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-revision 75 by nigel,
Sat Feb 24 21:40:37 2007 UTC
+revision 91 by nigel,
Sat Feb 24 21:41:34 2007 UTC
 Line 55 
 in the main
  page.
  </P>
  <P>
+ The remainder of this document discusses the patterns that are supported by
+ PCRE when its main matching function, <b>pcre_exec()</b>, is used.
+ From release 6.0, PCRE offers a second matching function,
+ <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
+ Perl-compatible. The advantages and disadvantages of the alternative function,
+ and how it differs from the normal function, are discussed in the
+ <a href="pcrematching.html"><b>pcrematching</b></a>
+ page.
+ </P>
+ <P>
  A regular expression is a pattern that is matched against a subject string from
  left to right. Most characters stand for themselves in a pattern, and match the
  corresponding characters in the subject. As a trivial example, the pattern
  <pre>
    The quick brown fox
  </pre>
- matches a portion of a subject string that is identical to itself. The power of
+ matches a portion of a subject string that is identical to itself. When
- regular expressions comes from the ability to include alternatives and
+ caseless matching is specified (the PCRE_CASELESS option), letters are matched
- repetitions in the pattern. These are encoded in the pattern by the use of
+ independently of case. In UTF-8 mode, PCRE always understands the concept of
+ case for characters whose values are less than 128, so caseless matching is
+ always possible. For characters with higher values, the concept of case is
+ supported if PCRE is compiled with Unicode property support, but not otherwise.
+ If you want to use caseless matching for characters 128 and above, you must
+ ensure that PCRE is compiled with Unicode property support as well as with
+ UTF-8 support.
+ </P>
+ <P>
+ The power of regular expressions comes from the ability to include alternatives
+ and repetitions in the pattern. These are encoded in the pattern by the use of
  <i>metacharacters</i>, which do not stand for themselves but instead are
  interpreted in some special way.
  </P>
-Line 103 
 The following sections describe the use
+Line 123 
 The following sections describe the use
  <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>
  <P>
  The backslash character has several uses. Firstly, if it is followed by a
- non-alphanumeric character, it takes away any special meaning that character may
+ non-alphanumeric character, it takes away any special meaning that character
- have. This use of backslash as an escape character applies both inside and
+ may have. This use of backslash as an escape character applies both inside and
  outside character classes.
  </P>
  <P>
-Line 117 
 particular, if you want to match a backs
+Line 137 
 particular, if you want to match a backs
  <P>
  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
  pattern (other than in a character class) and characters between a # outside
- a character class and the next newline character are ignored. An escaping
+ a character class and the next newline are ignored. An escaping backslash can
- backslash can be used to include a whitespace or # character as part of the
+ be used to include a whitespace or # character as part of the pattern.
- pattern.
  </P>
  <P>
  If you want to remove the special meaning from a sequence of characters, you
-Line 155 
 represents:
+Line 174 
 represents:
    \t        tab (hex 09)
    \ddd      character with octal code ddd, or backreference
    \xhh      character with hex code hh
-   \x{hhh..} character with hex code hhh... (UTF-8 mode only)
+   \x{hhh..} character with hex code hhh..
  </pre>
  The precise effect of \cx is as follows: if x is a lower case letter, it
  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
-Line 164 
 Thus \cz becomes hex 1A, but \c{ becomes
+Line 183 
 Thus \cz becomes hex 1A, but \c{ becomes
  </P>
  <P>
  After \x, from zero to two hexadecimal digits are read (letters can be in
- upper or lower case). In UTF-8 mode, any number of hexadecimal digits may
+ upper or lower case). Any number of hexadecimal digits may appear between \x{
- appear between \x{ and }, but the value of the character code must be less
+ and }, but the value of the character code must be less than 256 in non-UTF-8
- than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
+ mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
- other than hexadecimal digits appear between \x{ and }, or if there is no
+ is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{
- terminating }, this form of escape is not recognized. Instead, the initial
+ and }, or if there is no terminating }, this form of escape is not recognized.
- \x will be interpreted as a basic hexadecimal escape, with no following
+ Instead, the initial \x will be interpreted as a basic hexadecimal escape,
- digits, giving a character whose value is zero.
+ with no following digits, giving a character whose value is zero.
  </P>
  <P>
  Characters whose value is less than 256 can be defined by either of the two
- syntaxes for \x when PCRE is in UTF-8 mode. There is no difference in the
+ syntaxes for \x. There is no difference in the way they are handled. For
- way they are handled. For example, \xdc is exactly the same as \x{dc}.
+ example, \xdc is exactly the same as \x{dc}.
  </P>
  <P>
- After \0 up to two further octal digits are read. In both cases, if there
+ After \0 up to two further octal digits are read. If there are fewer than two
- are fewer than two digits, just those that are present are used. Thus the
+ digits, just those that are present are used. Thus the sequence \0\x\07
- sequence \0\x\07 specifies two binary zeros followed by a BEL character
+ specifies two binary zeros followed by a BEL character (code value 7). Make
- (code value 7). Make sure you supply two digits after the initial zero if the
+ sure you supply two digits after the initial zero if the pattern character that
- pattern character that follows is itself an octal digit.
+ follows is itself an octal digit.
  </P>
  <P>
  The handling of a backslash followed by a digit other than 0 is complicated.
-Line 197 
 following the discussion of
+Line 216 
 following the discussion of
  <P>
  Inside a character class, or if the decimal number is greater than 9 and there
  have not been that many capturing subpatterns, PCRE re-reads up to three octal
- digits following the backslash, and generates a single byte from the least
+ digits following the backslash, ane uses them to generate a data character. Any
- significant 8 bits of the value. Any subsequent digits stand for themselves.
+ subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
- For example:
+ character specified in octal must be less than \400. In UTF-8 mode, values up
+ to \777 are permitted. For example:
  <pre>
    \040   is another way of writing a space
    \40    is the same, provided there are fewer than 40 previous capturing subpatterns
-Line 215 
 Note that octal values of 100 or greater
+Line 235 
 Note that octal values of 100 or greater
  zero, because no more than three octal digits are ever read.
  </P>
  <P>
- All the sequences that define a single byte value or a single UTF-8 character
+ All the sequences that define a single character value can be used both inside
- (in UTF-8 mode) can be used both inside and outside character classes. In
+ and outside character classes. In addition, inside a character class, the
- addition, inside a character class, the sequence \b is interpreted as the
+ sequence \b is interpreted as the backspace character (hex 08), and the
- backspace character (hex 08), and the sequence \X is interpreted as the
+ sequence \X is interpreted as the character "X". Outside a character class,
- character "X". Outside a character class, these sequences have different
+ these sequences have different meanings
- meanings
  <a href="#uniextseq">(see below).</a>
  </P>
  <br><b>
-Line 249 
 there is no character to match.
+Line 268 
 there is no character to match.
  <P>
  For compatibility with Perl, \s does not match the VT character (code 11).
  This makes it different from the the POSIX "space" class. The \s characters
- are HT (9), LF (10), FF (12), CR (13), and space (32).
+ are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
+ included in a Perl script, \s may match the VT character. In PCRE, it never
+ does.)
  </P>
  <P>
  A "word" character is an underscore or any character less than 256 that is a
-Line 265 
 greater than 128 are used for accented l
+Line 286 
 greater than 128 are used for accented l
  <P>
  In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
  \w, and always match \D, \S, and \W. This is true even when Unicode
- character property support is available.
+ character property support is available. The use of locales with Unicode is
+ discouraged.
  <a name="uniextseq"></a></P>
  <br><b>
  Unicode character properties
  </b><br>
  <P>
  When PCRE is built with Unicode character property support, three additional
- escape sequences to match generic character types are available when UTF-8 mode
+ escape sequences to match character properties are available when UTF-8 mode
  is selected. They are:
  <pre>
-  \p{<i>xx</i>}   a character with the <i>xx</i> property
+   \p{<i>xx</i>}   a character with the <i>xx</i> property
-  \P{<i>xx</i>}   a character without the <i>xx</i> property
+   \P{<i>xx</i>}   a character without the <i>xx</i> property
-  \X       an extended Unicode sequence
+   \X       an extended Unicode sequence
  </pre>
- The property names represented by <i>xx</i> above are limited to the
+ The property names represented by <i>xx</i> above are limited to the Unicode
- Unicode general category properties. Each character has exactly one such
+ script names, the general category properties, and "Any", which matches any
- property, specified by a two-letter abbreviation. For compatibility with Perl,
+ character (including newline). Other properties such as "InMusicalSymbols" are
- negation can be specified by including a circumflex between the opening brace
+ not currently supported by PCRE. Note that \P{Any} does not match any
- and the property name. For example, \p{^Lu} is the same as \P{Lu}.
+ characters, so always causes a match failure.
  </P>
  <P>
- If only one letter is specified with \p or \P, it includes all the properties
+ Sets of Unicode characters are defined as belonging to certain scripts. A
- that start with that letter. In this case, in the absence of negation, the
+ character from one of these sets can be matched using a script name. For
- curly brackets in the escape sequence are optional; these two examples have
+ example:
- the same effect:
+ <pre>
+   \p{Greek}
+   \P{Han}
+ </pre>
+ Those that are not part of an identified script are lumped together as
+ "Common". The current list of scripts is:
+ </P>
+ <P>
+ Arabic,
+ Armenian,
+ Bengali,
+ Bopomofo,
+ Braille,
+ Buginese,
+ Buhid,
+ Canadian_Aboriginal,
+ Cherokee,
+ Common,
+ Coptic,
+ Cypriot,
+ Cyrillic,
+ Deseret,
+ Devanagari,
+ Ethiopic,
+ Georgian,
+ Glagolitic,
+ Gothic,
+ Greek,
+ Gujarati,
+ Gurmukhi,
+ Han,
+ Hangul,
+ Hanunoo,
+ Hebrew,
+ Hiragana,
+ Inherited,
+ Kannada,
+ Katakana,
+ Kharoshthi,
+ Khmer,
+ Lao,
+ Latin,
+ Limbu,
+ Linear_B,
+ Malayalam,
+ Mongolian,
+ Myanmar,
+ New_Tai_Lue,
+ Ogham,
+ Old_Italic,
+ Old_Persian,
+ Oriya,
+ Osmanya,
+ Runic,
+ Shavian,
+ Sinhala,
+ Syloti_Nagri,
+ Syriac,
+ Tagalog,
+ Tagbanwa,
+ Tai_Le,
+ Tamil,
+ Telugu,
+ Thaana,
+ Thai,
+ Tibetan,
+ Tifinagh,
+ Ugaritic,
+ Yi.
+ </P>
+ <P>
+ Each character has exactly one general category property, specified by a
+ two-letter abbreviation. For compatibility with Perl, negation can be specified
+ by including a circumflex between the opening brace and the property name. For
+ example, \p{^Lu} is the same as \P{Lu}.
+ </P>
+ <P>
+ If only one letter is specified with \p or \P, it includes all the general
+ category properties that start with that letter. In this case, in the absence
+ of negation, the curly brackets in the escape sequence are optional; these two
+ examples have the same effect:
  <pre>
    \p{L}
    \pL
  </pre>
- The following property codes are supported:
+ The following general category property codes are supported:
  <pre>
    C     Other
    Cc    Control
-Line 340 
 The following property codes are support
+Line 442 
 The following property codes are support
    Zp    Paragraph separator
    Zs    Space separator
  </pre>
- Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
+ The special property L& is also supported: it matches a character that has
- PCRE.
+ the Lu, Ll, or Lt property, in other words, a letter that is not classified as
+ a modifier or "other".
+ </P>
+ <P>
+ The long synonyms for these properties that Perl supports (such as \p{Letter})
+ are not supported by PCRE, nor is it permitted to prefix any of these
+ properties with "Is".
+ </P>
+ <P>
+ No character that is in the Unicode table has the Cn (unassigned) property.
+ Instead, this property is assumed for any code point that is not in the
+ Unicode table.
  </P>
  <P>
  Specifying caseless matching does not affect these escape sequences. For
-Line 375 
 specifies a condition that has to be met
+Line 488 
 specifies a condition that has to be met
  without consuming any characters from the subject string. The use of
  subpatterns for more complicated assertions is described
  <a href="#bigassertions">below.</a>
- The backslashed
+ The backslashed assertions are:
- assertions are:
  <pre>
    \b     matches at a word boundary
    \B     matches when not at a word boundary
-Line 403 
 PCRE_NOTBOL or PCRE_NOTEOL options, whic
+Line 515 
 PCRE_NOTBOL or PCRE_NOTEOL options, whic
  circumflex and dollar metacharacters. However, if the <i>startoffset</i>
  argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
  at a point other than the beginning of the subject, \A can never match. The
- difference between \Z and \z is that \Z matches before a newline that is the
+ difference between \Z and \z is that \Z matches before a newline at the end
- last character of the string as well as at the end of the string, whereas \z
+ of the string as well as at the very end, whereas \z matches only at the end.
- matches only at the end.
  </P>
  <P>
  The \G assertion is true only when the current matching position is at the
-Line 449 
 to be anchored.)
+Line 560 
 to be anchored.)
  <P>
  A dollar character is an assertion that is true only if the current matching
  point is at the end of the subject string, or immediately before a newline
- character that is the last character in the string (by default). Dollar need
+ at the end of the string (by default). Dollar need not be the last character of
- not be the last character of the pattern if a number of alternatives are
+ the pattern if a number of alternatives are involved, but it should be the last
- involved, but it should be the last item in any branch in which it appears.
+ item in any branch in which it appears. Dollar has no special meaning in a
- Dollar has no special meaning in a character class.
+ character class.
  </P>
  <P>
  The meaning of dollar can be changed so that it matches only at the very end of
-Line 461 
 does not affect the \Z assertion.
+Line 572 
 does not affect the \Z assertion.
  </P>
  <P>
  The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set. When this is the case, they match immediately
+ PCRE_MULTILINE option is set. When this is the case, a circumflex matches
- after and immediately before an internal newline character, respectively, in
+ immediately after internal newlines as well as at the start of the subject
- addition to matching at the start and end of the subject string. For example,
+ string. It does not match after a newline that ends the string. A dollar
- the pattern /^abc$/ matches the subject string "def\nabc" (where \n
+ matches before any newlines in the string, as well as at the very end, when
- represents a newline character) in multiline mode, but not otherwise.
+ PCRE_MULTILINE is set. When newline is specified as the two-character
- Consequently, patterns that are anchored in single line mode because all
+ sequence CRLF, isolated CR and LF characters do not indicate newlines.
- branches start with ^ are not anchored in multiline mode, and a match for
+ </P>
- circumflex is possible when the <i>startoffset</i> argument of <b>pcre_exec()</b>
+ <P>
- is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
+ For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
- set.
+ \n represents a newline) in multiline mode, but not otherwise. Consequently,
+ patterns that are anchored in single line mode because all branches start with
+ ^ are not anchored in multiline mode, and a match for circumflex is possible
+ when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
+ PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
  </P>
  <P>
  Note that the sequences \A, \Z, and \z can be used to match the start and
  end of the subject in both modes, and if all branches of a pattern start with
- \A it is always anchored, whether PCRE_MULTILINE is set or not.
+ \A it is always anchored, whether or not PCRE_MULTILINE is set.
  </P>
  <br><a name="SEC4" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
  <P>
  Outside a character class, a dot in the pattern matches any one character in
- the subject, including a non-printing character, but not (by default) newline.
+ the subject string except (by default) a character that signifies the end of a
- In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
+ line. In UTF-8 mode, the matched character may be more than one byte long. When
- byte long, except (by default) newline. If the PCRE_DOTALL option is set,
+ a line ending is defined as a single character (CR or LF), dot never matches
- dots match newlines as well. The handling of dot is entirely independent of the
+ that character; when the two-character sequence CRLF is used, dot does not
- handling of circumflex and dollar, the only relationship being that they both
+ match CR if it is immediately followed by LF, but otherwise it matches all
- involve newline characters. Dot has no special meaning in a character class.
+ characters (including isolated CRs and LFs).
+ </P>
+ <P>
+ The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
+ option is set, a dot matches any one character, without exception. If newline
+ is defined as the two-character sequence CRLF, it takes two dots to match it.
+ </P>
+ <P>
+ The handling of dot is entirely independent of the handling of circumflex and
+ dollar, the only relationship being that they both involve newlines. Dot has no
+ special meaning in a character class.
  </P>
  <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
  <P>
  Outside a character class, the escape sequence \C matches any one byte, both
- in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
+ in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The
- provided in Perl in order to match individual bytes in UTF-8 mode. Because it
+ feature is provided in Perl in order to match individual bytes in UTF-8 mode.
- breaks up UTF-8 characters into individual bytes, what remains in the string
+ Because it breaks up UTF-8 characters into individual bytes, what remains in
- may be a malformed UTF-8 string. For this reason, the \C escape sequence is
+ the string may be a malformed UTF-8 string. For this reason, the \C escape
- best avoided.
+ sequence is best avoided.
  </P>
  <P>
  PCRE does not allow \C to appear in lookbehind assertions
-Line 536 
 class as a literal string of bytes, or b
+Line 661 
 class as a literal string of bytes, or b
  When caseless matching is set, any letters in a class represent both their
  upper case and lower case versions, so for example, a caseless [aeiou] matches
  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
- caseful version would. When running in UTF-8 mode, PCRE supports the concept of
+ caseful version would. In UTF-8 mode, PCRE always understands the concept of
- case for characters with values greater than 128 only when it is compiled with
+ case for characters whose values are less than 128, so caseless matching is
- Unicode property support.
+ always possible. For characters with higher values, the concept of case is
+ supported if PCRE is compiled with Unicode property support, but not otherwise.
+ If you want to use caseless matching for characters 128 and above, you must
+ ensure that PCRE is compiled with Unicode property support as well as with
+ UTF-8 support.
  </P>
  <P>
- The newline character is never treated in any special way in character classes,
+ Characters that might indicate line breaks (CR and LF) are never treated in any
- whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
+ special way when matching character classes, whatever line-ending sequence is
- such as [^a] will always match a newline.
+ in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is
+ used. A class such as [^a] always matches one of these characters.
  </P>
  <P>
  The minus (hyphen) character can be used to specify a range of characters in a
-Line 646 
 the pattern
+Line 776 
 the pattern
    gilbert|sullivan
  </pre>
  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
- and an empty alternative is permitted (matching the empty string).
+ and an empty alternative is permitted (matching the empty string). The matching
- The matching process tries each alternative in turn, from left to right,
+ process tries each alternative in turn, from left to right, and the first one
- and the first one that succeeds is used. If the alternatives are within a
+ that succeeds is used. If the alternatives are within a subpattern
- subpattern
  <a href="#subpattern">(defined below),</a>
  "succeeds" means matching the rest of the main pattern as well as the
  alternative in the subpattern.
-Line 698 
 option settings happen at compile time.
+Line 827 
 option settings happen at compile time.
  behaviour otherwise.
  </P>
  <P>
- The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
+ The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
- same way as the Perl-compatible options by using the characters U and X
+ changed in the same way as the Perl-compatible options by using the characters
- respectively. The (?X) flag setting is special in that it must always occur
+ J, U and X respectively.
- earlier in the pattern than any of the additional features it turns on, even
- when it is at top level. It is best to put it at the start.
  <a name="subpattern"></a></P>
  <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>
  <P>
-Line 765 
 Identifying capturing parentheses by num
+Line 892 
 Identifying capturing parentheses by num
  to keep track of the numbers in complicated regular expressions. Furthermore,
  if an expression is modified, the numbers may change. To help with this
  difficulty, PCRE supports the naming of subpatterns, something that Perl does
- not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of
+ not provide. The Python syntax (?P&#60;name&#62;...) is used. References to capturing
- alphanumeric characters and underscores, and must be unique within a pattern.
+ parentheses from other parts of the pattern, such as
- </P>
+ <a href="#backreferences">backreferences,</a>
- <P>
+ <a href="#recursion">recursion,</a>
- Named capturing parentheses are still allocated numbers as well as names. The
+ and
- PCRE API provides function calls for extracting the name-to-number translation
+ <a href="#conditions">conditions,</a>
- table from a compiled pattern. There is also a convenience function for
+ can be made by name as well as by number.
- extracting a captured substring by name. For further details see the
+ </P>
+ <P>
+ Names consist of up to 32 alphanumeric characters and underscores. Named
+ capturing parentheses are still allocated numbers as well as names. The PCRE
+ API provides function calls for extracting the name-to-number translation table
+ from a compiled pattern. There is also a convenience function for extracting a
+ captured substring by name.
+ </P>
+ <P>
+ By default, a name must be unique within a pattern, but it is possible to relax
+ this constraint by setting the PCRE_DUPNAMES option at compile time. This can
+ be useful for patterns where only one instance of the named parentheses can
+ match. Suppose you want to match the name of a weekday, either as a 3-letter
+ abbreviation or as the full name, and in both cases you want to extract the
+ abbreviation. This pattern (ignoring the line breaks) does the job:
+ <pre>
+   (?P&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
+   (?P&#60;DN&#62;Tue)(?:sday)?|
+   (?P&#60;DN&#62;Wed)(?:nesday)?|
+   (?P&#60;DN&#62;Thu)(?:rsday)?|
+   (?P&#60;DN&#62;Sat)(?:urday)?
+ </pre>
+ There are five capturing substrings, but only one is ever set after a match.
+ The convenience function for extracting the data by name returns the substring
+ for the first, and in this example, the only, subpattern of that name that
+ matched. This saves searching to find which numbered subpattern it was. If you
+ make a reference to a non-unique named subpattern from elsewhere in the
+ pattern, the one that corresponds to the lowest number is used. For further
+ details of the interfaces for handling named subpatterns, see the
  <a href="pcreapi.html"><b>pcreapi</b></a>
  documentation.
  </P>
-Line 986 
 atomic group. However, there is no diffe
+Line 1141 
 atomic group. However, there is no diffe
  possessive quantifier and the equivalent atomic group.
  </P>
  <P>
- The possessive quantifier syntax is an extension to the Perl syntax. It
+ The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey
- originates in Sun's Java package.
+ Friedl originated the idea (and the name) in the first edition of his book.
+ Mike McCloskey liked it, so implemented it when he built Sun's Java package,
+ and PCRE copied it from there.
  </P>
  <P>
  When a pattern contains an unlimited repeat inside a subpattern that can itself
-Line 1028 
 However, if the decimal number following
+Line 1185 
 However, if the decimal number following
  always taken as a back reference, and causes an error only if there are not
  that many capturing left parentheses in the entire pattern. In other words, the
  parentheses that are referenced need not be to the left of the reference for
- numbers less than 10. See the subsection entitled "Non-printing characters"
+ numbers less than 10. A "forward back reference" of this type can make sense
+ when a repetition is involved and the subpattern to the right has participated
+ in an earlier iteration.
+ </P>
+ <P>
+ It is not possible to have a numerical "forward back reference" to subpattern
+ whose number is 10 or more. However, a back reference to any subpattern is
+ possible using named parentheses (see below). See also the subsection entitled
+ "Non-printing characters"
  <a href="#digitsafterbackslash">above</a>
  for further details of the handling of digits following a backslash.
  </P>
-Line 1054 
 capturing subpattern is matched caseless
+Line 1219 
 capturing subpattern is matched caseless
  Back references to named subpatterns use the Python syntax (?P=name). We could
  rewrite the above example as follows:
  <pre>
-   (?&#60;p1&#62;(?i)rah)\s+(?P=p1)
+   (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
  </pre>
+ A subpattern that is referenced by name may appear in the pattern before or
+ after the reference.
+ </P>
+ <P>
  There may be more than one back reference to the same subpattern. If a
  subpattern has not actually been used in a particular match, any back
  references to it always fail. For example, the pattern
-Line 1111 
 because it does not make sense for negat
+Line 1280 
 because it does not make sense for negat
  Lookahead assertions
  </b><br>
  <P>
- Lookahead assertions start
+ Lookahead assertions start with (?= for positive assertions and (?! for
- with (?= for positive assertions and (?! for negative assertions. For example,
+ negative assertions. For example,
  <pre>
    \w+(?=;)
  </pre>
-Line 1147 
 negative assertions. For example,
+Line 1316 
 negative assertions. For example,
  </pre>
  does find an occurrence of "bar" that is not preceded by "foo". The contents of
  a lookbehind assertion are restricted such that all the strings it matches must
- have a fixed length. However, if there are several alternatives, they do not
+ have a fixed length. However, if there are several top-level alternatives, they
- all have to have the same fixed length. Thus
+ do not all have to have the same fixed length. Thus
  <pre>
    (?&#60;=bullock|donkey)
  </pre>
-Line 1243 
 preceded by "foo", while
+Line 1412 
 preceded by "foo", while
  </pre>
  is another pattern that matches "foo" preceded by three digits and any three
  characters that are not "999".
- </P>
+ <a name="conditions"></a></P>
  <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
  <P>
  It is possible to cause the matching process to obey a subpattern
-Line 1260 
 subpattern, a compile-time error occurs.
+Line 1429 
 subpattern, a compile-time error occurs.
  </P>
  <P>
  There are three kinds of condition. If the text between the parentheses
- consists of a sequence of digits, the condition is satisfied if the capturing
+ consists of a sequence of digits, or a sequence of alphanumeric characters and
- subpattern of that number has previously matched. The number must be greater
+ underscores, the condition is satisfied if the capturing subpattern of that
- than zero. Consider the following pattern, which contains non-significant white
+ number or name has previously matched. There is a possible ambiguity here,
- space to make it more readable (assume the PCRE_EXTENDED option) and to divide
+ because subpattern names may consist entirely of digits. PCRE looks first for a
- it into three parts for ease of discussion:
+ named subpattern; if it cannot find one and the text consists entirely of
+ digits, it looks for a subpattern of that number, which must be greater than
+ zero. Using subpattern names that consist entirely of digits is not
+ recommended.
+ </P>
+ <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
+ three parts for ease of discussion:
  <pre>
    ( \( )?    [^()]+    (?(1) \) )
  </pre>
-Line 1276 
 or not. If they did, that is, if subject
+Line 1453 
 or not. If they did, that is, if subject
  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
- non-parentheses, optionally enclosed in parentheses.
+ non-parentheses, optionally enclosed in parentheses. Rewriting it to use a
- </P>
+ named subpattern gives this:
- <P>
+ <pre>
- If the condition is the string (R), it is satisfied if a recursive call to the
+   (?P&#60;OPEN&#62; \( )?    [^()]+    (?(OPEN) \) )
- pattern or subpattern has been made. At "top level", the condition is false.
+ </pre>
- This is a PCRE extension. Recursive patterns are described in the next section.
+ If the condition is the string (R), and there is no subpattern with the name R,
+ the condition is satisfied if a recursive call to the pattern or subpattern has
+ been made. At "top level", the condition is false. This is a PCRE extension.
+ Recursive patterns are described in the next section.
  </P>
  <P>
  If the condition is not a sequence of digits or (R), it must be an assertion.
-Line 1307 
 that make up a comment play no part in t
+Line 1487 
 that make up a comment play no part in t
  </P>
  <P>
  If the PCRE_EXTENDED option is set, an unescaped # character outside a
- character class introduces a comment that continues up to the next newline
+ character class introduces a comment that continues to immediately after the
- character in the pattern.
+ next newline in the pattern.
- </P>
+ <a name="recursion"></a></P>
  <br><a name="SEC18" href="#TOC1">RECURSIVE PATTERNS</a><br>
  <P>
  Consider the problem of matching a string in parentheses, allowing for
-Line 1336 
 number, provided that it occurs inside t
+Line 1516 
 number, provided that it occurs inside t
  (?R) is a recursive call of the entire regular expression.
  </P>
  <P>
- For example, this PCRE pattern solves the nested parentheses problem (assume
+ A recursive subpattern call is always treated as an atomic group. That is, once
- the PCRE_EXTENDED option is set so that white space is ignored):
+ it has matched some of the subject string, it is never re-entered, even if
+ it contains untried alternatives and there is a subsequent matching failure.
+ </P>
+ <P>
+ This PCRE pattern solves the nested parentheses problem (assume the
+ PCRE_EXTENDED option is set so that white space is ignored):
  <pre>
    \( ( (?&#62;[^()]+) | (?R) )* \)
  </pre>
  First it matches an opening parenthesis. Then it matches any number of
  substrings which can either be a sequence of non-parentheses, or a recursive
- match of the pattern itself (that is a correctly parenthesized substring).
+ match of the pattern itself (that is, a correctly parenthesized substring).
  Finally there is a closing parenthesis.
  </P>
  <P>
-Line 1423 
 matches "sense and sensibility" and "res
+Line 1608 
 matches "sense and sensibility" and "res
    (sens|respons)e and (?1)ibility
  </pre>
  is used, it does match "sense and responsibility" as well as the other two
- strings. Such references must, however, follow the subpattern to which they
+ strings. Such references, if given numerically, must follow the subpattern to
- refer.
+ which they refer. However, named references can refer to later subpatterns.
+ </P>
+ <P>
+ Like recursive subpatterns, a "subroutine" call is always treated as an atomic
+ group. That is, once it has matched some of the subject string, it is never
+ re-entered, even if it contains untried alternatives and there is a subsequent
+ matching failure.
  </P>
  <br><a name="SEC20" href="#TOC1">CALLOUTS</a><br>
  <P>
-Line 1462 
 description of the interface to the call
+Line 1653 
 description of the interface to the call
  documentation.
  </P>
  <P>
- Last updated: 09 September 2004
+ Last updated: 06 June 2006
  <br>
- Copyright &copy; 1997-2004 University of Cambridge.
+ Copyright &copy; 1997-2006 University of Cambridge.
  <p>
  Return to the <a href="index.html">PCRE index page</a>.
  </p>

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