Regular Expressions

Regular Expressions (REs) provide a mechanism to select specific strings from a set of character strings.

Regular expressions are a context-independent syntax that can represent a wide variety of character sets and character set orderings, where these character sets are interpreted according to the current locale. While many regular expressions can be interpreted differently depending on the current locale, many features, such as character class expressions, provide for contextual invariance across locales.

The Basic Regular Expression (BRE) notation and construction rules in 9.3 Basic Regular Expressions shall apply to most utilities supporting r…

Regular Expressions (REs) provide a mechanism to select specific strings from a set of character strings.

The Basic Regular Expression (BRE) notation and construction rules in 9.3 Basic Regular Expressions shall apply to most utilities supporting regular expressions. Some utilities, instead, support the Extended Regular Expressions (ERE) described in 9.4 Extended Regular Expressions; any exceptions for both cases are noted in the descriptions of the specific utilities using regular expressions. Both BREs and EREs are supported by the Regular Expression Matching interface in the System Interfaces volume of POSIX.1-2024 under regcomp(), regexec(), and related functions.

9.1 Regular Expression Definitions

For the purposes of this section, the following definitions shall apply:

entire regular expression

The concatenated set of one or more BREs or EREs that make up the pattern specified for string selection.

escape sequence

The escape character followed by any single character, which is thereby "escaped". The escape character is a <backslash> that is neither in a bracket expression nor itself escaped.

leftmost

The characters closest to the beginning of the string.

matched

A sequence of zero or more characters shall be said to be matched by a BRE or ERE when the characters in the sequence correspond to a sequence of characters defined by the pattern.

Matching shall be based on the bit pattern used for encoding the character, not on the graphic representation of the character. This means that if a character set contains two or more encodings for a graphic symbol, or if the strings searched contain text encoded in more than one codeset, no attempt is made to search for any other representation of the encoded symbol. If that is required, the user can specify equivalence classes containing all variations of the desired graphic symbol.

The search for a matching sequence starts at the beginning of a string and stops when the first sequence matching the expression is found, where "first" is defined to mean "begins earliest in the string". If the pattern permits a variable number of matching characters and thus there is more than one such sequence starting at that point, the longest such sequence is matched. For example, the BRE "bb*" matches the second to fourth characters of the string "abbbc", and the ERE "(wee|week)(knights|night)" matches all ten characters of the string "weeknights".

Consistent with the whole match being the longest of the leftmost matches, each subpattern, from left to right, shall match the longest possible string. For this purpose, a null string shall be considered to be longer than no match at all. For example, matching the BRE "(.*).*" against "abcdef", the subexpression "(\1)" is "abcdef", and matching the BRE "(a*)*" against "bc", the subexpression "(\1)" is the null string. However, matching the ERE "(.*?).*" against "abcdef", the subpattern "(.*?)" matches the empty string, since that is the longest possible match for the ERE ".*?".

When a multi-character collating element in a bracket expression (see 9.3.5 RE Bracket Expression) is involved, the longest sequence shall be measured in characters consumed from the string to be matched; that is, the collating element counts not as one element, but as the number of characters it matches.

BRE (ERE) matching a single character

A BRE or ERE that shall match either a single character or a single collating element.

Only a BRE or ERE of this type that includes a bracket expression (see 9.3.5 RE Bracket Expression) can match a collating element.

BRE (ERE) matching multiple characters

A BRE or ERE that shall match a concatenation of single characters or collating elements.

Such a BRE or ERE is made up from a BRE (ERE) matching a single character and BRE (ERE) special characters.

invalid

This section uses the term "invalid" for certain constructs or conditions. Invalid REs shall cause the utility or function using the RE to generate an error condition. When invalid is not used, violations of the specified syntax or semantics for REs produce undefined results: this may entail an error, enabling an extended syntax for that RE, or using the construct in error as literal characters to be matched. For example, the BRE construct "{1,2,3}" does not comply with the grammar. A conforming application cannot rely on it producing an error nor matching the literal characters "{1,2,3}".

9.2 Regular Expression General Requirements

The requirements in this section shall apply to both basic and extended regular expressions.

The use of regular expressions is generally associated with text processing. REs (BREs and EREs) operate on text strings; that is, zero or more characters followed by an end-of-string delimiter (typically NUL). Some utilities employing regular expressions limit the processing to lines; that is, zero or more characters followed by a <newline>.

In the functions processing regular expressions described in System Interfaces volume of POSIX.1-2024, the <newline> is regarded as an ordinary character and both a <period> and a non-matching list can match one. The Shell and Utilities volume of POSIX.1-2024 specifies within the individual descriptions of those standard utilities employing regular expressions whether they permit matching of <newline> characters; if not stated otherwise, the use of literal <newline> characters or any escape sequence equivalent in either patterns or matched text produces undefined results. Those utilities (like grep) that do not allow <newline> characters to match are responsible for eliminating any <newline> from strings before matching against the RE. The regcomp() function in the System Interfaces volume of POSIX.1-2024, however, can provide support for such processing without violating the rules of this section.

The interfaces specified in POSIX.1-2024 do not permit the inclusion of a NUL character in an RE or in the string to be matched. If during the operation of a standard utility a NUL is included in the text designated to be matched, that NUL may designate the end of the text string for the purposes of matching.

Some standard utilities and functions support case-insensitive regular expression matching. When this type of matching is in effect, the matching process shall be modified as described in 4.1 Case Insensitive Comparisons.

The implementation shall support any regular expression that does not exceed 256 bytes in length.

9.3 Basic Regular Expressions

9.3.1 BREs Matching a Single Character or Collating Element

When not inside a bracket expression, the following shall match a single character:

a BRE ordinary character

a BRE special character or ‘]’ preceded by an unescaped <backslash>

a <period>

A bracket expression shall match a single character or a single collating element.

9.3.2 BRE Ordinary Characters

An ordinary character is a BRE that matches itself: any character in the supported character set, except for the BRE special characters listed in 9.3.3 BRE Special Characters.

When not inside a bracket expression, the interpretation of an ordinary character preceded by an unescaped <backslash> is undefined, except for:

The characters ‘)’, ‘(’, ‘{’, and ‘}’

The digits 1 to 9 inclusive (see 9.3.6 BREs Matching Multiple Characters)

The ‘]’ character; "]" shall match a ‘]’ character

The ‘?’, ‘+’, and ‘|’ characters; it is implementation-defined whether "?", "+", and "|" each match the literal character ‘?’, ‘+’, or ‘|’, respectively, or behave as described for the ERE special characters ‘?’, ‘+’, and ‘|’, respectively (see 9.4.3 ERE Special Characters).

Note: A future version of this standard may require "?", "+", and "|" to behave as described for the ERE special characters ‘?’, ‘+’, and ‘|’, respectively.

9.3.3 BRE Special Characters

A BRE special character has special properties in certain contexts. Outside those contexts, or when preceded by an unescaped <backslash>, such a character is a BRE that matches the special character itself. The BRE special characters and the contexts in which they have their special meaning are as follows:

.[\ The <period>, <left-square-bracket>, and <backslash> shall be special except when used in a bracket expression (see 9.3.5 RE Bracket Expression). An expression containing a ‘[’ that is unescaped and is not part of a bracket expression produces undefined results. * The <asterisk> shall be special except when used:

In a bracket expression

As the first character of an entire BRE (after an initial ‘^’, if any)

Immediately following a "|" escape sequence (after an initial ‘^’, if any), if the implementation does not match the escape sequence "|" to the literal character ‘|’.

As the first character of a subexpression (after an initial ‘^’, if any); see 9.3.6 BREs Matching Multiple Characters ^ The <circumflex> shall be special when used as an anchor (see 9.3.8 BRE Expression Anchoring ). The <circumflex> shall signify a non-matching list expression when it occurs first in a list, immediately following a <left-square-bracket> (see 9.3.5 RE Bracket Expression). $ The <dollar-sign> shall be special when used as an anchor.

9.3.4 Periods in BREs

When not inside a bracket expression, a <period> (‘.’) is a BRE that shall match any character in the supported character set except NUL.

9.3.5 RE Bracket Expression

A bracket expression (an expression enclosed in square brackets, "[]") is an RE that shall match a specific set of single characters, and may match a specific set of multi-character collating elements, based on the non-empty set of list expressions contained in the bracket expression.

The following rules and definitions apply to bracket expressions:

A bracket expression is either a matching list expression or a non-matching list expression. It consists of one or more expressions: ordinary characters, collating elements, collating symbols, equivalence classes, character classes, or range expressions. The <right-square-bracket> (‘]’) shall lose its special meaning and represent itself in a bracket expression if it occurs first in the list (after an initial <circumflex> (‘^{’), if any). Otherwise, it shall terminate the bracket expression, unless it appears in a collating symbol (such as "[.].]") or is the ending <right-square-bracket> for a collating symbol, equivalence class, or character class. When the bracket expression appears within a BRE, the special characters ‘.’, ‘*’, ‘[’, and ‘\’ (<period>, <asterisk>, <left-square-bracket>, and <backslash>, respectively) shall lose their special meaning within the bracket expression. When the bracket expression appears within an ERE, the special characters ‘.’, ‘(’, ‘*’, ‘+’, ‘?’, ‘{’, ‘|’, ‘$’, ‘[’, and ‘\’ (<period>, <left-parenthesis>, <asterisk>, <plus-sign>, <question-mark>, <left-brace>, <vertical-line>, <dollar-sign>, <left-square-bracket>, and <backslash>, respectively) shall lose their special meaning within the bracket expression; <circumflex> (‘}’) shall lose its special meaning as an anchor. When the bracket expression appears within a shell pattern (see XCU 2.14 Pattern Matching Notation), the special characters ‘?’, ‘*’, and ‘[’ (<question-mark>, <asterisk>, and <left-square-bracket>, respectively) shall lose their special meaning within the bracket expression; whether or not <backslash> (‘\’) loses its special meaning as a pattern matching character is described in XCU 2.14.1 Patterns Matching a Single Character, but in contexts where a shell-quoting <backslash> can be used it shall retain its special meaning (see XCU 2.2 Quoting). For example:

$ ls
! $ - \ a b c
$ echo [a\-c]
- a c
$ echo [\!a]
! a
$ echo ["!\$a-c"]
! $ - a c
$ echo [!"\$a-c"]
! \ b
$ echo [!\]\\]
! $ - a b c

The character sequences "[.", "[=", and "[:" (<left-square-bracket> followed by a <period>, <equals-sign>, or <colon>) shall be special inside a bracket expression and are used to delimit collating symbols, equivalence class expressions, and character class expressions. These symbols shall be followed by a valid expression and the matching terminating sequence ".]", "=]", or ":]", as described in the following items. 1.

A matching list expression specifies a list that shall match any single character that is matched by one of the expressions represented in the list. The first character in the list cannot be the <circumflex>. An ordinary character in the list shall only match that character; for example, "[abc]" is an RE that only matches one of the characters ‘a’, ‘b’, or ‘c’.

It is unspecified whether a matching list expression matches a multi-character collating element that is matched by one of the expressions. 1.

A non-matching list expression begins with a <circumflex> (‘^’), and the matching behavior shall be the logical inverse of the corresponding matching list expression (the same bracket expression but without the leading <circumflex>). For example, since the RE "[abc]" only matches ‘a’, ‘b’, or ‘c’, it follows that "[^abc]" is an RE that matches any character except ‘a’, ‘b’, or ‘c’. It is unspecified whether a non-matching list expression matches a multi-character collating element that is not matched by any of the expressions. The <circumflex> shall have this special meaning only when it occurs first in the list, immediately following the <left-square-bracket>. 1.

A collating symbol is a collating element enclosed within bracket-period ("[." and ".]") delimiters. Collating elements are defined as described in 7.3.2.4 Collation Order. Conforming applications shall represent multi-character collating elements as collating symbols when it is necessary to distinguish them from a list of the individual characters that make up the multi-character collating element. For example, if the string "ch" is a collating element defined using the line:

collating-element <ch-digraph> from "<c><h>"

in the locale definition, the expression "[[.ch.]]" shall be treated as an RE containing the collating symbol ‘ch’, while "[ch]" shall be treated as an RE matching ‘c’ or ‘h’. Collating symbols are recognized only inside bracket expressions. If the string is not a collating element in the current locale, the expression is invalid. 1.

An equivalence class expression shall represent the set of collating elements belonging to an equivalence class, as described in 7.3.2.4 Collation Order. Only primary equivalence classes shall be recognized. The class shall be expressed by enclosing any one of the collating elements in the equivalence class within bracket-equal ("[=" and "=]") delimiters. For example, if ‘a’, ‘à’, and ‘â’ belong to the same equivalence class, then "[[=a=]b]", "[[=à=]b]", and "[[=â=]b]" are each equivalent to "[aàâb]". If the collating element does not belong to an equivalence class, the equivalence class expression shall be treated as a collating symbol. 1.

A character class expression shall represent the union of two sets:

The set of single characters that belong to the character class, as defined in the LC_CTYPE category in the current locale. 1.

An unspecified set of multi-character collating elements.

All character classes specified in the current locale shall be recognized. A character class expression is expressed as a character class name enclosed within bracket-<colon> ("[:" and ":]") delimiters.

The following character class expressions shall be supported in all locales:

[:alnum:]   [:cntrl:]   [:lower:]   [:space:]
[:alpha:]   [:digit:]   [:print:]   [:upper:]
[:blank:]   [:graph:]   [:punct:]   [:xdigit:]

In addition, character class expressions of the form:

[:name:]

are recognized in those locales where the name keyword has been given a charclass definition in the LC_CTYPE category. 1.

In the POSIX locale, a range expression represents the set of collating elements that fall between two elements in the collation sequence, inclusive. In other locales, a range expression has unspecified behavior: strictly conforming applications shall not rely on whether the range expression is valid, or on the set of collating elements matched. A range expression shall be expressed as the starting point and the ending point separated by a <hyphen-minus> (‘-’).

In the following, all examples assume the POSIX locale.

The starting range point and the ending range point shall be a collating element or collating symbol. An equivalence class expression used as a starting or ending point of a range expression produces unspecified results. An equivalence class can be used portably within a bracket expression, but only outside the range. If the represented set of collating elements is empty, it is unspecified whether the expression matches nothing, or is treated as invalid.

The interpretation of range expressions where the ending range point is also the starting range point of a subsequent range expression (for example, "[a-m-o]") is undefined.

The <hyphen-minus> character shall be treated as itself if it occurs first (after an initial ‘^{’, if any) or last in the list, or as an ending range point in a range expression. As examples, the expressions "[-ac]" and "[ac-]" are equivalent and match any of the characters ‘a’, ‘c’, or ‘-’; "[}-ac]" and "[^ac-]" are equivalent and match any characters except ‘a’, ‘c’, or ‘-’; the expression "[%–]" matches any of the characters between ‘%’ and ‘-’ inclusive; the expression "[–@]" matches any of the characters between ‘-’ and ‘@’ inclusive; and the expression "[a–@]" is either invalid or equivalent to ‘@’, because the letter ‘a’ follows the symbol ‘-’ in the POSIX locale. To use a <hyphen-minus> as the starting range point, it shall either come first in the bracket expression or be specified as a collating symbol; for example, "[][.-.]-0]", which matches either a <right-square-bracket> or any character or collating element that collates between <hyphen-minus> and 0, inclusive.

If a bracket expression specifies both ‘-’ and ‘]’, the ‘]’ shall be placed first (after the ‘^’, if any) and the ‘-’ last within the bracket expression. 1.

If a bracket expression contains at least three list elements, where the first and last list elements are the same single-character element of <period>, <equals-sign>, or <colon>, then it is unspecified whether the bracket expression will be treated as a collating symbol, equivalence class, or character class, respectively; treated as a matching list expression; or treated as an invalid bracket expression.

9.3.6 BREs Matching Multiple Characters

The following rules can be used to construct BREs matching multiple characters from BREs matching a single character:

The concatenation of BREs shall match the concatenation of the strings matched by each component of the BRE. 1.

A subexpression can be defined within a BRE by enclosing it between the character pairs "(" and ")". Such a subexpression shall match whatever it would have matched without the "(" and ")", except that anchoring within subexpressions is optional behavior; see 9.3.8 BRE Expression Anchoring. Subexpressions can be arbitrarily nested. 1.

The back-reference expression ‘\n’ shall match the same (possibly empty) string of characters as was matched by a subexpression enclosed between "(" and ")" preceding the ‘\n’. The character ‘n’ shall be a digit from 1 through 9, specifying the nth subexpression (the one that begins with the nth "(" from the beginning of the pattern and ends with the corresponding paired ")"). The expression is invalid if less than n subexpressions precede the ‘\n’. The string matched by a contained subexpression shall be within the string matched by the containing subexpression. If the containing subexpression does not match, or if there is no match for the contained subexpression within the string matched by the containing subexpression, then back-reference expressions corresponding to the contained subexpression shall not match. When a subexpression matches more than one string, a back-reference expression corresponding to the subexpression shall refer to the last matched string. For example, the expression "^{(.*)\1$" matches strings consisting of two adjacent appearances of the same substring, and the expression "(a)*\1" fails to match ‘a’, the expression "(a(b)*)*\2" fails to match ‘abab’, and the expression "}(ab*)*\1$" matches ‘ababbabb’, but fails to match ‘ababbab’. 1.

When a BRE matching a single character, a subexpression, or a back-reference is followed by the special character <asterisk> (‘*’), together with that <asterisk> it shall match what zero or more consecutive occurrences of the BRE would match. For example, "[ab]*" and "[ab][ab]" are equivalent when matching the string "ab". 1.

When a BRE matching a single character, a subexpression, or a back-reference is followed by an interval expression of the format "{m}", "{m,}", or "{m,n}", together with that interval expression it shall match what repeated consecutive occurrences of the BRE would match. The values of m and n are decimal integers in the range 0 <=m<=n<={RE_DUP_MAX}, where m specifies the exact or minimum number of occurrences and n specifies the maximum number of occurrences. The expression "{m}" shall match exactly m occurrences of the preceding BRE, "{m,}" shall match at least m occurrences, and "{m,n}" shall match any number of occurrences between m and n, inclusive.

For example, in the string "abababccccccd" the BRE "c{3}" is matched by characters seven to nine, the BRE "(ab){4,}" is not matched at all, and the BRE "c{1,3}d" is matched by characters ten to thirteen.

The behavior of multiple adjacent duplication symbols (‘*’ and intervals) produces undefined results.

A subexpression repeated by an <asterisk> (‘*’) or an interval expression shall not match a null expression unless this is the only match for the repetition or it is necessary to satisfy the exact or minimum number of occurrences for the interval expression.

9.3.7 BRE Precedence

The order of precedence shall be as shown in the following table:

BRE Precedence (from high to low)
Collation-related bracket symbols	[==] [::] [..]
Escaped characters	<special character>
Bracket expression	[]
Subexpressions/back-references	() \n
Single-character-BRE duplication	* {m,n}
Concatenation
Anchoring	^ $

9.3.8 BRE Expression Anchoring

A BRE can be limited to matching expressions that begin or end a string; this is called "anchoring". The <circumflex> and <dollar-sign> special characters shall be considered BRE anchors in the following contexts:

A <circumflex> (‘^’) shall be an anchor when used as the first character of an entire BRE and, if the implementation does not match the escape sequence "|" to the literal character ‘|’, when used immediately following a "|" escape sequence that is not inside a subexpression. The implementation may also treat a <circumflex> as an anchor when used inside a subexpression; in this case it shall be an anchor only when either of the following is true:

It is the first character of the subexpression.
It immediately follows a "|" escape sequence and the implementation does not match the escape sequence "|" to the literal character ‘|’.

The <circumflex> shall anchor the expression (or optionally subexpression) to the beginning of a string; only sequences starting at the first character of a string shall be matched by the BRE. For example, the BRE "^ab" matches "ab" in the string "abcdef", but fails to match in the string "cdefab". The BRE "(^ab)" may match the former string. A portable BRE shall escape a leading <circumflex> in a subexpression to match a literal <circumflex>.

A <dollar-sign> (‘$’) shall be an anchor when used as the last character of an entire BRE and, if the implementation does not match the escape sequence "|" to the literal character ‘|’, when used immediately preceding a "|" escape sequence that is not inside a subexpression. The implementation may also treat a <dollar-sign> as an anchor when used inside a subexpression; in this case it shall be an anchor only when either of the following is true:

It is the last character of the subexpression.
It immediately precedes a "|" escape sequence and the implementation does not match the escape sequence "|" to the literal character ‘|’.

The <dollar-sign> shall anchor the expression (or optionally subexpression) to the end of the string being matched; the <dollar-sign> can be said to match the end-of-string following the last character. A portable BRE shall escape a trailing <dollar-sign> in a subexpression to match a literal <dollar-sign>.

A BRE anchored by both ‘^’ and ‘$’ shall match only an entire string. For example, the BRE "^abcdef$" matches strings consisting only of "abcdef".

9.4 Extended Regular Expressions

The extended regular expression (ERE) notation and construction rules shall apply to utilities defined as using extended regular expressions; any exceptions to the following rules are noted in the descriptions of the specific utilities using EREs.

9.4.1 EREs Matching a Single Character or Collating Element

When not inside a bracket expression, the following shall match a single character:

an ERE ordinary character
an ERE special character, ‘]’, or ‘}’ preceded by an unescaped <backslash>
a <period>

A bracket expression shall match a single character or a single collating element. An ERE matching a single character enclosed in parentheses shall match the same as the ERE without parentheses would have matched.

9.4.2 ERE Ordinary Characters

An ordinary character is an ERE that matches itself. An ordinary character is any character in the supported character set, except for the ERE special characters listed in 9.4.3 ERE Special Characters. When not inside a bracket expression, the interpretation of an ordinary character preceded by an unescaped <backslash> is undefined, except for the ‘]’ and ‘}’ characters; "]" and "}" shall match the ‘]’ and ‘}’ characters, respectively.

9.4.3 ERE Special Characters

An ERE special character has special properties in certain contexts. Outside those contexts, or when preceded by an unescaped <backslash>, such a character shall be an ERE that matches the special character itself. The extended regular expression special characters and the contexts in which they shall have their special meaning are as follows:

.[( The <period>, <left-square-bracket>, <backslash>, and <left-parenthesis> shall be special except when used in a bracket expression (see 9.3.5 RE Bracket Expression). When not inside a bracket expression, an unescaped <left-parenthesis> immediately followed by a <right-parenthesis> produces undefined results. A <left-square-bracket> that is unescaped and is not part of a bracket expression also produces undefined results. ) The <right-parenthesis> shall be special when matched with a preceding <left-parenthesis>, both not inside a bracket expression. *+?{ The <asterisk>, <plus-sign>, <question-mark>, and <left-brace> shall be special except when used in a bracket expression (see 9.3.5 RE Bracket Expression). Any of the following uses produce undefined results:

If these characters appear first in an ERE, or immediately following an unescaped <vertical-line>, <circumflex>, <dollar-sign>, or <left-parenthesis>
If a <left-brace> is not part of a valid interval expression (see 9.4.6 EREs Matching Multiple Characters) | The <vertical-line> is special except when used in a bracket expression (see 9.3.5 RE Bracket Expression). A <vertical-line> appearing first or last in an ERE, or immediately following a <vertical-line> or a <left-parenthesis>, or immediately preceding a <right-parenthesis>, produces undefined results. ^ The <circumflex> shall be special when used as an anchor (see 9.4.9 ERE Expression Anchoring ). The <circumflex> shall signify a non-matching list expression when it occurs first in a list, immediately following a <left-square-bracket> (see 9.3.5 RE Bracket Expression). $ The <dollar-sign> shall be special when used as an anchor.

9.4.4 Periods in EREs

When not inside a bracket expression, a <period> (‘.’) is an ERE that shall match any character in the supported character set except NUL.

9.4.5 ERE Bracket Expression

The rules for ERE Bracket Expressions are the same as for Basic Regular Expressions; see 9.3.5 RE Bracket Expression.

9.4.6 EREs Matching Multiple Characters

The following rules shall be used to construct EREs matching multiple characters from EREs matching a single character:

A concatenation of EREs shall match the concatenation of the character sequences matched by each component of the ERE. A concatenation of EREs enclosed in parentheses shall match whatever the concatenation without the parentheses matches. For example, both the ERE "cd" and the ERE "(cd)" are matched by the third and fourth character of the string "abcdefabcdef".
When an ERE matching a single character or an ERE enclosed in parentheses is followed by the special character <plus-sign> (‘+’), together with that <plus-sign> it shall match what one or more consecutive occurrences of the ERE would match. For example, the ERE "b+(bc)" matches the fourth to seventh characters in the string "acabbbcde". And, "[ab]+" and "[ab][ab]*" are equivalent.
When an ERE matching a single character or an ERE enclosed in parentheses is followed by the special character <asterisk> (‘*’), together with that <asterisk> it shall match what zero or more consecutive occurrences of the ERE would match. For example, the ERE "b*c" matches the first character in the string "cabbbcde", and the ERE "b*cd" matches the third to seventh characters in the string "cabbbcdebbbbbbcdbc". And, "[ab]*" and "[ab][ab]" are equivalent when matching the string "ab".
When an ERE matching a single character or an ERE enclosed in parentheses is followed by the special character <question-mark> (‘?’), together with that <question-mark> it shall match what zero or one consecutive occurrences of the ERE would match. For example, the ERE "b?c" matches the second character in the string "acabbbcde".
When an ERE matching a single character or an ERE enclosed in parentheses is followed by an interval expression of the format "{m}", "{m,}", or "{m,n}", together with that interval expression it shall match what repeated consecutive occurrences of the ERE would match. The values of m and n are decimal integers in the range 0 <=m<=n<={RE_DUP_MAX}, where m specifies the exact or minimum number of occurrences and n specifies the maximum number of occurrences. The expression "{m}" matches exactly m occurrences of the preceding ERE, "{m,}" matches at least m occurrences, and "{m,n}" matches any number of occurrences between m and n, inclusive.

For example, in the string "abababccccccd" the ERE "c{3}" is matched by characters seven to nine and the ERE "(ab){2,}" is matched by characters one to six.

Each of the duplication symbols (‘+’, ‘*’, ‘?’, and intervals) can be suffixed by the repetition modifier ‘?’ (<question-mark>), in which case matching behavior for that repetition shall be changed from the leftmost longest possible match to the leftmost shortest possible match, including the null match (see A.9 Regular Expressions). For example, the ERE ".*c" matches up to and including the last character (‘c’) in the string "abc abc", whereas the ERE ".*?c" matches up to and including the first character ‘c’, the third character in the string.

If the REG_MINIMAL flag, defined in the <regex.h> header, is used when compiling an ERE via regcomp(), the leftmost shortest possible match shall be the default for all duplication symbols, and the repetition modifier ‘?’ can be used to select the leftmost longest possible match for the repetition it modifies.

The behavior of multiple adjacent duplication symbols (‘+’, ‘*’, ‘?’, and intervals, possibly suffixed by the repetition modifier ‘?’) produces undefined results.

An ERE matching a single character repeated by an ‘*’, ‘?’, or an interval expression shall not match a null expression unless this is the only match for the repetition or it is necessary to satisfy the exact or minimum number of occurrences for the interval expression.

9.4.7 ERE Alternation

Two EREs separated by the special character <vertical-line> (‘|’) shall match a string that is matched by either. For example, the ERE "a((bc)|d)" matches the string "abc" and the string "ad". Single characters, or expressions matching single characters, separated by the <vertical-line> and enclosed in parentheses, shall be treated as an ERE matching a single character.

9.4.8 ERE Precedence

The order of precedence shall be as shown in the following table:

ERE Precedence (from high to low)
Collation-related bracket symbols	[==] [::] [..]
Escaped characters	<special character>
Bracket expression	[]
Grouping	()
Single-character-ERE duplication	* + ? {m,n}
Concatenation
Anchoring	^ $
Alternation	\|

For example, the ERE "abba|cde" matches either the string "abba" or the string "cde" (rather than the string "abbade" or "abbcde", because concatenation has a higher order of precedence than alternation).

9.4.9 ERE Expression Anchoring

An ERE can be limited to matching expressions that begin or end a string; this is called "anchoring". The <circumflex> and <dollar-sign> special characters shall be considered ERE anchors when used anywhere except inside a bracket expression. This shall have the following effects:

When not inside a bracket expression, a <circumflex> (‘^’) shall anchor the expression or subexpression it begins to the beginning of a string; such an expression or subexpression can match only a sequence starting at the first character of a string. For example, the EREs "^ab" and "(^ab)" match "ab" in the string "abcdef", but fail to match in the string "cdefab", and the ERE "a^b" is valid, but can never match because the ‘a’ prevents the expression "^b" from matching starting at the first character.
When not inside a bracket expression, a <dollar-sign> (‘$’) shall anchor the expression or subexpression it ends to the end of a string; such an expression or subexpression can match only a sequence ending at the last character of a string. For example, the EREs "ef$" and "(ef$)" match "ef" in the string "abcdef", but fail to match in the string "cdefab", and the ERE "e$f" is valid, but can never match because the ‘f’ prevents the expression "e$" from matching ending at the last character.

9.5 Regular Expression Grammar

Grammars describing the syntax of both basic and extended regular expressions are presented in this section. The grammar takes precedence over the text. See XCU 1.3 Grammar Conventions .

9.5.1 BRE/ERE Grammar Lexical Conventions

The lexical conventions for regular expressions are as described in this section.

Except as noted, the longest possible token or delimiter beginning at a given point is recognized.

The following tokens are processed (in addition to those string constants shown in the grammar):

COLL_ELEM_SINGLE Any single-character collating element, unless it is a META_CHAR. COLL_ELEM_MULTI Any multi-character collating element. BACKREF Applicable only to basic regular expressions. The character string consisting of a <backslash> character followed by a single-digit numeral, ‘1’ to ‘9’. DUP_COUNT Represents a numeric constant. It shall be an integer in the range 0 <=DUP_COUNT <={RE_DUP_MAX}. This token is only recognized when the context of the grammar requires it. At all other times, digits not preceded by a <backslash> character are treated as ORD_CHAR. META_CHAR One of the characters:

^ When found first in a bracket expression - When found anywhere but first (after an initial ‘^{’, if any) or last in a bracket expression, or as the ending range point in a range expression ] When found anywhere but first (after an initial ‘}’, if any) in a bracket expression

L_ANCHOR Applicable only to basic regular expressions. The character ‘^{’ when it appears either as the first character of a basic regular expression or, if the implementation does not match the escape sequence "|" to the literal character ‘|’, when used immediately following a "|" escape sequence that is not inside a subexpression, and when not QUOTED_CHAR. The ‘}’ may be recognized as an anchor elsewhere; see 9.3.8 BRE Expression Anchoring. ORD_CHAR A character, other than one of the special characters in SPEC_CHAR. QUOTED_CHAR In a BRE, one of the character sequences:

\^    \.    \*    \[    \]    \$    \\

On implementations where the escape sequences "?", "+", and "|" match the literal characters ‘?’, ‘+’, and ‘|’, respectively, QUOTED_CHAR shall also include:

\?    \+    \|

In an ERE, one of the character sequences:

\^    \.    \[    \]    \$    \(    \)    \|
\*    \+    \?    \{    \}    \\

R_ANCHOR (Applicable only to basic regular expressions.) The character ‘$’ when it appears either as the last character of a basic regular expression or, if the implementation does not match the escape sequence "|" to the literal character ‘|’, when used immediately preceding a "|" escape sequence that is not inside a subexpression, and when not QUOTED_CHAR. The ‘$’ may be recognized as an anchor elsewhere; see 9.3.8 BRE Expression Anchoring. SPEC_CHAR For basic regular expressions, one of the following special characters:

. Anywhere except inside bracket expressions \ Anywhere except inside bracket expressions [ Anywhere except inside bracket expressions ^ When used as an anchor (see 9.3.8 BRE Expression Anchoring) $ When used as an anchor * Anywhere except first in an entire RE, anywhere in a bracket expression, directly following "(", directly following an anchoring ‘^’

For extended regular expressions, shall be one of the following special characters found anywhere except inside bracket expressions:

^    .    [    $    (    )    |
*    +    ?    {    \

The close-parenthesis shall be considered special in this context only if matched with a preceding open-parenthesis.

9.5.2 RE and Bracket Expression Grammar

This section presents the grammar for basic regular expressions, including the bracket expression grammar that is common to both BREs and EREs.

%token    ORD_CHAR QUOTED_CHAR DUP_COUNT

%token    BACKREF L_ANCHOR R_ANCHOR

%token    Back_open_paren  Back_close_paren
/*          '\('             '\)'        */

%token    Back_open_brace  Back_close_brace
/*          '\{'             '\}'        */

/* The following shall be tokens on implementations where
\?, \+, and \| are not included in QUOTED_CHAR */

%token    Back_qm  Back_plus  Back_bar
/*          '\?'     '\+'       '\|'     */

/* The following tokens are for the Bracket Expression
grammar common to both REs and EREs. */

%token    COLL_ELEM_SINGLE COLL_ELEM_MULTI META_CHAR

%token    Open_equal Equal_close Open_dot Dot_close Open_colon Colon_close
/*           '[='       '=]'        '[.'     '.]'      '[:'       ':]'  */

%token    class_name
/* class_name is a keyword to the LC_CTYPE locale category */
/* (representing a character class) in the current locale */
/* and is only recognized between [: and :] */

%start    basic_reg_exp
%%

/* --------------------------------------------
Basic Regular Expression
--------------------------------------------
*/
basic_reg_exp   :                        BRE_branch
| basic_reg_exp Back_bar BRE_branch /* if Back_bar
is a token */
;
BRE_branch      :            BRE_expression
| BRE_branch BRE_expression
;
BRE_expression  :          simple_BRE
| L_ANCHOR
|                     R_ANCHOR
| L_ANCHOR            R_ANCHOR
| L_ANCHOR simple_BRE
|          simple_BRE R_ANCHOR
| L_ANCHOR simple_BRE R_ANCHOR
;
simple_BRE      : nondupl_BRE
| nondupl_BRE BRE_dupl_symbol
;
nondupl_BRE     : one_char_or_coll_elem_BRE
| Back_open_paren basic_reg_exp Back_close_paren
| BACKREF
;
one_char_or_coll_elem_BRE  : ORD_CHAR
| QUOTED_CHAR
| '.'
| bracket_expression
;
BRE_dupl_symbol : '*'
| Back_qm   /* if Back_qm is a token */
| Back_plus /* if Back_plus is a token */
| Back_open_brace DUP_COUNT               Back_close_brace
| Back_open_brace DUP_COUNT ','           Back_close_brace
| Back_open_brace DUP_COUNT ',' DUP_COUNT Back_close_brace
;

/* --------------------------------------------
Bracket Expression
-------------------------------------------
*/
bracket_expression : '[' matching_list ']'
| '[' nonmatching_list ']'
;
matching_list  : bracket_list
;
nonmatching_list : '^' bracket_list
;
bracket_list   : follow_list
| follow_list '-'
;
follow_list    :             expression_term
| follow_list expression_term
;
expression_term : single_expression
| range_expression
;
single_expression : end_range
| character_class
| equivalence_class
;
range_expression : start_range end_range
| start_range '-'
;
start_range    : end_range '-'
;
end_range      : COLL_ELEM_SINGLE
| collating_symbol
;
collating_symbol : Open_dot COLL_ELEM_SINGLE Dot_close
| Open_dot COLL_ELEM_MULTI Dot_close
| Open_dot META_CHAR Dot_close
;
equivalence_class : Open_equal COLL_ELEM_SINGLE Equal_close
| Open_equal COLL_ELEM_MULTI Equal_close
;
character_class : Open_colon class_name Colon_close
;

Note that although the BRE grammar appears always to permit L_ANCHOR or R_ANCHOR inside "(" and ")", the lexical conventions (see 9.5.1 BRE/ERE Grammar Lexical Conventions) imply that ‘^{’ and ‘$’ may be ordinary characters there. This reflects the semantic limits on the application, as noted in 9.3.8 BRE Expression Anchoring. Since it is an implementation option whether to interpret ‘}’ and ‘$’ as anchors in these locations, conforming applications cannot use unescaped ‘^’ and ‘$’ in positions inside "(" and ")" that might be interpreted as anchors.

9.5.3 ERE Grammar

This section presents the grammar for extended regular expressions, excluding the bracket expression grammar.

Note: The bracket expression grammar and the associated %token lines are identical between BREs and EREs. It has been omitted from the ERE section to avoid unnecessary editorial duplication.

%token  ORD_CHAR QUOTED_CHAR DUP_COUNT
%start  extended_reg_exp
%%

/* --------------------------------------------
Extended Regular Expression
--------------------------------------------
*/
extended_reg_exp   :                      ERE_branch
| extended_reg_exp '|' ERE_branch
;
ERE_branch         :            ERE_expression
| ERE_branch ERE_expression
;
ERE_expression     : one_char_or_coll_elem_ERE
| '^'
| '$'
| '(' extended_reg_exp ')'
| ERE_expression ERE_dupl_symbol
;
one_char_or_coll_elem_ERE  : ORD_CHAR
| QUOTED_CHAR
| '.'
| bracket_expression
;
ERE_dupl_symbol    : '*'
| '+'
| '?'
| '{' DUP_COUNT               '}'
| '{' DUP_COUNT ','           '}'
| '{' DUP_COUNT ',' DUP_COUNT '}'
;

The ERE grammar does not permit several constructs that previous sections specify as having undefined results. Additionally, there are some constructs which the grammar permits but which still give undefined results:

ORD_CHAR preceded by an unescaped <backslash> character
One or more ERE_dupl_symbols appearing first in an ERE, or immediately following ‘|’, ‘^’, ‘(’, or ‘$’
‘{’ not part of a valid ERE_dupl_symbol
‘|’ appearing first or last in an ERE, or immediately following ‘|’ or ‘(’, or immediately preceding ‘)’

Implementations are permitted to extend the language to allow these. Strictly Conforming applications cannot use such constructs.

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