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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 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".
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 6.15 2025-05-17 regex(7)