/* Void Main's man pages */
{ phpMan } else { main(); }
REGEX(7) Linux Programmer's 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; 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 implementa-
tions.
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,
etc.
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 char-
acter(!) (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 ille-
gal(!) for two ranges to share an endpoint, for example, "a-c-e". Ranges are very collating-sequence-dependent, and por-
table 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 signifi-
cance 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 charac-
ters 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 charac-
ters 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. Subexpres-
sions 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 subex-
pressions 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 exam-
ple, "[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 ordi-
nary 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 subexpres-
sion (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 uninten-
tional 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 3.25 of the Linux man-pages project. A description of the project, and information about
reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
2009-01-12 REGEX(7)
