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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).