REGEX(7) (2020-08-13) 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; 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, sepa-
rated by aq|aq. 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(!) aq*aq,
aq+aq, aq?aq, or bound. An atom followed by aq*aq matches a
sequence of 0 or more matches of the atom. An atom followed
by aq+aq matches a sequence of 1 or more matches of the atom.
An atom followed by aq?aq matches a sequence of 0 or 1 matches
of the atom.
A bound is aq{aq followed by an unsigned decimal integer, pos-
sibly followed by aq,aq possibly followed by another unsigned
decimal integer, always followed by aq}aq. 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), aq.aq (matching any single character), aqhaaq (matching
the null string at the beginning of a line), aq$aq (matching
the null string at the end of a line), a aq\aq followed by one
of the characters "ha.[$()|*+?{\" (matching that character
taken as an ordinary character), a aq\aq followed by any other
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character(!) (matching that character taken as an ordinary
character, as if the aq\aq had not been present(!)), or a sin-
gle character with no other significance (matching that
character). A aq{aq 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 aq\aq.
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 aqhaaq, it
matches any single character (but see below) not from the
rest of the list. If two characters in the list are sepa-
rated by aq-aq, this is shorthand for the full range of char-
acters 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 end-
point, for example, "a-c-e". Ranges are very collating-
sequence-dependent, and portable programs should avoid rely-
ing on them.
To include a literal aq]aq in the list, make it the first
character (following a possible aqhaaq). To include a literal
aq-aq, make it the first or last character, or the second end-
point of a range. To use a literal aq-aq as the first end-
point of a range, enclose it in "[." and ".]" to make it a
collating element (see below). With the exception of these
and some combinations using aq[aq (see next paragraphs), all
other special characters, including aq\aq, lose their special
significance within a bracket expression.
Within a bracket expression, a collating element (a charac-
ter, 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 charac-
ters of that collating element. The sequence is a single
element of the bracket expression's list. A bracket expres-
sion 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 oha are the members of an equivalence class, then
"[[=o=]]", "[[=oha=]]", and "[ooha]" are all synonymous. An
equivalence class may not(!) be an endpoint of a range.
Within a bracket expression, the name of a character class
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enclosed in "[:" and ":]" stands for the list of all charac-
ters belonging to that class. Standard character class
names are:
l l l. 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 sub-
string 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 ele-
ments. 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 expres-
sion, it is effectively transformed into a bracket expres-
sion containing both cases, for example, aqxaq becomes "[xX]".
When it appears inside a bracket expression, all case coun-
terparts of it are added to the bracket expression, so that,
for example, "[x]" becomes "[xX]" and "[hax]" becomes
"[haxX]".
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. aq|aq, aq+aq, and aq?aq are ordinary characters and
there is no equivalent for their functionality. The
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delimiters for bounds are "\{" and "\}", with aq{aq and aq}aq by
themselves ordinary characters. The parentheses for nested
subexpressions are "\(" and "\)", with aq(aq and aq)aq by them-
selves ordinary characters. aqhaaq is an ordinary character
except at the beginning of the RE or(!) the beginning of a
parenthesized subexpression, aq$aq is an ordinary character
except at the end of the RE or(!) the end of a parenthesized
subexpression, and aq*aq is an ordinary character if it
appears at the beginning of the RE or the beginning of a
parenthesized subexpression (after a possible leading aqhaaq).
Finally, there is one new type of atom, a back reference:
aq\aq 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 aq)aq is an ordinary char-
acter in the absence of an unmatched aq(aq; this was an unin-
tentional 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).
COLOPHON
This page is part of release 5.10 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/.
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