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 ^ are the members of an
equivalence class, then "
[[=o=]]",
"
[[=^=]]", and "
[o^]" 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).