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

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-revision 91 by nigel,
Sat Feb 24 21:41:34 2007 UTC
+revision 139 by ph10,
Fri Mar 30 13:41:47 2007 UTC
 Line 36 
 and how it differs from the normal funct
  \fBpcrematching\fP
  .\"
  page.
- .P
+ .
+ .
+ .SH "CHARACTERS AND METACHARACTERS"
+ .rs
+ .sp
  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
-Line 60 
 interpreted in some special way.
+Line 64 
 interpreted in some special way.
  .P
  There are two different sets of metacharacters: those that are recognized
  anywhere in the pattern except within square brackets, and those that are
- recognized in square brackets. Outside square brackets, the metacharacters are
+ recognized within square brackets. Outside square brackets, the metacharacters
- as follows:
+ are as follows:
  .sp
    \e      general escape character with several uses
    ^      assert start of string (or line, in multiline mode)
-Line 92 
 a character class the only metacharacter
+Line 96 
 a character class the only metacharacter
  .sp
  The following sections describe the use of each of the metacharacters.
  .
+ .
  .SH BACKSLASH
  .rs
  .sp
-Line 190 
 parenthesized subpatterns.
+Line 195 
 parenthesized subpatterns.
  .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, ane uses them to generate a data character. Any
+ digits following the backslash, and uses them to generate a data character. Any
  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
  character specified in octal must be less than \e400. In UTF-8 mode, values up
  to \e777 are permitted. For example:
-Line 221 
 zero, because no more than three octal d
+Line 226 
 zero, because no more than three octal d
  All the sequences that define a single character value can be used both inside
  and outside character classes. In addition, inside a character class, the
  sequence \eb is interpreted as the backspace character (hex 08), and the
- sequence \eX is interpreted as the character "X". Outside a character class,
+ sequences \eR and \eX are interpreted as the characters "R" and "X",
- these sequences have different meanings
+ respectively. Outside a character class, these sequences have different
+ meanings
  .\" HTML <a href="#uniextseq">
  .\" </a>
  (see below).
  .\"
  .
  .
+ .SS "Absolute and relative back references"
+ .rs
+ .sp
+ The sequence \eg followed by a positive or negative number, optionally enclosed
+ in braces, is an absolute or relative back reference. Back references are
+ discussed
+ .\" HTML <a href="#backreferences">
+ .\" </a>
+ later,
+ .\"
+ following the discussion of
+ .\" HTML <a href="#subpattern">
+ .\" </a>
+ parenthesized subpatterns.
+ .\"
+ .
+ .
  .SS "Generic character types"
  .rs
  .sp
- The third use of backslash is for specifying generic character types. The
+ Another use of backslash is for specifying generic character types. The
  following are always recognized:
  .sp
    \ed     any decimal digit
-Line 268 
 in the
+Line 291 
 in the
  .\" HREF
  \fBpcreapi\fP
  .\"
- page). For example, in the "fr_FR" (French) locale, some character codes
+ page). For example, in a French locale such as "fr_FR" in Unix-like systems,
- greater than 128 are used for accented letters, and these are matched by \ew.
+ or "french" in Windows, some character codes greater than 128 are used for
+ accented letters, and these are matched by \ew.
  .P
  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
-Line 277 
 character property support is available.
+Line 301 
 character property support is available.
  discouraged.
  .
  .
+ .SS "Newline sequences"
+ .rs
+ .sp
+ Outside a character class, the escape sequence \eR matches any Unicode newline
+ sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
+ the following:
+ .sp
+   (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
+ .sp
+ This is an example of an "atomic group", details of which are given
+ .\" HTML <a href="#atomicgroup">
+ .\" </a>
+ below.
+ .\"
+ This particular group matches either the two-character sequence CR followed by
+ LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
+ U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
+ line, U+0085). The two-character sequence is treated as a single unit that
+ cannot be split.
+ .P
+ In UTF-8 mode, two additional characters whose codepoints are greater than 255
+ are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
+ Unicode character property support is not needed for these characters to be
+ recognized.
+ .P
+ Inside a character class, \eR matches the letter "R".
+ .
+ .
  .\" HTML <a name="uniextseq"></a>
  .SS Unicode character properties
  .rs
-Line 307 
 Those that are not part of an identified
+Line 359 
 Those that are not part of an identified
  .P
  Arabic,
  Armenian,
+ Balinese,
  Bengali,
  Bopomofo,
  Braille,
-Line 316 
 Canadian_Aboriginal,
+Line 369 
 Canadian_Aboriginal,
  Cherokee,
  Common,
  Coptic,
+ Cuneiform,
  Cypriot,
  Cyrillic,
  Deseret,
-Line 345 
 Malayalam,
+Line 399 
 Malayalam,
  Mongolian,
  Myanmar,
  New_Tai_Lue,
+ Nko,
  Ogham,
  Old_Italic,
  Old_Persian,
  Oriya,
  Osmanya,
+ Phags_Pa,
+ Phoenician,
  Runic,
  Shavian,
  Sinhala,
-Line 466 
 properties in PCRE.
+Line 523 
 properties in PCRE.
  .SS "Simple assertions"
  .rs
  .sp
- The fourth use of backslash is for certain simple assertions. An assertion
+ The final use of backslash is for certain simple assertions. An assertion
  specifies a condition that has to be met at a particular point in a match,
  without consuming any characters from the subject string. The use of
  subpatterns for more complicated assertions is described
-Line 478 
 The backslashed assertions are:
+Line 535 
 The backslashed assertions are:
  .sp
    \eb     matches at a word boundary
    \eB     matches when not at a word boundary
-   \eA     matches at start of subject
+   \eA     matches at the start of the subject
-   \eZ     matches at end of subject or before newline at end
+   \eZ     matches at the end of the subject
-   \ez     matches at end of subject
+           also matches before a newline at the end of the subject
-   \eG     matches at first matching position in subject
+   \ez     matches only at the end of the subject
+   \eG     matches at the first matching position in the subject
  .sp
  These assertions may not appear in character classes (but note that \eb has a
  different meaning, namely the backspace character, inside a character class).
-Line 578 
 end of the subject in both modes, and if
+Line 636 
 end of the subject in both modes, and if
  .sp
  Outside a character class, a dot in the pattern matches any one character in
  the subject string except (by default) a character that signifies the end of a
- line. In UTF-8 mode, the matched character may be more than one byte long. When
+ line. In UTF-8 mode, the matched character may be more than one byte long.
- a line ending is defined as a single character (CR or LF), dot never matches
+ .P
- that character; when the two-character sequence CRLF is used, dot does not
+ When a line ending is defined as a single character, dot never matches that
- match CR if it is immediately followed by LF, but otherwise it matches all
+ character; when the two-character sequence CRLF is used, dot does not match CR
- characters (including isolated CRs and LFs).
+ if it is immediately followed by LF, but otherwise it matches all characters
+ (including isolated CRs and LFs). When any Unicode line endings are being
+ recognized, dot does not match CR or LF or any of the other line ending
+ characters.
  .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
+ option is set, a dot matches any one character, without exception. If the
- is defined as the two-character sequence CRLF, it takes two dots to match it.
+ two-character sequence CRLF is present in the subject string, it takes two dots
+ to match it.
  .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
-Line 597 
 special meaning in a character class.
+Line 659 
 special meaning in a character class.
  .rs
  .sp
  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 CR and LF. The
+ in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
- feature is provided in Perl in order to match individual bytes in UTF-8 mode.
+ characters. The feature is provided in Perl in order to match individual bytes
- Because it breaks up UTF-8 characters into individual bytes, what remains in
+ in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
- the string may be a malformed UTF-8 string. For this reason, the \eC escape
+ what remains in the string may be a malformed UTF-8 string. For this reason,
- sequence is best avoided.
+ the \eC escape sequence is best avoided.
  .P
  PCRE does not allow \eC to appear in lookbehind assertions
  .\" HTML <a href="#lookbehind">
-Line 652 
 If you want to use caseless matching for
+Line 714 
 If you want to use caseless matching for
  ensure that PCRE is compiled with Unicode property support as well as with
  UTF-8 support.
  .P
- Characters that might indicate line breaks (CR and LF) are never treated in any
+ Characters that might indicate line breaks are never treated in any special way
- special way when matching character classes, whatever line-ending sequence is
+ when matching character classes, whatever line-ending sequence is in use, and
- in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is
+ whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
- used. A class such as [^a] always matches one of these characters.
+ such as [^a] always matches one of these characters.
  .P
  The minus (hyphen) character can be used to specify a range of characters in a
  character class. For example, [d-m] matches any letter between d and m,
-Line 679 
 example [\ex{100}-\ex{2ff}].
+Line 741 
 example [\ex{100}-\ex{2ff}].
  If a range that includes letters is used when caseless matching is set, it
  matches the letters in either case. For example, [W-c] is equivalent to
  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
- tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E
+ tables for a French locale are in use, [\exc8-\excb] matches accented E
  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
  characters with values greater than 128 only when it is compiled with Unicode
  property support.
-Line 790 
 If the change is placed right at the sta
+Line 852 
 If the change is placed right at the sta
  the global options (and it will therefore show up in data extracted by the
  \fBpcre_fullinfo()\fP function).
  .P
- An option change within a subpattern affects only that part of the current
+ An option change within a subpattern (see below for a description of
- pattern that follows it, so
+ subpatterns) affects only that part of the current pattern that follows it, so
  .sp
    (a(?i)b)c
  .sp
-Line 824 
 Turning part of a pattern into a subpatt
+Line 886 
 Turning part of a pattern into a subpatt
    cat(aract|erpillar|)
  .sp
  matches one of the words "cat", "cataract", or "caterpillar". Without the
- parentheses, it would match "cataract", "erpillar" or the empty string.
+ parentheses, it would 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
-Line 849 
 the string "the white queen" is matched
+Line 911 
 the string "the white queen" is matched
    the ((?:red|white) (king|queen))
  .sp
  the captured substrings are "white queen" and "queen", and are numbered 1 and
-. The maximum number of capturing subpatterns is 65535, and the maximum depth
+. The maximum number of capturing subpatterns is 65535.
- of nesting of all subpatterns, both capturing and non-capturing, is 200.
  .P
  As a convenient shorthand, if any option settings are required at the start of
  a non-capturing subpattern, the option letters may appear between the "?" and
-Line 871 
 the above patterns match "SUNDAY" as wel
+Line 932 
 the above patterns match "SUNDAY" as wel
  Identifying capturing parentheses by number is simple, but it can be very hard
  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
+ difficulty, PCRE supports the naming of subpatterns. This feature was not
- not provide. The Python syntax (?P<name>...) is used. References to capturing
+ added to Perl until release 5.10. Python had the feature earlier, and PCRE
+ introduced it at release 4.0, using the Python syntax. PCRE now supports both
+ the Perl and the Python syntax.
+ .P
+ In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
+ (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
  parentheses from other parts of the pattern, such as
  .\" HTML <a href="#backreferences">
  .\" </a>
-Line 890 
 conditions,
+Line 956 
 conditions,
  can be made by name as well as by number.
  .P
  Names consist of up to 32 alphanumeric characters and underscores. Named
- capturing parentheses are still allocated numbers as well as names. The PCRE
+ capturing parentheses are still allocated numbers as well as names, exactly as
- API provides function calls for extracting the name-to-number translation table
+ if the names were not present. The PCRE API provides function calls for
- from a compiled pattern. There is also a convenience function for extracting a
+ extracting the name-to-number translation table from a compiled pattern. There
- captured substring by name.
+ is also a convenience function for extracting a captured substring by name.
  .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
-Line 902 
 match. Suppose you want to match the nam
+Line 968 
 match. Suppose you want to match the nam
  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:
  .sp
-   (?P<DN>Mon|Fri|Sun)(?:day)?|
+   (?<DN>Mon|Fri|Sun)(?:day)?|
-   (?P<DN>Tue)(?:sday)?|
+   (?<DN>Tue)(?:sday)?|
-   (?P<DN>Wed)(?:nesday)?|
+   (?<DN>Wed)(?:nesday)?|
-   (?P<DN>Thu)(?:rsday)?|
+   (?<DN>Thu)(?:rsday)?|
-   (?P<DN>Sat)(?:urday)?
+   (?<DN>Sat)(?:urday)?
  .sp
  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
+ 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
-Line 928 
 Repetition is specified by quantifiers,
+Line 994 
 Repetition is specified by quantifiers,
  items:
  .sp
    a literal data character
-   the . metacharacter
+   the dot metacharacter
    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
    a character class
    a back reference (see next section)
-Line 968 
 which may be several bytes long (and the
+Line 1035 
 which may be several bytes long (and the
  The quantifier {0} is permitted, causing the expression to behave as if the
  previous item and the quantifier were not present.
  .P
- For convenience (and historical compatibility) the three most common
+ For convenience, the three most common quantifiers have single-character
- quantifiers have single-character abbreviations:
+ abbreviations:
  .sp
    *    is equivalent to {0,}
    +    is equivalent to {1,}
-Line 1017 
 own right. Because it has two uses, it c
+Line 1084 
 own right. Because it has two uses, it c
  which matches one digit by preference, but can match two if that is the only
  way the rest of the pattern matches.
  .P
- If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
+ If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
  the quantifiers are not greedy by default, but individual ones can be made
  greedy by following them with a question mark. In other words, it inverts the
  default behaviour.
-Line 1027 
 is greater than 1 or with a limited maxi
+Line 1094 
 is greater than 1 or with a limited maxi
  compiled pattern, in proportion to the size of the minimum or maximum.
  .P
  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
- to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
+ to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
  implicitly anchored, because whatever follows will be tried against every
  character position in the subject string, so there is no point in retrying the
  overall match at any position after the first. PCRE normally treats such a
-Line 1039 
 alternatively using ^ to indicate anchor
+Line 1106 
 alternatively using ^ to indicate anchor
  .P
  However, there is one situation where the optimization cannot be used. When .*
  is inside capturing parentheses that are the subject of a backreference
- elsewhere in the pattern, a match at the start may fail, and a later one
+ elsewhere in the pattern, a match at the start may fail where a later one
- succeed. Consider, for example:
+ succeeds. Consider, for example:
  .sp
    (.*)abc\e1
  .sp
-Line 1066 
 matches "aba" the value of the second ca
+Line 1133 
 matches "aba" the value of the second ca
  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
  .rs
  .sp
- With both maximizing and minimizing repetition, failure of what follows
+ With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- normally causes the repeated item to be re-evaluated to see if a different
+ repetition, failure of what follows normally causes the repeated item to be
- number of repeats allows the rest of the pattern to match. Sometimes it is
+ re-evaluated to see if a different number of repeats allows the rest of the
- useful to prevent this, either to change the nature of the match, or to cause
+ pattern to match. Sometimes it is useful to prevent this, either to change the
- it fail earlier than it otherwise might, when the author of the pattern knows
+ nature of the match, or to cause it fail earlier than it otherwise might, when
- there is no point in carrying on.
+ the author of the pattern knows there is no point in carrying on.
  .P
  Consider, for example, the pattern \ed+foo when applied to the subject line
  .sp
-Line 1083 
 item, and then with 4, and so on, before
+Line 1150 
 item, and then with 4, and so on, before
  (a term taken from Jeffrey Friedl's book) provides the means for specifying
  that once a subpattern has matched, it is not to be re-evaluated in this way.
  .P
- If we use atomic grouping for the previous example, the matcher would give up
+ If we use atomic grouping for the previous example, the matcher gives up
  immediately on failing to match "foo" the first time. The notation is a kind of
  special parenthesis, starting with (?> as in this example:
  .sp
-Line 1115 
 previous example can be rewritten as
+Line 1182 
 previous example can be rewritten as
  .sp
  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
  option is ignored. They are a convenient notation for the simpler forms of
- atomic group. However, there is no difference in the meaning or processing of a
+ atomic group. However, there is no difference in the meaning of a possessive
- possessive quantifier and the equivalent atomic group.
+ quantifier and the equivalent atomic group, though there may be a performance
- .P
+ difference; possessive quantifiers should be slightly faster.
- The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey
+ .P
- Friedl originated the idea (and the name) in the first edition of his book.
+ The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
- Mike McCloskey liked it, so implemented it when he built Sun's Java package,
+ Jeffrey Friedl originated the idea (and the name) in the first edition of his
- and PCRE copied it from there.
+ book. Mike McCloskey liked it, so implemented it when he built Sun's Java
+ package, and PCRE copied it from there. It ultimately found its way into Perl
+ at release 5.10.
+ .P
+ PCRE has an optimization that automatically "possessifies" certain simple
+ pattern constructs. For example, the sequence A+B is treated as A++B because
+ there is no point in backtracking into a sequence of A's when B must follow.
  .P
  When a pattern contains an unlimited repeat inside a subpattern that can itself
  be repeated an unlimited number of times, the use of an atomic group is the
-Line 1167 
 numbers less than 10. A "forward back re
+Line 1240 
 numbers less than 10. A "forward back re
  when a repetition is involved and the subpattern to the right has participated
  in an earlier iteration.
  .P
- It is not possible to have a numerical "forward back reference" to subpattern
+ It is not possible to have a numerical "forward back reference" to a subpattern
- whose number is 10 or more. However, a back reference to any subpattern is
+ whose number is 10 or more using this syntax because a sequence such as \e50 is
- possible using named parentheses (see below). See also the subsection entitled
+ interpreted as a character defined in octal. See the subsection entitled
  "Non-printing characters"
  .\" HTML <a href="#digitsafterbackslash">
  .\" </a>
  above
  .\"
- for further details of the handling of digits following a backslash.
+ for further details of the handling of digits following a backslash. There is
+ no such problem when named parentheses are used. A back reference to any
+ 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
+ Perl 5.10. This escape must be followed by a positive or a negative number,
+ optionally enclosed in braces. These examples are all identical:
+ .sp
+   (ring), \e1
+   (ring), \eg1
+   (ring), \eg{1}
+ .sp
+ A positive number specifies an absolute reference without the ambiguity that is
+ present in the older syntax. It is also useful when literal digits follow the
+ reference. A negative number is a relative reference. Consider this example:
+ .sp
+   (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}
+ would be equivalent to \e1. The use of relative references can be helpful in
+ long patterns, and also in patterns that are created by 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 1197 
 back reference, the case of letters is r
+Line 1293 
 back reference, the case of letters is r
  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
  capturing subpattern is matched caselessly.
  .P
- Back references to named subpatterns use the Python syntax (?P=name). We could
+ Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'
- rewrite the above example as follows:
+ or the Python syntax (?P=name). We could rewrite the above example in either of
+ the following ways:
  .sp
+   (?<p1>(?i)rah)\es+\ek<p1>
    (?P<p1>(?i)rah)\es+(?P=p1)
  .sp
  A subpattern that is referenced by name may appear in the pattern before or
-Line 1324 
 lengths, but it is acceptable if rewritt
+Line 1422 
 lengths, but it is acceptable if rewritt
    (?<=abc|abde)
  .sp
  The implementation of lookbehind assertions is, for each alternative, to
- temporarily move the current position back by the fixed width and then try to
+ temporarily move the current position back by the fixed length and then try to
  match. If there are insufficient characters before the current position, the
- match is deemed to fail.
+ assertion fails.
  .P
  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
  to appear in lookbehind assertions, because it makes it impossible to calculate
- the length of the lookbehind. The \eX escape, which can match different numbers
+ the length of the lookbehind. The \eX and \eR escapes, which can match
- of bytes, is also not permitted.
+ different numbers of bytes, are also not permitted.
  .P
- Atomic groups can be used in conjunction with lookbehind assertions to specify
+ Possessive quantifiers can be used in conjunction with lookbehind assertions to
- efficient matching at the end of the subject string. Consider a simple pattern
+ specify efficient matching at the end of the subject string. Consider a simple
- such as
+ pattern such as
  .sp
    abcd$
  .sp
-Line 1351 
 then all but the last two characters, an
+Line 1449 
 then all but the last two characters, an
  covers the entire string, from right to left, so we are no better off. However,
  if the pattern is written as
  .sp
-   ^(?>.*)(?<=abcd)
- .sp
- or, equivalently, using the possessive quantifier syntax,
- .sp
    ^.*+(?<=abcd)
  .sp
- there can be no backtracking for the .* item; it can match only the entire
+ there can be no backtracking for the .*+ item; it can match only the entire
  string. The subsequent lookbehind assertion does a single test on the last four
  characters. If it fails, the match fails immediately. For long strings, this
  approach makes a significant difference to the processing time.
-Line 1413 
 If the condition is satisfied, the yes-p
+Line 1507 
 If the condition is satisfied, the yes-p
  no-pattern (if present) is used. If there are more than two alternatives in the
  subpattern, a compile-time error occurs.
  .P
- There are three kinds of condition. If the text between the parentheses
+ There are four kinds of condition: references to subpatterns, references to
- consists of a sequence of digits, or a sequence of alphanumeric characters and
+ recursion, a pseudo-condition called DEFINE, and assertions.
- underscores, the condition is satisfied if the capturing subpattern of that
+ .
- number or name has previously matched. There is a possible ambiguity here,
+ .SS "Checking for a used subpattern by number"
- because subpattern names may consist entirely of digits. PCRE looks first for a
+ .rs
- named subpattern; if it cannot find one and the text consists entirely of
+ .sp
- digits, it looks for a subpattern of that number, which must be greater than
+ If the text between the parentheses consists of a sequence of digits, the
- zero. Using subpattern names that consist entirely of digits is not
+ condition is true if the capturing subpattern of that number has previously
- recommended.
+ matched.
  .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 1437 
 or not. If they did, that is, if subject
+Line 1531 
 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. Rewriting it to use a
+ non-parentheses, optionally enclosed in parentheses.
- named subpattern gives this:
+ .
+ .SS "Checking for a used subpattern by name"
+ .rs
+ .sp
+ Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
+ subpattern by name. For compatibility with earlier versions of PCRE, which had
+ this facility before Perl, the syntax (?(name)...) is also recognized. However,
+ there is a possible ambiguity with this syntax, because subpattern names may
+ consist entirely of digits. PCRE looks first for a named subpattern; if it
+ cannot find one and the name consists entirely of digits, PCRE 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
+ Rewriting the above example to use a named subpattern gives this:
  .sp
-   (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )
+   (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
+ .sp
+ .
+ .SS "Checking for pattern recursion"
+ .rs
  .sp
  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
+ the condition is true if a recursive call to the whole pattern or any
- been made. At "top level", the condition is false. This is a PCRE extension.
+ subpattern has been made. If digits or a name preceded by ampersand follow the
- Recursive patterns are described in the next section.
+ letter R, for example:
+ .sp
+   (?(R3)...) or (?(R&name)...)
+ .sp
+ the condition is true if the most recent recursion is into the subpattern whose
+ number or name is given. This condition does not check the entire recursion
+ stack.
  .P
- If the condition is not a sequence of digits or (R), it must be an assertion.
+ At "top level", all these recursion test conditions are false. Recursive
+ patterns are described below.
+ .
+ .SS "Defining subpatterns for use by reference only"
+ .rs
+ .sp
+ If the condition is the string (DEFINE), and there is no subpattern with the
+ name DEFINE, the condition is always false. In this case, there may be only one
+ alternative in the subpattern. It is always skipped if control reaches this
+ point in the pattern; the idea of DEFINE is that it can be used to define
+ "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
+ is described below.) For example, a pattern to match an IPv4 address 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
+ .sp
+ The first part of the pattern is a DEFINE group inside which a another group
+ named "byte" is defined. This matches an individual component of an IPv4
+ address (a number less than 256). When matching takes place, this part of the
+ pattern is skipped because DEFINE acts like a false condition.
+ .P
+ The rest of the pattern uses references to the named group to match the four
+ dot-separated components of an IPv4 address, insisting on a word boundary at
+ each end.
+ .
+ .SS "Assertion conditions"
+ .rs
+ .sp
+ If the condition is not in any of the above formats, it must be an assertion.
  This may be a positive or negative lookahead or lookbehind assertion. Consider
  this pattern, again containing non-significant white space, and with the two
  alternatives on the second line:
-Line 1483 
 next newline in the pattern.
+Line 1629 
 next newline in the pattern.
  Consider the problem of matching a string in parentheses, allowing for
  unlimited nested parentheses. Without the use of recursion, the best that can
  be done is to use a pattern that matches up to some fixed depth of nesting. It
- is not possible to handle an arbitrary nesting depth. Perl provides a facility
+ is not possible to handle an arbitrary nesting depth.
- that allows regular expressions to recurse (amongst other things). It does this
+ .P
- by interpolating Perl code in the expression at run time, and the code can
+ For some time, Perl has provided a facility that allows regular expressions to
- refer to the expression itself. A Perl pattern to solve the parentheses problem
+ recurse (amongst other things). It does this by interpolating Perl code in the
- can be created like this:
+ expression at run time, and the code can refer to the expression itself. A Perl
+ pattern using code interpolation to solve the parentheses problem can be
+ created like this:
  .sp
    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
  .sp
  The (?p{...}) item interpolates Perl code at run time, and in this case refers
- recursively to the pattern in which it appears. Obviously, PCRE cannot support
+ recursively to the pattern in which it appears.
- the interpolation of Perl code. Instead, it supports some special syntax for
+ .P
- recursion of the entire pattern, and also for individual subpattern recursion.
+ Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
- .P
+ supports special syntax for recursion of the entire pattern, and also for
- The special item that consists of (? followed by a number greater than zero and
+ individual subpattern recursion. After its introduction in PCRE and Python,
- a closing parenthesis is a recursive call of the subpattern of the given
+ this kind of recursion was introduced into Perl at release 5.10.
- number, provided that it occurs inside that subpattern. (If not, it is a
+ .P
- "subroutine" call, which is described in the next section.) The special item
+ A special item that consists of (? followed by a number greater than zero and a
- (?R) is a recursive call of the entire regular expression.
+ closing parenthesis is a recursive call of the subpattern of the given number,
- .P
+ provided that it occurs inside that subpattern. (If not, it is a "subroutine"
- A recursive subpattern call is always treated as an atomic group. That is, once
+ call, which is described in the next section.) The special item (?R) or (?0) is
- it has matched some of the subject string, it is never re-entered, even if
+ a recursive call of the entire regular expression.
- it contains untried alternatives and there is a subsequent matching failure.
+ .P
+ In PCRE (like Python, but unlike Perl), a recursive subpattern 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
  This PCRE pattern solves the nested parentheses problem (assume the
  PCRE_EXTENDED option is set so that white space is ignored):
-Line 1524 
 pattern, so instead you could use this:
+Line 1676 
 pattern, so instead you could use this:
  We have put the pattern into parentheses, and caused the recursion to refer to
  them instead of the whole pattern. In a larger pattern, keeping track of
  parenthesis numbers can be tricky. It may be more convenient to use named
- parentheses instead. For this, PCRE uses (?P>name), which is an extension to
+ parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier
- the Python syntax that PCRE uses for named parentheses (Perl does not provide
+ syntax (?P>name) is also supported. We could rewrite the above example as
- named parentheses). We could rewrite the above example as follows:
+ follows:
  .sp
-   (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
+   (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
  .sp
- This particular example pattern contains nested unlimited repeats, and so the
+ If there is more than one subpattern with the same name, the earliest one is
- use of atomic grouping for matching strings of non-parentheses is important
+ used. This particular example pattern contains nested unlimited repeats, and so
+ the use of atomic grouping for matching strings of non-parentheses is important
  when applying the pattern to strings that do not match. For example, when this
  pattern is applied to
  .sp
-Line 1545 
 before failure can be reported.
+Line 1698 
 before failure can be reported.
  At the end of a match, the values set for any capturing subpatterns are those
  from the outermost level of the recursion at which the subpattern value is set.
  If you want to obtain intermediate values, a callout function can be used (see
- the next section and the
+ below and the
  .\" HREF
  \fBpcrecallout\fP
  .\"
-Line 1584 
 is the actual recursive call.
+Line 1737 
 is the actual recursive call.
  .sp
  If the syntax for a recursive subpattern reference (either by number or by
  name) is used outside the parentheses to which it refers, it operates like a
- subroutine in a programming language. An earlier example pointed out that the
+ subroutine in a programming language. The "called" subpattern may be defined
- pattern
+ before or after the reference. An earlier example pointed out that the pattern
  .sp
    (sens|respons)e and \e1ibility
  .sp
-Line 1595 
 matches "sense and sensibility" and "res
+Line 1748 
 matches "sense and sensibility" and "res
    (sens|respons)e and (?1)ibility
  .sp
  is used, it does match "sense and responsibility" as well as the other two
- strings. Such references, if given numerically, must follow the subpattern to
+ strings. Another example is given in the discussion of DEFINE above.
- which they refer. However, named references can refer to later subpatterns.
  .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
+ When a subpattern is used as a subroutine, processing options such as
+ case-independence are fixed when the subpattern is defined. They cannot be
+ changed for different calls. For example, consider this pattern:
+ .sp
+   (abc)(?i:(?1))
+ .sp
+ It matches "abcabc". It does not match "abcABC" because the change of
+ processing option does not affect the called subpattern.
  .
  .
  .SH CALLOUTS
-Line 1638 
 description of the interface to the call
+Line 1799 
 description of the interface to the call
  \fBpcrecallout\fP
  .\"
  documentation.
- .P
+ .
- .in 0
+ .
- Last updated: 06 June 2006
+ .SH "SEE ALSO"
- .br
+ .rs
- Copyright (c) 1997-2006 University of Cambridge.
+ .sp
+ \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
+ .
+ .
+ .SH AUTHOR
+ .rs
+ .sp
+ .nf
+ Philip Hazel
+ University Computing Service
+ Cambridge CB2 3QH, England.
+ .fi
+ .
+ .
+ .SH REVISION
+ .rs
+ .sp
+ .nf
+ Last updated: 06 March 2007
+ Copyright (c) 1997-2007 University of Cambridge.
+ .fi

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