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# Pattern Syntax

Pattern Syntax -- Describes PCRE regex syntax

## Description

The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5, with just a few differences (see below). The current implementation corresponds to Perl 5.005.

## Differences From Perl

The differences described here are with respect to Perl 5.005.

1. By default, a whitespace character is any character that the C library function isspace() recognizes, though it is possible to compile PCRE with alternative character type tables. Normally isspace() matches space, formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 no longer includes vertical tab in its set of whitespace characters. The \v escape that was in the Perl documentation for a long time was never in fact recognized. However, the character itself was treated as whitespace at least up to 5.002. In 5.004 and 5.005 it does not match \s.

2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits them, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times.

3. Capturing subpatterns that occur inside negative looka- head assertions are counted, but their entries in the offsets vector are never set. Perl sets its numerical vari- ables from any such patterns that are matched before the assertion fails to match something (thereby succeeding), but only if the negative lookahead assertion contains just one branch.

4. Though binary zero characters are supported in the subject string, they are not allowed in a pattern string because it is passed as a normal C string, terminated by zero. The escape sequence "\\x00" can be used in the pattern to represent a binary zero.

5. The following Perl escape sequences are not supported: \l, \u, \L, \U, \E, \Q. In fact these are implemented by Perl's general string-handling and are not part of its pat- tern matching engine.

6. The Perl \G assertion is not supported as it is not relevant to single pattern matches.

7. Fairly obviously, PCRE does not support the (?{code}) construction.

8. There are at the time of writing some oddities in Perl 5.005_02 concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set. In Perl 5.004 $2 is set in both cases, and that is also `TRUE` of PCRE. If in the future Perl changes to a consistent state that is different, PCRE may change to follow.

9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.

10. PCRE provides some extensions to the Perl regular expression facilities:

    1. Although lookbehind assertions must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl 5.005 requires them all to have the same length.

    2. If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- character matches only at the very end of the string.

    3. If PCRE_EXTRA is set, a backslash followed by a letter with no special meaning is faulted.

    4. If PCRE_UNGREEDY is set, the greediness of the repeti- tion quantifiers is inverted, that is, by default they are not greedy, but if followed by a question mark they are.

## Regular Expression Details

### Introduction

The syntax and semantics of the regular expressions sup- ported 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 (ISBN 1-56592-257-3), covers them in great detail. The description here is intended as reference documentation. 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 charac- ters 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.

### Meta-characters

The power of regular expressions comes from the ability to include alternatives and repetitions in the pat- tern. These are encoded in the pattern by the use of meta- characters, 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 subject (or line, in multiline mode)

$

  assert end of subject (or line, in multiline mode)

.

  match any character except newline (by default)

[

  start character class definition

]

  end 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

{

  start min/max quantifier

}

  end 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

]

  terminates the character class

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

### backslash

The backslash character has several uses. Firstly, if it is followed by a non-alphanumeric character, it takes away any special meaning that character may 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 applies whether or not the follow- ing character would otherwise be interpreted as a meta- character, so it is always safe to precede a non-alphanumeric with "\" to specify that it stands for itself. In particu- lar, if you want to match a backslash, you write "\\".

If a pattern is compiled with the PCRE_EXTENDED option, whi- tespace 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.

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)

\xhh

  character with hex code hh

\ddd

  character with octal code ddd, or backreference

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`", up to two hexadecimal digits are read (letters can be in upper or lower case).

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. 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 back reference. 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

  is the character with octal code 113 (since there can be no more than 99 back references)

\377

  is a 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 intro- duced by a leading zero, because no more than three octal digits are ever read.

All the sequences that define a single byte value 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 charac- ter 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.

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 "Locale support" above). For example, in the "fr" (French) locale, some char- acter 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 asser- tions. 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

  word boundary

\B

  not a word boundary

\A

  start of subject (independent of multiline mode)

\Z

  end of subject or newline at end (independent of multiline mode)

\z

  end of subject(independent of multiline mode)

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. They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. 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.

### Circumflex and dollar

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. 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 alter-
natives 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 con-
structs 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 or matching 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 "\n" 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. Conse-
quently, patterns that are anchored in single line mode
because all branches start with "^" are not anchored in mul-
tiline mode. 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 is it always anchored,
whether PCRE_MULTILINE is set or not.

### FULL STOP

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. If the PCRE_DOTALL
option is set, then dots match newlines as well. The han-
dling of dot is entirely independent of the handling of cir-
cumflex and dollar, the only relationship being that they
both involve newline characters. Dot has no special meaning
in a character class.

### Square brackets

An opening square bracket introduces a character class, ter-
minated 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 cir-
cumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject;
the character must be in the set of characters defined by
the class, unless the first character in the class is a cir-
cumflex, 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 con-
venient 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.
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 case-
ful version would.
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 inter-
preted 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 "-") fol-
lowed 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 inter-
preted 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 ASCII collating sequence. They can also be
used for characters specified numerically, for example
[\000-\037]. 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 res-
tricted set of characters than the matching lower case type.
For example, the class [^\W_] matches any letter or digit,
but not underscore.
All non-alphanumeric characters other than \, -, ^ (at the
start) and the terminating ] are non-special in character
classes, but it does no harm if they are escaped.

### Vertical bar

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.

### Internal option setting

The settings of PCRE_CASELESS ,
PCRE_MULTILINE ,
PCRE_DOTALL ,
and PCRE_EXTENDED 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.
The scope of these option changes depends on where in the
pattern the setting occurs. For settings that are outside
any subpattern (defined below), the effect is the same as if
the options were set or unset at the start of matching. The
following patterns all behave in exactly the same way:
(?i)abc
a(?i)bc
ab(?i)c
abc(?i)
which in turn is the same as compiling the pattern abc with
PCRE_CASELESS set.
In other words, such "top level" settings apply to the whole
pattern (unless there are other changes inside subpatterns).
If there is more than one setting of the same option at top level,
the rightmost setting is used.
If an option change occurs inside a subpattern, the effect
is different. This is a change of behaviour in Perl 5.005.
An option change inside a subpattern affects only that part
of the subpattern 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.

### subpatterns

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 pat-
tern
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 ovector
argument of
pcre_exec(). 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.
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 "?:", 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 captured substrings
is 99, and the maximum number 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".

### Repetition

Repetition is specified by quantifiers, which can follow any
of the following items:
a single character, possibly escaped
the . metacharacter
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion -
see below)
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.
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 due to the
greediness of the .* item.
However, if a quantifier is followed by a question mark,
then 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 ques-
tion 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) then 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, then 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 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 when the pattern begins with .* in order to
obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.
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".

### BACK REFERENCES

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 (i.e. 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. 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, then
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.
There may be more than one back reference to the same subpattern.
If a subpattern has not actually been used in a
particular match, then 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 up to 99 back references, all digits
following the backslash are taken as part of a potential
back reference number. If the pattern continues with a digit
character, then 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", "ababaa" 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.

### Assertions

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, \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.
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 5.005, 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. Lookbehinds in conjunction with
once-only subpatterns can be particularly useful for matching
at the ends of strings; an example is given at the end
of the section on once-only subpatterns.
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,
then there is a check that the same three characters are not
"999". This pattern does not 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.
Assertions count towards the maximum of 200 parenthesized
subpatterns.

### Once-only subpatterns

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. Once-only subpatterns provide the
means for specifying that once a portion of the pattern has
matched, it is not to be re-evaluated in this way, so the
matcher would give up immediately on failing to match "foo"
the first time. The notation is another kind of special
parenthesis, starting with (?> as in this example:
(?>\d+)bar
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. Back-
tracking 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.
Once-only 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.
This construction can of course contain arbitrarily complicated
subpatterns, and it can be nested.
Once-only subpatterns can be used in conjunction with look-
behind 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 which 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$
then 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)
then 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.
When a pattern contains an unlimited repeat inside a subpattern
that can itself be repeated an unlimited number of
times, the use of a once-only subpattern 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.

### Conditional subpatterns

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 two kinds of condition. If the text between the
parentheses consists of a sequence of digits, then the
condition is satisfied if the capturing subpattern of that
number has previously matched. 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 not a sequence of digits, 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.

### Comments

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 contin-
ues up to the next newline character in the pattern.

### Recursive patterns

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 5.6 has
provided an experimental facility that allows regular
expressions to recurse (amongst other things). The special
item (?R) is provided for the specific case of recursion.
This PCRE pattern solves the 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
(i.e. a correctly parenthesized substring). Finally there is
a closing parenthesis.
This particular example pattern contains nested unlimited
repeats, and so the use of a once-only subpattern for matching
strings of non-parentheses is important when applying
the pattern to strings that do not match. For example, when
it is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a once-only subpattern
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.
The values set for any capturing subpatterns are those from
the outermost level of the recursion at which the subpattern
value is set. 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
\( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
^ ^ then 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 pcre_malloc, freeing it
via pcre_free afterwards. If no memory can be obtained, it
saves data for the first 15 capturing parentheses only, as
there is no way to give an out-of-memory error from within a
recursion.

### Performances

Certain items that may appear in patterns are more efficient
than others. It is more efficient to use a character class
like [aeiou] than a set of alternatives such as (a|e|i|o|u).
In general, the simplest construction that provides the
required behaviour is usually the most efficient. Jeffrey
Friedl's book contains a lot of discussion about optimizing
regular expressions for efficient performance.
When a pattern begins with .* and the PCRE_DOTALL option is
set, the pattern is implicitly anchored by PCRE, since it
can match only at the start of a subject string. However, if
PCRE_DOTALL is not set, PCRE cannot make this optimization,
because the . metacharacter does not then match a newline,
and if the subject string contains newlines, the pattern may
match from the character immediately following one of them
instead of from the very start. For example, the pattern
(.*) second
matches the subject "first\nand second" (where \n stands for
a newline character) with the first captured substring being
"and". In order to do this, PCRE has to retry the match
starting after every newline in the subject.
If you are using such a pattern with subject strings that do
not contain newlines, the best performance is obtained by
setting PCRE_DOTALL , or starting the pattern with ^.* to
indicate explicit anchoring. That saves PCRE from having to
scan along the subject looking for a newline to restart at.
Beware of patterns that contain nested indefinite repeats.
These can take a long time to run when applied to a string
that does not match. Consider the pattern fragment
(a+)*
This can match "aaaa" in 33 different ways, and this number
increases very rapidly as the string gets longer. (The *
repeat can match 0, 1, 2, 3, or 4 times, and for each of
those cases other than 0, the + repeats can match different
numbers of times.) When the remainder of the pattern is such
that the entire match is going to fail, PCRE has in principle
to try every possible variation, and this can take an
extremely long time.
An optimization catches some of the more simple cases such
as
(a+)*b
where a literal character follows. Before embarking on the
standard matching procedure, PCRE checks that there is a "b"
later in the subject string, and if there is not, it fails
the match immediately. However, when there is no following
literal this optimization cannot be used. You can see the
difference by comparing the behaviour of
(a+)*\d
with the pattern above. The former gives a failure almost
instantly when applied to a whole line of "a" characters,
whereas the latter takes an appreciable time with strings
longer than about 20 characters.

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