# re

### MODULE

re

### MODULE SUMMARY

Perl like regular expressions for Erlang

### DESCRIPTION

Warning

This is an experimental module and the interface is subject to change. The purpose is to be a reference implementation of what's described in the EEP "Built in regular expressions in Erlang", available at www.erlang.org for review. No production code should be based on this module until the draft is accepted and the module corrected accordingly.

This module contains functions for regular expression matching for strings and binaries.

The regular expression syntax and semantics resemble that of Perl. This library in many ways replaces the old regexp library written purely in Erlang, as it has a richer syntax as well as many more options. The library is also many times faster than the pure Erlang implementation.

Although the library's matching algorithms are currently based on the PCRE library, it is not to be viewed as an Erlang to PCRE mapping. Only parts of the PCRE library is interfaced and the re library in some ways extend PCRE. The PCRE documentation contains many parts of no interest to the Erlang programmer, why only the relevant part of the documentation is included here. There should bee no need to go directly to the PCRE library documentation.

### DATA TYPES

        iodata() = iolist() | binary()
        iolist() = [char() | binary() | iolist()]
          - a binary is allowed as the tail of the list

        mp() = Opaque datatype containing a compiled regular expression.

### EXPORTS

compile(Regexp) -> {ok, MP} | {error, ErrSpec}

Types:

Regexp = iodata()

The same as compile(Regexp,[])

compile(Regexp,Options) -> {ok, MP} | {error, ErrSpec}

Types:

Regexp = iodata()
Options = [ Option ]
Option = anchored | caseless | dollar_endonly | dotall | extended | firstline | multiline | no_auto_capture | dupnames | ungreedy | {newline, NLSpec}
NLSpec = cr | crlf | lf | anycrlf
MP = mp()
ErrSpec = {ErrString, Position}
ErrString = string()
Position = int()

This function compiles a regular expression with the syntax described below into an internal format to be used later as a parameter to the run/2,3 functions.

Compiling the regular expression before matching is useful if the same expression is to be used in matching against multiple subjects during the program's lifetime. Compiling once and executing many times is far more efficient than compiling each time one wants to match.

The options have the following meanings:

anchored
  The pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself.
caseless
  Letters in the pattern match both upper and lower case letters. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. Uppercase and lowercase letters are defined as in the ISO-8859-1 character set.
dollar_endonly
  A dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The dollar_endonly option is ignored if multiline is given. There is no equivalent option in Perl, and no way to set it within a pattern.
dotall
  A dot maturate in the pattern matches all characters, including those that indicate newline. Without it, a dot does not match when the current position is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A negative class such as [^a] always matches newline characters, independent of the setting of this option.
extended
  Whitespace data characters in the pattern are ignored except when escaped or inside a character class. Whitespace does not include the VT character (ASCII 11). In addition, characters between an unescaped # outside a character class and the next newline, inclusive, are also ignored. This is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting. This option makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.
firstline
  An unanchored pattern is required to match before or at the first newline in the subject string, though the matched text may continue over the newline.
multiline
  By default, PCRE treats the subject string as consisting of a single line of characters (even if it actually contains newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless dollar_endonly is given). This is the same as Perl.
  When multiline it is given, the "start of line" and "end of line" constructs match immediately following or immediately before internal newlines in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option, and it can be changed within a pattern by a (?m) option setting. If there are no newlines in a subject string, or no occurrences of ^ or $ in a pattern, setting multiline has no effect.
no_auto_capture
  Disables the use of numbered capturing parentheses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). There is no equivalent of this option in Perl.
dupnames
  Names used to identify capturing subpatterns need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named subpattern can ever be matched. There are more details of named subpatterns below
ungreedy
  This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern.
{newline, NLSpec}
  Override the default definition of a newline in the subject string, which is LF (ASCII 10) in Erlang.
  cr
    Newline is indicated by a single character CR (ASCII 13)
  lf
    Newline is indicated by a single character LF (ASCII 10), the default
  crlf
    Newline is indicated by the two-character CRLF (ASCII 13 followed by ASCII 10) sequence.
  anycrlf
    Any of the three preceding sequences should be recognized.

run(Subject,RE) -> {match, Captured} | nomatch | {error, ErrSpec}

Types:

RE = mp() | iodata()
Subject = iodata()
Captured = [ CaptureData ]
CaptureData = {int(),int()} | string() | binary()
ErrSpec = {ErrString, Position}
ErrString = string()
Position = int()

The same as run(RE,Subject,[]).

run(Subject,RE,Options) -> {match, Captured} | nomatch | {error, ErrSpec}

Types:

RE = mp() | iodata()
Subject = iodata()
Options = [ Option ]
Option = anchored | notbol | noteol | notempty | {offset, int()} | {newline, NLSpec} | {capture, ValueSpec} | {capture, ValueSpec, Type} | CompileOpt
Type = index | list | binary
ValueSpec = all | all_but_first | first | ValueList
ValueList = [ ValueID ]
ValueID = int() | string() | atom()
CompileOpt = see compile/2 above
NLSpec = cr | crlf | lf | anycrlf
Captured = [ CaptureData ]
CaptureData = {int(),int()} | string() | binary()
ErrSpec = {ErrString, Position}
ErrString = string()
Position = int()

Executes a regexp matching, returning {match, Captured} or nomatch. The regular expression can be given either as iodata() in which case it is automatically compiled (as by re:compile/2) and executed, or as a pre compiled mp() in which case it is executed against the subject directly.

When compilation is involved, the function may return compilation errors as when compiling separately ({error, {string(),int()}}); when only matching, no errors are returned.

The option list can only contain the options anchored, notbol, noteol, notempty, {offset, int()}, {newline, NLSpec} and {capture, ValueSpec}/{capture, ValueSpec, Type} if the regular expression is previously compiled, otherwise all options valid for the re:compile/2 function are allowed as well. Options allowed both for compilation and execution of a match, namely anchored and {newline, NLSpec}, will affect both the compilation and execution if present together with a non pre-compiled regular expression.

The {capture, ValueSpec}/{capture, ValueSpec, Type} defines what to return from the function upon successful matching. The capture tuple may contain both a value specification telling which of the captured substrings are to be returned, and a type specification, telling how captured substrings are to be returned (as index tuples, lists or binaries). The capture option makes the function quite flexible and powerful. The different options are described in detail below

If the capture options describe that no substring capturing at all is to be done ({capture, none}), the function will return the single atom match upon successful matching, otherwise the tuple {match, ValueList} is returned. Disabling capturing can be done either by specifying none or an empty list as ValueSpec.

A description of all the options relevant for execution follows:

anchored
  Limits re:run/3 to matching at the first matching position. If a pattern was compiled with anchored, or turned out to be anchored by virtue of its contents, it cannot be made unachored at matching time, hence there is no unanchored option.
notempty
  An empty string is not considered to be a valid match if this option is given. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern

          a?b?

  is applied to a string not beginning with "a" or "b", it matches the empty string at the start of the subject. With notempty given, this match is not valid, so re:run/3 searches further into the string for occurrences of "a" or "b".
  Perl has no direct equivalent of notempty, but it does make a special case of a pattern match of the empty string within its split() function, and when using the /g modifier. It is possible to emulate Perl's behavior after matching a null string by first trying the match again at the same offset with notempty and anchored, and then if that fails by advancing the starting offset (see below) and trying an ordinary match again.
notbol
  This option specifies that the first character of the subject string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without multiline (at compile time) causes circumflex never to match. This option affects only the behavior of the circumflex metacharacter. It does not affect \A.
noteol
  This option specifies that the end of the subject string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this without multiline (at compile time) causes dollar never to match. This option affects only the behavior of the dollar metacharacter. It does not affect \Z or \z.
{offset, int()}
  Start matching at the offset (position) given in the subject string. The offset is zero-based, so that the default is {offset,0} (all of the subject string).
{newline, NLSpec}
  Override the default definition of a newline in the subject string, which is LF (ASCII 10) in Erlang.
  cr
    Newline is indicated by a single character CR (ASCII 13)
  lf
    Newline is indicated by a single character LF (ASCII 10), the default
  crlf
    Newline is indicated by the two-character CRLF (ASCII 13 followed by ASCII 10) sequence.
  anycrlf
    Any of the three preceding sequences should be recognized.
{capture, ValueSpec}/{capture, ValueSpec, Type}
  Specifies which captured substrings are returned and in what format. By default, re:run/3 captures all of the matching part of the substring as well as all capturing subpatterns (all of the pattern is automatically captured). The default return type is (zero-based) indexes of the captured parts of the string, given as {Offset,Length} pairs (the index Type of capturing).
  As an example of the default behavior, the following call:

          re:run("ABCabcdABC","abcd",[]).

  returns, as first and only captured string the matching part of the subject ("abcd" in the middle) as a index pair {3,4}, where character positions are zero based, just as in offsets. The return value of the call above would then be:

          {match,[{3,4}]}

  Another (and quite common) case is where the regular expression matches all of the subject, as in:

          re:run("ABCabcdABC",".*abcd.*",[]).

  where the return value correspondingly will point out all of the string, beginning at index 0 and being 10 characters long:

          {match,[{0,10}]}

  If the regular expression contains capturing subpatterns, like in the following case:

          re:run("ABCabcdABC",".*(abcd).*",[]).

  all of the matched subject is captured, as well as the captured substrings:

          {match,[{0,10},{3,4}]}

  the complete matching pattern always giving the first return value in the list and the rest of the subpatterns being added in the order they occurred in the regular expression.
  The capture tuple is built up as follows:
  ValueSpec
    Specifies which captured (sub)patterns are to be returned. The ValueSpec can either be an atom describing a predefined set of return values, or a list containing either the indexes or the names of specific subpatterns to return.
    The predefined sets of subpatterns are:
    all
      All captured subpatterns including the complete matching string. This is the default.
    first
      Only the first captured subpattern, which is always the complete matching part of the subject. All explicitly captured subpatterns are discarded.
    all_but_first
      All but the first matching subpattern, i.e. all explicitly captured subpatterns, but not the complete matching part of the subject string. This is useful if the regular expression as a whole matches a large part of the subject, but the part you're interested in is in an explicitly captured subpattern. If the return type is list or binary, not returning subpatterns you're not interested in is a good way to optimize.
    none
      Do not return matching subpatterns at all, yielding the single atom match as the return value of the function when matching successfully instead of the {match, list()} return. Specifying an empty list gives the same behavior.
    The value list is a list of indexes for the subpatterns to return, where index 0 is for all of the pattern, and 1 is for the first explicit capturing subpattern in the regular expression, and so forth. When using named captured subpatterns (see below) in the regular expression, one can use atom()'s or string()'s to specify the subpatterns to be returned. This deserves an example, consider the following regular expression:

            ".*(abcd).*"

    matched against the string ""ABCabcdABC", capturing only the "abcd" part (the first explicit subpattern):

            re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).

    The call will yield the following result:

            {match,[{3,4}]}

    as the first explicitly captured subpattern is "(abcd)", matching "abcd" in the subject, at (zero-based) position 3, of length 4.
    Now consider the same regular expression, but with the subpattern explicitly named 'FOO':

            ".*(?<FOO>abcd).*"

    With this expression, we could still give the index of the subpattern with the following call:

            re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,[1]}]).

    giving the same result as before. But as the subpattern is named, we can also give its name in the value list:

            re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).

    which would yield the same result as the earlier examples, namely:

            {match,[{3,4}]}

    The values list might specify indexes or names not present in the regular expression, in which case the return values vary depending on the type. If the type is index, the tuple {-1,0} is returned for values having no corresponding subpattern in the regexp, but for the other types (binary and list), the values are the empty binary or list respectively. This makes it impossible to differentiate between a empty matching subpattern and an invalid subpattern name in the return values for those types. If that differentiation is necessary, use the type index and do the conversion to the final type in Erlang code.
  In general, subpatterns that got assigned no value in the match are returned as the tuple {-1,0} when type is index. Unasigned subpatterns are returned as the empty binary or list respectively for other return types. Consider the regular expression:

          ".*((?<FOO>abdd)|a(..d)).*"

  There are three explicitly capturing subpatterns, where the opening parenthesis position determines the order in the result, hence ((?<FOO>abdd)|a(..d)) is subpattern index 1, (?<FOO>abdd) is subpattern index 2 and (..d) is subpattern index 3. When matched against the following string:

          "ABCabcdABC"

  the subpattern at index 2 won't match, as "abdd" is not present in the string, but the complete pattern matches (due to the alternative a(..d). The subpattern at index 2 is therefore unassigned and the default return value will be:

          {match,[{0,10},{3,4},{-1,0},{4,3}]}

  Setting the capture Type to binary would give the following:

          {match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}

  where the empty binary (<<>>) represents the unassigned subpattern. In the binary case, some information about the matching is therefore lost, the <<>> might just as well be an empty string captured.
Type
  Optionally specifies how captured substrings are to be returned. If omitted, the default of index is used. The Type can be one of the following:
  index
    Return captured substrings as pairs of byte indexes into the subject string and length of the matching string in the subject (as if the subject string was flattened with iolist_to_binary prior to matching). This is the default.
  list
    Return matching substrings as lists of characters (Erlang string()'s).
  binary
    Return matching substrings as binaries.

The options solely affecting the compilation step are described in the re:compile/2 function.

### PERL LIKE REGULAR EXPRESSIONS SYNTAX

The following sections contain reference material for the regular expressions used by this module. The regular expression reference is taken from the PCRE documentation, but converted as needed.

The documentation is altered where appropriate and where the re module behaves differently than the PCRE library.

### PCRE regular expression details

The syntax and semantics of the regular expressions that are supported by PCRE are described in detail below. Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This description of PCRE's regular expressions is intended as reference material.

The reference material is divided into the following sections:

- Newline conventions
- Characters and metacharacters
- Backslash
- Circumflex and dollar
- Full stop (period, dot)
- Square brackets and character classes
- Posix character classes
- Vertical bar
- Internal option setting
- Subpatterns
- Duplicate subpattern numbers
- Named subpatterns
- Repetition
- Atomic grouping and possessive quantifiers
- Back references
- Assertions
- Conditional subpatterns
- Comments
- Recursive patterns
- Subpatterns as subroutines
- Backtracking control

### Newline conventions

PCRE supports four different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (linefeed) character, the two-character sequence CRLF or any of the three preceding.

It is also possible to specify a newline convention by starting a pattern string with one of the following five sequences:

(*CR)
  carriage return
(*LF)
  linefeed
(*CRLF)
  carriage return, followed by linefeed
(*ANYCRLF)
  any of the three above

These override the default and the options given to re:compile/2. For example, the pattern:

> (*CR)a.b

changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline. Note that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used.

### Characters and metacharacters

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. When caseless matching is specified (the caseless option), letters are matched independently of case.

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 metacharacters, which do not stand for themselves but instead are interpreted in some special way.

There are two different sets of metacharacters: those that are recognized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters 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 metacharacters 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 metacharacters.

### 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 escaping action applies whether or not the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric 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 extended option, whitespace in the pattern (other than in a character class) and characters between a # outside a character class and the next newline 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.

Non-printing characters

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
  linefeed (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..

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). Any number of hexadecimal digits may appear between \x{ and }, but the value of the character code must be less than 256.

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 character whose value is zero.

Characters whose value is less than 256 can be defined by either of the two syntaxes for \x. 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. 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 pattern 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 uses them to generate a data character. Any subsequent digits stand for themselves. The value of a character specified in octal must be less than \400. 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 character 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), and the sequences \R and \X are interpreted as the characters "R" and "X", respectively. Outside a character class, these sequences have different meanings (see below).

Absolute and relative back references

The sequence \g followed by an unsigned or a negative number, optionally enclosed in braces, is an absolute or relative back reference. A named back reference can be coded as \g{name}. Back references are discussed later, following the discussion of parenthesized subpatterns.

Generic character types

Another use of backslash is for specifying generic character types. The following are always recognized:

\d
  any decimal digit
\D
  any character that is not a decimal digit
\h
  any horizontal whitespace character
\H
  any character that is not a horizontal whitespace character
\s
  any whitespace character
\S
  any character that is not a whitespace character
\v
  any vertical whitespace character
\V
  any character that is not a vertical 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.

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.

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). If "use locale;" is included in a Perl script, \s may match the VT character. In PCRE, it never does.

A "word" character is an underscore or any character less than 256 that is a letter or digit. The definition of letters and digits is controlled by PCRE's low-valued character tables, which are always ISO-8859-1.

Newline sequences

The \R sequence is equivalent to the following:

> (?>\r\n|\n|\x0b|\f|\r|\x85)

This is an example of an "atomic group", details of which are given below.

This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next line, U+0085). The two-character sequence is treated as a single unit that cannot be split.

Inside a character class, \R matches the letter "R".

Resetting the match start

The escape sequence \K, which is a Perl 5.10 feature, causes any previously matched characters not to be included in the final matched sequence. For example, the pattern:

> foo\Kbar

matches "foobar", but reports that it has matched "bar". This feature is similar to a lookbehind assertion (described below). However, in this case, the part of the subject before the real match does not have to be of fixed length, as lookbehind assertions do. The use of \K does not interfere with the setting of captured substrings. For example, when the pattern

> (foo)\Kbar

matches "foobar", the first substring is still set to "foo".

Simple assertions

The final use of backslash is for certain simple assertions. An assertion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are:

\b
  matches at a word boundary
\B
  matches when not at a word boundary
\A
  matches at the start of the subject
\Z
  matches at the end of the subject also matches before a newline at the end of the subject
\z
  matches only at the end of the subject
\G
  matches at the first matching position in the 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 in the next section) 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. These three assertions are not affected by the notbol or noteol options, which affect only the behaviour of the circumflex and dollar metacharacters. However, if the startoffset argument of re:run/3 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 at the end of the string as well as at the very end, 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 startoffset argument of re:run/3. It differs from \A when the value of startoffset is non-zero. By calling re:run/3 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.

### Circumflex and dollar

Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If the startoffset argument of re:run/3 is non-zero, circumflex can never match if the 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 that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of 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 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 multiline option is set. When this is the case, a circumflex matches immediately after internal newlines as well as at the start of the subject string. It does not match after a newline that ends the string. A dollar matches before any newlines in the string, as well as at the very end, when multiline is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines.

For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of re:run/3 is non-zero. The dollar_endonly option is ignored if 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 or not multiline is set.

### Full stop (period, dot)

Outside a character class, a dot in the pattern matches any one character in the subject string except (by default) a character that signifies the end of a line.

When a line ending is defined as a single character, dot never matches that character; when the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs).

The behaviour of dot with regard to newlines can be changed. If the dotall option is set, a dot matches any one character, without exception. If the two-character sequence CRLF is present in the subject string, it takes two dots to match it.

The handling of dot is entirely independent of the handling of circumflex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class.

### Square brackets and character classes

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. 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 that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion: it still consumes a character from the subject string, and therefore it 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 caseful version would. Caselessness is always in regard to the ISO-8859-1 character set in Erlang.

Characters that might indicate line breaks are never treated in any special way when matching character classes, whatever line-ending sequence is in use, and whatever setting of the dotall and multiline options is used. A class such as [^a] always matches one of these characters.

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 class containing a range followed by two other 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].

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 .

The character types \d, \D, \p, \P, \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.

The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name - see the next section), and the terminating closing square bracket. However, escaping other non-alphanumeric characters does no harm.

### Posix character classes

Perl supports the POSIX notation for character classes. This 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
  whitespace (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.

### 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 the caseless, multiline, dotall, and extended options (which are Perl-compatible) can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are

i
  for caseless
m
  for multiline
s
  for dotall
x
  for 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 caseless and multiline while unsetting dotall and extended, is also permitted. If a letter appears both before and after the hyphen, the option is unset.

The PCRE-specific options dupnames, ungreedy, and extra can be changed in the same way as the Perl-compatible options by using the characters J, U and X respectively.

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

An option change within a subpattern (see below for a description of subpatterns) affects only that part of the current pattern that follows it, so

> (a(?i)b)c

matches abc and aBc and no other strings (assuming 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.

Note: There are other PCRE-specific options that can be set by the application when the compile or match functions are called. In some cases the pattern can contain special leading sequences to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above.

### Subpatterns

Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into 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 an empty string.

2. It sets up the subpattern as a capturing subpattern. This means that, when the complete pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller via the return value of re:run/3. Opening parentheses are counted from left to right (starting from 1) to obtain numbers for 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.

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".

### Duplicate subpattern numbers

Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider this pattern:

> (?|(Sat)ur|(Sun))day

Because the two alternatives are inside a (?| group, both sets of capturing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, parentheses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing buffers that follow the subpattern start after the highest number used in any branch. The following example is taken from the Perl documentation. The numbers underneath show in which buffer the captured content will be stored.

      # before  ---------------branch-reset----------- after
      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
      # 1            2         2  3        2     3     4

A backreference or a recursive call to a numbered subpattern always refers to the first one in the pattern with the given number.

An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section.

### Named subpatterns

Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expressions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of subpatterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the Perl and the Python syntax.

In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as backreferences, recursion, and conditions, can be made by name as well as by number.

Names consist of up to 32 alphanumeric characters and underscores. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The capture specification to re:run/3 can use named values if they are present in the regular expression.

By default, a name must be unique within a pattern, but it is possible to relax this constraint by setting the dupnames option at compile time. This can be useful for patterns where only one instance of the named parentheses can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job:

      (?<DN>Mon|Fri|Sun)(?:day)?|
      (?<DN>Tue)(?:sday)?|
      (?<DN>Wed)(?:nesday)?|
      (?<DN>Thu)(?:rsday)?|
      (?<DN>Sat)(?:urday)?

There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.)

In case of capturing named subpatterns which are not unique, the first occurence is returned from re:exec/3, if the name is specified int the values part of the capture statement.

### Repetition

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

- a literal data character
- the dot metacharacter
- the \C escape sequence
- the \X escape sequence
- the \R escape sequence
- an escape such as \d that matches a single character
- 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.

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

For convenience, 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 /* and */ and within the comment, individual * and / characters may appear. An attempt to match C comments by applying the pattern

> /\*.*\*/

to the string

> /* first comment */ 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 ungreedy option is set (an option that is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. In other words, it inverts the default behaviour.

When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more memory 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 dotall option (equivalent to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position after the first. PCRE normally treats such a 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 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 where a later one succeeds. 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".

### Atomic grouping and possessive quantifiers

With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 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 gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example:

> (?>\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++foo

Note that a possessive quantifier can be used with an entire group, for example:

> (abc|xyz){2,3}+

Possessive quantifiers are always greedy; the setting of the ungreedy option is ignored. They are a convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, though there may be a performance difference; possessive quantifiers should be slightly faster.

The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE copied it from there. It ultimately found its way into Perl at release 5.10.

PCRE has an optimization that automatically "possessifies" certain simple pattern constructs. For example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of A's when B must follow.

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 internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The example uses [!?] 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 so that it uses an atomic group, like this:

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

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

### 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 (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. A "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier iteration.

It is not possible to have a numerical "forward back reference" to a subpattern whose number is 10 or more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further details of the handling of digits following a backslash. There is no such problem when named parentheses are used. A back reference to any subpattern is possible using named parentheses (see below).

Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence, which is a feature introduced in Perl 5.10. This escape must be followed by an unsigned number or a negative number, optionally enclosed in braces. These examples are all identical:

- (ring), \1
- (ring), \g1
- (ring), \g{1}

An unsigned number specifies an absolute reference without the ambiguity that is present in the older syntax. It is also useful when literal digits follow the reference. A negative number is a relative reference. Consider this example:

> (abc(def)ghi)\g{-1}

The sequence \g{-1} is a reference to the most recently started capturing subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2} would be equivalent to \1. The use of relative references can be helpful in long patterns, and also in patterns that are created by joining together fragments that contain references within themselves.

A back reference matches whatever actually matched the capturing subpattern in the current subject string, rather than anything matching the subpattern itself (see "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.

There are several different ways of writing back references to named subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified back reference syntax, in which \g can be used for both numeric and named references, is also supported. We could rewrite the above example in any of the following ways:

- (?<p1>(?i)rah)\s+\k<p1>
- (?'p1'(?i)rah)\s+\k{p1}
- (?P<p1>(?i)rah)\s+(?P=p1)
- (?<p1>(?i)rah)\s+\g{p1}

A subpattern that is referenced by name may appear in the pattern before or after the reference.

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 extended option is set, this can be whitespace. Otherwise an empty comment (see "Comments" below) 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.

### 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, \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.

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.

Lookahead assertions

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 the other 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

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 top-level 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)

In some cases, the Perl 5.10 escape sequence \K (see above) can be used instead of a lookbehind assertion; this is not restricted to a fixed-length.

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

Possessive quantifiers 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)

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.

Using multiple assertions

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 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 that matches "foo" preceded by three digits and any three characters that are not "999".

### 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 four kinds of condition: references to subpatterns, references to recursion, a pseudo-condition called DEFINE, and assertions.

Checking for a used subpattern by number

If the text between the parentheses consists of a sequence of digits, the condition is true if the capturing subpattern of that number has previously matched. An alternative notation is to precede the digits with a plus or minus sign. In this case, the subpattern number is relative rather than absolute. The most recently opened parentheses can be referenced by (?(-1), the next most recent by (?(-2), and so on. In looping constructs it can also make sense to refer to subsequent groups with constructs such as (?(+2).

Consider the following pattern, which contains non-significant whitespace to make it more readable (assume the 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 you were embedding this pattern in a larger one, you could use a relative reference:

> ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...

This makes the fragment independent of the parentheses in the larger pattern.

Checking for a used subpattern by name

Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by name. For compatibility with earlier versions of PCRE, which had this facility before Perl, the syntax (?(name)...) is also recognized. However, there is a possible ambiguity with this syntax, because subpattern names may consist entirely of digits. PCRE looks first for a named subpattern; if it cannot find one and the name consists entirely of digits, PCRE looks for a subpattern of that number, which must be greater than zero. Using subpattern names that consist entirely of digits is not recommended.

Rewriting the above example to use a named subpattern gives this:

> (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )

Checking for pattern recursion

If the condition is the string (R), and there is no subpattern with the name R, the condition is true if a recursive call to the whole pattern or any subpattern has been made. If digits or a name preceded by ampersand follow the letter R, for example:

> (?(R3)...) or (?(R&name)...)

the condition is true if the most recent recursion is into the subpattern whose number or name is given. This condition does not check the entire recursion stack.

At "top level", all these recursion test conditions are false. Recursive patterns are described below.

Defining subpatterns for use by reference only

If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE, the condition is always false. In this case, there may be only one alternative in the subpattern. It is always skipped if control reaches this point in the pattern; the idea of DEFINE is that it can be used to define "subroutines" that can be referenced from elsewhere. (The use of "subroutines" is described below.) For example, a pattern to match an IPv4 address could be written like this (ignore whitespace and line breaks):

> (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b

The first part of the pattern is a DEFINE group inside which a another group named "byte" is defined. This matches an individual component of an IPv4 address (a number less than 256). When matching takes place, this part of the pattern is skipped because DEFINE acts like a false condition.

The rest of the pattern uses references to the named group to match the four dot-separated components of an IPv4 address, insisting on a word boundary at each end.

Assertion conditions

If the condition is not in any of the above formats, it must be an assertion. This may be a positive or negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant whitespace, 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 that 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 extended option is set, an unescaped # character outside a character class introduces a comment that continues to immediately after the next newline 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.

For some time, Perl has provided a 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 using code interpolation 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 special syntax for recursion of the entire pattern, and also for individual subpattern recursion. After its introduction in PCRE and Python, this kind of recursion was introduced into Perl at release 5.10.

A 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) or (?0) is a recursive call of the entire regular expression.

In PCRE (like Python, but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure.

This PCRE pattern solves the nested parentheses problem (assume the extended option is set so that whitespace 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. This is made easier by the use of relative references. (A Perl 5.10 feature.) Instead of (?1) in the pattern above you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. In other words, a negative number counts capturing parentheses leftwards from the point at which it is encountered.

It is also possible to refer to subsequently opened parentheses, by writing references such as (?+2). However, these cannot be recursive because the reference is not inside the parentheses that are referenced. They are always "subroutine" calls, as described in the next section.

An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We could rewrite the above example as follows:

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

If there is more than one subpattern with the same name, the earliest one is used.

This particular example pattern that we have been looking at 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 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.

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.

### Subpatterns as subroutines

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. The "called" subpattern may be defined before or after the reference. A numbered reference can be absolute or relative, as in these examples:

- (...(absolute)...)...(?2)...
- (...(relative)...)...(?-1)...
- (...(?+1)...(relative)...

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. Another example is given in the discussion of DEFINE above.

Like recursive subpatterns, a "subroutine" call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure.

When a subpattern is used as a subroutine, processing options such as case-independence are fixed when the subpattern is defined. They cannot be changed for different calls. For example, consider this pattern:

> (abc)(?i:(?-1))

It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called subpattern.

### Backtracking control

Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are described in the Perl documentation as "experimental and subject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section.

The new verbs make use of what was previously invalid syntax: an opening parenthesis followed by an asterisk. In Perl, they are generally of the form (*VERB:ARG) but PCRE does not support the use of arguments, so its general form is just (*VERB). Any number of these verbs may occur in a pattern. There are two kinds:

Verbs that act immediately

The following verbs act as soon as they are encountered:

> (*ACCEPT)

This verb causes the match to end successfully, skipping the remainder of the pattern. When inside a recursion, only the innermost pattern is ended immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is captured: in PCRE no data is captured. For example:

> A(A|B(*ACCEPT)|C)D

This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is captured.

> (*FAIL) or (*F)

This verb causes the match to fail, forcing backtracking to occur. It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE. The nearest equivalent is the callout feature, as for example in this pattern:

> a+(?C)(*FAIL)

A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times).

Verbs that act after backtracking

The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is no subsequent match, a failure is forced. The verbs differ in exactly what kind of failure occurs.

> (*COMMIT)

This verb causes the whole match to fail outright if the rest of the pattern does not match. Even if the pattern is unanchored, no further attempts to find a match by advancing the start point take place. Once (*COMMIT) has been passed, re:run/3 is committed to finding a match at the current starting point, or not at all. For example:

> a+(*COMMIT)b

This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish."

> (*PRUNE)

This verb causes the match to fail at the current position if the rest of the pattern does not match. If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then happens. Backtracking can occur as usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way.

> (*SKIP)

This verb is like (*PRUNE), except that if the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text was matched leading up to it cannot be part of a successful match. Consider:

> a+(*SKIP)b

If the subject is "aaaac...", after the first match attempt fails (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Note that a possessive quantifer does not have the same effect in this example; although it would suppress backtracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c".

> (*THEN)

This verb causes a skip to the next alternation if the rest of the pattern does not match. That is, it cancels pending backtracking, but only within the current alternation. Its name comes from the observation that it can be used for a pattern-based if-then-else block:

> ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds); on failure the matcher skips to the second alternative and tries COND2, without backtracking into COND1. If (*THEN) is used outside of any alternation, it acts exactly like (*PRUNE).

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