& $/ = ''; # paragraph mode |
| 540 | \& $pat = shift; |
| 541 | \& while (<>) { |
| 542 | \& print if /$pat/o; |
| 543 | \& } |
| 544 | .Ve |
| 545 | .Sh "How do I use a regular expression to strip C style comments from a file?" |
| 546 | .IX Subsection "How do I use a regular expression to strip C style comments from a file?" |
| 547 | While this actually can be done, it's much harder than you'd think. |
| 548 | For example, this one-liner |
| 549 | .PP |
| 550 | .Vb 1 |
| 551 | \& perl -0777 -pe 's{/\e*.*?\e*/}{}gs' foo.c |
| 552 | .Ve |
| 553 | .PP |
| 554 | will work in many but not all cases. You see, it's too simple-minded for |
| 555 | certain kinds of C programs, in particular, those with what appear to be |
| 556 | comments in quoted strings. For that, you'd need something like this, |
| 557 | created by Jeffrey Friedl and later modified by Fred Curtis. |
| 558 | .PP |
| 559 | .Vb 4 |
| 560 | \& $/ = undef; |
| 561 | \& $_ = <>; |
| 562 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#defined $2 ? $2 : ""#gse; |
| 563 | \& print; |
| 564 | .Ve |
| 565 | .PP |
| 566 | This could, of course, be more legibly written with the \f(CW\*(C`/x\*(C'\fR modifier, adding |
| 567 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
| 568 | .PP |
| 569 | .Vb 8 |
| 570 | \& s{ |
| 571 | \& /\e* ## Start of /* ... */ comment |
| 572 | \& [^*]*\e*+ ## Non-* followed by 1-or-more *'s |
| 573 | \& ( |
| 574 | \& [^/*][^*]*\e*+ |
| 575 | \& )* ## 0-or-more things which don't start with / |
| 576 | \& ## but do end with '*' |
| 577 | \& / ## End of /* ... */ comment |
| 578 | .Ve |
| 579 | .PP |
| 580 | .Vb 1 |
| 581 | \& | ## OR various things which aren't comments: |
| 582 | .Ve |
| 583 | .PP |
| 584 | .Vb 8 |
| 585 | \& ( |
| 586 | \& " ## Start of " ... " string |
| 587 | \& ( |
| 588 | \& \e\e. ## Escaped char |
| 589 | \& | ## OR |
| 590 | \& [^"\e\e] ## Non "\e |
| 591 | \& )* |
| 592 | \& " ## End of " ... " string |
| 593 | .Ve |
| 594 | .PP |
| 595 | .Vb 1 |
| 596 | \& | ## OR |
| 597 | .Ve |
| 598 | .PP |
| 599 | .Vb 7 |
| 600 | \& ' ## Start of ' ... ' string |
| 601 | \& ( |
| 602 | \& \e\e. ## Escaped char |
| 603 | \& | ## OR |
| 604 | \& [^'\e\e] ## Non '\e |
| 605 | \& )* |
| 606 | \& ' ## End of ' ... ' string |
| 607 | .Ve |
| 608 | .PP |
| 609 | .Vb 1 |
| 610 | \& | ## OR |
| 611 | .Ve |
| 612 | .PP |
| 613 | .Vb 4 |
| 614 | \& . ## Anything other char |
| 615 | \& [^/"'\e\e]* ## Chars which doesn't start a comment, string or escape |
| 616 | \& ) |
| 617 | \& }{defined $2 ? $2 : ""}gxse; |
| 618 | .Ve |
| 619 | .PP |
| 620 | A slight modification also removes \*(C+ comments: |
| 621 | .PP |
| 622 | .Vb 1 |
| 623 | \& s#/\e*[^*]*\e*+([^/*][^*]*\e*+)*/|//[^\en]*|("(\e\e.|[^"\e\e])*"|'(\e\e.|[^'\e\e])*'|.[^/"'\e\e]*)#defined $2 ? $2 : ""#gse; |
| 624 | .Ve |
| 625 | .Sh "Can I use Perl regular expressions to match balanced text?" |
| 626 | .IX Xref "regex, matching balanced test regexp, matching balanced test regular expression, matching balanced test" |
| 627 | .IX Subsection "Can I use Perl regular expressions to match balanced text?" |
| 628 | Historically, Perl regular expressions were not capable of matching |
| 629 | balanced text. As of more recent versions of perl including 5.6.1 |
| 630 | experimental features have been added that make it possible to do this. |
| 631 | Look at the documentation for the (??{ }) construct in recent perlre manual |
| 632 | pages to see an example of matching balanced parentheses. Be sure to take |
| 633 | special notice of the warnings present in the manual before making use |
| 634 | of this feature. |
| 635 | .PP |
| 636 | \&\s-1CPAN\s0 contains many modules that can be useful for matching text |
| 637 | depending on the context. Damian Conway provides some useful |
| 638 | patterns in Regexp::Common. The module Text::Balanced provides a |
| 639 | general solution to this problem. |
| 640 | .PP |
| 641 | One of the common applications of balanced text matching is working |
| 642 | with \s-1XML\s0 and \s-1HTML\s0. There are many modules available that support |
| 643 | these needs. Two examples are HTML::Parser and XML::Parser. There |
| 644 | are many others. |
| 645 | .PP |
| 646 | An elaborate subroutine (for 7\-bit \s-1ASCII\s0 only) to pull out balanced |
| 647 | 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, |
| 648 | or \f(CW\*(C`(\*(C'\fR and \f(CW\*(C`)\*(C'\fR can be found in |
| 649 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . |
| 650 | .PP |
| 651 | The C::Scan module from \s-1CPAN\s0 also contains such subs for internal use, |
| 652 | but they are undocumented. |
| 653 | .Sh "What does it mean that regexes are greedy? How can I get around it?" |
| 654 | .IX Xref "greedy greediness" |
| 655 | .IX Subsection "What does it mean that regexes are greedy? How can I get around it?" |
| 656 | Most people mean that greedy regexes match as much as they can. |
| 657 | Technically speaking, it's actually the quantifiers (\f(CW\*(C`?\*(C'\fR, \f(CW\*(C`*\*(C'\fR, \f(CW\*(C`+\*(C'\fR, |
| 658 | \&\f(CW\*(C`{}\*(C'\fR) that are greedy rather than the whole pattern; Perl prefers local |
| 659 | greed and immediate gratification to overall greed. To get non-greedy |
| 660 | 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). |
| 661 | .PP |
| 662 | An example: |
| 663 | .PP |
| 664 | .Vb 3 |
| 665 | \& $s1 = $s2 = "I am very very cold"; |
| 666 | \& $s1 =~ s/ve.*y //; # I am cold |
| 667 | \& $s2 =~ s/ve.*?y //; # I am very cold |
| 668 | .Ve |
| 669 | .PP |
| 670 | Notice how the second substitution stopped matching as soon as it |
| 671 | encountered \*(L"y \*(R". The \f(CW\*(C`*?\*(C'\fR quantifier effectively tells the regular |
| 672 | expression engine to find a match as quickly as possible and pass |
| 673 | control on to whatever is next in line, like you would if you were |
| 674 | playing hot potato. |
| 675 | .Sh "How do I process each word on each line?" |
| 676 | .IX Xref "word" |
| 677 | .IX Subsection "How do I process each word on each line?" |
| 678 | Use the split function: |
| 679 | .PP |
| 680 | .Vb 5 |
| 681 | \& while (<>) { |
| 682 | \& foreach $word ( split ) { |
| 683 | \& # do something with $word here |
| 684 | \& } |
| 685 | \& } |
| 686 | .Ve |
| 687 | .PP |
| 688 | Note that this isn't really a word in the English sense; it's just |
| 689 | chunks of consecutive non-whitespace characters. |
| 690 | .PP |
| 691 | To work with only alphanumeric sequences (including underscores), you |
| 692 | might consider |
| 693 | .PP |
| 694 | .Vb 5 |
| 695 | \& while (<>) { |
| 696 | \& foreach $word (m/(\ew+)/g) { |
| 697 | \& # do something with $word here |
| 698 | \& } |
| 699 | \& } |
| 700 | .Ve |
| 701 | .Sh "How can I print out a word-frequency or line-frequency summary?" |
| 702 | .IX Subsection "How can I print out a word-frequency or line-frequency summary?" |
| 703 | To do this, you have to parse out each word in the input stream. We'll |
| 704 | pretend that by word you mean chunk of alphabetics, hyphens, or |
| 705 | apostrophes, rather than the non-whitespace chunk idea of a word given |
| 706 | in the previous question: |
| 707 | .PP |
| 708 | .Vb 8 |
| 709 | \& while (<>) { |
| 710 | \& while ( /(\eb[^\eW_\ed][\ew'-]+\eb)/g ) { # misses "`sheep'" |
| 711 | \& $seen{$1}++; |
| 712 | \& } |
| 713 | \& } |
| 714 | \& while ( ($word, $count) = each %seen ) { |
| 715 | \& print "$count $word\en"; |
| 716 | \& } |
| 717 | .Ve |
| 718 | .PP |
| 719 | If you wanted to do the same thing for lines, you wouldn't need a |
| 720 | regular expression: |
| 721 | .PP |
| 722 | .Vb 6 |
| 723 | \& while (<>) { |
| 724 | \& $seen{$_}++; |
| 725 | \& } |
| 726 | \& while ( ($line, $count) = each %seen ) { |
| 727 | \& print "$count $line"; |
| 728 | \& } |
| 729 | .Ve |
| 730 | .PP |
| 731 | If you want these output in a sorted order, see perlfaq4: \*(L"How do I |
| 732 | sort a hash (optionally by value instead of key)?\*(R". |
| 733 | .Sh "How can I do approximate matching?" |
| 734 | .IX Xref "match, approximate matching, approximate" |
| 735 | .IX Subsection "How can I do approximate matching?" |
| 736 | See the module String::Approx available from \s-1CPAN\s0. |
| 737 | .Sh "How do I efficiently match many regular expressions at once?" |
| 738 | .IX Xref "regex, efficiency regexp, efficiency regular expression, efficiency" |
| 739 | .IX Subsection "How do I efficiently match many regular expressions at once?" |
| 740 | ( contributed by brian d foy ) |
| 741 | .PP |
| 742 | Avoid asking Perl to compile a regular expression every time |
| 743 | you want to match it. In this example, perl must recompile |
| 744 | the regular expression for every iteration of the \fIforeach()\fR |
| 745 | loop since it has no way to know what \f(CW$pattern\fR will be. |
| 746 | .PP |
| 747 | .Vb 1 |
| 748 | \& @patterns = qw( foo bar baz ); |
| 749 | .Ve |
| 750 | .PP |
| 751 | .Vb 8 |
| 752 | \& LINE: while( <> ) |
| 753 | \& { |
| 754 | \& foreach $pattern ( @patterns ) |
| 755 | \& { |
| 756 | \& print if /\eb$pattern\eb/i; |
| 757 | \& next LINE; |
| 758 | \& } |
| 759 | \& } |
| 760 | .Ve |
| 761 | .PP |
| 762 | The qr// operator showed up in perl 5.005. It compiles a |
| 763 | regular expression, but doesn't apply it. When you use the |
| 764 | pre-compiled version of the regex, perl does less work. In |
| 765 | this example, I inserted a \fImap()\fR to turn each pattern into |
| 766 | its pre-compiled form. The rest of the script is the same, |
| 767 | but faster. |
| 768 | .PP |
| 769 | .Vb 1 |
| 770 | \& @patterns = map { qr/\eb$_\eb/i } qw( foo bar baz ); |
| 771 | .Ve |
| 772 | .PP |
| 773 | .Vb 8 |
| 774 | \& LINE: while( <> ) |
| 775 | \& { |
| 776 | \& foreach $pattern ( @patterns ) |
| 777 | \& { |
| 778 | \& print if /\eb$pattern\eb/i; |
| 779 | \& next LINE; |
| 780 | \& } |
| 781 | \& } |
| 782 | .Ve |
| 783 | .PP |
| 784 | In some cases, you may be able to make several patterns into |
| 785 | a single regular expression. Beware of situations that require |
| 786 | backtracking though. |
| 787 | .PP |
| 788 | .Vb 1 |
| 789 | \& $regex = join '|', qw( foo bar baz ); |
| 790 | .Ve |
| 791 | .PP |
| 792 | .Vb 4 |
| 793 | \& LINE: while( <> ) |
| 794 | \& { |
| 795 | \& print if /\eb(?:$regex)\eb/i; |
| 796 | \& } |
| 797 | .Ve |
| 798 | .PP |
| 799 | For more details on regular expression efficiency, see Mastering |
| 800 | Regular Expressions by Jeffrey Freidl. He explains how regular |
| 801 | expressions engine work and why some patterns are surprisingly |
| 802 | inefficient. Once you understand how perl applies regular |
| 803 | expressions, you can tune them for individual situations. |
| 804 | .ie n .Sh "Why don't word-boundary searches with ""\eb"" work for me?" |
| 805 | .el .Sh "Why don't word-boundary searches with \f(CW\eb\fP work for me?" |
| 806 | .IX Xref "\b" |
| 807 | .IX Subsection "Why don't word-boundary searches with b work for me?" |
| 808 | (contributed by brian d foy) |
| 809 | .PP |
| 810 | Ensure that you know what \eb really does: it's the boundary between a |
| 811 | word character, \ew, and something that isn't a word character. That |
| 812 | thing that isn't a word character might be \eW, but it can also be the |
| 813 | start or end of the string. |
| 814 | .PP |
| 815 | It's not (not!) the boundary between whitespace and non\-whitespace, |
| 816 | and it's not the stuff between words we use to create sentences. |
| 817 | .PP |
| 818 | In regex speak, a word boundary (\eb) is a \*(L"zero width assertion\*(R", |
| 819 | meaning that it doesn't represent a character in the string, but a |
| 820 | condition at a certain position. |
| 821 | .PP |
| 822 | For the regular expression, /\ebPerl\eb/, there has to be a word |
| 823 | boundary before the \*(L"P\*(R" and after the \*(L"l\*(R". As long as something other |
| 824 | than a word character precedes the \*(L"P\*(R" and succeeds the \*(L"l\*(R", the |
| 825 | pattern will match. These strings match /\ebPerl\eb/. |
| 826 | .PP |
| 827 | .Vb 4 |
| 828 | \& "Perl" # no word char before P or after l |
| 829 | \& "Perl " # same as previous (space is not a word char) |
| 830 | \& "'Perl'" # the ' char is not a word char |
| 831 | \& "Perl's" # no word char before P, non-word char after "l" |
| 832 | .Ve |
| 833 | .PP |
| 834 | These strings do not match /\ebPerl\eb/. |
| 835 | .PP |
| 836 | .Vb 2 |
| 837 | \& "Perl_" # _ is a word char! |
| 838 | \& "Perler" # no word char before P, but one after l |
| 839 | .Ve |
| 840 | .PP |
| 841 | You don't have to use \eb to match words though. You can look for |
| 842 | non-word characters surrounded by word characters. These strings |
| 843 | match the pattern /\eb'\eb/. |
| 844 | .PP |
| 845 | .Vb 2 |
| 846 | \& "don't" # the ' char is surrounded by "n" and "t" |
| 847 | \& "qep'a'" # the ' char is surrounded by "p" and "a" |
| 848 | .Ve |
| 849 | .PP |
| 850 | These strings do not match /\eb'\eb/. |
| 851 | .PP |
| 852 | .Vb 1 |
| 853 | \& "foo'" # there is no word char after non-word ' |
| 854 | .Ve |
| 855 | .PP |
| 856 | You can also use the complement of \eb, \eB, to specify that there |
| 857 | should not be a word boundary. |
| 858 | .PP |
| 859 | In the pattern /\eBam\eB/, there must be a word character before the \*(L"a\*(R" |
| 860 | and after the \*(L"m\*(R". These patterns match /\eBam\eB/: |
| 861 | .PP |
| 862 | .Vb 2 |
| 863 | \& "llama" # "am" surrounded by word chars |
| 864 | \& "Samuel" # same |
| 865 | .Ve |
| 866 | .PP |
| 867 | These strings do not match /\eBam\eB/ |
| 868 | .PP |
| 869 | .Vb 2 |
| 870 | \& "Sam" # no word boundary before "a", but one after "m" |
| 871 | \& "I am Sam" # "am" surrounded by non-word chars |
| 872 | .Ve |
| 873 | .Sh "Why does using $&, $`, or $' slow my program down?" |
| 874 | .IX Xref "$MATCH $& $POSTMATCH $' $PREMATCH $`" |
| 875 | .IX Subsection "Why does using $&, $`, or $' slow my program down?" |
| 876 | (contributed by Anno Siegel) |
| 877 | .PP |
| 878 | Once Perl sees that you need one of these variables anywhere in the |
| 879 | program, it provides them on each and every pattern match. That means |
| 880 | that on every pattern match the entire string will be copied, part of it |
| 881 | to $`, part to $&, and part to $'. Thus the penalty is most severe with |
| 882 | long strings and patterns that match often. Avoid $&, $', and $` if you |
| 883 | can, but if you can't, once you've used them at all, use them at will |
| 884 | because you've already paid the price. Remember that some algorithms |
| 885 | really appreciate them. As of the 5.005 release, the $& variable is no |
| 886 | longer \*(L"expensive\*(R" the way the other two are. |
| 887 | .PP |
| 888 | Since Perl 5.6.1 the special variables @\- and @+ can functionally replace |
| 889 | $`, $& and $'. These arrays contain pointers to the beginning and end |
| 890 | of each match (see perlvar for the full story), so they give you |
| 891 | essentially the same information, but without the risk of excessive |
| 892 | string copying. |
| 893 | .ie n .Sh "What good is ""\eG"" in a regular expression?" |
| 894 | .el .Sh "What good is \f(CW\eG\fP in a regular expression?" |
| 895 | .IX Xref "\G" |
| 896 | .IX Subsection "What good is G in a regular expression?" |
| 897 | You use the \f(CW\*(C`\eG\*(C'\fR anchor to start the next match on the same |
| 898 | string where the last match left off. The regular |
| 899 | expression engine cannot skip over any characters to find |
| 900 | the next match with this anchor, so \f(CW\*(C`\eG\*(C'\fR is similar to the |
| 901 | beginning of string anchor, \f(CW\*(C`^\*(C'\fR. The \f(CW\*(C`\eG\*(C'\fR anchor is typically |
| 902 | used with the \f(CW\*(C`g\*(C'\fR flag. It uses the value of \fIpos()\fR |
| 903 | as the position to start the next match. As the match |
| 904 | operator makes successive matches, it updates \fIpos()\fR with the |
| 905 | position of the next character past the last match (or the |
| 906 | first character of the next match, depending on how you like |
| 907 | to look at it). Each string has its own \fIpos()\fR value. |
| 908 | .PP |
| 909 | Suppose you want to match all of consective pairs of digits |
| 910 | in a string like \*(L"1122a44\*(R" and stop matching when you |
| 911 | encounter non\-digits. You want to match \f(CW11\fR and \f(CW22\fR but |
| 912 | the letter <a> shows up between \f(CW22\fR and \f(CW44\fR and you want |
| 913 | to stop at \f(CW\*(C`a\*(C'\fR. Simply matching pairs of digits skips over |
| 914 | the \f(CW\*(C`a\*(C'\fR and still matches \f(CW44\fR. |
| 915 | .PP |
| 916 | .Vb 2 |
| 917 | \& $_ = "1122a44"; |
| 918 | \& my @pairs = m/(\ed\ed)/g; # qw( 11 22 44 ) |
| 919 | .Ve |
| 920 | .PP |
| 921 | If you use the \eG anchor, you force the match after \f(CW22\fR to |
| 922 | start with the \f(CW\*(C`a\*(C'\fR. The regular expression cannot match |
| 923 | there since it does not find a digit, so the next match |
| 924 | fails and the match operator returns the pairs it already |
| 925 | found. |
| 926 | .PP |
| 927 | .Vb 2 |
| 928 | \& $_ = "1122a44"; |
| 929 | \& my @pairs = m/\eG(\ed\ed)/g; # qw( 11 22 ) |
| 930 | .Ve |
| 931 | .PP |
| 932 | You can also use the \f(CW\*(C`\eG\*(C'\fR anchor in scalar context. You |
| 933 | still need the \f(CW\*(C`g\*(C'\fR flag. |
| 934 | .PP |
| 935 | .Vb 5 |
| 936 | \& $_ = "1122a44"; |
| 937 | \& while( m/\eG(\ed\ed)/g ) |
| 938 | \& { |
| 939 | \& print "Found $1\en"; |
| 940 | \& } |
| 941 | .Ve |
| 942 | .PP |
| 943 | After the match fails at the letter \f(CW\*(C`a\*(C'\fR, perl resets \fIpos()\fR |
| 944 | and the next match on the same string starts at the beginning. |
| 945 | .PP |
| 946 | .Vb 5 |
| 947 | \& $_ = "1122a44"; |
| 948 | \& while( m/\eG(\ed\ed)/g ) |
| 949 | \& { |
| 950 | \& print "Found $1\en"; |
| 951 | \& } |
| 952 | .Ve |
| 953 | .PP |
| 954 | .Vb 1 |
| 955 | \& print "Found $1 after while" if m/(\ed\ed)/g; # finds "11" |
| 956 | .Ve |
| 957 | .PP |
| 958 | You can disable \fIpos()\fR resets on fail with the \f(CW\*(C`c\*(C'\fR flag. |
| 959 | Subsequent matches start where the last successful match |
| 960 | ended (the value of \fIpos()\fR) even if a match on the same |
| 961 | string as failed in the meantime. In this case, the match |
| 962 | after the \fIwhile()\fR loop starts at the \f(CW\*(C`a\*(C'\fR (where the last |
| 963 | match stopped), and since it does not use any anchor it can |
| 964 | skip over the \f(CW\*(C`a\*(C'\fR to find \*(L"44\*(R". |
| 965 | .PP |
| 966 | .Vb 5 |
| 967 | \& $_ = "1122a44"; |
| 968 | \& while( m/\eG(\ed\ed)/gc ) |
| 969 | \& { |
| 970 | \& print "Found $1\en"; |
| 971 | \& } |
| 972 | .Ve |
| 973 | .PP |
| 974 | .Vb 1 |
| 975 | \& print "Found $1 after while" if m/(\ed\ed)/g; # finds "44" |
| 976 | .Ve |
| 977 | .PP |
| 978 | Typically you use the \f(CW\*(C`\eG\*(C'\fR anchor with the \f(CW\*(C`c\*(C'\fR flag |
| 979 | when you want to try a different match if one fails, |
| 980 | such as in a tokenizer. Jeffrey Friedl offers this example |
| 981 | which works in 5.004 or later. |
| 982 | .PP |
| 983 | .Vb 9 |
| 984 | \& while (<>) { |
| 985 | \& chomp; |
| 986 | \& PARSER: { |
| 987 | \& m/ \eG( \ed+\eb )/gcx && do { print "number: $1\en"; redo; }; |
| 988 | \& m/ \eG( \ew+ )/gcx && do { print "word: $1\en"; redo; }; |
| 989 | \& m/ \eG( \es+ )/gcx && do { print "space: $1\en"; redo; }; |
| 990 | \& m/ \eG( [^\ew\ed]+ )/gcx && do { print "other: $1\en"; redo; }; |
| 991 | \& } |
| 992 | \& } |
| 993 | .Ve |
| 994 | .PP |
| 995 | For each line, the \s-1PARSER\s0 loop first tries to match a series |
| 996 | of digits followed by a word boundary. This match has to |
| 997 | start at the place the last match left off (or the beginning |
| 998 | of the string on the first match). Since \f(CW\*(C`m/ \eG( \ed+\eb |
| 999 | )/gcx\*(C'\fR uses the \f(CW\*(C`c\*(C'\fR flag, if the string does not match that |
| 1000 | regular expression, perl does not reset \fIpos()\fR and the next |
| 1001 | match starts at the same position to try a different |
| 1002 | pattern. |
| 1003 | .Sh "Are Perl regexes DFAs or NFAs? Are they \s-1POSIX\s0 compliant?" |
| 1004 | .IX Xref "DFA NFA POSIX" |
| 1005 | .IX Subsection "Are Perl regexes DFAs or NFAs? Are they POSIX compliant?" |
| 1006 | While it's true that Perl's regular expressions resemble the DFAs |
| 1007 | (deterministic finite automata) of the \fIegrep\fR\|(1) program, they are in |
| 1008 | fact implemented as NFAs (non\-deterministic finite automata) to allow |
| 1009 | backtracking and backreferencing. And they aren't POSIX-style either, |
| 1010 | because those guarantee worst-case behavior for all cases. (It seems |
| 1011 | that some people prefer guarantees of consistency, even when what's |
| 1012 | guaranteed is slowness.) See the book \*(L"Mastering Regular Expressions\*(R" |
| 1013 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever |
| 1014 | hope to know on these matters (a full citation appears in |
| 1015 | perlfaq2). |
| 1016 | .Sh "What's wrong with using grep in a void context?" |
| 1017 | .IX Xref "grep" |
| 1018 | .IX Subsection "What's wrong with using grep in a void context?" |
| 1019 | The problem is that grep builds a return list, regardless of the context. |
| 1020 | This means you're making Perl go to the trouble of building a list that |
| 1021 | you then just throw away. If the list is large, you waste both time and space. |
| 1022 | If your intent is to iterate over the list, then use a for loop for this |
| 1023 | purpose. |
| 1024 | .PP |
| 1025 | In perls older than 5.8.1, map suffers from this problem as well. |
| 1026 | But since 5.8.1, this has been fixed, and map is context aware \- in void |
| 1027 | context, no lists are constructed. |
| 1028 | .Sh "How can I match strings with multibyte characters?" |
| 1029 | .IX Xref "regex, and multibyte characters regexp, and multibyte characters regular expression, and multibyte characters" |
| 1030 | .IX Subsection "How can I match strings with multibyte characters?" |
| 1031 | Starting from Perl 5.6 Perl has had some level of multibyte character |
| 1032 | support. Perl 5.8 or later is recommended. Supported multibyte |
| 1033 | character repertoires include Unicode, and legacy encodings |
| 1034 | through the Encode module. See perluniintro, perlunicode, |
| 1035 | and Encode. |
| 1036 | .PP |
| 1037 | If you are stuck with older Perls, you can do Unicode with the |
| 1038 | \&\f(CW\*(C`Unicode::String\*(C'\fR module, and character conversions using the |
| 1039 | \&\f(CW\*(C`Unicode::Map8\*(C'\fR and \f(CW\*(C`Unicode::Map\*(C'\fR modules. If you are using |
| 1040 | Japanese encodings, you might try using the jperl 5.005_03. |
| 1041 | .PP |
| 1042 | Finally, the following set of approaches was offered by Jeffrey |
| 1043 | Friedl, whose article in issue #5 of The Perl Journal talks about |
| 1044 | this very matter. |
| 1045 | .PP |
| 1046 | Let's suppose you have some weird Martian encoding where pairs of |
| 1047 | \&\s-1ASCII\s0 uppercase letters encode single Martian letters (i.e. the two |
| 1048 | bytes \*(L"\s-1CV\s0\*(R" make a single Martian letter, as do the two bytes \*(L"\s-1SG\s0\*(R", |
| 1049 | \&\*(L"\s-1VS\s0\*(R", \*(L"\s-1XX\s0\*(R", etc.). Other bytes represent single characters, just like |
| 1050 | \&\s-1ASCII\s0. |
| 1051 | .PP |
| 1052 | So, the string of Martian \*(L"I am \s-1CVSGXX\s0!\*(R" uses 12 bytes to encode the |
| 1053 | nine characters 'I', ' ', 'a', 'm', ' ', '\s-1CV\s0', '\s-1SG\s0', '\s-1XX\s0', '!'. |
| 1054 | .PP |
| 1055 | Now, say you want to search for the single character \f(CW\*(C`/GX/\*(C'\fR. Perl |
| 1056 | doesn't know about Martian, so it'll find the two bytes \*(L"\s-1GX\s0\*(R" in the \*(L"I |
| 1057 | am \s-1CVSGXX\s0!\*(R" string, even though that character isn't there: it just |
| 1058 | looks like it is because \*(L"\s-1SG\s0\*(R" is next to \*(L"\s-1XX\s0\*(R", but there's no real |
| 1059 | \&\*(L"\s-1GX\s0\*(R". This is a big problem. |
| 1060 | .PP |
| 1061 | Here are a few ways, all painful, to deal with it: |
| 1062 | .PP |
| 1063 | .Vb 3 |
| 1064 | \& $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent "martian" |
| 1065 | \& # bytes are no longer adjacent. |
| 1066 | \& print "found GX!\en" if $martian =~ /GX/; |
| 1067 | .Ve |
| 1068 | .PP |
| 1069 | Or like this: |
| 1070 | .PP |
| 1071 | .Vb 6 |
| 1072 | \& @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
| 1073 | \& # above is conceptually similar to: @chars = $text =~ m/(.)/g; |
| 1074 | \& # |
| 1075 | \& foreach $char (@chars) { |
| 1076 | \& print "found GX!\en", last if $char eq 'GX'; |
| 1077 | \& } |
| 1078 | .Ve |
| 1079 | .PP |
| 1080 | Or like this: |
| 1081 | .PP |
| 1082 | .Vb 3 |
| 1083 | \& while ($martian =~ m/\eG([A-Z][A-Z]|.)/gs) { # \eG probably unneeded |
| 1084 | \& print "found GX!\en", last if $1 eq 'GX'; |
| 1085 | \& } |
| 1086 | .Ve |
| 1087 | .PP |
| 1088 | Here's another, slightly less painful, way to do it from Benjamin |
| 1089 | Goldberg, who uses a zero-width negative look-behind assertion. |
| 1090 | .PP |
| 1091 | .Vb 5 |
| 1092 | \& print "found GX!\en" if $martian =~ m/ |
| 1093 | \& (?<![A-Z]) |
| 1094 | \& (?:[A-Z][A-Z])*? |
| 1095 | \& GX |
| 1096 | \& /x; |
| 1097 | .Ve |
| 1098 | .PP |
| 1099 | This succeeds if the \*(L"martian\*(R" character \s-1GX\s0 is in the string, and fails |
| 1100 | otherwise. If you don't like using (?<!), a zero-width negative |
| 1101 | look-behind assertion, you can replace (?<![A\-Z]) with (?:^|[^A\-Z]). |
| 1102 | .PP |
| 1103 | It does have the drawback of putting the wrong thing in $\-[0] and $+[0], |
| 1104 | but this usually can be worked around. |
| 1105 | .Sh "How do I match a pattern that is supplied by the user?" |
| 1106 | .IX Subsection "How do I match a pattern that is supplied by the user?" |
| 1107 | Well, if it's really a pattern, then just use |
| 1108 | .PP |
| 1109 | .Vb 2 |
| 1110 | \& chomp($pattern = <STDIN>); |
| 1111 | \& if ($line =~ /$pattern/) { } |
| 1112 | .Ve |
| 1113 | .PP |
| 1114 | Alternatively, since you have no guarantee that your user entered |
| 1115 | a valid regular expression, trap the exception this way: |
| 1116 | .PP |
| 1117 | .Vb 1 |
| 1118 | \& if (eval { $line =~ /$pattern/ }) { } |
| 1119 | .Ve |
| 1120 | .PP |
| 1121 | If all you really want is to search for a string, not a pattern, |
| 1122 | then you should either use the \fIindex()\fR function, which is made for |
| 1123 | string searching, or, if you can't be disabused of using a pattern |
| 1124 | match on a non\-pattern, then be sure to use \f(CW\*(C`\eQ\*(C'\fR...\f(CW\*(C`\eE\*(C'\fR, documented |
| 1125 | in perlre. |
| 1126 | .PP |
| 1127 | .Vb 1 |
| 1128 | \& $pattern = <STDIN>; |
| 1129 | .Ve |
| 1130 | .PP |
| 1131 | .Vb 5 |
| 1132 | \& open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; |
| 1133 | \& while (<FILE>) { |
| 1134 | \& print if /\eQ$pattern\eE/; |
| 1135 | \& } |
| 1136 | \& close FILE; |
| 1137 | .Ve |
| 1138 | .SH "AUTHOR AND COPYRIGHT" |
| 1139 | .IX Header "AUTHOR AND COPYRIGHT" |
| 1140 | Copyright (c) 1997\-2006 Tom Christiansen, Nathan Torkington, and |
| 1141 | other authors as noted. All rights reserved. |
| 1142 | .PP |
| 1143 | This documentation is free; you can redistribute it and/or modify it |
| 1144 | under the same terms as Perl itself. |
| 1145 | .PP |
| 1146 | Irrespective of its distribution, all code examples in this file |
| 1147 | are hereby placed into the public domain. You are permitted and |
| 1148 | encouraged to use this code in your own programs for fun |
| 1149 | or for profit as you see fit. A simple comment in the code giving |
| 1150 | credit would be courteous but is not required. |