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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLREQUICK 1" |
| 132 | .TH PERLREQUICK 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlrequick \- Perl regular expressions quick start |
| 135 | .SH "DESCRIPTION" |
| 136 | .IX Header "DESCRIPTION" |
| 137 | This page covers the very basics of understanding, creating and |
| 138 | using regular expressions ('regexes') in Perl. |
| 139 | .SH "The Guide" |
| 140 | .IX Header "The Guide" |
| 141 | .Sh "Simple word matching" |
| 142 | .IX Subsection "Simple word matching" |
| 143 | The simplest regex is simply a word, or more generally, a string of |
| 144 | characters. A regex consisting of a word matches any string that |
| 145 | contains that word: |
| 146 | .PP |
| 147 | .Vb 1 |
| 148 | \& "Hello World" =~ /World/; # matches |
| 149 | .Ve |
| 150 | .PP |
| 151 | In this statement, \f(CW\*(C`World\*(C'\fR is a regex and the \f(CW\*(C`//\*(C'\fR enclosing |
| 152 | \&\f(CW\*(C`/World/\*(C'\fR tells perl to search a string for a match. The operator |
| 153 | \&\f(CW\*(C`=~\*(C'\fR associates the string with the regex match and produces a true |
| 154 | value if the regex matched, or false if the regex did not match. In |
| 155 | our case, \f(CW\*(C`World\*(C'\fR matches the second word in \f(CW"Hello World"\fR, so the |
| 156 | expression is true. This idea has several variations. |
| 157 | .PP |
| 158 | Expressions like this are useful in conditionals: |
| 159 | .PP |
| 160 | .Vb 1 |
| 161 | \& print "It matches\en" if "Hello World" =~ /World/; |
| 162 | .Ve |
| 163 | .PP |
| 164 | The sense of the match can be reversed by using \f(CW\*(C`!~\*(C'\fR operator: |
| 165 | .PP |
| 166 | .Vb 1 |
| 167 | \& print "It doesn't match\en" if "Hello World" !~ /World/; |
| 168 | .Ve |
| 169 | .PP |
| 170 | The literal string in the regex can be replaced by a variable: |
| 171 | .PP |
| 172 | .Vb 2 |
| 173 | \& $greeting = "World"; |
| 174 | \& print "It matches\en" if "Hello World" =~ /$greeting/; |
| 175 | .Ve |
| 176 | .PP |
| 177 | If you're matching against \f(CW$_\fR, the \f(CW\*(C`$_ =~\*(C'\fR part can be omitted: |
| 178 | .PP |
| 179 | .Vb 2 |
| 180 | \& $_ = "Hello World"; |
| 181 | \& print "It matches\en" if /World/; |
| 182 | .Ve |
| 183 | .PP |
| 184 | Finally, the \f(CW\*(C`//\*(C'\fR default delimiters for a match can be changed to |
| 185 | arbitrary delimiters by putting an \f(CW'm'\fR out front: |
| 186 | .PP |
| 187 | .Vb 4 |
| 188 | \& "Hello World" =~ m!World!; # matches, delimited by '!' |
| 189 | \& "Hello World" =~ m{World}; # matches, note the matching '{}' |
| 190 | \& "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin', |
| 191 | \& # '/' becomes an ordinary char |
| 192 | .Ve |
| 193 | .PP |
| 194 | Regexes must match a part of the string \fIexactly\fR in order for the |
| 195 | statement to be true: |
| 196 | .PP |
| 197 | .Vb 3 |
| 198 | \& "Hello World" =~ /world/; # doesn't match, case sensitive |
| 199 | \& "Hello World" =~ /o W/; # matches, ' ' is an ordinary char |
| 200 | \& "Hello World" =~ /World /; # doesn't match, no ' ' at end |
| 201 | .Ve |
| 202 | .PP |
| 203 | perl will always match at the earliest possible point in the string: |
| 204 | .PP |
| 205 | .Vb 2 |
| 206 | \& "Hello World" =~ /o/; # matches 'o' in 'Hello' |
| 207 | \& "That hat is red" =~ /hat/; # matches 'hat' in 'That' |
| 208 | .Ve |
| 209 | .PP |
| 210 | Not all characters can be used 'as is' in a match. Some characters, |
| 211 | called \fBmetacharacters\fR, are reserved for use in regex notation. |
| 212 | The metacharacters are |
| 213 | .PP |
| 214 | .Vb 1 |
| 215 | \& {}[]()^$.|*+?\e |
| 216 | .Ve |
| 217 | .PP |
| 218 | A metacharacter can be matched by putting a backslash before it: |
| 219 | .PP |
| 220 | .Vb 4 |
| 221 | \& "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter |
| 222 | \& "2+2=4" =~ /2\e+2/; # matches, \e+ is treated like an ordinary + |
| 223 | \& 'C:\eWIN32' =~ /C:\e\eWIN/; # matches |
| 224 | \& "/usr/bin/perl" =~ /\e/usr\e/local\e/bin\e/perl/; # matches |
| 225 | .Ve |
| 226 | .PP |
| 227 | In the last regex, the forward slash \f(CW'/'\fR is also backslashed, |
| 228 | because it is used to delimit the regex. |
| 229 | .PP |
| 230 | Non-printable \s-1ASCII\s0 characters are represented by \fBescape sequences\fR. |
| 231 | Common examples are \f(CW\*(C`\et\*(C'\fR for a tab, \f(CW\*(C`\en\*(C'\fR for a newline, and \f(CW\*(C`\er\*(C'\fR |
| 232 | for a carriage return. Arbitrary bytes are represented by octal |
| 233 | escape sequences, e.g., \f(CW\*(C`\e033\*(C'\fR, or hexadecimal escape sequences, |
| 234 | e.g., \f(CW\*(C`\ex1B\*(C'\fR: |
| 235 | .PP |
| 236 | .Vb 2 |
| 237 | \& "1000\et2000" =~ m(0\et2) # matches |
| 238 | \& "cat" =~ /\e143\ex61\ex74/ # matches, but a weird way to spell cat |
| 239 | .Ve |
| 240 | .PP |
| 241 | Regexes are treated mostly as double quoted strings, so variable |
| 242 | substitution works: |
| 243 | .PP |
| 244 | .Vb 3 |
| 245 | \& $foo = 'house'; |
| 246 | \& 'cathouse' =~ /cat$foo/; # matches |
| 247 | \& 'housecat' =~ /${foo}cat/; # matches |
| 248 | .Ve |
| 249 | .PP |
| 250 | With all of the regexes above, if the regex matched anywhere in the |
| 251 | string, it was considered a match. To specify \fIwhere\fR it should |
| 252 | match, we would use the \fBanchor\fR metacharacters \f(CW\*(C`^\*(C'\fR and \f(CW\*(C`$\*(C'\fR. The |
| 253 | anchor \f(CW\*(C`^\*(C'\fR means match at the beginning of the string and the anchor |
| 254 | \&\f(CW\*(C`$\*(C'\fR means match at the end of the string, or before a newline at the |
| 255 | end of the string. Some examples: |
| 256 | .PP |
| 257 | .Vb 5 |
| 258 | \& "housekeeper" =~ /keeper/; # matches |
| 259 | \& "housekeeper" =~ /^keeper/; # doesn't match |
| 260 | \& "housekeeper" =~ /keeper$/; # matches |
| 261 | \& "housekeeper\en" =~ /keeper$/; # matches |
| 262 | \& "housekeeper" =~ /^housekeeper$/; # matches |
| 263 | .Ve |
| 264 | .Sh "Using character classes" |
| 265 | .IX Subsection "Using character classes" |
| 266 | A \fBcharacter class\fR allows a set of possible characters, rather than |
| 267 | just a single character, to match at a particular point in a regex. |
| 268 | Character classes are denoted by brackets \f(CW\*(C`[...]\*(C'\fR, with the set of |
| 269 | characters to be possibly matched inside. Here are some examples: |
| 270 | .PP |
| 271 | .Vb 3 |
| 272 | \& /cat/; # matches 'cat' |
| 273 | \& /[bcr]at/; # matches 'bat', 'cat', or 'rat' |
| 274 | \& "abc" =~ /[cab]/; # matches 'a' |
| 275 | .Ve |
| 276 | .PP |
| 277 | In the last statement, even though \f(CW'c'\fR is the first character in |
| 278 | the class, the earliest point at which the regex can match is \f(CW'a'\fR. |
| 279 | .PP |
| 280 | .Vb 3 |
| 281 | \& /[yY][eE][sS]/; # match 'yes' in a case-insensitive way |
| 282 | \& # 'yes', 'Yes', 'YES', etc. |
| 283 | \& /yes/i; # also match 'yes' in a case-insensitive way |
| 284 | .Ve |
| 285 | .PP |
| 286 | The last example shows a match with an \f(CW'i'\fR \fBmodifier\fR, which makes |
| 287 | the match case\-insensitive. |
| 288 | .PP |
| 289 | Character classes also have ordinary and special characters, but the |
| 290 | sets of ordinary and special characters inside a character class are |
| 291 | different than those outside a character class. The special |
| 292 | characters for a character class are \f(CW\*(C`\-]\e^$\*(C'\fR and are matched using an |
| 293 | escape: |
| 294 | .PP |
| 295 | .Vb 5 |
| 296 | \& /[\e]c]def/; # matches ']def' or 'cdef' |
| 297 | \& $x = 'bcr'; |
| 298 | \& /[$x]at/; # matches 'bat, 'cat', or 'rat' |
| 299 | \& /[\e$x]at/; # matches '$at' or 'xat' |
| 300 | \& /[\e\e$x]at/; # matches '\eat', 'bat, 'cat', or 'rat' |
| 301 | .Ve |
| 302 | .PP |
| 303 | The special character \f(CW'\-'\fR acts as a range operator within character |
| 304 | classes, so that the unwieldy \f(CW\*(C`[0123456789]\*(C'\fR and \f(CW\*(C`[abc...xyz]\*(C'\fR |
| 305 | become the svelte \f(CW\*(C`[0\-9]\*(C'\fR and \f(CW\*(C`[a\-z]\*(C'\fR: |
| 306 | .PP |
| 307 | .Vb 2 |
| 308 | \& /item[0-9]/; # matches 'item0' or ... or 'item9' |
| 309 | \& /[0-9a-fA-F]/; # matches a hexadecimal digit |
| 310 | .Ve |
| 311 | .PP |
| 312 | If \f(CW'\-'\fR is the first or last character in a character class, it is |
| 313 | treated as an ordinary character. |
| 314 | .PP |
| 315 | The special character \f(CW\*(C`^\*(C'\fR in the first position of a character class |
| 316 | denotes a \fBnegated character class\fR, which matches any character but |
| 317 | those in the brackets. Both \f(CW\*(C`[...]\*(C'\fR and \f(CW\*(C`[^...]\*(C'\fR must match a |
| 318 | character, or the match fails. Then |
| 319 | .PP |
| 320 | .Vb 4 |
| 321 | \& /[^a]at/; # doesn't match 'aat' or 'at', but matches |
| 322 | \& # all other 'bat', 'cat, '0at', '%at', etc. |
| 323 | \& /[^0-9]/; # matches a non-numeric character |
| 324 | \& /[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary |
| 325 | .Ve |
| 326 | .PP |
| 327 | Perl has several abbreviations for common character classes: |
| 328 | .IP "\(bu" 4 |
| 329 | \&\ed is a digit and represents [0\-9] |
| 330 | .IP "\(bu" 4 |
| 331 | \&\es is a whitespace character and represents [\e \et\er\en\ef] |
| 332 | .IP "\(bu" 4 |
| 333 | \&\ew is a word character (alphanumeric or _) and represents [0\-9a\-zA\-Z_] |
| 334 | .IP "\(bu" 4 |
| 335 | \&\eD is a negated \ed; it represents any character but a digit [^0\-9] |
| 336 | .IP "\(bu" 4 |
| 337 | \&\eS is a negated \es; it represents any non-whitespace character [^\es] |
| 338 | .IP "\(bu" 4 |
| 339 | \&\eW is a negated \ew; it represents any non-word character [^\ew] |
| 340 | .IP "\(bu" 4 |
| 341 | The period '.' matches any character but \*(L"\en\*(R" |
| 342 | .PP |
| 343 | The \f(CW\*(C`\ed\es\ew\eD\eS\eW\*(C'\fR abbreviations can be used both inside and outside |
| 344 | of character classes. Here are some in use: |
| 345 | .PP |
| 346 | .Vb 7 |
| 347 | \& /\ed\ed:\ed\ed:\ed\ed/; # matches a hh:mm:ss time format |
| 348 | \& /[\ed\es]/; # matches any digit or whitespace character |
| 349 | \& /\ew\eW\ew/; # matches a word char, followed by a |
| 350 | \& # non-word char, followed by a word char |
| 351 | \& /..rt/; # matches any two chars, followed by 'rt' |
| 352 | \& /end\e./; # matches 'end.' |
| 353 | \& /end[.]/; # same thing, matches 'end.' |
| 354 | .Ve |
| 355 | .PP |
| 356 | The \fBword\ anchor\fR\ \f(CW\*(C`\eb\*(C'\fR matches a boundary between a word |
| 357 | character and a non-word character \f(CW\*(C`\ew\eW\*(C'\fR or \f(CW\*(C`\eW\ew\*(C'\fR: |
| 358 | .PP |
| 359 | .Vb 4 |
| 360 | \& $x = "Housecat catenates house and cat"; |
| 361 | \& $x =~ /\ebcat/; # matches cat in 'catenates' |
| 362 | \& $x =~ /cat\eb/; # matches cat in 'housecat' |
| 363 | \& $x =~ /\ebcat\eb/; # matches 'cat' at end of string |
| 364 | .Ve |
| 365 | .PP |
| 366 | In the last example, the end of the string is considered a word |
| 367 | boundary. |
| 368 | .Sh "Matching this or that" |
| 369 | .IX Subsection "Matching this or that" |
| 370 | We can match different character strings with the \fBalternation\fR |
| 371 | metacharacter \f(CW'|'\fR. To match \f(CW\*(C`dog\*(C'\fR or \f(CW\*(C`cat\*(C'\fR, we form the regex |
| 372 | \&\f(CW\*(C`dog|cat\*(C'\fR. As before, perl will try to match the regex at the |
| 373 | earliest possible point in the string. At each character position, |
| 374 | perl will first try to match the first alternative, \f(CW\*(C`dog\*(C'\fR. If |
| 375 | \&\f(CW\*(C`dog\*(C'\fR doesn't match, perl will then try the next alternative, \f(CW\*(C`cat\*(C'\fR. |
| 376 | If \f(CW\*(C`cat\*(C'\fR doesn't match either, then the match fails and perl moves to |
| 377 | the next position in the string. Some examples: |
| 378 | .PP |
| 379 | .Vb 2 |
| 380 | \& "cats and dogs" =~ /cat|dog|bird/; # matches "cat" |
| 381 | \& "cats and dogs" =~ /dog|cat|bird/; # matches "cat" |
| 382 | .Ve |
| 383 | .PP |
| 384 | Even though \f(CW\*(C`dog\*(C'\fR is the first alternative in the second regex, |
| 385 | \&\f(CW\*(C`cat\*(C'\fR is able to match earlier in the string. |
| 386 | .PP |
| 387 | .Vb 2 |
| 388 | \& "cats" =~ /c|ca|cat|cats/; # matches "c" |
| 389 | \& "cats" =~ /cats|cat|ca|c/; # matches "cats" |
| 390 | .Ve |
| 391 | .PP |
| 392 | At a given character position, the first alternative that allows the |
| 393 | regex match to succeed will be the one that matches. Here, all the |
| 394 | alternatives match at the first string position, so th first matches. |
| 395 | .Sh "Grouping things and hierarchical matching" |
| 396 | .IX Subsection "Grouping things and hierarchical matching" |
| 397 | The \fBgrouping\fR metacharacters \f(CW\*(C`()\*(C'\fR allow a part of a regex to be |
| 398 | treated as a single unit. Parts of a regex are grouped by enclosing |
| 399 | them in parentheses. The regex \f(CW\*(C`house(cat|keeper)\*(C'\fR means match |
| 400 | \&\f(CW\*(C`house\*(C'\fR followed by either \f(CW\*(C`cat\*(C'\fR or \f(CW\*(C`keeper\*(C'\fR. Some more examples |
| 401 | are |
| 402 | .PP |
| 403 | .Vb 2 |
| 404 | \& /(a|b)b/; # matches 'ab' or 'bb' |
| 405 | \& /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere |
| 406 | .Ve |
| 407 | .PP |
| 408 | .Vb 3 |
| 409 | \& /house(cat|)/; # matches either 'housecat' or 'house' |
| 410 | \& /house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or |
| 411 | \& # 'house'. Note groups can be nested. |
| 412 | .Ve |
| 413 | .PP |
| 414 | .Vb 2 |
| 415 | \& "20" =~ /(19|20|)\ed\ed/; # matches the null alternative '()\ed\ed', |
| 416 | \& # because '20\ed\ed' can't match |
| 417 | .Ve |
| 418 | .Sh "Extracting matches" |
| 419 | .IX Subsection "Extracting matches" |
| 420 | The grouping metacharacters \f(CW\*(C`()\*(C'\fR also allow the extraction of the |
| 421 | parts of a string that matched. For each grouping, the part that |
| 422 | matched inside goes into the special variables \f(CW$1\fR, \f(CW$2\fR, etc. |
| 423 | They can be used just as ordinary variables: |
| 424 | .PP |
| 425 | .Vb 5 |
| 426 | \& # extract hours, minutes, seconds |
| 427 | \& $time =~ /(\ed\ed):(\ed\ed):(\ed\ed)/; # match hh:mm:ss format |
| 428 | \& $hours = $1; |
| 429 | \& $minutes = $2; |
| 430 | \& $seconds = $3; |
| 431 | .Ve |
| 432 | .PP |
| 433 | In list context, a match \f(CW\*(C`/regex/\*(C'\fR with groupings will return the |
| 434 | list of matched values \f(CW\*(C`($1,$2,...)\*(C'\fR. So we could rewrite it as |
| 435 | .PP |
| 436 | .Vb 1 |
| 437 | \& ($hours, $minutes, $second) = ($time =~ /(\ed\ed):(\ed\ed):(\ed\ed)/); |
| 438 | .Ve |
| 439 | .PP |
| 440 | If the groupings in a regex are nested, \f(CW$1\fR gets the group with the |
| 441 | leftmost opening parenthesis, \f(CW$2\fR the next opening parenthesis, |
| 442 | etc. For example, here is a complex regex and the matching variables |
| 443 | indicated below it: |
| 444 | .PP |
| 445 | .Vb 2 |
| 446 | \& /(ab(cd|ef)((gi)|j))/; |
| 447 | \& 1 2 34 |
| 448 | .Ve |
| 449 | .PP |
| 450 | Associated with the matching variables \f(CW$1\fR, \f(CW$2\fR, ... are |
| 451 | the \fBbackreferences\fR \f(CW\*(C`\e1\*(C'\fR, \f(CW\*(C`\e2\*(C'\fR, ... Backreferences are |
| 452 | matching variables that can be used \fIinside\fR a regex: |
| 453 | .PP |
| 454 | .Vb 1 |
| 455 | \& /(\ew\ew\ew)\es\e1/; # find sequences like 'the the' in string |
| 456 | .Ve |
| 457 | .PP |
| 458 | \&\f(CW$1\fR, \f(CW$2\fR, ... should only be used outside of a regex, and \f(CW\*(C`\e1\*(C'\fR, |
| 459 | \&\f(CW\*(C`\e2\*(C'\fR, ... only inside a regex. |
| 460 | .Sh "Matching repetitions" |
| 461 | .IX Subsection "Matching repetitions" |
| 462 | The \fBquantifier\fR metacharacters \f(CW\*(C`?\*(C'\fR, \f(CW\*(C`*\*(C'\fR, \f(CW\*(C`+\*(C'\fR, and \f(CW\*(C`{}\*(C'\fR allow us |
| 463 | to determine the number of repeats of a portion of a regex we |
| 464 | consider to be a match. Quantifiers are put immediately after the |
| 465 | character, character class, or grouping that we want to specify. They |
| 466 | have the following meanings: |
| 467 | .IP "\(bu" 4 |
| 468 | \&\f(CW\*(C`a?\*(C'\fR = match 'a' 1 or 0 times |
| 469 | .IP "\(bu" 4 |
| 470 | \&\f(CW\*(C`a*\*(C'\fR = match 'a' 0 or more times, i.e., any number of times |
| 471 | .IP "\(bu" 4 |
| 472 | \&\f(CW\*(C`a+\*(C'\fR = match 'a' 1 or more times, i.e., at least once |
| 473 | .IP "\(bu" 4 |
| 474 | \&\f(CW\*(C`a{n,m}\*(C'\fR = match at least \f(CW\*(C`n\*(C'\fR times, but not more than \f(CW\*(C`m\*(C'\fR |
| 475 | times. |
| 476 | .IP "\(bu" 4 |
| 477 | \&\f(CW\*(C`a{n,}\*(C'\fR = match at least \f(CW\*(C`n\*(C'\fR or more times |
| 478 | .IP "\(bu" 4 |
| 479 | \&\f(CW\*(C`a{n}\*(C'\fR = match exactly \f(CW\*(C`n\*(C'\fR times |
| 480 | .PP |
| 481 | Here are some examples: |
| 482 | .PP |
| 483 | .Vb 6 |
| 484 | \& /[a-z]+\es+\ed*/; # match a lowercase word, at least some space, and |
| 485 | \& # any number of digits |
| 486 | \& /(\ew+)\es+\e1/; # match doubled words of arbitrary length |
| 487 | \& $year =~ /\ed{2,4}/; # make sure year is at least 2 but not more |
| 488 | \& # than 4 digits |
| 489 | \& $year =~ /\ed{4}|\ed{2}/; # better match; throw out 3 digit dates |
| 490 | .Ve |
| 491 | .PP |
| 492 | These quantifiers will try to match as much of the string as possible, |
| 493 | while still allowing the regex to match. So we have |
| 494 | .PP |
| 495 | .Vb 5 |
| 496 | \& $x = 'the cat in the hat'; |
| 497 | \& $x =~ /^(.*)(at)(.*)$/; # matches, |
| 498 | \& # $1 = 'the cat in the h' |
| 499 | \& # $2 = 'at' |
| 500 | \& # $3 = '' (0 matches) |
| 501 | .Ve |
| 502 | .PP |
| 503 | The first quantifier \f(CW\*(C`.*\*(C'\fR grabs as much of the string as possible |
| 504 | while still having the regex match. The second quantifier \f(CW\*(C`.*\*(C'\fR has |
| 505 | no string left to it, so it matches 0 times. |
| 506 | .Sh "More matching" |
| 507 | .IX Subsection "More matching" |
| 508 | There are a few more things you might want to know about matching |
| 509 | operators. In the code |
| 510 | .PP |
| 511 | .Vb 4 |
| 512 | \& $pattern = 'Seuss'; |
| 513 | \& while (<>) { |
| 514 | \& print if /$pattern/; |
| 515 | \& } |
| 516 | .Ve |
| 517 | .PP |
| 518 | perl has to re-evaluate \f(CW$pattern\fR each time through the loop. If |
| 519 | \&\f(CW$pattern\fR won't be changing, use the \f(CW\*(C`//o\*(C'\fR modifier, to only |
| 520 | perform variable substitutions once. If you don't want any |
| 521 | substitutions at all, use the special delimiter \f(CW\*(C`m''\*(C'\fR: |
| 522 | .PP |
| 523 | .Vb 2 |
| 524 | \& $pattern = 'Seuss'; |
| 525 | \& m'$pattern'; # matches '$pattern', not 'Seuss' |
| 526 | .Ve |
| 527 | .PP |
| 528 | The global modifier \f(CW\*(C`//g\*(C'\fR allows the matching operator to match |
| 529 | within a string as many times as possible. In scalar context, |
| 530 | successive matches against a string will have \f(CW\*(C`//g\*(C'\fR jump from match |
| 531 | to match, keeping track of position in the string as it goes along. |
| 532 | You can get or set the position with the \f(CW\*(C`pos()\*(C'\fR function. |
| 533 | For example, |
| 534 | .PP |
| 535 | .Vb 4 |
| 536 | \& $x = "cat dog house"; # 3 words |
| 537 | \& while ($x =~ /(\ew+)/g) { |
| 538 | \& print "Word is $1, ends at position ", pos $x, "\en"; |
| 539 | \& } |
| 540 | .Ve |
| 541 | .PP |
| 542 | prints |
| 543 | .PP |
| 544 | .Vb 3 |
| 545 | \& Word is cat, ends at position 3 |
| 546 | \& Word is dog, ends at position 7 |
| 547 | \& Word is house, ends at position 13 |
| 548 | .Ve |
| 549 | .PP |
| 550 | A failed match or changing the target string resets the position. If |
| 551 | you don't want the position reset after failure to match, add the |
| 552 | \&\f(CW\*(C`//c\*(C'\fR, as in \f(CW\*(C`/regex/gc\*(C'\fR. |
| 553 | .PP |
| 554 | In list context, \f(CW\*(C`//g\*(C'\fR returns a list of matched groupings, or if |
| 555 | there are no groupings, a list of matches to the whole regex. So |
| 556 | .PP |
| 557 | .Vb 4 |
| 558 | \& @words = ($x =~ /(\ew+)/g); # matches, |
| 559 | \& # $word[0] = 'cat' |
| 560 | \& # $word[1] = 'dog' |
| 561 | \& # $word[2] = 'house' |
| 562 | .Ve |
| 563 | .Sh "Search and replace" |
| 564 | .IX Subsection "Search and replace" |
| 565 | Search and replace is performed using \f(CW\*(C`s/regex/replacement/modifiers\*(C'\fR. |
| 566 | The \f(CW\*(C`replacement\*(C'\fR is a Perl double quoted string that replaces in the |
| 567 | string whatever is matched with the \f(CW\*(C`regex\*(C'\fR. The operator \f(CW\*(C`=~\*(C'\fR is |
| 568 | also used here to associate a string with \f(CW\*(C`s///\*(C'\fR. If matching |
| 569 | against \f(CW$_\fR, the \f(CW\*(C`$_\ =~\*(C'\fR\ can be dropped. If there is a match, |
| 570 | \&\f(CW\*(C`s///\*(C'\fR returns the number of substitutions made, otherwise it returns |
| 571 | false. Here are a few examples: |
| 572 | .PP |
| 573 | .Vb 5 |
| 574 | \& $x = "Time to feed the cat!"; |
| 575 | \& $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!" |
| 576 | \& $y = "'quoted words'"; |
| 577 | \& $y =~ s/^'(.*)'$/$1/; # strip single quotes, |
| 578 | \& # $y contains "quoted words" |
| 579 | .Ve |
| 580 | .PP |
| 581 | With the \f(CW\*(C`s///\*(C'\fR operator, the matched variables \f(CW$1\fR, \f(CW$2\fR, etc. |
| 582 | are immediately available for use in the replacement expression. With |
| 583 | the global modifier, \f(CW\*(C`s///g\*(C'\fR will search and replace all occurrences |
| 584 | of the regex in the string: |
| 585 | .PP |
| 586 | .Vb 4 |
| 587 | \& $x = "I batted 4 for 4"; |
| 588 | \& $x =~ s/4/four/; # $x contains "I batted four for 4" |
| 589 | \& $x = "I batted 4 for 4"; |
| 590 | \& $x =~ s/4/four/g; # $x contains "I batted four for four" |
| 591 | .Ve |
| 592 | .PP |
| 593 | The evaluation modifier \f(CW\*(C`s///e\*(C'\fR wraps an \f(CW\*(C`eval{...}\*(C'\fR around the |
| 594 | replacement string and the evaluated result is substituted for the |
| 595 | matched substring. Some examples: |
| 596 | .PP |
| 597 | .Vb 3 |
| 598 | \& # reverse all the words in a string |
| 599 | \& $x = "the cat in the hat"; |
| 600 | \& $x =~ s/(\ew+)/reverse $1/ge; # $x contains "eht tac ni eht tah" |
| 601 | .Ve |
| 602 | .PP |
| 603 | .Vb 3 |
| 604 | \& # convert percentage to decimal |
| 605 | \& $x = "A 39% hit rate"; |
| 606 | \& $x =~ s!(\ed+)%!$1/100!e; # $x contains "A 0.39 hit rate" |
| 607 | .Ve |
| 608 | .PP |
| 609 | The last example shows that \f(CW\*(C`s///\*(C'\fR can use other delimiters, such as |
| 610 | \&\f(CW\*(C`s!!!\*(C'\fR and \f(CW\*(C`s{}{}\*(C'\fR, and even \f(CW\*(C`s{}//\*(C'\fR. If single quotes are used |
| 611 | \&\f(CW\*(C`s'''\*(C'\fR, then the regex and replacement are treated as single quoted |
| 612 | strings. |
| 613 | .Sh "The split operator" |
| 614 | .IX Subsection "The split operator" |
| 615 | \&\f(CW\*(C`split /regex/, string\*(C'\fR splits \f(CW\*(C`string\*(C'\fR into a list of substrings |
| 616 | and returns that list. The regex determines the character sequence |
| 617 | that \f(CW\*(C`string\*(C'\fR is split with respect to. For example, to split a |
| 618 | string into words, use |
| 619 | .PP |
| 620 | .Vb 4 |
| 621 | \& $x = "Calvin and Hobbes"; |
| 622 | \& @word = split /\es+/, $x; # $word[0] = 'Calvin' |
| 623 | \& # $word[1] = 'and' |
| 624 | \& # $word[2] = 'Hobbes' |
| 625 | .Ve |
| 626 | .PP |
| 627 | To extract a comma-delimited list of numbers, use |
| 628 | .PP |
| 629 | .Vb 4 |
| 630 | \& $x = "1.618,2.718, 3.142"; |
| 631 | \& @const = split /,\es*/, $x; # $const[0] = '1.618' |
| 632 | \& # $const[1] = '2.718' |
| 633 | \& # $const[2] = '3.142' |
| 634 | .Ve |
| 635 | .PP |
| 636 | If the empty regex \f(CW\*(C`//\*(C'\fR is used, the string is split into individual |
| 637 | characters. If the regex has groupings, then list produced contains |
| 638 | the matched substrings from the groupings as well: |
| 639 | .PP |
| 640 | .Vb 6 |
| 641 | \& $x = "/usr/bin"; |
| 642 | \& @parts = split m!(/)!, $x; # $parts[0] = '' |
| 643 | \& # $parts[1] = '/' |
| 644 | \& # $parts[2] = 'usr' |
| 645 | \& # $parts[3] = '/' |
| 646 | \& # $parts[4] = 'bin' |
| 647 | .Ve |
| 648 | .PP |
| 649 | Since the first character of \f(CW$x\fR matched the regex, \f(CW\*(C`split\*(C'\fR prepended |
| 650 | an empty initial element to the list. |
| 651 | .SH "BUGS" |
| 652 | .IX Header "BUGS" |
| 653 | None. |
| 654 | .SH "SEE ALSO" |
| 655 | .IX Header "SEE ALSO" |
| 656 | This is just a quick start guide. For a more in-depth tutorial on |
| 657 | regexes, see perlretut and for the reference page, see perlre. |
| 658 | .SH "AUTHOR AND COPYRIGHT" |
| 659 | .IX Header "AUTHOR AND COPYRIGHT" |
| 660 | Copyright (c) 2000 Mark Kvale |
| 661 | All rights reserved. |
| 662 | .PP |
| 663 | This document may be distributed under the same terms as Perl itself. |
| 664 | .Sh "Acknowledgments" |
| 665 | .IX Subsection "Acknowledgments" |
| 666 | The author would like to thank Mark-Jason Dominus, Tom Christiansen, |
| 667 | Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful |
| 668 | comments. |