Annotation of parser3/src/lib/pcre/pcre.3.txt, revision 1.1
1.1 ! paf 1: NAME
! 2: pcre - Perl-compatible regular expressions.
! 3:
! 4:
! 5:
! 6: SYNOPSIS
! 7: #include <pcre.h>
! 8:
! 9: pcre *pcre_compile(const char *pattern, int options,
! 10: const char **errptr, int *erroffset,
! 11: const unsigned char *tableptr);
! 12:
! 13: pcre_extra *pcre_study(const pcre *code, int options,
! 14: const char **errptr);
! 15:
! 16: int pcre_exec(const pcre *code, const pcre_extra *extra,
! 17: const char *subject, int length, int startoffset,
! 18: int options, int *ovector, int ovecsize);
! 19:
! 20: int pcre_copy_substring(const char *subject, int *ovector,
! 21: int stringcount, int stringnumber, char *buffer,
! 22: int buffersize);
! 23:
! 24: int pcre_get_substring(const char *subject, int *ovector,
! 25: int stringcount, int stringnumber,
! 26: const char **stringptr);
! 27:
! 28: int pcre_get_substring_list(const char *subject,
! 29: int *ovector, int stringcount, const char ***listptr);
! 30:
! 31: const unsigned char *pcre_maketables(void);
! 32:
! 33: int pcre_info(const pcre *code, int *optptr, *firstcharptr);
! 34:
! 35: char *pcre_version(void);
! 36:
! 37: void *(*pcre_malloc)(size_t);
! 38:
! 39: void (*pcre_free)(void *);
! 40:
! 41:
! 42:
! 43:
! 44: DESCRIPTION
! 45: The PCRE library is a set of functions that implement regu-
! 46: lar expression pattern matching using the same syntax and
! 47: semantics as Perl 5, with just a few differences (see
! 48: below). The current implementation corresponds to Perl
! 49: 5.005.
! 50:
! 51: PCRE has its own native API, which is described in this
! 52: document. There is also a set of wrapper functions that
! 53: correspond to the POSIX API. These are described in the
! 54: pcreposix documentation.
! 55: The native API function prototypes are defined in the header
! 56: file pcre.h, and on Unix systems the library itself is
! 57: called libpcre.a, so can be accessed by adding -lpcre to the
! 58: command for linking an application which calls it.
! 59:
! 60: The functions pcre_compile(), pcre_study(), and pcre_exec()
! 61: are used for compiling and matching regular expressions,
! 62: while pcre_copy_substring(), pcre_get_substring(), and
! 63: pcre_get_substring_list() are convenience functions for
! 64: extracting captured substrings from a matched subject
! 65: string. The function pcre_maketables() is used (optionally)
! 66: to build a set of character tables in the current locale for
! 67: passing to pcre_compile().
! 68:
! 69: The function pcre_info() is used to find out information
! 70: about a compiled pattern, while the function pcre_version()
! 71: returns a pointer to a string containing the version of PCRE
! 72: and its date of release.
! 73:
! 74: The global variables pcre_malloc and pcre_free initially
! 75: contain the entry points of the standard malloc() and free()
! 76: functions respectively. PCRE calls the memory management
! 77: functions via these variables, so a calling program can
! 78: replace them if it wishes to intercept the calls. This
! 79: should be done before calling any PCRE functions.
! 80:
! 81:
! 82:
! 83: MULTI-THREADING
! 84: The PCRE functions can be used in multi-threading applica-
! 85: tions, with the proviso that the memory management functions
! 86: pointed to by pcre_malloc and pcre_free are shared by all
! 87: threads.
! 88:
! 89: The compiled form of a regular expression is not altered
! 90: during matching, so the same compiled pattern can safely be
! 91: used by several threads at once.
! 92:
! 93:
! 94:
! 95: COMPILING A PATTERN
! 96: The function pcre_compile() is called to compile a pattern
! 97: into an internal form. The pattern is a C string terminated
! 98: by a binary zero, and is passed in the argument pattern. A
! 99: pointer to a single block of memory that is obtained via
! 100: pcre_malloc is returned. This contains the compiled code and
! 101: related data. The pcre type is defined for this for conveni-
! 102: ence, but in fact pcre is just a typedef for void, since the
! 103: contents of the block are not externally defined. It is up
! 104: to the caller to free the memory when it is no longer
! 105: required.
! 106:
! 107: The size of a compiled pattern is roughly proportional to
! 108: the length of the pattern string, except that each character
! 109: class (other than those containing just a single character,
! 110: negated or not) requires 33 bytes, and repeat quantifiers
! 111: with a minimum greater than one or a bounded maximum cause
! 112: the relevant portions of the compiled pattern to be repli-
! 113: cated.
! 114:
! 115: The options argument contains independent bits that affect
! 116: the compilation. It should be zero if no options are
! 117: required. Some of the options, in particular, those that are
! 118: compatible with Perl, can also be set and unset from within
! 119: the pattern (see the detailed description of regular expres-
! 120: sions below). For these options, the contents of the options
! 121: argument specifies their initial settings at the start of
! 122: compilation and execution. The PCRE_ANCHORED option can be
! 123: set at the time of matching as well as at compile time.
! 124:
! 125: If errptr is NULL, pcre_compile() returns NULL immediately.
! 126: Otherwise, if compilation of a pattern fails, pcre_compile()
! 127: returns NULL, and sets the variable pointed to by errptr to
! 128: point to a textual error message. The offset from the start
! 129: of the pattern to the character where the error was
! 130: discovered is placed in the variable pointed to by
! 131: erroffset, which must not be NULL. If it is, an immediate
! 132: error is given.
! 133:
! 134: If the final argument, tableptr, is NULL, PCRE uses a
! 135: default set of character tables which are built when it is
! 136: compiled, using the default C locale. Otherwise, tableptr
! 137: must be the result of a call to pcre_maketables(). See the
! 138: section on locale support below.
! 139:
! 140: The following option bits are defined in the header file:
! 141:
! 142: PCRE_ANCHORED
! 143:
! 144: If this bit is set, the pattern is forced to be "anchored",
! 145: that is, it is constrained to match only at the start of the
! 146: string which is being searched (the "subject string"). This
! 147: effect can also be achieved by appropriate constructs in the
! 148: pattern itself, which is the only way to do it in Perl.
! 149:
! 150: PCRE_CASELESS
! 151:
! 152: If this bit is set, letters in the pattern match both upper
! 153: and lower case letters. It is equivalent to Perl's /i
! 154: option.
! 155:
! 156: PCRE_DOLLAR_ENDONLY
! 157:
! 158: If this bit is set, a dollar metacharacter in the pattern
! 159: matches only at the end of the subject string. Without this
! 160: option, a dollar also matches immediately before the final
! 161: character if it is a newline (but not before any other new-
! 162: lines). The PCRE_DOLLAR_ENDONLY option is ignored if
! 163: PCRE_MULTILINE is set. There is no equivalent to this option
! 164: in Perl.
! 165:
! 166: PCRE_DOTALL
! 167:
! 168: If this bit is set, a dot metacharater in the pattern
! 169: matches all characters, including newlines. Without it, new-
! 170: lines are excluded. This option is equivalent to Perl's /s
! 171: option. A negative class such as [^a] always matches a new-
! 172: line character, independent of the setting of this option.
! 173:
! 174: PCRE_EXTENDED
! 175:
! 176: If this bit is set, whitespace data characters in the pat-
! 177: tern are totally ignored except when escaped or inside a
! 178: character class, and characters between an unescaped # out-
! 179: side a character class and the next newline character,
! 180: inclusive, are also ignored. This is equivalent to Perl's /x
! 181: option, and makes it possible to include comments inside
! 182: complicated patterns. Note, however, that this applies only
! 183: to data characters. Whitespace characters may never appear
! 184: within special character sequences in a pattern, for example
! 185: within the sequence (?( which introduces a conditional sub-
! 186: pattern.
! 187:
! 188: PCRE_EXTRA
! 189:
! 190: This option turns on additional functionality of PCRE that
! 191: is incompatible with Perl. Any backslash in a pattern that
! 192: is followed by a letter that has no special meaning causes
! 193: an error, thus reserving these combinations for future
! 194: expansion. By default, as in Perl, a backslash followed by a
! 195: letter with no special meaning is treated as a literal.
! 196: There are at present no other features controlled by this
! 197: option.
! 198:
! 199: PCRE_MULTILINE
! 200:
! 201: By default, PCRE treats the subject string as consisting of
! 202: a single "line" of characters (even if it actually contains
! 203: several newlines). The "start of line" metacharacter (^)
! 204: matches only at the start of the string, while the "end of
! 205: line" metacharacter ($) matches only at the end of the
! 206: string, or before a terminating newline (unless
! 207: PCRE_DOLLAR_ENDONLY is set). This is the same as Perl.
! 208:
! 209: When PCRE_MULTILINE it is set, the "start of line" and "end
! 210: of line" constructs match immediately following or
! 211: immediately before any newline in the subject string,
! 212: respectively, as well as at the very start and end. This is
! 213: equivalent to Perl's /m option. If there are no "\n" charac-
! 214: ters in a subject string, or no occurrences of ^ or $ in a
! 215: pattern, setting PCRE_MULTILINE has no effect.
! 216:
! 217: PCRE_UNGREEDY
! 218:
! 219: This option inverts the "greediness" of the quantifiers so
! 220: that they are not greedy by default, but become greedy if
! 221: followed by "?". It is not compatible with Perl. It can also
! 222: be set by a (?U) option setting within the pattern.
! 223:
! 224:
! 225:
! 226: STUDYING A PATTERN
! 227: When a pattern is going to be used several times, it is
! 228: worth spending more time analyzing it in order to speed up
! 229: the time taken for matching. The function pcre_study() takes
! 230: a pointer to a compiled pattern as its first argument, and
! 231: returns a pointer to a pcre_extra block (another void
! 232: typedef) containing additional information about the pat-
! 233: tern; this can be passed to pcre_exec(). If no additional
! 234: information is available, NULL is returned.
! 235:
! 236: The second argument contains option bits. At present, no
! 237: options are defined for pcre_study(), and this argument
! 238: should always be zero.
! 239:
! 240: The third argument for pcre_study() is a pointer to an error
! 241: message. If studying succeeds (even if no data is returned),
! 242: the variable it points to is set to NULL. Otherwise it
! 243: points to a textual error message.
! 244:
! 245: At present, studying a pattern is useful only for non-
! 246: anchored patterns that do not have a single fixed starting
! 247: character. A bitmap of possible starting characters is
! 248: created.
! 249:
! 250:
! 251:
! 252: LOCALE SUPPORT
! 253: PCRE handles caseless matching, and determines whether char-
! 254: acters are letters, digits, or whatever, by reference to a
! 255: set of tables. The library contains a default set of tables
! 256: which is created in the default C locale when PCRE is com-
! 257: piled. This is used when the final argument of
! 258: pcre_compile() is NULL, and is sufficient for many applica-
! 259: tions.
! 260:
! 261: An alternative set of tables can, however, be supplied. Such
! 262: tables are built by calling the pcre_maketables() function,
! 263: which has no arguments, in the relevant locale. The result
! 264: can then be passed to pcre_compile() as often as necessary.
! 265: For example, to build and use tables that are appropriate
! 266: for the French locale (where accented characters with codes
! 267: greater than 128 are treated as letters), the following code
! 268: could be used:
! 269:
! 270: setlocale(LC_CTYPE, "fr");
! 271: tables = pcre_maketables();
! 272: re = pcre_compile(..., tables);
! 273:
! 274: The tables are built in memory that is obtained via
! 275: pcre_malloc. The pointer that is passed to pcre_compile is
! 276: saved with the compiled pattern, and the same tables are
! 277: used via this pointer by pcre_study() and pcre_exec(). Thus
! 278: for any single pattern, compilation, studying and matching
! 279: all happen in the same locale, but different patterns can be
! 280: compiled in different locales. It is the caller's responsi-
! 281: bility to ensure that the memory containing the tables
! 282: remains available for as long as it is needed.
! 283:
! 284:
! 285:
! 286: INFORMATION ABOUT A PATTERN
! 287: The pcre_info() function returns information about a com-
! 288: piled pattern. Its yield is the number of capturing subpat-
! 289: terns, or one of the following negative numbers:
! 290:
! 291: PCRE_ERROR_NULL the argument code was NULL
! 292: PCRE_ERROR_BADMAGIC the "magic number" was not found
! 293:
! 294: If the optptr argument is not NULL, a copy of the options
! 295: with which the pattern was compiled is placed in the integer
! 296: it points to. These option bits are those specified in the
! 297: call to pcre_compile(), modified by any top-level option
! 298: settings within the pattern itself, and with the
! 299: PCRE_ANCHORED bit set if the form of the pattern implies
! 300: that it can match only at the start of a subject string.
! 301:
! 302: If the pattern is not anchored and the firstcharptr argument
! 303: is not NULL, it is used to pass back information about the
! 304: first character of any matched string. If there is a fixed
! 305: first character, e.g. from a pattern such as
! 306: (cat|cow|coyote), then it is returned in the integer pointed
! 307: to by firstcharptr. Otherwise, if either
! 308:
! 309: (a) the pattern was compiled with the PCRE_MULTILINE option,
! 310: and every branch starts with "^", or
! 311:
! 312: (b) every branch of the pattern starts with ".*" and
! 313: PCRE_DOTALL is not set (if it were set, the pattern would be
! 314: anchored),
! 315: then -1 is returned, indicating that the pattern matches
! 316: only at the start of a subject string or after any "\n"
! 317: within the string. Otherwise -2 is returned.
! 318:
! 319:
! 320:
! 321: MATCHING A PATTERN
! 322: The function pcre_exec() is called to match a subject string
! 323: against a pre-compiled pattern, which is passed in the code
! 324: argument. If the pattern has been studied, the result of the
! 325: study should be passed in the extra argument. Otherwise this
! 326: must be NULL.
! 327:
! 328: The PCRE_ANCHORED option can be passed in the options argu-
! 329: ment, whose unused bits must be zero. However, if a pattern
! 330: was compiled with PCRE_ANCHORED, or turned out to be
! 331: anchored by virtue of its contents, it cannot be made
! 332: unachored at matching time.
! 333:
! 334: There are also three further options that can be set only at
! 335: matching time:
! 336:
! 337: PCRE_NOTBOL
! 338:
! 339: The first character of the string is not the beginning of a
! 340: line, so the circumflex metacharacter should not match
! 341: before it. Setting this without PCRE_MULTILINE (at compile
! 342: time) causes circumflex never to match.
! 343:
! 344: PCRE_NOTEOL
! 345:
! 346: The end of the string is not the end of a line, so the dol-
! 347: lar metacharacter should not match it nor (except in multi-
! 348: line mode) a newline immediately before it. Setting this
! 349: without PCRE_MULTILINE (at compile time) causes dollar never
! 350: to match.
! 351:
! 352: PCRE_NOTEMPTY
! 353:
! 354: An empty string is not considered to be a valid match if
! 355: this option is set. If there are alternatives in the pat-
! 356: tern, they are tried. If all the alternatives match the
! 357: empty string, the entire match fails. For example, if the
! 358: pattern
! 359:
! 360: a?b?
! 361:
! 362: is applied to a string not beginning with "a" or "b", it
! 363: matches the empty string at the start of the subject. With
! 364: PCRE_NOTEMPTY set, this match is not valid, so PCRE searches
! 365: further into the string for occurrences of "a" or "b". Perl
! 366: has no direct equivalent of this option, but it makes a
! 367: special case of a pattern match of the empty string within
! 368: its split() function, or when using the /g modifier. Using
! 369: PCRE_NOTEMPTY it is possible to emulate this behaviour.
! 370:
! 371: The subject string is passed as a pointer in subject, a
! 372: length in length, and a starting offset in startoffset.
! 373: Unlike the pattern string, it may contain binary zero char-
! 374: acters. When the starting offset is zero, the search for a
! 375: match starts at the beginning of the subject, and this is by
! 376: far the most common case.
! 377:
! 378: A non-zero starting offset is useful when searching for
! 379: another match in the same subject by calling pcre_exec()
! 380: again after a previous success. Setting startoffset differs
! 381: from just passing over a shortened string and setting
! 382: PCRE_NOTBOL in the case of a pattern that begins with any
! 383: kind of lookbehind. For example, consider the pattern
! 384:
! 385: \Biss\B
! 386:
! 387: which finds occurrences of "iss" in the middle of words. (\B
! 388: matches only if the current position in the subject is not a
! 389: word boundary.) When applied to the string "Mississipi" the
! 390: first call to pcre_exec() finds the first occurrence. If
! 391: pcre_exec() is called again with just the remainder of the
! 392: subject, namely "issipi", it does not match, because \B is
! 393: always false at the start of the subject, which is deemed to
! 394: be a word boundary. However, if pcre_exec() is passed the
! 395: entire string again, but with startoffset set to 4, it finds
! 396: the second occurrence of "iss" because it is able to look
! 397: behind the starting point to discover that it is preceded by
! 398: a letter.
! 399:
! 400: If a non-zero starting offset is passed when the pattern is
! 401: anchored, one attempt to match at the given offset is tried.
! 402: This can only succeed if the pattern does not require the
! 403: match to be at the start of the subject.
! 404:
! 405: In general, a pattern matches a certain portion of the sub-
! 406: ject, and in addition, further substrings from the subject
! 407: may be picked out by parts of the pattern. Following the
! 408: usage in Jeffrey Friedl's book, this is called "capturing"
! 409: in what follows, and the phrase "capturing subpattern" is
! 410: used for a fragment of a pattern that picks out a substring.
! 411: PCRE supports several other kinds of parenthesized subpat-
! 412: tern that do not cause substrings to be captured.
! 413:
! 414: Captured substrings are returned to the caller via a vector
! 415: of integer offsets whose address is passed in ovector. The
! 416: number of elements in the vector is passed in ovecsize. The
! 417: first two-thirds of the vector is used to pass back captured
! 418: substrings, each substring using a pair of integers. The
! 419: remaining third of the vector is used as workspace by
! 420: pcre_exec() while matching capturing subpatterns, and is not
! 421: available for passing back information. The length passed in
! 422: ovecsize should always be a multiple of three. If it is not,
! 423: it is rounded down.
! 424:
! 425: When a match has been successful, information about captured
! 426: substrings is returned in pairs of integers, starting at the
! 427: beginning of ovector, and continuing up to two-thirds of its
! 428: length at the most. The first element of a pair is set to
! 429: the offset of the first character in a substring, and the
! 430: second is set to the offset of the first character after the
! 431: end of a substring. The first pair, ovector[0] and ovec-
! 432: tor[1], identify the portion of the subject string matched
! 433: by the entire pattern. The next pair is used for the first
! 434: capturing subpattern, and so on. The value returned by
! 435: pcre_exec() is the number of pairs that have been set. If
! 436: there are no capturing subpatterns, the return value from a
! 437: successful match is 1, indicating that just the first pair
! 438: of offsets has been set.
! 439:
! 440: Some convenience functions are provided for extracting the
! 441: captured substrings as separate strings. These are described
! 442: in the following section.
! 443:
! 444: It is possible for an capturing subpattern number n+1 to
! 445: match some part of the subject when subpattern n has not
! 446: been used at all. For example, if the string "abc" is
! 447: matched against the pattern (a|(z))(bc) subpatterns 1 and 3
! 448: are matched, but 2 is not. When this happens, both offset
! 449: values corresponding to the unused subpattern are set to -1.
! 450:
! 451: If a capturing subpattern is matched repeatedly, it is the
! 452: last portion of the string that it matched that gets
! 453: returned.
! 454:
! 455: If the vector is too small to hold all the captured sub-
! 456: strings, it is used as far as possible (up to two-thirds of
! 457: its length), and the function returns a value of zero. In
! 458: particular, if the substring offsets are not of interest,
! 459: pcre_exec() may be called with ovector passed as NULL and
! 460: ovecsize as zero. However, if the pattern contains back
! 461: references and the ovector isn't big enough to remember the
! 462: related substrings, PCRE has to get additional memory for
! 463: use during matching. Thus it is usually advisable to supply
! 464: an ovector.
! 465:
! 466: Note that pcre_info() can be used to find out how many cap-
! 467: turing subpatterns there are in a compiled pattern. The
! 468: smallest size for ovector that will allow for n captured
! 469: substrings in addition to the offsets of the substring
! 470: matched by the whole pattern is (n+1)*3.
! 471: If pcre_exec() fails, it returns a negative number. The fol-
! 472: lowing are defined in the header file:
! 473:
! 474: PCRE_ERROR_NOMATCH (-1)
! 475:
! 476: The subject string did not match the pattern.
! 477:
! 478: PCRE_ERROR_NULL (-2)
! 479:
! 480: Either code or subject was passed as NULL, or ovector was
! 481: NULL and ovecsize was not zero.
! 482:
! 483: PCRE_ERROR_BADOPTION (-3)
! 484:
! 485: An unrecognized bit was set in the options argument.
! 486:
! 487: PCRE_ERROR_BADMAGIC (-4)
! 488:
! 489: PCRE stores a 4-byte "magic number" at the start of the com-
! 490: piled code, to catch the case when it is passed a junk
! 491: pointer. This is the error it gives when the magic number
! 492: isn't present.
! 493:
! 494: PCRE_ERROR_UNKNOWN_NODE (-5)
! 495:
! 496: While running the pattern match, an unknown item was encoun-
! 497: tered in the compiled pattern. This error could be caused by
! 498: a bug in PCRE or by overwriting of the compiled pattern.
! 499:
! 500: PCRE_ERROR_NOMEMORY (-6)
! 501:
! 502: If a pattern contains back references, but the ovector that
! 503: is passed to pcre_exec() is not big enough to remember the
! 504: referenced substrings, PCRE gets a block of memory at the
! 505: start of matching to use for this purpose. If the call via
! 506: pcre_malloc() fails, this error is given. The memory is
! 507: freed at the end of matching.
! 508:
! 509:
! 510:
! 511: EXTRACTING CAPTURED SUBSTRINGS
! 512: Captured substrings can be accessed directly by using the
! 513: offsets returned by pcre_exec() in ovector. For convenience,
! 514: the functions pcre_copy_substring(), pcre_get_substring(),
! 515: and pcre_get_substring_list() are provided for extracting
! 516: captured substrings as new, separate, zero-terminated
! 517: strings. A substring that contains a binary zero is
! 518: correctly extracted and has a further zero added on the end,
! 519: but the result does not, of course, function as a C string.
! 520:
! 521: The first three arguments are the same for all three func-
! 522: tions: subject is the subject string which has just been
! 523: successfully matched, ovector is a pointer to the vector of
! 524: integer offsets that was passed to pcre_exec(), and
! 525: stringcount is the number of substrings that were captured
! 526: by the match, including the substring that matched the
! 527: entire regular expression. This is the value returned by
! 528: pcre_exec if it is greater than zero. If pcre_exec()
! 529: returned zero, indicating that it ran out of space in ovec-
! 530: tor, then the value passed as stringcount should be the size
! 531: of the vector divided by three.
! 532:
! 533: The functions pcre_copy_substring() and pcre_get_substring()
! 534: extract a single substring, whose number is given as string-
! 535: number. A value of zero extracts the substring that matched
! 536: the entire pattern, while higher values extract the captured
! 537: substrings. For pcre_copy_substring(), the string is placed
! 538: in buffer, whose length is given by buffersize, while for
! 539: pcre_get_substring() a new block of store is obtained via
! 540: pcre_malloc, and its address is returned via stringptr. The
! 541: yield of the function is the length of the string, not
! 542: including the terminating zero, or one of
! 543:
! 544: PCRE_ERROR_NOMEMORY (-6)
! 545:
! 546: The buffer was too small for pcre_copy_substring(), or the
! 547: attempt to get memory failed for pcre_get_substring().
! 548:
! 549: PCRE_ERROR_NOSUBSTRING (-7)
! 550:
! 551: There is no substring whose number is stringnumber.
! 552:
! 553: The pcre_get_substring_list() function extracts all avail-
! 554: able substrings and builds a list of pointers to them. All
! 555: this is done in a single block of memory which is obtained
! 556: via pcre_malloc. The address of the memory block is returned
! 557: via listptr, which is also the start of the list of string
! 558: pointers. The end of the list is marked by a NULL pointer.
! 559: The yield of the function is zero if all went well, or
! 560:
! 561: PCRE_ERROR_NOMEMORY (-6)
! 562:
! 563: if the attempt to get the memory block failed.
! 564:
! 565: When any of these functions encounter a substring that is
! 566: unset, which can happen when capturing subpattern number n+1
! 567: matches some part of the subject, but subpattern n has not
! 568: been used at all, they return an empty string. This can be
! 569: distinguished from a genuine zero-length substring by
! 570: inspecting the appropriate offset in ovector, which is nega-
! 571: tive for unset substrings.
! 572:
! 573:
! 574:
! 575: LIMITATIONS
! 576: There are some size limitations in PCRE but it is hoped that
! 577: they will never in practice be relevant. The maximum length
! 578: of a compiled pattern is 65539 (sic) bytes. All values in
! 579: repeating quantifiers must be less than 65536. The maximum
! 580: number of capturing subpatterns is 99. The maximum number
! 581: of all parenthesized subpatterns, including capturing sub-
! 582: patterns, assertions, and other types of subpattern, is 200.
! 583:
! 584: The maximum length of a subject string is the largest posi-
! 585: tive number that an integer variable can hold. However, PCRE
! 586: uses recursion to handle subpatterns and indefinite repeti-
! 587: tion. This means that the available stack space may limit
! 588: the size of a subject string that can be processed by cer-
! 589: tain patterns.
! 590:
! 591:
! 592:
! 593: DIFFERENCES FROM PERL
! 594: The differences described here are with respect to Perl
! 595: 5.005.
! 596:
! 597: 1. By default, a whitespace character is any character that
! 598: the C library function isspace() recognizes, though it is
! 599: possible to compile PCRE with alternative character type
! 600: tables. Normally isspace() matches space, formfeed, newline,
! 601: carriage return, horizontal tab, and vertical tab. Perl 5 no
! 602: longer includes vertical tab in its set of whitespace char-
! 603: acters. The \v escape that was in the Perl documentation for
! 604: a long time was never in fact recognized. However, the char-
! 605: acter itself was treated as whitespace at least up to 5.002.
! 606: In 5.004 and 5.005 it does not match \s.
! 607:
! 608: 2. PCRE does not allow repeat quantifiers on lookahead
! 609: assertions. Perl permits them, but they do not mean what you
! 610: might think. For example, (?!a){3} does not assert that the
! 611: next three characters are not "a". It just asserts that the
! 612: next character is not "a" three times.
! 613:
! 614: 3. Capturing subpatterns that occur inside negative looka-
! 615: head assertions are counted, but their entries in the
! 616: offsets vector are never set. Perl sets its numerical vari-
! 617: ables from any such patterns that are matched before the
! 618: assertion fails to match something (thereby succeeding), but
! 619: only if the negative lookahead assertion contains just one
! 620: branch.
! 621:
! 622: 4. Though binary zero characters are supported in the sub-
! 623: ject string, they are not allowed in a pattern string
! 624: because it is passed as a normal C string, terminated by
! 625: zero. The escape sequence "\0" can be used in the pattern to
! 626: represent a binary zero.
! 627: 5. The following Perl escape sequences are not supported:
! 628: \l, \u, \L, \U, \E, \Q. In fact these are implemented by
! 629: Perl's general string-handling and are not part of its pat-
! 630: tern matching engine.
! 631:
! 632: 6. The Perl \G assertion is not supported as it is not
! 633: relevant to single pattern matches.
! 634:
! 635: 7. Fairly obviously, PCRE does not support the (?{code})
! 636: construction.
! 637:
! 638: 8. There are at the time of writing some oddities in Perl
! 639: 5.005_02 concerned with the settings of captured strings
! 640: when part of a pattern is repeated. For example, matching
! 641: "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
! 642: "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2
! 643: unset. However, if the pattern is changed to
! 644: /^(aa(b(b))?)+$/ then $2 (and $3) get set.
! 645:
! 646: In Perl 5.004 $2 is set in both cases, and that is also true
! 647: of PCRE. If in the future Perl changes to a consistent state
! 648: that is different, PCRE may change to follow.
! 649:
! 650: 9. Another as yet unresolved discrepancy is that in Perl
! 651: 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string
! 652: "a", whereas in PCRE it does not. However, in both Perl and
! 653: PCRE /^(a)?a/ matched against "a" leaves $1 unset.
! 654:
! 655: 10. PCRE provides some extensions to the Perl regular
! 656: expression facilities:
! 657:
! 658: (a) Although lookbehind assertions must match fixed length
! 659: strings, each alternative branch of a lookbehind assertion
! 660: can match a different length of string. Perl 5.005 requires
! 661: them all to have the same length.
! 662:
! 663: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not
! 664: set, the $ meta- character matches only at the very end of
! 665: the string.
! 666:
! 667: (c) If PCRE_EXTRA is set, a backslash followed by a letter
! 668: with no special meaning is faulted.
! 669:
! 670: (d) If PCRE_UNGREEDY is set, the greediness of the repeti-
! 671: tion quantifiers is inverted, that is, by default they are
! 672: not greedy, but if followed by a question mark they are.
! 673:
! 674: (e) PCRE_ANCHORED can be used to force a pattern to be tried
! 675: only at the start of the subject.
! 676:
! 677: (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options
! 678: for pcre_exec() have no Perl equivalents.
! 679:
! 680:
! 681:
! 682: REGULAR EXPRESSION DETAILS
! 683: The syntax and semantics of the regular expressions sup-
! 684: ported by PCRE are described below. Regular expressions are
! 685: also described in the Perl documentation and in a number of
! 686: other books, some of which have copious examples. Jeffrey
! 687: Friedl's "Mastering Regular Expressions", published by
! 688: O'Reilly (ISBN 1-56592-257-3), covers them in great detail.
! 689: The description here is intended as reference documentation.
! 690:
! 691: A regular expression is a pattern that is matched against a
! 692: subject string from left to right. Most characters stand for
! 693: themselves in a pattern, and match the corresponding charac-
! 694: ters in the subject. As a trivial example, the pattern
! 695:
! 696: The quick brown fox
! 697:
! 698: matches a portion of a subject string that is identical to
! 699: itself. The power of regular expressions comes from the
! 700: ability to include alternatives and repetitions in the pat-
! 701: tern. These are encoded in the pattern by the use of meta-
! 702: characters, which do not stand for themselves but instead
! 703: are interpreted in some special way.
! 704:
! 705: There are two different sets of meta-characters: those that
! 706: are recognized anywhere in the pattern except within square
! 707: brackets, and those that are recognized in square brackets.
! 708: Outside square brackets, the meta-characters are as follows:
! 709:
! 710: \ general escape character with several uses
! 711: ^ assert start of subject (or line, in multiline
! 712: mode)
! 713: $ assert end of subject (or line, in multiline mode)
! 714: . match any character except newline (by default)
! 715: [ start character class definition
! 716: | start of alternative branch
! 717: ( start subpattern
! 718: ) end subpattern
! 719: ? extends the meaning of (
! 720: also 0 or 1 quantifier
! 721: also quantifier minimizer
! 722: * 0 or more quantifier
! 723: + 1 or more quantifier
! 724: { start min/max quantifier
! 725:
! 726: Part of a pattern that is in square brackets is called a
! 727: "character class". In a character class the only meta-
! 728: characters are:
! 729:
! 730: \ general escape character
! 731: ^ negate the class, but only if the first character
! 732: - indicates character range
! 733: ] terminates the character class
! 734:
! 735: The following sections describe the use of each of the
! 736: meta-characters.
! 737:
! 738:
! 739:
! 740: BACKSLASH
! 741: The backslash character has several uses. Firstly, if it is
! 742: followed by a non-alphameric character, it takes away any
! 743: special meaning that character may have. This use of
! 744: backslash as an escape character applies both inside and
! 745: outside character classes.
! 746:
! 747: For example, if you want to match a "*" character, you write
! 748: "\*" in the pattern. This applies whether or not the follow-
! 749: ing character would otherwise be interpreted as a meta-
! 750: character, so it is always safe to precede a non-alphameric
! 751: with "\" to specify that it stands for itself. In particu-
! 752: lar, if you want to match a backslash, you write "\\".
! 753:
! 754: If a pattern is compiled with the PCRE_EXTENDED option, whi-
! 755: tespace in the pattern (other than in a character class) and
! 756: characters between a "#" outside a character class and the
! 757: next newline character are ignored. An escaping backslash
! 758: can be used to include a whitespace or "#" character as part
! 759: of the pattern.
! 760:
! 761: A second use of backslash provides a way of encoding non-
! 762: printing characters in patterns in a visible manner. There
! 763: is no restriction on the appearance of non-printing charac-
! 764: ters, apart from the binary zero that terminates a pattern,
! 765: but when a pattern is being prepared by text editing, it is
! 766: usually easier to use one of the following escape sequences
! 767: than the binary character it represents:
! 768:
! 769: \a alarm, that is, the BEL character (hex 07)
! 770: \cx "control-x", where x is any character
! 771: \e escape (hex 1B)
! 772: \f formfeed (hex 0C)
! 773: \n newline (hex 0A)
! 774: \r carriage return (hex 0D)
! 775: \t tab (hex 09)
! 776: \xhh character with hex code hh
! 777: \ddd character with octal code ddd, or backreference
! 778:
! 779: The precise effect of "\cx" is as follows: if "x" is a lower
! 780: case letter, it is converted to upper case. Then bit 6 of
! 781: the character (hex 40) is inverted. Thus "\cz" becomes hex
! 782: 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.
! 783:
! 784: After "\x", up to two hexadecimal digits are read (letters
! 785: can be in upper or lower case).
! 786:
! 787: After "\0" up to two further octal digits are read. In both
! 788: cases, if there are fewer than two digits, just those that
! 789: are present are used. Thus the sequence "\0\x\07" specifies
! 790: two binary zeros followed by a BEL character. Make sure you
! 791: supply two digits after the initial zero if the character
! 792: that follows is itself an octal digit.
! 793:
! 794: The handling of a backslash followed by a digit other than 0
! 795: is complicated. Outside a character class, PCRE reads it
! 796: and any following digits as a decimal number. If the number
! 797: is less than 10, or if there have been at least that many
! 798: previous capturing left parentheses in the expression, the
! 799: entire sequence is taken as a back reference. A description
! 800: of how this works is given later, following the discussion
! 801: of parenthesized subpatterns.
! 802:
! 803: Inside a character class, or if the decimal number is
! 804: greater than 9 and there have not been that many capturing
! 805: subpatterns, PCRE re-reads up to three octal digits follow-
! 806: ing the backslash, and generates a single byte from the
! 807: least significant 8 bits of the value. Any subsequent digits
! 808: stand for themselves. For example:
! 809:
! 810: \040 is another way of writing a space
! 811: \40 is the same, provided there are fewer than 40
! 812: previous capturing subpatterns
! 813: \7 is always a back reference
! 814: \11 might be a back reference, or another way of
! 815: writing a tab
! 816: \011 is always a tab
! 817: \0113 is a tab followed by the character "3"
! 818: \113 is the character with octal code 113 (since there
! 819: can be no more than 99 back references)
! 820: \377 is a byte consisting entirely of 1 bits
! 821: \81 is either a back reference, or a binary zero
! 822: followed by the two characters "8" and "1"
! 823:
! 824: Note that octal values of 100 or greater must not be intro-
! 825: duced by a leading zero, because no more than three octal
! 826: digits are ever read.
! 827:
! 828: All the sequences that define a single byte value can be
! 829: used both inside and outside character classes. In addition,
! 830: inside a character class, the sequence "\b" is interpreted
! 831: as the backspace character (hex 08). Outside a character
! 832: class it has a different meaning (see below).
! 833:
! 834: The third use of backslash is for specifying generic charac-
! 835: ter types:
! 836:
! 837: \d any decimal digit
! 838: \D any character that is not a decimal digit
! 839: any whitespace character
! 840: \S any character that is not a whitespace character
! 841: \w any "word" character
! 842: \W any "non-word" character
! 843:
! 844: Each pair of escape sequences partitions the complete set of
! 845: characters into two disjoint sets. Any given character
! 846: matches one, and only one, of each pair.
! 847:
! 848: A "word" character is any letter or digit or the underscore
! 849: character, that is, any character which can be part of a
! 850: Perl "word". The definition of letters and digits is con-
! 851: trolled by PCRE's character tables, and may vary if locale-
! 852: specific matching is taking place (see "Locale support"
! 853: above). For example, in the "fr" (French) locale, some char-
! 854: acter codes greater than 128 are used for accented letters,
! 855: and these are matched by \w.
! 856:
! 857: These character type sequences can appear both inside and
! 858: outside character classes. They each match one character of
! 859: the appropriate type. If the current matching point is at
! 860: the end of the subject string, all of them fail, since there
! 861: is no character to match.
! 862:
! 863: The fourth use of backslash is for certain simple asser-
! 864: tions. An assertion specifies a condition that has to be met
! 865: at a particular point in a match, without consuming any
! 866: characters from the subject string. The use of subpatterns
! 867: for more complicated assertions is described below. The
! 868: backslashed assertions are
! 869:
! 870: \b word boundary
! 871: \B not a word boundary
! 872: \A start of subject (independent of multiline mode)
! 873: \Z end of subject or newline at end (independent of
! 874: multiline mode)
! 875: \z end of subject (independent of multiline mode)
! 876:
! 877: These assertions may not appear in character classes (but
! 878: note that "\b" has a different meaning, namely the backspace
! 879: character, inside a character class).
! 880:
! 881: A word boundary is a position in the subject string where
! 882: the current character and the previous character do not both
! 883: match \w or \W (i.e. one matches \w and the other matches
! 884: \W), or the start or end of the string if the first or last
! 885: character matches \w, respectively.
! 886:
! 887: The \A, \Z, and \z assertions differ from the traditional
! 888: circumflex and dollar (described below) in that they only
! 889: ever match at the very start and end of the subject string,
! 890: whatever options are set. They are not affected by the
! 891: PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-
! 892: ment of pcre_exec() is non-zero, \A can never match. The
! 893: difference between \Z and \z is that \Z matches before a
! 894: newline that is the last character of the string as well as
! 895: at the end of the string, whereas \z matches only at the
! 896: end.
! 897:
! 898:
! 899:
! 900: CIRCUMFLEX AND DOLLAR
! 901: Outside a character class, in the default matching mode, the
! 902: circumflex character is an assertion which is true only if
! 903: the current matching point is at the start of the subject
! 904: string. If the startoffset argument of pcre_exec() is non-
! 905: zero, circumflex can never match. Inside a character class,
! 906: circumflex has an entirely different meaning (see below).
! 907:
! 908: Circumflex need not be the first character of the pattern if
! 909: a number of alternatives are involved, but it should be the
! 910: first thing in each alternative in which it appears if the
! 911: pattern is ever to match that branch. If all possible alter-
! 912: natives start with a circumflex, that is, if the pattern is
! 913: constrained to match only at the start of the subject, it is
! 914: said to be an "anchored" pattern. (There are also other con-
! 915: structs that can cause a pattern to be anchored.)
! 916:
! 917: A dollar character is an assertion which is true only if the
! 918: current matching point is at the end of the subject string,
! 919: or immediately before a newline character that is the last
! 920: character in the string (by default). Dollar need not be the
! 921: last character of the pattern if a number of alternatives
! 922: are involved, but it should be the last item in any branch
! 923: in which it appears. Dollar has no special meaning in a
! 924: character class.
! 925:
! 926: The meaning of dollar can be changed so that it matches only
! 927: at the very end of the string, by setting the
! 928: PCRE_DOLLAR_ENDONLY option at compile or matching time. This
! 929: does not affect the \Z assertion.
! 930:
! 931: The meanings of the circumflex and dollar characters are
! 932: changed if the PCRE_MULTILINE option is set. When this is
! 933: the case, they match immediately after and immediately
! 934: before an internal "\n" character, respectively, in addition
! 935: to matching at the start and end of the subject string. For
! 936: example, the pattern /^abc$/ matches the subject string
! 937: "def\nabc" in multiline mode, but not otherwise. Conse-
! 938: quently, patterns that are anchored in single line mode
! 939: because all branches start with "^" are not anchored in mul-
! 940: tiline mode, and a match for circumflex is possible when the
! 941: startoffset argument of pcre_exec() is non-zero. The
! 942: PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
! 943: set.
! 944:
! 945: Note that the sequences \A, \Z, and \z can be used to match
! 946: the start and end of the subject in both modes, and if all
! 947: branches of a pattern start with \A is it always anchored,
! 948: whether PCRE_MULTILINE is set or not.
! 949:
! 950:
! 951:
! 952: FULL STOP (PERIOD, DOT)
! 953: Outside a character class, a dot in the pattern matches any
! 954: one character in the subject, including a non-printing char-
! 955: acter, but not (by default) newline. If the PCRE_DOTALL
! 956: option is set, then dots match newlines as well. The han-
! 957: dling of dot is entirely independent of the handling of cir-
! 958: cumflex and dollar, the only relationship being that they
! 959: both involve newline characters. Dot has no special meaning
! 960: in a character class.
! 961:
! 962:
! 963:
! 964: SQUARE BRACKETS
! 965: An opening square bracket introduces a character class, ter-
! 966: minated by a closing square bracket. A closing square
! 967: bracket on its own is not special. If a closing square
! 968: bracket is required as a member of the class, it should be
! 969: the first data character in the class (after an initial cir-
! 970: cumflex, if present) or escaped with a backslash.
! 971:
! 972: A character class matches a single character in the subject;
! 973: the character must be in the set of characters defined by
! 974: the class, unless the first character in the class is a cir-
! 975: cumflex, in which case the subject character must not be in
! 976: the set defined by the class. If a circumflex is actually
! 977: required as a member of the class, ensure it is not the
! 978: first character, or escape it with a backslash.
! 979:
! 980: For example, the character class [aeiou] matches any lower
! 981: case vowel, while [^aeiou] matches any character that is not
! 982: a lower case vowel. Note that a circumflex is just a con-
! 983: venient notation for specifying the characters which are in
! 984: the class by enumerating those that are not. It is not an
! 985: assertion: it still consumes a character from the subject
! 986: string, and fails if the current pointer is at the end of
! 987: the string.
! 988:
! 989: When caseless matching is set, any letters in a class
! 990: represent both their upper case and lower case versions, so
! 991: for example, a caseless [aeiou] matches "A" as well as "a",
! 992: and a caseless [^aeiou] does not match "A", whereas a case-
! 993: ful version would.
! 994:
! 995: The newline character is never treated in any special way in
! 996: character classes, whatever the setting of the PCRE_DOTALL
! 997: or PCRE_MULTILINE options is. A class such as [^a] will
! 998: always match a newline.
! 999:
! 1000: The minus (hyphen) character can be used to specify a range
! 1001: of characters in a character class. For example, [d-m]
! 1002: matches any letter between d and m, inclusive. If a minus
! 1003: character is required in a class, it must be escaped with a
! 1004: backslash or appear in a position where it cannot be inter-
! 1005: preted as indicating a range, typically as the first or last
! 1006: character in the class.
! 1007:
! 1008: It is not possible to have the literal character "]" as the
! 1009: end character of a range. A pattern such as [W-]46] is
! 1010: interpreted as a class of two characters ("W" and "-") fol-
! 1011: lowed by a literal string "46]", so it would match "W46]" or
! 1012: "-46]". However, if the "]" is escaped with a backslash it
! 1013: is interpreted as the end of range, so [W-\]46] is inter-
! 1014: preted as a single class containing a range followed by two
! 1015: separate characters. The octal or hexadecimal representation
! 1016: of "]" can also be used to end a range.
! 1017:
! 1018: Ranges operate in ASCII collating sequence. They can also be
! 1019: used for characters specified numerically, for example
! 1020: [\000-\037]. If a range that includes letters is used when
! 1021: caseless matching is set, it matches the letters in either
! 1022: case. For example, [W-c] is equivalent to [][\^_`wxyzabc],
! 1023: matched caselessly, and if character tables for the "fr"
! 1024: locale are in use, [\xc8-\xcb] matches accented E characters
! 1025: in both cases.
! 1026:
! 1027: The character types \d, \D, \s, \S, \w, and \W may also
! 1028: appear in a character class, and add the characters that
! 1029: they match to the class. For example, [\dABCDEF] matches any
! 1030: hexadecimal digit. A circumflex can conveniently be used
! 1031: with the upper case character types to specify a more res-
! 1032: tricted set of characters than the matching lower case type.
! 1033: For example, the class [^\W_] matches any letter or digit,
! 1034: but not underscore.
! 1035:
! 1036: All non-alphameric characters other than \, -, ^ (at the
! 1037: start) and the terminating ] are non-special in character
! 1038: classes, but it does no harm if they are escaped.
! 1039:
! 1040:
! 1041:
! 1042: VERTICAL BAR
! 1043: Vertical bar characters are used to separate alternative
! 1044: patterns. For example, the pattern
! 1045:
! 1046: gilbert|sullivan
! 1047:
! 1048: matches either "gilbert" or "sullivan". Any number of alter-
! 1049: natives may appear, and an empty alternative is permitted
! 1050: (matching the empty string). The matching process tries
! 1051: each alternative in turn, from left to right, and the first
! 1052: one that succeeds is used. If the alternatives are within a
! 1053: subpattern (defined below), "succeeds" means matching the
! 1054: rest of the main pattern as well as the alternative in the
! 1055: subpattern.
! 1056:
! 1057:
! 1058:
! 1059: INTERNAL OPTION SETTING
! 1060: The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL,
! 1061: and PCRE_EXTENDED can be changed from within the pattern by
! 1062: a sequence of Perl option letters enclosed between "(?" and
! 1063: ")". The option letters are
! 1064:
! 1065: i for PCRE_CASELESS
! 1066: m for PCRE_MULTILINE
! 1067: s for PCRE_DOTALL
! 1068: x for PCRE_EXTENDED
! 1069:
! 1070: For example, (?im) sets caseless, multiline matching. It is
! 1071: also possible to unset these options by preceding the letter
! 1072: with a hyphen, and a combined setting and unsetting such as
! 1073: (?im-sx), which sets PCRE_CASELESS and PCRE_MULTILINE while
! 1074: unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted.
! 1075: If a letter appears both before and after the hyphen, the
! 1076: option is unset.
! 1077:
! 1078: The scope of these option changes depends on where in the
! 1079: pattern the setting occurs. For settings that are outside
! 1080: any subpattern (defined below), the effect is the same as if
! 1081: the options were set or unset at the start of matching. The
! 1082: following patterns all behave in exactly the same way:
! 1083:
! 1084: (?i)abc
! 1085: a(?i)bc
! 1086: ab(?i)c
! 1087: abc(?i)
! 1088:
! 1089: which in turn is the same as compiling the pattern abc with
! 1090: PCRE_CASELESS set. In other words, such "top level" set-
! 1091: tings apply to the whole pattern (unless there are other
! 1092: changes inside subpatterns). If there is more than one set-
! 1093: ting of the same option at top level, the rightmost setting
! 1094: is used.
! 1095:
! 1096: If an option change occurs inside a subpattern, the effect
! 1097: is different. This is a change of behaviour in Perl 5.005.
! 1098: An option change inside a subpattern affects only that part
! 1099: of the subpattern that follows it, so
! 1100:
! 1101: (a(?i)b)c
! 1102:
! 1103: matches abc and aBc and no other strings (assuming
! 1104: PCRE_CASELESS is not used). By this means, options can be
! 1105: made to have different settings in different parts of the
! 1106: pattern. Any changes made in one alternative do carry on
! 1107: into subsequent branches within the same subpattern. For
! 1108: example,
! 1109:
! 1110: (a(?i)b|c)
! 1111:
! 1112: matches "ab", "aB", "c", and "C", even though when matching
! 1113: "C" the first branch is abandoned before the option setting.
! 1114: This is because the effects of option settings happen at
! 1115: compile time. There would be some very weird behaviour oth-
! 1116: erwise.
! 1117:
! 1118: The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can
! 1119: be changed in the same way as the Perl-compatible options by
! 1120: using the characters U and X respectively. The (?X) flag
! 1121: setting is special in that it must always occur earlier in
! 1122: the pattern than any of the additional features it turns on,
! 1123: even when it is at top level. It is best put at the start.
! 1124:
! 1125:
! 1126:
! 1127: SUBPATTERNS
! 1128: Subpatterns are delimited by parentheses (round brackets),
! 1129: which can be nested. Marking part of a pattern as a subpat-
! 1130: tern does two things:
! 1131:
! 1132: 1. It localizes a set of alternatives. For example, the pat-
! 1133: tern
! 1134:
! 1135: cat(aract|erpillar|)
! 1136:
! 1137: matches one of the words "cat", "cataract", or "caterpil-
! 1138: lar". Without the parentheses, it would match "cataract",
! 1139: "erpillar" or the empty string.
! 1140:
! 1141: 2. It sets up the subpattern as a capturing subpattern (as
! 1142: defined above). When the whole pattern matches, that por-
! 1143: tion of the subject string that matched the subpattern is
! 1144: passed back to the caller via the ovector argument of
! 1145: pcre_exec(). Opening parentheses are counted from left to
! 1146: right (starting from 1) to obtain the numbers of the captur-
! 1147: ing subpatterns.
! 1148:
! 1149: For example, if the string "the red king" is matched against
! 1150: the pattern
! 1151:
! 1152: the ((red|white) (king|queen))
! 1153:
! 1154: the captured substrings are "red king", "red", and "king",
! 1155: and are numbered 1, 2, and 3.
! 1156:
! 1157: The fact that plain parentheses fulfil two functions is not
! 1158: always helpful. There are often times when a grouping sub-
! 1159: pattern is required without a capturing requirement. If an
! 1160: opening parenthesis is followed by "?:", the subpattern does
! 1161: not do any capturing, and is not counted when computing the
! 1162: number of any subsequent capturing subpatterns. For example,
! 1163: if the string "the white queen" is matched against the pat-
! 1164: tern
! 1165:
! 1166: the ((?:red|white) (king|queen))
! 1167:
! 1168: the captured substrings are "white queen" and "queen", and
! 1169: are numbered 1 and 2. The maximum number of captured sub-
! 1170: strings is 99, and the maximum number of all subpatterns,
! 1171: both capturing and non-capturing, is 200.
! 1172:
! 1173: As a convenient shorthand, if any option settings are
! 1174: required at the start of a non-capturing subpattern, the
! 1175: option letters may appear between the "?" and the ":". Thus
! 1176: the two patterns
! 1177:
! 1178: (?i:saturday|sunday)
! 1179: (?:(?i)saturday|sunday)
! 1180:
! 1181: match exactly the same set of strings. Because alternative
! 1182: branches are tried from left to right, and options are not
! 1183: reset until the end of the subpattern is reached, an option
! 1184: setting in one branch does affect subsequent branches, so
! 1185: the above patterns match "SUNDAY" as well as "Saturday".
! 1186:
! 1187:
! 1188:
! 1189: REPETITION
! 1190: Repetition is specified by quantifiers, which can follow any
! 1191: of the following items:
! 1192:
! 1193: a single character, possibly escaped
! 1194: the . metacharacter
! 1195: a character class
! 1196: a back reference (see next section)
! 1197: a parenthesized subpattern (unless it is an assertion -
! 1198: see below)
! 1199:
! 1200: The general repetition quantifier specifies a minimum and
! 1201: maximum number of permitted matches, by giving the two
! 1202: numbers in curly brackets (braces), separated by a comma.
! 1203: The numbers must be less than 65536, and the first must be
! 1204: less than or equal to the second. For example:
! 1205:
! 1206: z{2,4}
! 1207:
! 1208: matches "zz", "zzz", or "zzzz". A closing brace on its own
! 1209: is not a special character. If the second number is omitted,
! 1210: but the comma is present, there is no upper limit; if the
! 1211: second number and the comma are both omitted, the quantifier
! 1212: specifies an exact number of required matches. Thus
! 1213:
! 1214: [aeiou]{3,}
! 1215:
! 1216: matches at least 3 successive vowels, but may match many
! 1217: more, while
! 1218:
! 1219: \d{8}
! 1220:
! 1221: matches exactly 8 digits. An opening curly bracket that
! 1222: appears in a position where a quantifier is not allowed, or
! 1223: one that does not match the syntax of a quantifier, is taken
! 1224: as a literal character. For example, {,6} is not a quantif-
! 1225: ier, but a literal string of four characters.
! 1226:
! 1227: The quantifier {0} is permitted, causing the expression to
! 1228: behave as if the previous item and the quantifier were not
! 1229: present.
! 1230:
! 1231: For convenience (and historical compatibility) the three
! 1232: most common quantifiers have single-character abbreviations:
! 1233:
! 1234: * is equivalent to {0,}
! 1235: + is equivalent to {1,}
! 1236: ? is equivalent to {0,1}
! 1237:
! 1238: It is possible to construct infinite loops by following a
! 1239: subpattern that can match no characters with a quantifier
! 1240: that has no upper limit, for example:
! 1241:
! 1242: (a?)*
! 1243:
! 1244: Earlier versions of Perl and PCRE used to give an error at
! 1245: compile time for such patterns. However, because there are
! 1246: cases where this can be useful, such patterns are now
! 1247: accepted, but if any repetition of the subpattern does in
! 1248: fact match no characters, the loop is forcibly broken.
! 1249:
! 1250: By default, the quantifiers are "greedy", that is, they
! 1251: match as much as possible (up to the maximum number of per-
! 1252: mitted times), without causing the rest of the pattern to
! 1253: fail. The classic example of where this gives problems is in
! 1254: trying to match comments in C programs. These appear between
! 1255: the sequences /* and */ and within the sequence, individual
! 1256: * and / characters may appear. An attempt to match C com-
! 1257: ments by applying the pattern
! 1258:
! 1259: /\*.*\*/
! 1260:
! 1261: to the string
! 1262:
! 1263: /* first command */ not comment /* second comment */
! 1264:
! 1265: fails, because it matches the entire string due to the
! 1266: greediness of the .* item.
! 1267:
! 1268: However, if a quantifier is followed by a question mark,
! 1269: then it ceases to be greedy, and instead matches the minimum
! 1270: number of times possible, so the pattern
! 1271:
! 1272: /\*.*?\*/
! 1273:
! 1274: does the right thing with the C comments. The meaning of the
! 1275: various quantifiers is not otherwise changed, just the pre-
! 1276: ferred number of matches. Do not confuse this use of ques-
! 1277: tion mark with its use as a quantifier in its own right.
! 1278: Because it has two uses, it can sometimes appear doubled, as
! 1279: in
! 1280:
! 1281: \d??\d
! 1282:
! 1283: which matches one digit by preference, but can match two if
! 1284: that is the only way the rest of the pattern matches.
! 1285:
! 1286: If the PCRE_UNGREEDY option is set (an option which is not
! 1287: available in Perl) then the quantifiers are not greedy by
! 1288: default, but individual ones can be made greedy by following
! 1289: them with a question mark. In other words, it inverts the
! 1290: default behaviour.
! 1291:
! 1292: When a parenthesized subpattern is quantified with a minimum
! 1293: repeat count that is greater than 1 or with a limited max-
! 1294: imum, more store is required for the compiled pattern, in
! 1295: proportion to the size of the minimum or maximum.
! 1296:
! 1297: If a pattern starts with .* or .{0,} and the PCRE_DOTALL
! 1298: option (equivalent to Perl's /s) is set, thus allowing the .
! 1299: to match newlines, then the pattern is implicitly anchored,
! 1300: because whatever follows will be tried against every charac-
! 1301: ter position in the subject string, so there is no point in
! 1302: retrying the overall match at any position after the first.
! 1303: PCRE treats such a pattern as though it were preceded by \A.
! 1304: In cases where it is known that the subject string contains
! 1305: no newlines, it is worth setting PCRE_DOTALL when the pat-
! 1306: tern begins with .* in order to obtain this optimization, or
! 1307: alternatively using ^ to indicate anchoring explicitly.
! 1308:
! 1309: When a capturing subpattern is repeated, the value captured
! 1310: is the substring that matched the final iteration. For
! 1311: example, after
! 1312:
! 1313: (tweedle[dume]{3}\s*)+
! 1314:
! 1315: has matched "tweedledum tweedledee" the value of the cap-
! 1316: tured substring is "tweedledee". However, if there are
! 1317: nested capturing subpatterns, the corresponding captured
! 1318: values may have been set in previous iterations. For exam-
! 1319: ple, after
! 1320:
! 1321: /(a|(b))+/
! 1322:
! 1323: matches "aba" the value of the second captured substring is
! 1324: "b".
! 1325:
! 1326:
! 1327:
! 1328: BACK REFERENCES
! 1329: Outside a character class, a backslash followed by a digit
! 1330: greater than 0 (and possibly further digits) is a back
! 1331: reference to a capturing subpattern earlier (i.e. to its
! 1332: left) in the pattern, provided there have been that many
! 1333: previous capturing left parentheses.
! 1334:
! 1335: However, if the decimal number following the backslash is
! 1336: less than 10, it is always taken as a back reference, and
! 1337: causes an error only if there are not that many capturing
! 1338: left parentheses in the entire pattern. In other words, the
! 1339: parentheses that are referenced need not be to the left of
! 1340: the reference for numbers less than 10. See the section
! 1341: entitled "Backslash" above for further details of the han-
! 1342: dling of digits following a backslash.
! 1343:
! 1344: A back reference matches whatever actually matched the cap-
! 1345: turing subpattern in the current subject string, rather than
! 1346: anything matching the subpattern itself. So the pattern
! 1347:
! 1348: (sens|respons)e and \1ibility
! 1349:
! 1350: matches "sense and sensibility" and "response and responsi-
! 1351: bility", but not "sense and responsibility". If caseful
! 1352: matching is in force at the time of the back reference, then
! 1353: the case of letters is relevant. For example,
! 1354:
! 1355: ((?i)rah)\s+\1
! 1356:
! 1357: matches "rah rah" and "RAH RAH", but not "RAH rah", even
! 1358: though the original capturing subpattern is matched case-
! 1359: lessly.
! 1360:
! 1361: There may be more than one back reference to the same sub-
! 1362: pattern. If a subpattern has not actually been used in a
! 1363: particular match, then any back references to it always
! 1364: fail. For example, the pattern
! 1365:
! 1366: (a|(bc))\2
! 1367:
! 1368: always fails if it starts to match "a" rather than "bc".
! 1369: Because there may be up to 99 back references, all digits
! 1370: following the backslash are taken as part of a potential
! 1371: back reference number. If the pattern continues with a digit
! 1372: character, then some delimiter must be used to terminate the
! 1373: back reference. If the PCRE_EXTENDED option is set, this can
! 1374: be whitespace. Otherwise an empty comment can be used.
! 1375:
! 1376: A back reference that occurs inside the parentheses to which
! 1377: it refers fails when the subpattern is first used, so, for
! 1378: example, (a\1) never matches. However, such references can
! 1379: be useful inside repeated subpatterns. For example, the pat-
! 1380: tern
! 1381:
! 1382: (a|b\1)+
! 1383:
! 1384: matches any number of "a"s and also "aba", "ababaa" etc. At
! 1385: each iteration of the subpattern, the back reference matches
! 1386: the character string corresponding to the previous itera-
! 1387: tion. In order for this to work, the pattern must be such
! 1388: that the first iteration does not need to match the back
! 1389: reference. This can be done using alternation, as in the
! 1390: example above, or by a quantifier with a minimum of zero.
! 1391:
! 1392:
! 1393:
! 1394: ASSERTIONS
! 1395: An assertion is a test on the characters following or
! 1396: preceding the current matching point that does not actually
! 1397: consume any characters. The simple assertions coded as \b,
! 1398: \B, \A, \Z, \z, ^ and $ are described above. More compli-
! 1399: cated assertions are coded as subpatterns. There are two
! 1400: kinds: those that look ahead of the current position in the
! 1401: subject string, and those that look behind it.
! 1402:
! 1403: An assertion subpattern is matched in the normal way, except
! 1404: that it does not cause the current matching position to be
! 1405: changed. Lookahead assertions start with (?= for positive
! 1406: assertions and (?! for negative assertions. For example,
! 1407:
! 1408: \w+(?=;)
! 1409:
! 1410: matches a word followed by a semicolon, but does not include
! 1411: the semicolon in the match, and
! 1412:
! 1413: foo(?!bar)
! 1414:
! 1415: matches any occurrence of "foo" that is not followed by
! 1416: "bar". Note that the apparently similar pattern
! 1417:
! 1418: (?!foo)bar
! 1419:
! 1420: does not find an occurrence of "bar" that is preceded by
! 1421: something other than "foo"; it finds any occurrence of "bar"
! 1422: whatsoever, because the assertion (?!foo) is always true
! 1423: when the next three characters are "bar". A lookbehind
! 1424: assertion is needed to achieve this effect.
! 1425:
! 1426: Lookbehind assertions start with (?<= for positive asser-
! 1427: tions and (?<! for negative assertions. For example,
! 1428:
! 1429: (?<!foo)bar
! 1430:
! 1431: does find an occurrence of "bar" that is not preceded by
! 1432: "foo". The contents of a lookbehind assertion are restricted
! 1433: such that all the strings it matches must have a fixed
! 1434: length. However, if there are several alternatives, they do
! 1435: not all have to have the same fixed length. Thus
! 1436:
! 1437: (?<=bullock|donkey)
! 1438:
! 1439: is permitted, but
! 1440:
! 1441: (?<!dogs?|cats?)
! 1442:
! 1443: causes an error at compile time. Branches that match dif-
! 1444: ferent length strings are permitted only at the top level of
! 1445: a lookbehind assertion. This is an extension compared with
! 1446: Perl 5.005, which requires all branches to match the same
! 1447: length of string. An assertion such as
! 1448:
! 1449: (?<=ab(c|de))
! 1450:
! 1451: is not permitted, because its single top-level branch can
! 1452: match two different lengths, but it is acceptable if rewrit-
! 1453: ten to use two top-level branches:
! 1454:
! 1455: (?<=abc|abde)
! 1456:
! 1457: The implementation of lookbehind assertions is, for each
! 1458: alternative, to temporarily move the current position back
! 1459: by the fixed width and then try to match. If there are
! 1460: insufficient characters before the current position, the
! 1461: match is deemed to fail. Lookbehinds in conjunction with
! 1462: once-only subpatterns can be particularly useful for match-
! 1463: ing at the ends of strings; an example is given at the end
! 1464: of the section on once-only subpatterns.
! 1465:
! 1466: Several assertions (of any sort) may occur in succession.
! 1467: For example,
! 1468:
! 1469: (?<=\d{3})(?<!999)foo
! 1470:
! 1471: matches "foo" preceded by three digits that are not "999".
! 1472: Notice that each of the assertions is applied independently
! 1473: at the same point in the subject string. First there is a
! 1474: check that the previous three characters are all digits,
! 1475: then there is a check that the same three characters are not
! 1476: "999". This pattern does not match "foo" preceded by six
! 1477: characters, the first of which are digits and the last three
! 1478: of which are not "999". For example, it doesn't match
! 1479: "123abcfoo". A pattern to do that is
! 1480:
! 1481: (?<=\d{3}...)(?<!999)foo
! 1482:
! 1483: This time the first assertion looks at the preceding six
! 1484: characters, checking that the first three are digits, and
! 1485: then the second assertion checks that the preceding three
! 1486: characters are not "999".
! 1487:
! 1488: Assertions can be nested in any combination. For example,
! 1489:
! 1490: (?<=(?<!foo)bar)baz
! 1491:
! 1492: matches an occurrence of "baz" that is preceded by "bar"
! 1493: which in turn is not preceded by "foo", while
! 1494:
! 1495: (?<=\d{3}(?!999)...)foo
! 1496:
! 1497: is another pattern which matches "foo" preceded by three
! 1498: digits and any three characters that are not "999".
! 1499:
! 1500: Assertion subpatterns are not capturing subpatterns, and may
! 1501: not be repeated, because it makes no sense to assert the
! 1502: same thing several times. If any kind of assertion contains
! 1503: capturing subpatterns within it, these are counted for the
! 1504: purposes of numbering the capturing subpatterns in the whole
! 1505: pattern. However, substring capturing is carried out only
! 1506: for positive assertions, because it does not make sense for
! 1507: negative assertions.
! 1508:
! 1509: Assertions count towards the maximum of 200 parenthesized
! 1510: subpatterns.
! 1511:
! 1512:
! 1513:
! 1514: ONCE-ONLY SUBPATTERNS
! 1515: With both maximizing and minimizing repetition, failure of
! 1516: what follows normally causes the repeated item to be re-
! 1517: evaluated to see if a different number of repeats allows the
! 1518: rest of the pattern to match. Sometimes it is useful to
! 1519: prevent this, either to change the nature of the match, or
! 1520: to cause it fail earlier than it otherwise might, when the
! 1521: author of the pattern knows there is no point in carrying
! 1522: on.
! 1523:
! 1524: Consider, for example, the pattern \d+foo when applied to
! 1525: the subject line
! 1526:
! 1527: 123456bar
! 1528:
! 1529: After matching all 6 digits and then failing to match "foo",
! 1530: the normal action of the matcher is to try again with only 5
! 1531: digits matching the \d+ item, and then with 4, and so on,
! 1532: before ultimately failing. Once-only subpatterns provide the
! 1533: means for specifying that once a portion of the pattern has
! 1534: matched, it is not to be re-evaluated in this way, so the
! 1535: matcher would give up immediately on failing to match "foo"
! 1536: the first time. The notation is another kind of special
! 1537: parenthesis, starting with (?> as in this example:
! 1538:
! 1539: (?>\d+)bar
! 1540:
! 1541: This kind of parenthesis "locks up" the part of the pattern
! 1542: it contains once it has matched, and a failure further into
! 1543: the pattern is prevented from backtracking into it. Back-
! 1544: tracking past it to previous items, however, works as nor-
! 1545: mal.
! 1546:
! 1547: An alternative description is that a subpattern of this type
! 1548: matches the string of characters that an identical stan-
! 1549: dalone pattern would match, if anchored at the current point
! 1550: in the subject string.
! 1551:
! 1552: Once-only subpatterns are not capturing subpatterns. Simple
! 1553: cases such as the above example can be thought of as a max-
! 1554: imizing repeat that must swallow everything it can. So,
! 1555: while both \d+ and \d+? are prepared to adjust the number of
! 1556: digits they match in order to make the rest of the pattern
! 1557: match, (?>\d+) can only match an entire sequence of digits.
! 1558:
! 1559: This construction can of course contain arbitrarily compli-
! 1560: cated subpatterns, and it can be nested.
! 1561:
! 1562: Once-only subpatterns can be used in conjunction with look-
! 1563: behind assertions to specify efficient matching at the end
! 1564: of the subject string. Consider a simple pattern such as
! 1565:
! 1566: abcd$
! 1567:
! 1568: when applied to a long string which does not match it.
! 1569: Because matching proceeds from left to right, PCRE will look
! 1570: for each "a" in the subject and then see if what follows
! 1571: matches the rest of the pattern. If the pattern is specified
! 1572: as
! 1573:
! 1574: ^.*abcd$
! 1575:
! 1576: then the initial .* matches the entire string at first, but
! 1577: when this fails, it backtracks to match all but the last
! 1578: character, then all but the last two characters, and so on.
! 1579: Once again the search for "a" covers the entire string, from
! 1580: right to left, so we are no better off. However, if the pat-
! 1581: tern is written as
! 1582:
! 1583: ^(?>.*)(?<=abcd)
! 1584:
! 1585: then there can be no backtracking for the .* item; it can
! 1586: match only the entire string. The subsequent lookbehind
! 1587: assertion does a single test on the last four characters. If
! 1588: it fails, the match fails immediately. For long strings,
! 1589: this approach makes a significant difference to the process-
! 1590: ing time.
! 1591:
! 1592:
! 1593:
! 1594: CONDITIONAL SUBPATTERNS
! 1595: It is possible to cause the matching process to obey a sub-
! 1596: pattern conditionally or to choose between two alternative
! 1597: subpatterns, depending on the result of an assertion, or
! 1598: whether a previous capturing subpattern matched or not. The
! 1599: two possible forms of conditional subpattern are
! 1600:
! 1601: (?(condition)yes-pattern)
! 1602: (?(condition)yes-pattern|no-pattern)
! 1603:
! 1604: If the condition is satisfied, the yes-pattern is used; oth-
! 1605: erwise the no-pattern (if present) is used. If there are
! 1606: more than two alternatives in the subpattern, a compile-time
! 1607: error occurs.
! 1608:
! 1609: There are two kinds of condition. If the text between the
! 1610: parentheses consists of a sequence of digits, then the con-
! 1611: dition is satisfied if the capturing subpattern of that
! 1612: number has previously matched. Consider the following pat-
! 1613: tern, which contains non-significant white space to make it
! 1614: more readable (assume the PCRE_EXTENDED option) and to
! 1615: divide it into three parts for ease of discussion:
! 1616:
! 1617: ( \( )? [^()]+ (?(1) \) )
! 1618:
! 1619: The first part matches an optional opening parenthesis, and
! 1620: if that character is present, sets it as the first captured
! 1621: substring. The second part matches one or more characters
! 1622: that are not parentheses. The third part is a conditional
! 1623: subpattern that tests whether the first set of parentheses
! 1624: matched or not. If they did, that is, if subject started
! 1625: with an opening parenthesis, the condition is true, and so
! 1626: the yes-pattern is executed and a closing parenthesis is
! 1627: required. Otherwise, since no-pattern is not present, the
! 1628: subpattern matches nothing. In other words, this pattern
! 1629: matches a sequence of non-parentheses, optionally enclosed
! 1630: in parentheses.
! 1631:
! 1632: If the condition is not a sequence of digits, it must be an
! 1633: assertion. This may be a positive or negative lookahead or
! 1634: lookbehind assertion. Consider this pattern, again contain-
! 1635: ing non-significant white space, and with the two alterna-
! 1636: tives on the second line:
! 1637:
! 1638: (?(?=[^a-z]*[a-z])
! 1639: \d{2}[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
! 1640:
! 1641: The condition is a positive lookahead assertion that matches
! 1642: an optional sequence of non-letters followed by a letter. In
! 1643: other words, it tests for the presence of at least one
! 1644: letter in the subject. If a letter is found, the subject is
! 1645: matched against the first alternative; otherwise it is
! 1646: matched against the second. This pattern matches strings in
! 1647: one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
! 1648: letters and dd are digits.
! 1649:
! 1650:
! 1651:
! 1652: COMMENTS
! 1653: The sequence (?# marks the start of a comment which contin-
! 1654: ues up to the next closing parenthesis. Nested parentheses
! 1655: are not permitted. The characters that make up a comment
! 1656: play no part in the pattern matching at all.
! 1657:
! 1658: If the PCRE_EXTENDED option is set, an unescaped # character
! 1659: outside a character class introduces a comment that contin-
! 1660: ues up to the next newline character in the pattern.
! 1661:
! 1662:
! 1663:
! 1664: PERFORMANCE
! 1665: Certain items that may appear in patterns are more efficient
! 1666: than others. It is more efficient to use a character class
! 1667: like [aeiou] than a set of alternatives such as (a|e|i|o|u).
! 1668: In general, the simplest construction that provides the
! 1669: required behaviour is usually the most efficient. Jeffrey
! 1670: Friedl's book contains a lot of discussion about optimizing
! 1671: regular expressions for efficient performance.
! 1672:
! 1673: When a pattern begins with .* and the PCRE_DOTALL option is
! 1674: set, the pattern is implicitly anchored by PCRE, since it
! 1675: can match only at the start of a subject string. However, if
! 1676: PCRE_DOTALL is not set, PCRE cannot make this optimization,
! 1677: because the . metacharacter does not then match a newline,
! 1678: and if the subject string contains newlines, the pattern may
! 1679: match from the character immediately following one of them
! 1680: instead of from the very start. For example, the pattern
! 1681:
! 1682: (.*) second
! 1683:
! 1684: matches the subject "first\nand second" (where \n stands for
! 1685: a newline character) with the first captured substring being
! 1686: "and". In order to do this, PCRE has to retry the match
! 1687: starting after every newline in the subject.
! 1688:
! 1689: If you are using such a pattern with subject strings that do
! 1690: not contain newlines, the best performance is obtained by
! 1691: setting PCRE_DOTALL, or starting the pattern with ^.* to
! 1692: indicate explicit anchoring. That saves PCRE from having to
! 1693: scan along the subject looking for a newline to restart at.
! 1694:
! 1695: Beware of patterns that contain nested indefinite repeats.
! 1696: These can take a long time to run when applied to a string
! 1697: that does not match. Consider the pattern fragment
! 1698:
! 1699: (a+)*
! 1700:
! 1701: This can match "aaaa" in 33 different ways, and this number
! 1702: increases very rapidly as the string gets longer. (The *
! 1703: repeat can match 0, 1, 2, 3, or 4 times, and for each of
! 1704: those cases other than 0, the + repeats can match different
! 1705: numbers of times.) When the remainder of the pattern is such
! 1706: that the entire match is going to fail, PCRE has in princi-
! 1707: ple to try every possible variation, and this can take an
! 1708: extremely long time.
! 1709:
! 1710: An optimization catches some of the more simple cases such
! 1711: as
! 1712:
! 1713: (a+)*b
! 1714:
! 1715: where a literal character follows. Before embarking on the
! 1716: standard matching procedure, PCRE checks that there is a "b"
! 1717: later in the subject string, and if there is not, it fails
! 1718: the match immediately. However, when there is no following
! 1719: literal this optimization cannot be used. You can see the
! 1720: difference by comparing the behaviour of
! 1721:
! 1722: (a+)*\d
! 1723:
! 1724: with the pattern above. The former gives a failure almost
! 1725: instantly when applied to a whole line of "a" characters,
! 1726: whereas the latter takes an appreciable time with strings
! 1727: longer than about 20 characters.
! 1728:
! 1729:
! 1730:
! 1731: AUTHOR
! 1732: Philip Hazel <ph10@cam.ac.uk>
! 1733: University Computing Service,
! 1734: New Museums Site,
! 1735: Cambridge CB2 3QG, England.
! 1736: Phone: +44 1223 334714
! 1737:
! 1738: Last updated: 29 July 1999
! 1739: Copyright (c) 1997-1999 University of Cambridge.
E-mail:
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