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regex(7) Miscellaneous Information Manual regex(7)
regex - POSIX.2 regular expressions
Regular expressions ("RE"s), as defined in POSIX.2, come in two
forms: modern REs (roughly those of egrep(1); POSIX.2 calls these
"extended" REs) and obsolete REs (roughly those of ed(1); POSIX.2
"basic" REs). Obsolete REs mostly exist for backward
compatibility in some old programs; they will be discussed at the
end. POSIX.2 leaves some aspects of RE syntax and semantics open;
"(!)" marks decisions on these aspects that may not be fully
portable to other POSIX.2 implementations.
A (modern) RE is one(!) or more nonempty(!) branches, separated
by '|'. It matches anything that matches one of the branches.
A branch is one(!) or more pieces, concatenated. It matches a
match for the first, followed by a match for the second, and so
on.
A piece is an atom possibly followed by a single(!) '*', '+', '?',
or bound. An atom followed by '*' matches a sequence of 0 or more
matches of the atom. An atom followed by '+' matches a sequence
of 1 or more matches of the atom. An atom followed by '?' matches
a sequence of 0 or 1 matches of the atom.
A bound is '{' followed by an unsigned decimal integer, possibly
followed by ',' possibly followed by another unsigned decimal
integer, always followed by '}'. The integers must lie between 0
and RE_DUP_MAX (255(!)) inclusive, and if there are two of them,
the first may not exceed the second. An atom followed by a bound
containing one integer i and no comma matches a sequence of
exactly i matches of the atom. An atom followed by a bound
containing one integer i and a comma matches a sequence of i or
more matches of the atom. An atom followed by a bound containing
two integers i and j matches a sequence of i through j (inclusive)
matches of the atom.
An atom is a regular expression enclosed in "()" (matching a match
for the regular expression), an empty set of "()" (matching the
null string)(!), a bracket expression (see below), '.' (matching
any single character), '^' (matching the null string at the
beginning of a line), '$' (matching the null string at the end of
a line), a '\' followed by one of the characters "^.[$()|*+?{\"
(matching that character taken as an ordinary character), a '\'
followed by any other character(!) (matching that character taken
as an ordinary character, as if the '\' had not been present(!)),
or a single character with no other significance (matching that
character). A '{' followed by a character other than a digit is
an ordinary character, not the beginning of a bound(!). It is
illegal to end an RE with '\'.
A bracket expression is a list of characters enclosed in "[]". It
normally matches any single character from the list (but see
below). If the list begins with '^', it matches any single
character (but see below) not from the rest of the list. If two
characters in the list are separated by '-', this is shorthand for
the full range of characters between those two (inclusive) in the
collating sequence, for example, "[0-9]" in ASCII matches any
decimal digit. It is illegal(!) for two ranges to share an
endpoint, for example, "a-c-e". Ranges are very collating-
sequence-dependent, and portable programs should avoid relying on
them.
To include a literal ']' in the list, make it the first character
(following a possible '^'). To include a literal '-', make it the
first or last character, or the second endpoint of a range. To
use a literal '-' as the first endpoint of a range, enclose it in
"[." and ".]" to make it a collating element (see below). With
the exception of these and some combinations using '[' (see next
paragraphs), all other special characters, including '\', lose
their special significance within a bracket expression.
Within a bracket expression, a collating element (a character, a
multicharacter sequence that collates as if it were a single
character, or a collating-sequence name for either) enclosed in
"[." and ".]" stands for the sequence of characters of that
collating element. The sequence is a single element of the
bracket expression's list. A bracket expression containing a
multicharacter collating element can thus match more than one
character, for example, if the collating sequence includes a "ch"
collating element, then the RE "[[.ch.]]*c" matches the first five
characters of "chchcc".
Within a bracket expression, a collating element enclosed in "[="
and "=]" is an equivalence class, standing for the sequences of
characters of all collating elements equivalent to that one,
including itself. (If there are no other equivalent collating
elements, the treatment is as if the enclosing delimiters were
"[." and ".]".) For example, if o and ô are the members of an
equivalence class, then "[[=o=]]", "[[=ô=]]", and "[oô]" are all
synonymous. An equivalence class may not(!) be an endpoint of a
range.
Within a bracket expression, the name of a character class
enclosed in "[:" and ":]" stands for the list of all characters
belonging to that class. Standard character class names are:
alnum digit punct
alpha graph space
blank lower upper
cntrl print xdigit
These stand for the character classes defined in wctype(3). A
locale may provide others. A character class may not be used as
an endpoint of a range.
In the event that an RE could match more than one substring of a
given string, the RE matches the one starting earliest in the
string. If the RE could match more than one substring starting at
that point, it matches the longest. Subexpressions also match the
longest possible substrings, subject to the constraint that the
whole match be as long as possible, with subexpressions starting
earlier in the RE taking priority over ones starting later. Note
that higher-level subexpressions thus take priority over their
lower-level component subexpressions.
Match lengths are measured in characters, not collating elements.
A null string is considered longer than no match at all. For
example, "bb*" matches the three middle characters of "abbbc",
"(wee|week)(knights|nights)" matches all ten characters of
"weeknights", when "(.*).*" is matched against "abc" the
parenthesized subexpression matches all three characters, and when
"(a*)*" is matched against "bc" both the whole RE and the
parenthesized subexpression match the null string.
If case-independent matching is specified, the effect is much as
if all case distinctions had vanished from the alphabet. When an
alphabetic that exists in multiple cases appears as an ordinary
character outside a bracket expression, it is effectively
transformed into a bracket expression containing both cases, for
example, 'x' becomes "[xX]". When it appears inside a bracket
expression, all case counterparts of it are added to the bracket
expression, so that, for example, "[x]" becomes "[xX]" and "[^x]"
becomes "[^xX]".
No particular limit is imposed on the length of REs(!). Programs
intended to be portable should not employ REs longer than 256
bytes, as an implementation can refuse to accept such REs and
remain POSIX-compliant.
Obsolete ("basic") regular expressions differ in several respects.
'|', '+', and '?' are ordinary characters and there is no
equivalent for their functionality. The delimiters for bounds are
"\{" and "\}", with '{' and '}' by themselves ordinary characters.
The parentheses for nested subexpressions are "\(" and "\)", with
'(' and ')' by themselves ordinary characters. '^' is an ordinary
character except at the beginning of the RE or(!) the beginning of
a parenthesized subexpression, '$' is an ordinary character except
at the end of the RE or(!) the end of a parenthesized
subexpression, and '*' is an ordinary character if it appears at
the beginning of the RE or the beginning of a parenthesized
subexpression (after a possible leading '^').
Finally, there is one new type of atom, a back reference: '\'
followed by a nonzero decimal digit d matches the same sequence of
characters matched by the dth parenthesized subexpression
(numbering subexpressions by the positions of their opening
parentheses, left to right), so that, for example, "\([bc]\)\1"
matches "bb" or "cc" but not "bc".
Having two kinds of REs is a botch.
The current POSIX.2 spec says that ')' is an ordinary character in
the absence of an unmatched '('; this was an unintentional result
of a wording error, and change is likely. Avoid relying on it.
Back references are a dreadful botch, posing major problems for
efficient implementations. They are also somewhat vaguely defined
(does "a\(\(b\)*\2\)*d" match "abbbd"?). Avoid using them.
POSIX.2's specification of case-independent matching is vague.
The "one case implies all cases" definition given above is current
consensus among implementors as to the right interpretation.
This page was taken from Henry Spencer's regex package.
grep(1), regex(3)
POSIX.2, section 2.8 (Regular Expression Notation).
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Linux man-pages 6.15 2025-05-17 regex(7)
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