REGEX(7) Linux Programmer's 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; 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".
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).
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
2009-01-12 REGEX(7)
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