qpdf/external-libs/pcre/doc/pcrepattern.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH PCRE REGULAR EXPRESSION DETAILS
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The syntax and semantics of the regular expressions supported by PCRE are
described below. Regular expressions are also described in the Perl
documentation and in a number of other books, some of which have copious
examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
O'Reilly, covers them in great detail. The description here is intended as
reference documentation.

The basic operation of PCRE is on strings of bytes. However, there is also
support for UTF-8 character strings. To use this support you must build PCRE to
include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8
option. How this affects the pattern matching is mentioned in several places
below. There is also a summary of UTF-8 features in the
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page.

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

  The quick brown fox

matches a portion of a subject string that is identical to itself. 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
\fImeta-characters\fR, which do not stand for themselves but instead are
interpreted in some special way.

There are two different sets of meta-characters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized in square brackets. Outside square brackets, the meta-characters are
as follows:

  \\      general escape character with several uses
  ^      assert start of string (or line, in multiline mode)
  $      assert end of string (or line, in multiline mode)
  .      match any character except newline (by default)
  [      start character class definition
  |      start of alternative branch
  (      start subpattern
  )      end subpattern
  ?      extends the meaning of (
         also 0 or 1 quantifier
         also quantifier minimizer
  *      0 or more quantifier
  +      1 or more quantifier
         also "possessive quantifier"
  {      start min/max quantifier

Part of a pattern that is in square brackets is called a "character class". In
a character class the only meta-characters are:

  \\      general escape character
  ^      negate the class, but only if the first character
  -      indicates character range
  [      POSIX character class (only if followed by POSIX
           syntax)
  ]      terminates the character class

The following sections describe the use of each of the meta-characters.

.SH BACKSLASH
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The backslash character has several uses. Firstly, if it is followed by a
non-alphameric character, it takes away any special meaning that character may
have. This use of backslash as an escape character applies both inside and
outside character classes.

For example, if you want to match a * character, you write \\* in the pattern.
This escaping action applies whether or not the following character would
otherwise be interpreted as a meta-character, so it is always safe to precede a
non-alphameric with backslash to specify that it stands for itself. In
particular, if you want to match a backslash, you write \\\\.

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
backslash can be used to include a whitespace or # character as part of the
pattern.

If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \\Q and \\E. This is different from Perl in
that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in
Perl, $ and @ cause variable interpolation. Note the following examples:

  Pattern            PCRE matches   Perl matches

  \\Qabc$xyz\\E        abc$xyz        abc followed by the
                                      contents of $xyz
  \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz
  \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz

The \\Q...\\E sequence is recognized both inside and outside character classes.

A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually easier to
use one of the following escape sequences than the binary character it
represents:

  \\a        alarm, that is, the BEL character (hex 07)
  \\cx       "control-x", where x is any character
  \\e        escape (hex 1B)
  \\f        formfeed (hex 0C)
  \\n        newline (hex 0A)
  \\r        carriage return (hex 0D)
  \\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)

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.
Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex
7B.

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
appear between \\x{ and }, but the value of the character code must be less
than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters
other than hexadecimal digits appear between \\x{ and }, or if there is no
terminating }, this form of escape is not recognized. Instead, the initial
\\x will be interpreted as a basic hexadecimal escape, with no following
digits, giving a byte whose value is zero.

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
way they are handled. For example, \\xdc is exactly the same as \\x{dc}.

After \\0 up to two further octal digits are read. In both cases, if there
are fewer than two digits, just those that are present are used. Thus the
sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character
(code value 7). Make sure you supply two digits after the initial zero if the
character that follows is itself an octal digit.

The handling of a backslash followed by a digit other than 0 is complicated.
Outside a character class, PCRE reads it and any following digits as a decimal
number. If the number is less than 10, or if there have been at least that many
previous capturing left parentheses in the expression, the entire sequence is
taken as a \fIback reference\fR. A description of how this works is given
later, following the discussion of parenthesized subpatterns.

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
significant 8 bits of the value. Any subsequent digits stand for themselves.
For example:

  \\040   is another way of writing a space
  \\40    is the same, provided there are fewer than 40
            previous capturing subpatterns
  \\7     is always a back reference
  \\11    might be a back reference, or another way of
            writing a tab
  \\011   is always a tab
  \\0113  is a tab followed by the character "3"
  \\113   might be a back reference, otherwise the
            character with octal code 113
  \\377   might be a back reference, otherwise
            the byte consisting entirely of 1 bits
  \\81    is either a back reference, or a binary zero
            followed by the two characters "8" and "1"

Note that octal values of 100 or greater must not be introduced by a leading
zero, because no more than three octal digits are ever read.

All the sequences that define a single byte value or a single UTF-8 character
(in UTF-8 mode) can be used both inside and outside character classes. In
addition, inside a character class, the sequence \\b is interpreted as the
backspace character (hex 08). Outside a character class it has a different
meaning (see below).

The third use of backslash is for specifying generic character types:

  \\d     any decimal digit
  \\D     any character that is not a decimal digit
  \\s     any whitespace character
  \\S     any character that is not a whitespace character
  \\w     any "word" character
  \\W     any "non-word" character

Each pair of escape sequences partitions the complete set of characters into
two disjoint sets. Any given character matches one, and only one, of each pair.

In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or
\\w, and always match \\D, \\S, and \\W.

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).

A "word" character is any letter or digit or the underscore character, that is,
any character which can be part of a Perl "word". The definition of letters and
digits is controlled by PCRE's character tables, and may vary if locale-
specific matching is taking place (see
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page). For example, in the "fr" (French) locale, some character codes greater
than 128 are used for accented letters, and these are matched by \\w.

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
there is no character to match.

The fourth 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 below. The backslashed
assertions are

  \\b     matches at a word boundary
  \\B     matches when not at a word boundary
  \\A     matches at start of subject
  \\Z     matches at end of subject or before newline at end
  \\z     matches at end of subject
  \\G     matches at first matching position in subject

These assertions may not appear in character classes (but note that \\b has a
different meaning, namely the backspace character, inside a character class).

A word boundary is a position in the subject string where the current character
and the previous character do not both match \\w or \\W (i.e. one matches
\\w and the other matches \\W), or the start or end of the string if the
first or last character matches \\w, respectively.

The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
dollar (described below) in that they only ever match at the very start and end
of the subject string, whatever options are set. Thus, they are independent of
multiline mode.

They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the
\fIstartoffset\fR argument of \fBpcre_exec()\fR 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 last character of the string as well as at the end of the
string, whereas \\z matches only at the end.

The \\G assertion is true only when the current matching position is at the
start point of the match, as specified by the \fIstartoffset\fR argument of
\fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is
non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate
arguments, you can mimic Perl's /g option, and it is in this kind of
implementation where \\G can be useful.

Note, however, that PCRE's interpretation of \\G, as the start of the current
match, is subtly different from Perl's, which defines it as the end of the
previous match. In Perl, these can be different when the previously matched
string was empty. Because PCRE does just one match at a time, it cannot
reproduce this behaviour.

If all the alternatives of a pattern begin with \\G, the expression is anchored
to the starting match position, and the "anchored" flag is set in the compiled
regular expression.

.SH CIRCUMFLEX AND DOLLAR
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Outside a character class, in the default matching mode, the circumflex
character is an assertion which is true only if the current matching point is
at the start of the subject string. If the \fIstartoffset\fR argument of
\fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE
option is unset. Inside a character class, circumflex has an entirely different
meaning (see below).

Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)

A dollar character is an assertion which 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
not be the last character of the pattern if a number of alternatives are
involved, but it should be the last item in any branch in which it appears.
Dollar has no special meaning in a character class.

The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
does not affect the \\Z assertion.

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
after and immediately before an internal newline character, respectively, in
addition to matching at the start and end of the subject string. For example,
the pattern /^abc$/ matches the subject string "def\\nabc" 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 \fIstartoffset\fR argument of
\fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.

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.

.SH FULL STOP (PERIOD, DOT)
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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.
In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
byte long, except (by default) for newline. If the PCRE_DOTALL option is set,
dots match newlines as well. The handling of dot is entirely independent of the
handling of circumflex and dollar, the only relationship being that they both
involve newline characters. Dot has no special meaning in a character class.

.SH MATCHING A SINGLE BYTE
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Outside a character class, the escape sequence \\C matches any one byte, both
in and out of UTF-8 mode. Unlike a dot, it always matches a newline. 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, what remains in
the string may be a malformed UTF-8 string. For this reason it is best avoided.

PCRE does not allow \\C to appear in lookbehind assertions (see below), because
in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.

.SH SQUARE BRACKETS
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An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special. If a
closing square bracket is required as a member of the class, it should be the
first data character in the class (after an initial circumflex, if present) or
escaped with a backslash.

A character class matches a single character in the subject. In UTF-8 mode, the
character may occupy more than one byte. A matched character must be in the set
of characters defined by the class, unless the first character in the class
definition is a circumflex, in which case the subject character must not be in
the set defined by the class. If a circumflex is actually required as a member
of the class, ensure it is not the first character, or escape it with a
backslash.

For example, the character class [aeiou] matches any lower case vowel, while
[^aeiou] matches any character that is not a lower case vowel. Note that a
circumflex is just a convenient notation for specifying the characters which
are in the class by enumerating those that are not. It is not an assertion: it
still consumes a character from the subject string, and fails if the current
pointer is at the end of the string.

In UTF-8 mode, characters with values greater than 255 can be included in a
class as a literal string of bytes, or by using the \\x{ escaping mechanism.

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. PCRE does not support the concept of case for characters
with values greater than 255.

The newline character is never treated in any special way in character classes,
whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
such as [^a] will always match a newline.

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,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class.

It is not possible to have the literal character "]" as the end character of a
range. A pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\\]46] is interpreted as a single class containing a
range followed by two separate characters. The octal or hexadecimal
representation of "]" can also be used to end a range.

Ranges operate in the collating sequence of character values. They can also be
used for characters specified numerically, for example [\\000-\\037]. In UTF-8
mode, ranges can include characters whose values are greater than 255, for
example [\\x{100}-\\x{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
[][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"
locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.

The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
character class, and add the characters that they match to the class. For
example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
conveniently be used with the upper case character types to specify a more
restricted set of characters than the matching lower case type. For example,
the class [^\\W_] matches any letter or digit, but not underscore.

All non-alphameric characters other than \\, -, ^ (at the start) and the
terminating ] are non-special in character classes, but it does no harm if they
are escaped.

.SH POSIX CHARACTER CLASSES
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Perl supports the POSIX notation for character classes, which uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also supports
this notation. For example,

  [01[:alpha:]%]

matches "0", "1", any alphabetic character, or "%". The supported class names
are

  alnum    letters and digits
  alpha    letters
  ascii    character codes 0 - 127
  blank    space or tab only
  cntrl    control characters
  digit    decimal digits (same as \\d)
  graph    printing characters, excluding space
  lower    lower case letters
  print    printing characters, including space
  punct    printing characters, excluding letters and digits
  space    white space (not quite the same as \\s)
  upper    upper case letters
  word     "word" characters (same as \\w)
  xdigit   hexadecimal digits

The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32). Notice that this list includes the VT character (code 11). This
makes "space" different to \\s, which does not include VT (for Perl
compatibility).

The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
5.8. Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,

  [12[:^digit:]]

matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
supported, and an error is given if they are encountered.

In UTF-8 mode, characters with values greater than 255 do not match any of
the POSIX character classes.

.SH VERTICAL BAR
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Vertical bar characters are used to separate alternative patterns. For example,
the pattern

  gilbert|sullivan

matches either "gilbert" or "sullivan". Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string).
The matching process tries each alternative in turn, from left to right,
and the first one that succeeds is used. If the alternatives are within a
subpattern (defined below), "succeeds" means matching the rest of the main
pattern as well as the alternative in the subpattern.

.SH INTERNAL OPTION SETTING
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The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options can be changed from within the pattern by a sequence of
Perl option letters enclosed between "(?" and ")". The option letters are

  i  for PCRE_CASELESS
  m  for PCRE_MULTILINE
  s  for PCRE_DOTALL
  x  for PCRE_EXTENDED

For example, (?im) sets caseless, multiline matching. It is also possible to
unset these options by preceding the letter with a hyphen, and a combined
setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
permitted. If a letter appears both before and after the hyphen, the option is
unset.

When an option change occurs at top level (that is, not inside subpattern
parentheses), the change applies to the remainder of the pattern that follows.
If the change is placed right at the start of a pattern, PCRE extracts it into
the global options (and it will therefore show up in data extracted by the
\fBpcre_fullinfo()\fR function).

An option change within a subpattern affects only that part of the current
pattern that follows it, so

  (a(?i)b)c

matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
By this means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same subpattern. For example,

  (a(?i)b|c)

matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.

The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
same way as the Perl-compatible options by using the characters U and X
respectively. The (?X) flag setting is special in that it must always occur
earlier in the pattern than any of the additional features it turns on, even
when it is at top level. It is best put at the start.

.SH SUBPATTERNS
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Subpatterns are delimited by parentheses (round brackets), which can be nested.
Marking part of a pattern as a subpattern does two things:

1. It localizes a set of alternatives. For example, the pattern

  cat(aract|erpillar|)

matches one of the words "cat", "cataract", or "caterpillar". Without the
parentheses, it would match "cataract", "erpillar" or the empty string.

2. It sets up the subpattern as a capturing subpattern (as defined above).
When the whole pattern matches, that portion of the subject string that matched
the subpattern is passed back to the caller via the \fIovector\fR argument of
\fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
from 1) to obtain the numbers of the capturing subpatterns.

For example, if the string "the red king" is matched against the pattern

  the ((red|white) (king|queen))

the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3, respectively.

The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by a question mark
and a colon, the subpattern does not do any capturing, and is not counted when
computing the number of any subsequent capturing subpatterns. For example, if
the string "the white queen" is matched against the pattern

  the ((?:red|white) (king|queen))

the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of capturing subpatterns is 65535, and the maximum depth
of nesting of all subpatterns, both capturing and non-capturing, is 200.

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
the ":". Thus the two patterns

  (?i:saturday|sunday)
  (?:(?i)saturday|sunday)

match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the subpattern
is reached, an option setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".

.SH NAMED SUBPATTERNS
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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 the
difficulty, PCRE supports the naming of subpatterns, something that Perl does
not provide. The Python syntax (?P<name>...) is used. Names consist of
alphanumeric characters and underscores, and must be unique within a pattern.

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. For further details see the
.\" HREF
\fBpcreapi\fR
.\"
documentation.

.SH REPETITION
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Repetition is specified by quantifiers, which can follow any of the following
items:

  a literal data character
  the . metacharacter
  the \\C escape sequence
  escapes such as \\d that match single characters
  a character class
  a back reference (see next section)
  a parenthesized subpattern (unless it is an assertion)

The general repetition quantifier specifies a minimum and maximum number of
permitted matches, by giving the two numbers in curly brackets (braces),
separated by a comma. The numbers must be less than 65536, and the first must
be less than or equal to the second. For example:

  z{2,4}

matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus

  [aeiou]{3,}

matches at least 3 successive vowels, but may match many more, while

  \\d{8}

matches exactly 8 digits. An opening curly bracket that appears in a position
where a quantifier is not allowed, or one that does not match the syntax of a
quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.

In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of
which is represented by a two-byte sequence.

The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present.

For convenience (and historical compatibility) the three most common
quantifiers have single-character abbreviations:

  *    is equivalent to {0,}
  +    is equivalent to {1,}
  ?    is equivalent to {0,1}

It is possible to construct infinite loops by following a subpattern that can
match no characters with a quantifier that has no upper limit, for example:

  (a?)*

Earlier versions of Perl and PCRE used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but if any repetition of the subpattern does in fact
match no characters, the loop is forcibly broken.

By default, the quantifiers are "greedy", that is, they match as much as
possible (up to the maximum number of permitted times), without causing the
rest of the pattern to fail. The classic example of where this gives problems
is in trying to match comments in C programs. These appear between the
sequences /* and */ and within the sequence, individual * and / characters may
appear. An attempt to match C comments by applying the pattern

  /\\*.*\\*/

to the string

  /* first command */  not comment  /* second comment */

fails, because it matches the entire string owing to the greediness of the .*
item.

However, if a quantifier is followed by a question mark, it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern

  /\\*.*?\\*/

does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of matches.
Do not confuse this use of question mark with its use as a quantifier in its
own right. Because it has two uses, it can sometimes appear doubled, as in

  \\d??\\d

which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.

If the PCRE_UNGREEDY option is set (an option which 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.

When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more store is required for the
compiled pattern, in proportion to the size of the minimum or maximum.

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
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
pattern as though it were preceded by \\A.

In cases where it is known that the subject string contains no newlines, it is
worth setting PCRE_DOTALL in order to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.

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
succeed. Consider, for example:

  (.*)abc\\1

If the subject is "xyz123abc123" the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.

When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after

  (tweedle[dume]{3}\\s*)+

has matched "tweedledum tweedledee" the value of the captured substring is
"tweedledee". However, if there are nested capturing subpatterns, the
corresponding captured values may have been set in previous iterations. For
example, after

  /(a|(b))+/

matches "aba" the value of the second captured substring is "b".

.SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
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With both maximizing and minimizing repetition, failure of what follows
normally causes the repeated item to be re-evaluated to see if a different
number of repeats allows the rest of the pattern to match. Sometimes it is
useful to prevent this, either to change the nature of the match, or to cause
it fail earlier than it otherwise might, when the author of the pattern knows
there is no point in carrying on.

Consider, for example, the pattern \\d+foo when applied to the subject line

  123456bar

After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \\d+
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(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.

If we use atomic grouping for the previous example, the matcher would give up
immediately on failing to match "foo" the first time. The notation is a kind of
special parenthesis, starting with (?> as in this example:

  (?>\\d+)foo

This kind of parenthesis "locks up" the  part of the pattern it contains once
it has matched, and a failure further into the pattern is prevented from
backtracking into it. Backtracking past it to previous items, however, works as
normal.

An alternative description is that a subpattern of this type matches the string
of characters that an identical standalone pattern would match, if anchored at
the current point in the subject string.

Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
the above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\\d+) can only match an entire sequence of digits.

Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional + character following a quantifier. Using this notation, the
previous example can be rewritten as

  \\d++bar

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
possessive quantifier and the equivalent atomic group.

The possessive quantifier syntax is an extension to the Perl syntax. It
originates in Sun's Java package.

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
only way to avoid some failing matches taking a very long time indeed. The
pattern

  (\\D+|<\\d+>)*[!?]

matches an unlimited number of substrings that either consist of non-digits, or
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to

  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

it takes a long time before reporting failure. This is because the string can
be divided between the two repeats in a large number of ways, and all have to
be tried. (The example used [!?] rather than a single character at the end,
because both PCRE and Perl have an optimization that allows for fast failure
when a single character is used. They remember the last single character that
is required for a match, and fail early if it is not present in the string.)
If the pattern is changed to

  ((?>\\D+)|<\\d+>)*[!?]

sequences of non-digits cannot be broken, and failure happens quickly.

.SH BACK REFERENCES
.rs
.sp
Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a back reference to a capturing subpattern earlier
(that is, to its left) in the pattern, provided there have been that many
previous capturing left parentheses.

However, if the decimal number following the backslash is less than 10, it is
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 section entitled "Backslash" above for further
details of the handling of digits following a backslash.

A back reference matches whatever actually matched the capturing subpattern in
the current subject string, rather than anything matching the subpattern
itself (see
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"Subpatterns as subroutines"
.\"
below for a way of doing that). So the pattern

  (sens|respons)e and \\1ibility

matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If caseful matching is in force at the time of the
back reference, the case of letters is relevant. For example,

  ((?i)rah)\\s+\\1

matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly.

Back references to named subpatterns use the Python syntax (?P=name). We could
rewrite the above example as follows:

  (?<p1>(?i)rah)\\s+(?P=p1)

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

  (a|(bc))\\2

always fails if it starts to match "a" rather than "bc". Because there may be
many capturing parentheses in a pattern, all digits following the backslash are
taken as part of a potential back reference number. If the pattern continues
with a digit character, some delimiter must be used to terminate the back
reference. If the PCRE_EXTENDED option is set, this can be whitespace.
Otherwise an empty comment can be used.

A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\\1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern

  (a|b\\1)+

matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
the subpattern, the back reference matches the character string corresponding
to the previous iteration. In order for this to work, the pattern must be such
that the first iteration does not need to match the back reference. This can be
done using alternation, as in the example above, or by a quantifier with a
minimum of zero.

.SH ASSERTIONS
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An assertion is a test on the characters following or preceding the current
matching point that does not actually consume any characters. The simple
assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.
More complicated assertions are coded as subpatterns. There are two kinds:
those that look ahead of the current position in the subject string, and those
that look behind it.

An assertion subpattern is matched in the normal way, except that it does not
cause the current matching position to be changed. Lookahead assertions start
with (?= for positive assertions and (?! for negative assertions. For example,

  \\w+(?=;)

matches a word followed by a semicolon, but does not include the semicolon in
the match, and

  foo(?!bar)

matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern

  (?!foo)bar

does not find an occurrence of "bar" that is preceded by something other than
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
(?!foo) is always true when the next three characters are "bar". A
lookbehind assertion is needed to achieve this effect.

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.

Lookbehind assertions start with (?<= for positive assertions and (?<! for
negative assertions. For example,

  (?<!foo)bar

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
all have to have the same fixed length. Thus

  (?<=bullock|donkey)

is permitted, but

  (?<!dogs?|cats?)

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 (at least for 5.8), which requires all branches to
match the same length of string. An assertion such as

  (?<=ab(c|de))

is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable if rewritten to use two top-level branches:

  (?<=abc|abde)

The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed width and then try to
match. If there are insufficient characters before the current position, the
match is deemed to fail.

PCRE does not allow the \\C 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.

Atomic groups can be used in conjunction with lookbehind assertions to specify
efficient matching at the end of the subject string. Consider a simple pattern
such as

  abcd$

when applied to a long string that does not match. Because matching proceeds
from left to right, PCRE will look for each "a" in the subject and then see if
what follows matches the rest of the pattern. If the pattern is specified as

  ^.*abcd$

the initial .* matches the entire string at first, but when this fails (because
there is no following "a"), it backtracks to match all but the last character,
then all but the last two characters, and so on. Once again the search for "a"
covers the entire string, from right to left, so we are no better off. However,
if the pattern is written as

  ^(?>.*)(?<=abcd)

or, equivalently,

  ^.*+(?<=abcd)

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.

Several assertions (of any sort) may occur in succession. For example,

  (?<=\\d{3})(?<!999)foo

matches "foo" preceded by three digits that are not "999". Notice that each of
the assertions is applied independently at the same point in the subject
string. First there is a check that the previous three characters are all
digits, and then there is a check that the same three characters are not "999".
This pattern does \fInot\fR match "foo" preceded by six characters, the first
of which are digits and the last three of which are not "999". For example, it
doesn't match "123abcfoo". A pattern to do that is

  (?<=\\d{3}...)(?<!999)foo

This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not "999".

Assertions can be nested in any combination. For example,

  (?<=(?<!foo)bar)baz

matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo", while

  (?<=\\d{3}(?!999)...)foo

is another pattern which matches "foo" preceded by three digits and any three
characters that are not "999".

Assertion subpatterns are not capturing subpatterns, and may not be repeated,
because it makes no sense to assert the same thing several times. If any kind
of assertion contains capturing subpatterns within it, these are counted for
the purposes of numbering the capturing subpatterns in the whole pattern.
However, substring capturing is carried out only for positive assertions,
because it does not make sense for negative assertions.

.SH CONDITIONAL SUBPATTERNS
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It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns, depending on
the result of an assertion, or whether a previous capturing subpattern matched
or not. The two possible forms of conditional subpattern are

  (?(condition)yes-pattern)
  (?(condition)yes-pattern|no-pattern)

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.

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
subpattern of that number has previously matched. The number must be greater
than zero. 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:

  ( \\( )?    [^()]+    (?(1) \\) )

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
or not. 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
non-parentheses, optionally enclosed in parentheses.

If the condition is the string (R), it 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.

If the condition is not a sequence of digits or (R), 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:

  (?(?=[^a-z]*[a-z])
  \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )

The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.

.SH COMMENTS
.rs
.sp
The sequence (?# marks the start of a comment which continues up to the next
closing parenthesis. Nested parentheses are not permitted. The characters
that make up a comment play no part in the pattern matching at all.

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 in the pattern.

.SH RECURSIVE PATTERNS
.rs
.sp
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 has provided an
experimental facility that allows regular expressions to recurse (amongst other
things). It does this by interpolating Perl code in the expression at run time,
and the code can refer to the expression itself. A Perl pattern to solve the
parentheses problem can be created like this:

  $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;

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
the interpolation of Perl code. Instead, it supports some special syntax for
recursion of the entire pattern, and also for individual subpattern recursion.

The special item that consists of (? followed by a number greater than zero and
a closing parenthesis is a recursive call of the subpattern of the given
number, provided that it occurs inside that subpattern. (If not, it is a
"subroutine" call, which is described in the next section.) The special item
(?R) is a recursive call of the entire regular expression.

For example, this PCRE pattern solves the nested parentheses problem (assume
the PCRE_EXTENDED option is set so that white space is ignored):

  \\( ( (?>[^()]+) | (?R) )* \\)

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).
Finally there is a closing parenthesis.

If this were part of a larger pattern, you would not want to recurse the entire
pattern, so instead you could use this:

  ( \\( ( (?>[^()]+) | (?1) )* \\) )

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
the Python syntax that PCRE uses for named parentheses (Perl does not provide
named parentheses). We could rewrite the above example as follows:

  (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )

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

  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

it yields "no match" quickly. However, if atomic grouping is not used,
the match runs for a very long time indeed because there are so many different
ways the + and * repeats can carve up the subject, and all have to be tested
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
below and the
.\" HREF
\fBpcrecallout\fR
.\"
documentation). If the pattern above is matched against

  (ab(cd)ef)

the value for the capturing parentheses is "ef", which is the last value taken
on at the top level. If additional parentheses are added, giving

  \\( ( ( (?>[^()]+) | (?R) )* ) \\)
     ^                        ^
     ^                        ^

the string they capture is "ab(cd)ef", the contents of the top level
parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
has to obtain extra memory to store data during a recursion, which it does by
using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.

Do not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.

  < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >

In this pattern, (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases. The (?R) item
is the actual recursive call.

.\" HTML <a name="subpatternsassubroutines"></a>
.SH SUBPATTERNS AS SUBROUTINES
.rs
.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
pattern

  (sens|respons)e and \\1ibility

matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If instead the pattern

  (sens|respons)e and (?1)ibility

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
refer.

.SH CALLOUTS
.rs
.sp
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
code to be obeyed in the middle of matching a regular expression. This makes it
possible, amongst other things, to extract different substrings that match the
same pair of parentheses when there is a repetition.

PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE provides an external
function by putting its entry point in the global variable \fIpcre_callout\fR.
By default, this variable contains NULL, which disables all calling out.

Within a regular expression, (?C) indicates the points at which the external
function is to be called. If you want to identify different callout points, you
can put a number less than 256 after the letter C. The default value is zero.
For example, this pattern has two callout points:

  (?C1)\dabc(?C2)def

During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is
set), the external function is called. It is provided with the number of the
callout, and, optionally, one item of data originally supplied by the caller of
\fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to
fail altogether. A complete description of the interface to the callout
function is given in the
.\" HREF
\fBpcrecallout\fR
.\"
documentation.

.in 0
Last updated: 03 February 2003
.br
Copyright (c) 1997-2003 University of Cambridge.