/[pcre]/code/trunk/doc/pcrepattern.3

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-revision 517 by ph10,
Wed May  5 10:44:20 2010 UTC
+revision 572 by ph10,
Wed Nov 17 17:55:57 2010 UTC
 Line 42 
 in the main
  .\"
  page.
  .P
+ Another special sequence that may appear at the start of a pattern or in
+ combination with (*UTF8) is:
+ .sp
+   (*UCP)
+ .sp
+ This has the same effect as setting the PCRE_UCP option: it causes sequences
+ such as \ed and \ew to use Unicode properties to determine character types,
+ instead of recognizing only characters with codes less than 128 via a lookup
+ table.
+ .P
  The remainder of this document discusses the patterns that are supported by
  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
  From release 6.0, PCRE offers a second matching function,
-Line 56 
 discussed in the
+Line 66 
 discussed in the
  page.
  .
  .
+ .\" HTML <a name="newlines"></a>
  .SH "NEWLINE CONVENTIONS"
  .rs
  .sp
-Line 200 
 Perl, $ and @ cause variable interpolati
+Line 211 
 Perl, $ and @ cause variable interpolati
    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
  .sp
  The \eQ...\eE sequence is recognized both inside and outside character classes.
+ An isolated \eE that is not preceded by \eQ is ignored.
  .
  .
  .\" HTML <a name="digitsafterbackslash"></a>
-Line 340 
 subroutine
+Line 352 
 subroutine
  call.
  .
  .
+ .\" HTML <a name="genericchartypes"></a>
  .SS "Generic character types"
  .rs
  .sp
-Line 356 
 Another use of backslash is for specifyi
+Line 369 
 Another use of backslash is for specifyi
    \ew     any "word" character
    \eW     any "non-word" character
  .sp
  There is also the single sequence \eN, which matches a non-newline character.
  This is the same as
  .\" HTML <a href="#fullstopdot">
  .\" </a>
  the "." metacharacter
  .\"
  when PCRE_DOTALL is not set.
  .P
  Each pair of lower and upper case escape sequences partitions the complete set
  of characters into two disjoint sets. Any given character matches one, and only
- one, of each pair.
+ one, of each pair. The sequences can appear both inside and outside character
- .P
- These character type sequences can appear both inside and outside character
  classes. They each match one character of the appropriate type. If the current
- matching point is at the end of the subject string, all of them fail, since
+ matching point is at the end of the subject string, all of them fail, because
  there is no character to match.
  .P
  For compatibility with Perl, \es does not match the VT character (code 11).
-Line 379 
 are HT (9), LF (10), FF (12), CR (13), a
+Line 390 
 are HT (9), LF (10), FF (12), CR (13), a
  included in a Perl script, \es may match the VT character. In PCRE, it never
  does.
  .P
- In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
+ A "word" character is an underscore or any character that is a letter or digit.
- \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
+ By default, the definition of letters and digits is controlled by PCRE's
- character property support is available. These sequences retain their original
+ low-valued character tables, and may vary if locale-specific matching is taking
- meanings from before UTF-8 support was available, mainly for efficiency
+ place (see
- reasons. Note that this also affects \eb, because it is defined in terms of \ew
+ .\" HTML <a href="pcreapi.html#localesupport">
- and \eW.
+ .\" </a>
- .P
+ "Locale support"
- The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
+ .\"
- other sequences, these do match certain high-valued codepoints in UTF-8 mode.
+ in the
- The horizontal space characters are:
+ .\" HREF
+ \fBpcreapi\fP
+ .\"
+ page). For example, in a French locale such as "fr_FR" in Unix-like systems,
+ or "french" in Windows, some character codes greater than 128 are used for
+ accented letters, and these are then matched by \ew. The use of locales with
+ Unicode is discouraged.
+ .P
+ By default, in UTF-8 mode, characters with values greater than 128 never match
+ \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
+ their original meanings from before UTF-8 support was available, mainly for
+ efficiency reasons. However, if PCRE is compiled with Unicode property support,
+ and the PCRE_UCP option is set, the behaviour is changed so that Unicode
+ properties are used to determine character types, as follows:
+ .sp
+   \ed  any character that \ep{Nd} matches (decimal digit)
+   \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
+   \ew  any character that \ep{L} or \ep{N} matches, plus underscore
+ .sp
+ The upper case escapes match the inverse sets of characters. Note that \ed
+ matches only decimal digits, whereas \ew matches any Unicode digit, as well as
+ any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
+ \eB because they are defined in terms of \ew and \eW. Matching these sequences
+ is noticeably slower when PCRE_UCP is set.
+ .P
+ The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
+ release 5.10. In contrast to the other sequences, which match only ASCII
+ characters by default, these always match certain high-valued codepoints in
+ UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
+ are:
  .sp
    U+0009     Horizontal tab
    U+0020     Space
-Line 419 
 The vertical space characters are:
+Line 459 
 The vertical space characters are:
    U+0085     Next line
    U+2028     Line separator
    U+2029     Paragraph separator
- .P
- A "word" character is an underscore or any character less than 256 that is a
- letter or digit. The definition of letters and digits is controlled by PCRE's
- low-valued character tables, and may vary if locale-specific matching is taking
- place (see
- .\" HTML <a href="pcreapi.html#localesupport">
- .\" </a>
- "Locale support"
- .\"
- in the
- .\" HREF
- \fBpcreapi\fP
- .\"
- page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- or "french" in Windows, some character codes greater than 128 are used for
- accented letters, and these are matched by \ew. The use of locales with Unicode
- is discouraged.
  .
  .
  .\" HTML <a name="newlineseq"></a>
-Line 443 
 is discouraged.
+Line 466 
 is discouraged.
  .rs
  .sp
  Outside a character class, by default, the escape sequence \eR matches any
- Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
+ Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
- equivalent to the following:
  .sp
    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
  .sp
-Line 481 
 These override the default and the optio
+Line 503 
 These override the default and the optio
  which are not Perl-compatible, are recognized only at the very start of a
  pattern, and that they must be in upper case. If more than one of them is
  present, the last one is used. They can be combined with a change of newline
- convention, for example, a pattern can start with:
+ convention; for example, a pattern can start with:
  .sp
    (*ANY)(*BSR_ANYCRLF)
  .sp
- Inside a character class, \eR is treated as an unrecognized escape sequence,
+ They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
- and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is
+ a character class, \eR is treated as an unrecognized escape sequence, and so
- set.
+ matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
  .
  .
  .\" HTML <a name="uniextseq"></a>
-Line 507 
 The extra escape sequences are:
+Line 529 
 The extra escape sequences are:
  The property names represented by \fIxx\fP above are limited to the Unicode
  script names, the general category properties, "Any", which matches any
  character (including newline), and some special PCRE properties (described
  in the
  .\" HTML <a href="#extraprops">
  .\" </a>
  next section).
  .\"
  Other Perl properties such as "InMusicalSymbols" are not currently supported by
  PCRE. Note that \eP{Any} does not match any characters, so always causes a
-Line 721 
 non-UTF-8 mode \eX matches any one chara
+Line 743 
 non-UTF-8 mode \eX matches any one chara
  Matching characters by Unicode property is not fast, because PCRE has to search
  a structure that contains data for over fifteen thousand characters. That is
  why the traditional escape sequences such as \ed and \ew do not use Unicode
- properties in PCRE.
+ properties in PCRE by default, though you can make them do so by setting the
+ PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
+ (*UCP).
  .
  .
  .\" HTML <a name="extraprops"></a>
  .SS PCRE's additional properties
  .rs
  .sp
  As well as the standard Unicode properties described in the previous
  section, PCRE supports four more that make it possible to convert traditional
  escape sequences such as \ew and \es and POSIX character classes to use Unicode
- properties. These are:
+ properties. PCRE uses these non-standard, non-Perl properties internally when
+ PCRE_UCP is set. They are:
  .sp
    Xan   Any alphanumeric character
    Xps   Any POSIX space character
    Xsp   Any Perl space character
    Xwd   Any Perl "word" character
  .sp
  Xan matches characters that have either the L (letter) or the N (number)
  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
  carriage return, and any other character that has the Z (separator) property.
  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
  same characters as Xan, plus underscore.
  .
  .
-Line 749 
 same characters as Xan, plus underscore.
+Line 774 
 same characters as Xan, plus underscore.
  .SS "Resetting the match start"
  .rs
  .sp
- The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
+ The escape sequence \eK causes any previously matched characters not to be
- matched characters not to be included in the final matched sequence. For
+ included in the final matched sequence. For example, the pattern:
- example, the pattern:
  .sp
    foo\eKbar
  .sp
-Line 802 
 The backslashed assertions are:
+Line 826 
 The backslashed assertions are:
    \eG     matches at the first matching position in the subject
  .sp
  Inside a character class, \eb has a different meaning; it matches the backspace
  character. If any other of these assertions appears in a character class, by
  default it matches the corresponding literal character (for example, \eB
  matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
  escape sequence" error is generated instead.
-Line 810 
 escape sequence" error is generated inst
+Line 834 
 escape sequence" error is generated inst
  A word boundary is a position in the subject string where the current character
  and the previous character do not both match \ew or \eW (i.e. one matches
  \ew and the other matches \eW), or the start or end of the string if the
- first or last character matches \ew, respectively. Neither PCRE nor Perl has a
+ first or last character matches \ew, respectively. In UTF-8 mode, the meanings
- separte "start of word" or "end of word" metasequence. However, whatever
+ of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
- follows \eb normally determines which it is. For example, the fragment
+ done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
- \eba matches "a" at the start of a word.
+ of word" or "end of word" metasequence. However, whatever follows \eb normally
+ determines which it is. For example, the fragment \eba matches "a" at the start
+ of a word.
  .P
  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
  dollar (described in the next section) in that they only ever match at the very
-Line 921 
 The handling of dot is entirely independ
+Line 947 
 The handling of dot is entirely independ
  dollar, the only relationship being that they both involve newlines. Dot has no
  special meaning in a character class.
  .P
- The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
+ The escape sequence \eN behaves like a dot, except that it is not affected by
- set. In other words, it matches any one character except one that signifies the
+ the PCRE_DOTALL option. In other words, it matches any character except one
- end of a line.
+ that signifies the end of a line.
  .
  .
  .SH "MATCHING A SINGLE BYTE"
-Line 932 
 end of a line.
+Line 958 
 end of a line.
  Outside a character class, the escape sequence \eC matches any one byte, both
  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
  characters. The feature is provided in Perl in order to match individual bytes
- in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
+ in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
- what remains in the string may be a malformed UTF-8 string. For this reason,
+ rest of the string may start with a malformed UTF-8 character. For this reason,
  the \eC escape sequence is best avoided.
  .P
  PCRE does not allow \eC to appear in lookbehind assertions
-Line 1018 
 characters in both cases. In UTF-8 mode,
+Line 1044 
 characters in both cases. In UTF-8 mode,
  characters with values greater than 128 only when it is compiled with Unicode
  property support.
  .P
- The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
+ The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
- in a character class, and add the characters that they match to the class. For
+ \eW may also appear in a character class, and add the characters that they
- example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
+ match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
- conveniently be used with the upper case character types to specify a more
+ circumflex can conveniently be used with the upper case character types to
- restricted set of characters than the matching lower case type. For example,
+ specify a more restricted set of characters than the matching lower case type.
- the class [^\eW_] matches any letter or digit, but not underscore.
+ For example, the class [^\eW_] matches any letter or digit, but not underscore.
  .P
  The only metacharacters that are recognized in character classes are backslash,
  hyphen (only where it can be interpreted as specifying a range), circumflex
-Line 1043 
 this notation. For example,
+Line 1069 
 this notation. For example,
    [01[:alpha:]%]
  .sp
  matches "0", "1", any alphabetic character, or "%". The supported class names
- are
+ are:
  .sp
    alnum    letters and digits
    alpha    letters
-Line 1054 
 are
+Line 1080 
 are
    graph    printing characters, excluding space
    lower    lower case letters
    print    printing characters, including space
-   punct    printing characters, excluding letters and digits
+   punct    printing characters, excluding letters and digits and space
    space    white space (not quite the same as \es)
    upper    upper case letters
    word     "word" characters (same as \ew)
-Line 1075 
 matches "1", "2", or any non-digit. PCRE
+Line 1101 
 matches "1", "2", or any non-digit. PCRE
  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
  supported, and an error is given if they are encountered.
  .P
- In UTF-8 mode, characters with values greater than 128 do not match any of
+ By default, in UTF-8 mode, characters with values greater than 128 do not match
- the POSIX character classes.
+ any of the POSIX character classes. However, if the PCRE_UCP option is passed
+ to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
+ character properties are used. This is achieved by replacing the POSIX classes
+ by other sequences, as follows:
+ .sp
+   [:alnum:]  becomes  \ep{Xan}
+   [:alpha:]  becomes  \ep{L}
+   [:blank:]  becomes  \eh
+   [:digit:]  becomes  \ep{Nd}
+   [:lower:]  becomes  \ep{Ll}
+   [:space:]  becomes  \ep{Xps}
+   [:upper:]  becomes  \ep{Lu}
+   [:word:]   becomes  \ep{Xwd}
+ .sp
+ Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
+ classes are unchanged, and match only characters with code points less than
+.
  .
  .
  .SH "VERTICAL BAR"
-Line 1130 
 extracts it into the global options (and
+Line 1172 
 extracts it into the global options (and
  extracted by the \fBpcre_fullinfo()\fP function).
  .P
  An option change within a subpattern (see below for a description of
- subpatterns) affects only that part of the current pattern that follows it, so
+ subpatterns) affects only that part of the subpattern that follows it, so
  .sp
    (a(?i)b)c
  .sp
-Line 1155 
 section entitled
+Line 1197 
 section entitled
  .\" </a>
  "Newline sequences"
  .\"
- above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
+ above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
- mode; this is equivalent to setting the PCRE_UTF8 option.
+ to set UTF-8 and Unicode property modes; they are equivalent to setting the
+ PCRE_UTF8 and the PCRE_UCP options, respectively.
  .
  .
  .\" HTML <a name="subpattern"></a>
-Line 1170 
 Turning part of a pattern into a subpatt
+Line 1213 
 Turning part of a pattern into a subpatt
  .sp
    cat(aract|erpillar|)
  .sp
- matches one of the words "cat", "cataract", or "caterpillar". Without the
+ matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
- parentheses, it would match "cataract", "erpillar" or an empty string.
+ match "cataract", "erpillar" or an empty string.
  .sp
 . It sets up the subpattern as a capturing subpattern. This means that, when
  the whole pattern matches, that portion of the subject string that matched the
  subpattern is passed back to the caller via the \fIovector\fP argument of
  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
- from 1) to obtain numbers for the capturing subpatterns.
+ from 1) to obtain numbers for the capturing subpatterns. For example, if the
- .P
+ string "the red king" is matched against the pattern
- For example, if the string "the red king" is matched against the pattern
  .sp
    the ((red|white) (king|queen))
  .sp
-Line 1228 
 at captured substring number one, whiche
+Line 1270 
 at captured substring number one, whiche
  is useful when you want to capture part, but not all, of one of a number of
  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
  number is reset at the start of each branch. The numbers of any capturing
- buffers that follow the subpattern start after the highest number used in any
+ parentheses that follow the subpattern start after the highest number used in
- branch. The following example is taken from the Perl documentation.
+ any branch. The following example is taken from the Perl documentation. The
- The numbers underneath show in which buffer the captured content will be
+ numbers underneath show in which buffer the captured content will be stored.
- stored.
  .sp
    # before  ---------------branch-reset----------- after
    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
-Line 1358 
 items:
+Line 1399 
 items:
    the \eC escape sequence
    the \eX escape sequence (in UTF-8 mode with Unicode properties)
    the \eR escape sequence
-   an escape such as \ed that matches a single character
+   an escape such as \ed or \epL that matches a single character
    a character class
    a back reference (see next section)
    a parenthesized subpattern (unless it is an assertion)
-Line 1400 
 subpatterns that are referenced as
+Line 1441 
 subpatterns that are referenced as
  .\" </a>
  subroutines
  .\"
- from elsewhere in the pattern. Items other than subpatterns that have a {0}
+ from elsewhere in the pattern (but see also the section entitled
- quantifier are omitted from the compiled pattern.
+ .\" HTML <a href="#subdefine">
+ .\" </a>
+ "Defining subpatterns for use by reference only"
+ .\"
+ below). Items other than subpatterns that have a {0} quantifier are omitted
+ from the compiled pattern.
  .P
  For convenience, the three most common quantifiers have single-character
  abbreviations:
-Line 1626 
 no such problem when named parentheses a
+Line 1672 
 no such problem when named parentheses a
  subpattern is possible using named parentheses (see below).
  .P
  Another way of avoiding the ambiguity inherent in the use of digits following a
- backslash is to use the \eg escape sequence, which is a feature introduced in
+ backslash is to use the \eg escape sequence. This escape must be followed by an
- Perl 5.10. This escape must be followed by an unsigned number or a negative
+ unsigned number or a negative number, optionally enclosed in braces. These
- number, optionally enclosed in braces. These examples are all identical:
+ examples are all identical:
  .sp
    (ring), \e1
    (ring), \eg1
-Line 1642 
 example:
+Line 1688 
 example:
    (abc(def)ghi)\eg{-1}
  .sp
  The sequence \eg{-1} is a reference to the most recently started capturing
- subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
+ subpattern before \eg, that is, is it equivalent to \e2 in this example.
- would be equivalent to \e1. The use of relative references can be helpful in
+ Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
- long patterns, and also in patterns that are created by joining together
+ can be helpful in long patterns, and also in patterns that are created by
- fragments that contain references within themselves.
+ joining together fragments that contain references within themselves.
  .P
  A back reference matches whatever actually matched the capturing subpattern in
  the current subject string, rather than anything matching the subpattern
-Line 1781 
 lookbehind assertion is needed to achiev
+Line 1827 
 lookbehind assertion is needed to achiev
  If you want to force a matching failure at some point in a pattern, the most
  convenient way to do it is with (?!) because an empty string always matches, so
  an assertion that requires there not to be an empty string must always fail.
- The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
+ The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
- synonym for (?!).
  .
  .
  .\" HTML <a name="lookbehind"></a>
-Line 1807 
 is permitted, but
+Line 1852 
 is permitted, but
  .sp
  causes an error at compile time. Branches that match different length strings
  are permitted only at the top level of a lookbehind assertion. This is an
- extension compared with Perl (5.8 and 5.10), which requires all branches to
+ extension compared with Perl, which requires all branches to match the same
- match the same length of string. An assertion such as
+ length of string. An assertion such as
  .sp
    (?<=ab(c|de))
  .sp
-Line 1818 
 branches:
+Line 1863 
 branches:
  .sp
    (?<=abc|abde)
  .sp
- In some cases, the Perl 5.10 escape sequence \eK
+ In some cases, the escape sequence \eK
  .\" HTML <a href="#resetmatchstart">
  .\" </a>
  (see above)
-Line 1922 
 already been matched. The two possible f
+Line 1967 
 already been matched. The two possible f
  .sp
  If the condition is satisfied, the yes-pattern is used; otherwise the
  no-pattern (if present) is used. If there are more than two alternatives in the
- subpattern, a compile-time error occurs.
+ subpattern, a compile-time error occurs. Each of the two alternatives may
+ itself contain nested subpatterns of any form, including conditional
+ subpatterns; the restriction to two alternatives applies only at the level of
+ the condition. This pattern fragment is an example where the alternatives are
+ complex:
+ .sp
+   (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
+ .sp
  .P
  There are four kinds of condition: references to subpatterns, references to
  recursion, a pseudo-condition called DEFINE, and assertions.
-Line 1939 
 matched. If there is more than one captu
+Line 1991 
 matched. If there is more than one captu
  .\" </a>
  section about duplicate subpattern numbers),
  .\"
- the condition is true if any of them have been set. An alternative notation is
+ the condition is true if any of them have matched. An alternative notation is
  to precede the digits with a plus or minus sign. In this case, the subpattern
  number is relative rather than absolute. The most recently opened parentheses
- can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
+ can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
- looping constructs it can also make sense to refer to subsequent groups with
+ loops it can also make sense to refer to subsequent groups. The next
- constructs such as (?(+2).
+ parentheses to be opened can be referenced as (?(+1), and so on. (The value
+ zero in any of these forms is not used; it provokes a compile-time error.)
  .P
  Consider the following pattern, which contains non-significant white space to
  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
-Line 1955 
 three parts for ease of discussion:
+Line 2008 
 three parts for ease of discussion:
  The first part matches an optional opening parenthesis, and if that
  character is present, sets it as the first captured substring. The second part
  matches one or more characters that are not parentheses. The third part is a
- conditional subpattern that tests whether the first set of parentheses matched
+ conditional subpattern that tests whether or not the first set of parentheses
- or not. If they did, that is, if subject started with an opening parenthesis,
+ matched. If they did, that is, if subject started with an opening parenthesis,
  the condition is true, and so the yes-pattern is executed and a closing
  parenthesis is required. Otherwise, since no-pattern is not present, the
  subpattern matches nothing. In other words, this pattern matches a sequence of
-Line 2012 
 The syntax for recursive patterns
+Line 2065 
 The syntax for recursive patterns
  .\"
  is described below.
  .
+ .\" HTML <a name="subdefine"></a>
  .SS "Defining subpatterns for use by reference only"
  .rs
  .sp
-Line 2024 
 point in the pattern; the idea of DEFINE
+Line 2078 
 point in the pattern; the idea of DEFINE
  .\" </a>
  "subroutines"
  .\"
- 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 such as
- written like this (ignore whitespace and line breaks):
+ "192.168.23.245" could be written like this (ignore whitespace and line
+ breaks):
  .sp
    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
    \eb (?&byte) (\e.(?&byte)){3} \eb
-Line 2060 
 dd-aaa-dd or dd-dd-dd, where aaa are let
+Line 2115 
 dd-aaa-dd or dd-dd-dd, where aaa are let
  .SH COMMENTS
  .rs
  .sp
- The sequence (?# marks the start of a comment that continues up to the next
+ There are two ways of including comments in patterns that are processed by
- closing parenthesis. Nested parentheses are not permitted. The characters
+ PCRE. In both cases, the start of the comment must not be in a character class,
- that make up a comment play no part in the pattern matching at all.
+ nor in the middle of any other sequence of related characters such as (?: or a
+ subpattern name or number. The characters that make up a comment play no part
+ in the pattern matching.
  .P
- If the PCRE_EXTENDED option is set, an unescaped # character outside a
+ The sequence (?# marks the start of a comment that continues up to the next
- character class introduces a comment that continues to immediately after the
+ closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
- next newline in the pattern.
+ option is set, an unescaped # character also introduces a comment, which in
+ this case continues to immediately after the next newline character or
+ character sequence in the pattern. Which characters are interpreted as newlines
+ is controlled by the options passed to \fBpcre_compile()\fP or by a special
+ sequence at the start of the pattern, as described in the section entitled
+ .\" HTML <a href="#newlines">
+ .\" </a>
+ "Newline conventions"
+ .\"
+ above. Note that the end of this type of comment is a literal newline sequence
+ in the pattern; escape sequences that happen to represent a newline do not
+ count. For example, consider this pattern when PCRE_EXTENDED is set, and the
+ default newline convention is in force:
+ .sp
+   abc #comment \en still comment
+ .sp
+ On encountering the # character, \fBpcre_compile()\fP skips along, looking for
+ a newline in the pattern. The sequence \en is still literal at this stage, so
+ it does not terminate the comment. Only an actual character with the code value
+x0a (the default newline) does so.
  .
  .
  .\" HTML <a name="recursion"></a>
-Line 2124 
 We have put the pattern into parentheses
+Line 2200 
 We have put the pattern into parentheses
  them instead of the whole pattern.
  .P
  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
- is made easier by the use of relative references (a Perl 5.10 feature).
+ is made easier by the use of relative references. Instead of (?1) in the
- Instead of (?1) in the pattern above you can write (?-2) to refer to the second
+ pattern above you can write (?-2) to refer to the second most recently opened
- most recently opened parentheses preceding the recursion. In other words, a
+ parentheses preceding the recursion. In other words, a negative number counts
- negative number counts capturing parentheses leftwards from the point at which
+ capturing parentheses leftwards from the point at which it is encountered.
- it is encountered.
  .P
  It is also possible to refer to subsequently opened parentheses, by writing
  references such as (?+2). However, these cannot be recursive because the
-Line 2231 
 time we do have another alternative to t
+Line 2306 
 time we do have another alternative to t
  difference: in the previous case the remaining alternative is at a deeper
  recursion level, which PCRE cannot use.
  .P
- To change the pattern so that matches all palindromic strings, not just those
+ To change the pattern so that it matches all palindromic strings, not just
- with an odd number of characters, it is tempting to change the pattern to this:
+ those with an odd number of characters, it is tempting to change the pattern to
+ this:
  .sp
    ^((.)(?1)\e2|.?)$
  .sp
-Line 2588 
 matching name is found, normal "bumpalon
+Line 2664 
 matching name is found, normal "bumpalon
  .sp
    (*THEN) or (*THEN:NAME)
  .sp
- This verb causes a skip to the next alternation if the rest of the pattern does
+ This verb causes a skip to the next alternation in the innermost enclosing
- not match. That is, it cancels pending backtracking, but only within the
+ group if the rest of the pattern does not match. That is, it cancels pending
- current alternation. Its name comes from the observation that it can be used
+ backtracking, but only within the current alternation. Its name comes from the
- for a pattern-based if-then-else block:
+ observation that it can be used for a pattern-based if-then-else block:
  .sp
    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
  .sp
-Line 2602 
 behaviour of (*THEN:NAME) is exactly the
+Line 2678 
 behaviour of (*THEN:NAME) is exactly the
  overall match fails. If (*THEN) is not directly inside an alternation, it acts
  like (*PRUNE).
  .
+ .P
+ The above verbs provide four different "strengths" of control when subsequent
+ matching fails. (*THEN) is the weakest, carrying on the match at the next
+ alternation. (*PRUNE) comes next, failing the match at the current starting
+ position, but allowing an advance to the next character (for an unanchored
+ pattern). (*SKIP) is similar, except that the advance may be more than one
+ character. (*COMMIT) is the strongest, causing the entire match to fail.
+ .P
+ If more than one is present in a pattern, the "stongest" one wins. For example,
+ consider this pattern, where A, B, etc. are complex pattern fragments:
+ .sp
+   (A(*COMMIT)B(*THEN)C|D)
+ .sp
+ Once A has matched, PCRE is committed to this match, at the current starting
+ position. If subsequently B matches, but C does not, the normal (*THEN) action
+ of trying the next alternation (that is, D) does not happen because (*COMMIT)
+ overrides.
+ .
  .
  .SH "SEE ALSO"
  .rs
-Line 2624 
 Cambridge CB2 3QH, England.
+Line 2718 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 05 May 2010
+ Last updated: 17 November 2010
  Copyright (c) 1997-2010 University of Cambridge.
  .fi

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