Torok Edwin | ce0c81e | 2009-08-30 08:24:09 +0000 | [diff] [blame] | 1 | .\" $OpenBSD: re_format.7,v 1.14 2007/05/31 19:19:30 jmc Exp $ |
| 2 | .\" |
| 3 | .\" Copyright (c) 1997, Phillip F Knaack. All rights reserved. |
| 4 | .\" |
| 5 | .\" Copyright (c) 1992, 1993, 1994 Henry Spencer. |
| 6 | .\" Copyright (c) 1992, 1993, 1994 |
| 7 | .\" The Regents of the University of California. All rights reserved. |
| 8 | .\" |
| 9 | .\" This code is derived from software contributed to Berkeley by |
| 10 | .\" Henry Spencer. |
| 11 | .\" |
| 12 | .\" Redistribution and use in source and binary forms, with or without |
| 13 | .\" modification, are permitted provided that the following conditions |
| 14 | .\" are met: |
| 15 | .\" 1. Redistributions of source code must retain the above copyright |
| 16 | .\" notice, this list of conditions and the following disclaimer. |
| 17 | .\" 2. Redistributions in binary form must reproduce the above copyright |
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| 19 | .\" documentation and/or other materials provided with the distribution. |
| 20 | .\" 3. Neither the name of the University nor the names of its contributors |
| 21 | .\" may be used to endorse or promote products derived from this software |
| 22 | .\" without specific prior written permission. |
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| 24 | .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND |
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| 36 | .\" @(#)re_format.7 8.3 (Berkeley) 3/20/94 |
| 37 | .\" |
| 38 | .Dd $Mdocdate: May 31 2007 $ |
| 39 | .Dt RE_FORMAT 7 |
| 40 | .Os |
| 41 | .Sh NAME |
| 42 | .Nm re_format |
| 43 | .Nd POSIX regular expressions |
| 44 | .Sh DESCRIPTION |
| 45 | Regular expressions (REs), |
| 46 | as defined in |
| 47 | .St -p1003.1-2004 , |
| 48 | come in two forms: |
| 49 | basic regular expressions |
| 50 | (BREs) |
| 51 | and extended regular expressions |
| 52 | (EREs). |
| 53 | Both forms of regular expressions are supported |
| 54 | by the interfaces described in |
| 55 | .Xr regex 3 . |
| 56 | Applications dealing with regular expressions |
| 57 | may use one or the other form |
| 58 | (or indeed both). |
| 59 | For example, |
| 60 | .Xr ed 1 |
| 61 | uses BREs, |
| 62 | whilst |
| 63 | .Xr egrep 1 |
| 64 | talks EREs. |
| 65 | Consult the manual page for the specific application to find out which |
| 66 | it uses. |
| 67 | .Pp |
| 68 | POSIX leaves some aspects of RE syntax and semantics open; |
| 69 | .Sq ** |
| 70 | marks decisions on these aspects that |
| 71 | may not be fully portable to other POSIX implementations. |
| 72 | .Pp |
| 73 | This manual page first describes regular expressions in general, |
| 74 | specifically extended regular expressions, |
| 75 | and then discusses differences between them and basic regular expressions. |
| 76 | .Sh EXTENDED REGULAR EXPRESSIONS |
| 77 | An ERE is one** or more non-empty** |
| 78 | .Em branches , |
| 79 | separated by |
| 80 | .Sq \*(Ba . |
| 81 | It matches anything that matches one of the branches. |
| 82 | .Pp |
| 83 | A branch is one** or more |
| 84 | .Em pieces , |
| 85 | concatenated. |
| 86 | It matches a match for the first, followed by a match for the second, etc. |
| 87 | .Pp |
| 88 | A piece is an |
| 89 | .Em atom |
| 90 | possibly followed by a single** |
| 91 | .Sq * , |
| 92 | .Sq + , |
| 93 | .Sq ?\& , |
| 94 | or |
| 95 | .Em bound . |
| 96 | An atom followed by |
| 97 | .Sq * |
| 98 | matches a sequence of 0 or more matches of the atom. |
| 99 | An atom followed by |
| 100 | .Sq + |
| 101 | matches a sequence of 1 or more matches of the atom. |
| 102 | An atom followed by |
| 103 | .Sq ?\& |
| 104 | matches a sequence of 0 or 1 matches of the atom. |
| 105 | .Pp |
| 106 | A bound is |
| 107 | .Sq { |
| 108 | followed by an unsigned decimal integer, |
| 109 | possibly followed by |
| 110 | .Sq ,\& |
| 111 | possibly followed by another unsigned decimal integer, |
| 112 | always followed by |
| 113 | .Sq } . |
| 114 | The integers must lie between 0 and |
| 115 | .Dv RE_DUP_MAX |
| 116 | (255**) inclusive, |
| 117 | and if there are two of them, the first may not exceed the second. |
| 118 | An atom followed by a bound containing one integer |
| 119 | .Ar i |
| 120 | and no comma matches |
| 121 | a sequence of exactly |
| 122 | .Ar i |
| 123 | matches of the atom. |
| 124 | An atom followed by a bound |
| 125 | containing one integer |
| 126 | .Ar i |
| 127 | and a comma matches |
| 128 | a sequence of |
| 129 | .Ar i |
| 130 | or more matches of the atom. |
| 131 | An atom followed by a bound |
| 132 | containing two integers |
| 133 | .Ar i |
| 134 | and |
| 135 | .Ar j |
| 136 | matches a sequence of |
| 137 | .Ar i |
| 138 | through |
| 139 | .Ar j |
| 140 | (inclusive) matches of the atom. |
| 141 | .Pp |
| 142 | An atom is a regular expression enclosed in |
| 143 | .Sq () |
| 144 | (matching a part of the regular expression), |
| 145 | an empty set of |
| 146 | .Sq () |
| 147 | (matching the null string)**, |
| 148 | a |
| 149 | .Em bracket expression |
| 150 | (see below), |
| 151 | .Sq .\& |
| 152 | (matching any single character), |
| 153 | .Sq ^ |
| 154 | (matching the null string at the beginning of a line), |
| 155 | .Sq $ |
| 156 | (matching the null string at the end of a line), |
| 157 | a |
| 158 | .Sq \e |
| 159 | followed by one of the characters |
| 160 | .Sq ^.[$()|*+?{\e |
| 161 | (matching that character taken as an ordinary character), |
| 162 | a |
| 163 | .Sq \e |
| 164 | followed by any other character** |
| 165 | (matching that character taken as an ordinary character, |
| 166 | as if the |
| 167 | .Sq \e |
| 168 | had not been present**), |
| 169 | or a single character with no other significance (matching that character). |
| 170 | A |
| 171 | .Sq { |
| 172 | followed by a character other than a digit is an ordinary character, |
| 173 | not the beginning of a bound**. |
| 174 | It is illegal to end an RE with |
| 175 | .Sq \e . |
| 176 | .Pp |
| 177 | A bracket expression is a list of characters enclosed in |
| 178 | .Sq [] . |
| 179 | It normally matches any single character from the list (but see below). |
| 180 | If the list begins with |
| 181 | .Sq ^ , |
| 182 | it matches any single character |
| 183 | .Em not |
| 184 | from the rest of the list |
| 185 | (but see below). |
| 186 | If two characters in the list are separated by |
| 187 | .Sq - , |
| 188 | this is shorthand for the full |
| 189 | .Em range |
| 190 | of characters between those two (inclusive) in the |
| 191 | collating sequence, e.g.\& |
| 192 | .Sq [0-9] |
| 193 | in ASCII matches any decimal digit. |
| 194 | It is illegal** for two ranges to share an endpoint, e.g.\& |
| 195 | .Sq a-c-e . |
| 196 | Ranges are very collating-sequence-dependent, |
| 197 | and portable programs should avoid relying on them. |
| 198 | .Pp |
| 199 | To include a literal |
| 200 | .Sq ]\& |
| 201 | in the list, make it the first character |
| 202 | (following a possible |
| 203 | .Sq ^ ) . |
| 204 | To include a literal |
| 205 | .Sq - , |
| 206 | make it the first or last character, |
| 207 | or the second endpoint of a range. |
| 208 | To use a literal |
| 209 | .Sq - |
| 210 | as the first endpoint of a range, |
| 211 | enclose it in |
| 212 | .Sq [. |
| 213 | and |
| 214 | .Sq .] |
| 215 | to make it a collating element (see below). |
| 216 | With the exception of these and some combinations using |
| 217 | .Sq [ |
| 218 | (see next paragraphs), |
| 219 | all other special characters, including |
| 220 | .Sq \e , |
| 221 | lose their special significance within a bracket expression. |
| 222 | .Pp |
| 223 | Within a bracket expression, a collating element |
| 224 | (a character, |
| 225 | a multi-character sequence that collates as if it were a single character, |
| 226 | or a collating-sequence name for either) |
| 227 | enclosed in |
| 228 | .Sq [. |
| 229 | and |
| 230 | .Sq .] |
| 231 | stands for the sequence of characters of that collating element. |
| 232 | The sequence is a single element of the bracket expression's list. |
| 233 | A bracket expression containing a multi-character collating element |
| 234 | can thus match more than one character, |
| 235 | e.g. if the collating sequence includes a |
| 236 | .Sq ch |
| 237 | collating element, |
| 238 | then the RE |
| 239 | .Sq [[.ch.]]*c |
| 240 | matches the first five characters of |
| 241 | .Sq chchcc . |
| 242 | .Pp |
| 243 | Within a bracket expression, a collating element enclosed in |
| 244 | .Sq [= |
| 245 | and |
| 246 | .Sq =] |
| 247 | is an equivalence class, standing for the sequences of characters |
| 248 | of all collating elements equivalent to that one, including itself. |
| 249 | (If there are no other equivalent collating elements, |
| 250 | the treatment is as if the enclosing delimiters were |
| 251 | .Sq [. |
| 252 | and |
| 253 | .Sq .] . ) |
| 254 | For example, if |
| 255 | .Sq x |
| 256 | and |
| 257 | .Sq y |
| 258 | are the members of an equivalence class, |
| 259 | then |
| 260 | .Sq [[=x=]] , |
| 261 | .Sq [[=y=]] , |
| 262 | and |
| 263 | .Sq [xy] |
| 264 | are all synonymous. |
| 265 | An equivalence class may not** be an endpoint of a range. |
| 266 | .Pp |
| 267 | Within a bracket expression, the name of a |
| 268 | .Em character class |
| 269 | enclosed |
| 270 | in |
| 271 | .Sq [: |
| 272 | and |
| 273 | .Sq :] |
| 274 | stands for the list of all characters belonging to that class. |
| 275 | Standard character class names are: |
| 276 | .Bd -literal -offset indent |
| 277 | alnum digit punct |
| 278 | alpha graph space |
| 279 | blank lower upper |
| 280 | cntrl print xdigit |
| 281 | .Ed |
| 282 | .Pp |
| 283 | These stand for the character classes defined in |
| 284 | .Xr ctype 3 . |
| 285 | A locale may provide others. |
| 286 | A character class may not be used as an endpoint of a range. |
| 287 | .Pp |
| 288 | There are two special cases** of bracket expressions: |
| 289 | the bracket expressions |
| 290 | .Sq [[:<:]] |
| 291 | and |
| 292 | .Sq [[:>:]] |
| 293 | match the null string at the beginning and end of a word, respectively. |
| 294 | A word is defined as a sequence of |
| 295 | characters starting and ending with a word character |
| 296 | which is neither preceded nor followed by |
| 297 | word characters. |
| 298 | A word character is an |
| 299 | .Em alnum |
| 300 | character (as defined by |
| 301 | .Xr ctype 3 ) |
| 302 | or an underscore. |
| 303 | This is an extension, |
| 304 | compatible with but not specified by POSIX, |
| 305 | and should be used with |
| 306 | caution in software intended to be portable to other systems. |
| 307 | .Pp |
| 308 | In the event that an RE could match more than one substring of a given |
| 309 | string, |
| 310 | the RE matches the one starting earliest in the string. |
| 311 | If the RE could match more than one substring starting at that point, |
| 312 | it matches the longest. |
| 313 | Subexpressions also match the longest possible substrings, subject to |
| 314 | the constraint that the whole match be as long as possible, |
| 315 | with subexpressions starting earlier in the RE taking priority over |
| 316 | ones starting later. |
| 317 | Note that higher-level subexpressions thus take priority over |
| 318 | their lower-level component subexpressions. |
| 319 | .Pp |
| 320 | Match lengths are measured in characters, not collating elements. |
| 321 | A null string is considered longer than no match at all. |
| 322 | For example, |
| 323 | .Sq bb* |
| 324 | matches the three middle characters of |
| 325 | .Sq abbbc ; |
| 326 | .Sq (wee|week)(knights|nights) |
| 327 | matches all ten characters of |
| 328 | .Sq weeknights ; |
| 329 | when |
| 330 | .Sq (.*).* |
| 331 | is matched against |
| 332 | .Sq abc , |
| 333 | the parenthesized subexpression matches all three characters; |
| 334 | and when |
| 335 | .Sq (a*)* |
| 336 | is matched against |
| 337 | .Sq bc , |
| 338 | both the whole RE and the parenthesized subexpression match the null string. |
| 339 | .Pp |
| 340 | If case-independent matching is specified, |
| 341 | the effect is much as if all case distinctions had vanished from the |
| 342 | alphabet. |
| 343 | When an alphabetic that exists in multiple cases appears as an |
| 344 | ordinary character outside a bracket expression, it is effectively |
| 345 | transformed into a bracket expression containing both cases, |
| 346 | e.g.\& |
| 347 | .Sq x |
| 348 | becomes |
| 349 | .Sq [xX] . |
| 350 | When it appears inside a bracket expression, |
| 351 | all case counterparts of it are added to the bracket expression, |
| 352 | so that, for example, |
| 353 | .Sq [x] |
| 354 | becomes |
| 355 | .Sq [xX] |
| 356 | and |
| 357 | .Sq [^x] |
| 358 | becomes |
| 359 | .Sq [^xX] . |
| 360 | .Pp |
| 361 | No particular limit is imposed on the length of REs**. |
| 362 | Programs intended to be portable should not employ REs longer |
| 363 | than 256 bytes, |
| 364 | as an implementation can refuse to accept such REs and remain |
| 365 | POSIX-compliant. |
| 366 | .Pp |
| 367 | The following is a list of extended regular expressions: |
| 368 | .Bl -tag -width Ds |
| 369 | .It Ar c |
| 370 | Any character |
| 371 | .Ar c |
| 372 | not listed below matches itself. |
| 373 | .It \e Ns Ar c |
| 374 | Any backslash-escaped character |
| 375 | .Ar c |
| 376 | matches itself. |
| 377 | .It \&. |
| 378 | Matches any single character that is not a newline |
| 379 | .Pq Sq \en . |
| 380 | .It Bq Ar char-class |
| 381 | Matches any single character in |
| 382 | .Ar char-class . |
| 383 | To include a |
| 384 | .Ql \&] |
| 385 | in |
| 386 | .Ar char-class , |
| 387 | it must be the first character. |
| 388 | A range of characters may be specified by separating the end characters |
| 389 | of the range with a |
| 390 | .Ql - ; |
| 391 | e.g.\& |
| 392 | .Ar a-z |
| 393 | specifies the lower case characters. |
| 394 | The following literal expressions can also be used in |
| 395 | .Ar char-class |
| 396 | to specify sets of characters: |
| 397 | .Bd -unfilled -offset indent |
| 398 | [:alnum:] [:cntrl:] [:lower:] [:space:] |
| 399 | [:alpha:] [:digit:] [:print:] [:upper:] |
| 400 | [:blank:] [:graph:] [:punct:] [:xdigit:] |
| 401 | .Ed |
| 402 | .Pp |
| 403 | If |
| 404 | .Ql - |
| 405 | appears as the first or last character of |
| 406 | .Ar char-class , |
| 407 | then it matches itself. |
| 408 | All other characters in |
| 409 | .Ar char-class |
| 410 | match themselves. |
| 411 | .Pp |
| 412 | Patterns in |
| 413 | .Ar char-class |
| 414 | of the form |
| 415 | .Eo [. |
| 416 | .Ar col-elm |
| 417 | .Ec .]\& |
| 418 | or |
| 419 | .Eo [= |
| 420 | .Ar col-elm |
| 421 | .Ec =]\& , |
| 422 | where |
| 423 | .Ar col-elm |
| 424 | is a collating element, are interpreted according to |
| 425 | .Xr setlocale 3 |
| 426 | .Pq not currently supported . |
| 427 | .It Bq ^ Ns Ar char-class |
| 428 | Matches any single character, other than newline, not in |
| 429 | .Ar char-class . |
| 430 | .Ar char-class |
| 431 | is defined as above. |
| 432 | .It ^ |
| 433 | If |
| 434 | .Sq ^ |
| 435 | is the first character of a regular expression, then it |
| 436 | anchors the regular expression to the beginning of a line. |
| 437 | Otherwise, it matches itself. |
| 438 | .It $ |
| 439 | If |
| 440 | .Sq $ |
| 441 | is the last character of a regular expression, |
| 442 | it anchors the regular expression to the end of a line. |
| 443 | Otherwise, it matches itself. |
| 444 | .It [[:<:]] |
| 445 | Anchors the single character regular expression or subexpression |
| 446 | immediately following it to the beginning of a word. |
| 447 | .It [[:>:]] |
| 448 | Anchors the single character regular expression or subexpression |
| 449 | immediately following it to the end of a word. |
| 450 | .It Pq Ar re |
| 451 | Defines a subexpression |
| 452 | .Ar re . |
| 453 | Any set of characters enclosed in parentheses |
| 454 | matches whatever the set of characters without parentheses matches |
| 455 | (that is a long-winded way of saying the constructs |
| 456 | .Sq (re) |
| 457 | and |
| 458 | .Sq re |
| 459 | match identically). |
| 460 | .It * |
| 461 | Matches the single character regular expression or subexpression |
| 462 | immediately preceding it zero or more times. |
| 463 | If |
| 464 | .Sq * |
| 465 | is the first character of a regular expression or subexpression, |
| 466 | then it matches itself. |
| 467 | The |
| 468 | .Sq * |
| 469 | operator sometimes yields unexpected results. |
| 470 | For example, the regular expression |
| 471 | .Ar b* |
| 472 | matches the beginning of the string |
| 473 | .Qq abbb |
| 474 | (as opposed to the substring |
| 475 | .Qq bbb ) , |
| 476 | since a null match is the only leftmost match. |
| 477 | .It + |
| 478 | Matches the singular character regular expression |
| 479 | or subexpression immediately preceding it |
| 480 | one or more times. |
| 481 | .It ? |
| 482 | Matches the singular character regular expression |
| 483 | or subexpression immediately preceding it |
| 484 | 0 or 1 times. |
| 485 | .Sm off |
| 486 | .It Xo |
| 487 | .Pf { Ar n , m No }\ \& |
| 488 | .Pf { Ar n , No }\ \& |
| 489 | .Pf { Ar n No } |
| 490 | .Xc |
| 491 | .Sm on |
| 492 | Matches the single character regular expression or subexpression |
| 493 | immediately preceding it at least |
| 494 | .Ar n |
| 495 | and at most |
| 496 | .Ar m |
| 497 | times. |
| 498 | If |
| 499 | .Ar m |
| 500 | is omitted, then it matches at least |
| 501 | .Ar n |
| 502 | times. |
| 503 | If the comma is also omitted, then it matches exactly |
| 504 | .Ar n |
| 505 | times. |
| 506 | .It \*(Ba |
| 507 | Used to separate patterns. |
| 508 | For example, |
| 509 | the pattern |
| 510 | .Sq cat\*(Badog |
| 511 | matches either |
| 512 | .Sq cat |
| 513 | or |
| 514 | .Sq dog . |
| 515 | .El |
| 516 | .Sh BASIC REGULAR EXPRESSIONS |
| 517 | Basic regular expressions differ in several respects: |
| 518 | .Bl -bullet -offset 3n |
| 519 | .It |
| 520 | .Sq \*(Ba , |
| 521 | .Sq + , |
| 522 | and |
| 523 | .Sq ?\& |
| 524 | are ordinary characters and there is no equivalent |
| 525 | for their functionality. |
| 526 | .It |
| 527 | The delimiters for bounds are |
| 528 | .Sq \e{ |
| 529 | and |
| 530 | .Sq \e} , |
| 531 | with |
| 532 | .Sq { |
| 533 | and |
| 534 | .Sq } |
| 535 | by themselves ordinary characters. |
| 536 | .It |
| 537 | The parentheses for nested subexpressions are |
| 538 | .Sq \e( |
| 539 | and |
| 540 | .Sq \e) , |
| 541 | with |
| 542 | .Sq ( |
| 543 | and |
| 544 | .Sq )\& |
| 545 | by themselves ordinary characters. |
| 546 | .It |
| 547 | .Sq ^ |
| 548 | is an ordinary character except at the beginning of the |
| 549 | RE or** the beginning of a parenthesized subexpression. |
| 550 | .It |
| 551 | .Sq $ |
| 552 | is an ordinary character except at the end of the |
| 553 | RE or** the end of a parenthesized subexpression. |
| 554 | .It |
| 555 | .Sq * |
| 556 | is an ordinary character if it appears at the beginning of the |
| 557 | RE or the beginning of a parenthesized subexpression |
| 558 | (after a possible leading |
| 559 | .Sq ^ ) . |
| 560 | .It |
| 561 | Finally, there is one new type of atom, a |
| 562 | .Em back-reference : |
| 563 | .Sq \e |
| 564 | followed by a non-zero decimal digit |
| 565 | .Ar d |
| 566 | matches the same sequence of characters matched by the |
| 567 | .Ar d Ns th |
| 568 | parenthesized subexpression |
| 569 | (numbering subexpressions by the positions of their opening parentheses, |
| 570 | left to right), |
| 571 | so that, for example, |
| 572 | .Sq \e([bc]\e)\e1 |
| 573 | matches |
| 574 | .Sq bb\& |
| 575 | or |
| 576 | .Sq cc |
| 577 | but not |
| 578 | .Sq bc . |
| 579 | .El |
| 580 | .Pp |
| 581 | The following is a list of basic regular expressions: |
| 582 | .Bl -tag -width Ds |
| 583 | .It Ar c |
| 584 | Any character |
| 585 | .Ar c |
| 586 | not listed below matches itself. |
| 587 | .It \e Ns Ar c |
| 588 | Any backslash-escaped character |
| 589 | .Ar c , |
| 590 | except for |
| 591 | .Sq { , |
| 592 | .Sq } , |
| 593 | .Sq \&( , |
| 594 | and |
| 595 | .Sq \&) , |
| 596 | matches itself. |
| 597 | .It \&. |
| 598 | Matches any single character that is not a newline |
| 599 | .Pq Sq \en . |
| 600 | .It Bq Ar char-class |
| 601 | Matches any single character in |
| 602 | .Ar char-class . |
| 603 | To include a |
| 604 | .Ql \&] |
| 605 | in |
| 606 | .Ar char-class , |
| 607 | it must be the first character. |
| 608 | A range of characters may be specified by separating the end characters |
| 609 | of the range with a |
| 610 | .Ql - ; |
| 611 | e.g.\& |
| 612 | .Ar a-z |
| 613 | specifies the lower case characters. |
| 614 | The following literal expressions can also be used in |
| 615 | .Ar char-class |
| 616 | to specify sets of characters: |
| 617 | .Bd -unfilled -offset indent |
| 618 | [:alnum:] [:cntrl:] [:lower:] [:space:] |
| 619 | [:alpha:] [:digit:] [:print:] [:upper:] |
| 620 | [:blank:] [:graph:] [:punct:] [:xdigit:] |
| 621 | .Ed |
| 622 | .Pp |
| 623 | If |
| 624 | .Ql - |
| 625 | appears as the first or last character of |
| 626 | .Ar char-class , |
| 627 | then it matches itself. |
| 628 | All other characters in |
| 629 | .Ar char-class |
| 630 | match themselves. |
| 631 | .Pp |
| 632 | Patterns in |
| 633 | .Ar char-class |
| 634 | of the form |
| 635 | .Eo [. |
| 636 | .Ar col-elm |
| 637 | .Ec .]\& |
| 638 | or |
| 639 | .Eo [= |
| 640 | .Ar col-elm |
| 641 | .Ec =]\& , |
| 642 | where |
| 643 | .Ar col-elm |
| 644 | is a collating element, are interpreted according to |
| 645 | .Xr setlocale 3 |
| 646 | .Pq not currently supported . |
| 647 | .It Bq ^ Ns Ar char-class |
| 648 | Matches any single character, other than newline, not in |
| 649 | .Ar char-class . |
| 650 | .Ar char-class |
| 651 | is defined as above. |
| 652 | .It ^ |
| 653 | If |
| 654 | .Sq ^ |
| 655 | is the first character of a regular expression, then it |
| 656 | anchors the regular expression to the beginning of a line. |
| 657 | Otherwise, it matches itself. |
| 658 | .It $ |
| 659 | If |
| 660 | .Sq $ |
| 661 | is the last character of a regular expression, |
| 662 | it anchors the regular expression to the end of a line. |
| 663 | Otherwise, it matches itself. |
| 664 | .It [[:<:]] |
| 665 | Anchors the single character regular expression or subexpression |
| 666 | immediately following it to the beginning of a word. |
| 667 | .It [[:>:]] |
| 668 | Anchors the single character regular expression or subexpression |
| 669 | immediately following it to the end of a word. |
| 670 | .It \e( Ns Ar re Ns \e) |
| 671 | Defines a subexpression |
| 672 | .Ar re . |
| 673 | Subexpressions may be nested. |
| 674 | A subsequent backreference of the form |
| 675 | .Pf \e Ns Ar n , |
| 676 | where |
| 677 | .Ar n |
| 678 | is a number in the range [1,9], expands to the text matched by the |
| 679 | .Ar n Ns th |
| 680 | subexpression. |
| 681 | For example, the regular expression |
| 682 | .Ar \e(.*\e)\e1 |
| 683 | matches any string consisting of identical adjacent substrings. |
| 684 | Subexpressions are ordered relative to their left delimiter. |
| 685 | .It * |
| 686 | Matches the single character regular expression or subexpression |
| 687 | immediately preceding it zero or more times. |
| 688 | If |
| 689 | .Sq * |
| 690 | is the first character of a regular expression or subexpression, |
| 691 | then it matches itself. |
| 692 | The |
| 693 | .Sq * |
| 694 | operator sometimes yields unexpected results. |
| 695 | For example, the regular expression |
| 696 | .Ar b* |
| 697 | matches the beginning of the string |
| 698 | .Qq abbb |
| 699 | (as opposed to the substring |
| 700 | .Qq bbb ) , |
| 701 | since a null match is the only leftmost match. |
| 702 | .Sm off |
| 703 | .It Xo |
| 704 | .Pf \e{ Ar n , m No \e}\ \& |
| 705 | .Pf \e{ Ar n , No \e}\ \& |
| 706 | .Pf \e{ Ar n No \e} |
| 707 | .Xc |
| 708 | .Sm on |
| 709 | Matches the single character regular expression or subexpression |
| 710 | immediately preceding it at least |
| 711 | .Ar n |
| 712 | and at most |
| 713 | .Ar m |
| 714 | times. |
| 715 | If |
| 716 | .Ar m |
| 717 | is omitted, then it matches at least |
| 718 | .Ar n |
| 719 | times. |
| 720 | If the comma is also omitted, then it matches exactly |
| 721 | .Ar n |
| 722 | times. |
| 723 | .El |
| 724 | .Sh SEE ALSO |
| 725 | .Xr ctype 3 , |
| 726 | .Xr regex 3 |
| 727 | .Sh STANDARDS |
| 728 | .St -p1003.1-2004 : |
| 729 | Base Definitions, Chapter 9 (Regular Expressions). |
| 730 | .Sh BUGS |
| 731 | Having two kinds of REs is a botch. |
| 732 | .Pp |
| 733 | The current POSIX spec says that |
| 734 | .Sq )\& |
| 735 | is an ordinary character in the absence of an unmatched |
| 736 | .Sq ( ; |
| 737 | this was an unintentional result of a wording error, |
| 738 | and change is likely. |
| 739 | Avoid relying on it. |
| 740 | .Pp |
| 741 | Back-references are a dreadful botch, |
| 742 | posing major problems for efficient implementations. |
| 743 | They are also somewhat vaguely defined |
| 744 | (does |
| 745 | .Sq a\e(\e(b\e)*\e2\e)*d |
| 746 | match |
| 747 | .Sq abbbd ? ) . |
| 748 | Avoid using them. |
| 749 | .Pp |
| 750 | POSIX's specification of case-independent matching is vague. |
| 751 | The |
| 752 | .Dq one case implies all cases |
| 753 | definition given above |
| 754 | is the current consensus among implementors as to the right interpretation. |
| 755 | .Pp |
| 756 | The syntax for word boundaries is incredibly ugly. |