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 pro-
grams; 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 fol-
lowed by ',' possibly followed by another unsigned decimal integer, al-
ways 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 ex-
ceed 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 (in-
clusive) 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 signifi-
cance (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 nor-
mally matches any single character from the list (but see below). If
the list begins with '^', it matches any single character (but see be-
low) 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 be-
tween 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 col-
lating-sequence-dependent, and portable programs should avoid relying on
them.
To include a literal ']' in the list, make it the first character (fol-
lowing 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 multi-
character 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 char-
acters 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 end-
point 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 expres-
sion 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 in-
tended 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 begin-
ning 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 ap-
pears 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 word-
ing error, and change is likely. Avoid relying on it.
Back references are a dreadful botch, posing major problems for effi-
cient 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).
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