/[pcre]/code/trunk/doc/pcrepattern.3
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revision 453 by ph10, Fri Sep 18 19:12:35 2009 UTC revision 454 by ph10, Tue Sep 22 09:42:11 2009 UTC
# Line 333  syntax for referencing a subpattern as a Line 333  syntax for referencing a subpattern as a
333  later.  later.
334  .\"  .\"
335  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342  .  .
343  .  .
344  .SS "Generic character types"  .SS "Generic character types"
# Line 1669  is permitted, but Line 1674  is permitted, but
1674  .sp  .sp
1675  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1676  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1677  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1678  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1679  .sp  .sp
1680    (?<=ab(c|de))    (?<=ab(c|de))
1681  .sp  .sp
1682  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1683  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1684    branches:
1685  .sp  .sp
1686    (?<=abc|abde)    (?<=abc|abde)
1687  .sp  .sp
# Line 1684  In some cases, the Perl 5.10 escape sequ Line 1690  In some cases, the Perl 5.10 escape sequ
1690  .\" </a>  .\" </a>
1691  (see above)  (see above)
1692  .\"  .\"
1693  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1694  fixed-length.  restriction.
1695  .P  .P
1696  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1697  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1697  to appear in lookbehind assertions, beca Line 1703  to appear in lookbehind assertions, beca
1703  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1704  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1705  .P  .P
1706    .\" HTML <a href="#subpatternsassubroutines">
1707    .\" </a>
1708    "Subroutine"
1709    .\"
1710    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1711    as the subpattern matches a fixed-length string.
1712    .\" HTML <a href="#recursion">
1713    .\" </a>
1714    Recursion,
1715    .\"
1716    however, is not supported.
1717    .P
1718  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1719  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching at the end of the subject string. Consider a simple
1720  pattern such as  pattern such as
# Line 1841  the condition is true if the most recent Line 1859  the condition is true if the most recent
1859  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
1860  stack.  stack.
1861  .P  .P
1862  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1863  patterns are described below.  .\" HTML <a href="#recursion">
1864    .\" </a>
1865    Recursive patterns
1866    .\"
1867    are described below.
1868  .  .
1869  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1870  .rs  .rs
# Line 1851  If the condition is the string (DEFINE), Line 1873  If the condition is the string (DEFINE),
1873  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
1874  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1875  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
1876  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1877    .\" HTML <a href="#subpatternsassubroutines">
1878    .\" </a>
1879    "subroutines"
1880    .\"
1881  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
1882  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1883  .sp  .sp
# Line 1926  this kind of recursion was subsequently Line 1952  this kind of recursion was subsequently
1952  .P  .P
1953  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
1954  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
1955  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
1956    .\" HTML <a href="#subpatternsassubroutines">
1957    .\" </a>
1958    "subroutine"
1959    .\"
1960  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
1961  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
1962  .P  .P
# Line 1958  it is encountered. Line 1988  it is encountered.
1988  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
1989  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
1990  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
1991  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
1992    .\" </a>
1993    "subroutine"
1994    .\"
1995    calls, as described in the next section.
1996  .P  .P
1997  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
1998  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
# Line 2317  Cambridge CB2 3QH, England. Line 2351  Cambridge CB2 3QH, England.
2351  .rs  .rs
2352  .sp  .sp
2353  .nf  .nf
2354  Last updated: 18 September 2009  Last updated: 22 September 2009
2355  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2356  .fi  .fi
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