regex(7) Miscellaneous Information Manual regex(7)
NAME
regex - POSIX.2 regular expressions
DESCRIPTION
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 o are the members of an equivalence class, then "[[=o=]]",
"[[=o=]]", and "[oo]" 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".
BUGS
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.
AUTHOR
This page was taken from Henry Spencer's regex package.
SEE ALSO
grep(1), regex(3)
POSIX.2, section 2.8 (Regular Expression Notation).
Linux man-pages 6.14 2025-05-06 regex(7)