NAME
regex - POSIX 1003.2 regular expressions
DESCRIPTION
Regular expressions (``RE''s), as defined in POSIX 1003.2,
come in two forms: modern REs (roughly those of egrep;
1003.2 calls these ``extended'' REs) and obsolete REs
(roughly those of ed; 1003.2 ``basic'' REs). Obsolete REs
mostly exist for backward compatibility in some old pro-
grams; they will be discussed at the end. 1003.2 leaves
some aspects of RE syntax and semantics open; `-' marks
decisions on these aspects that may not be fully portable to
other 1003.2 implementations.
A (modern) RE is one- or more non-empty- 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, pos-
sibly 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 charac-
ter). 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, e.g. `[0-9]' in ASCII matches any decimal digit.
It is illegal- for two ranges to share an endpoint, e.g.
`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 end-
point of a range. To use a literal `-' 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 `[' (see next paragraphs), all
other special characters, including `\', lose their special
significance within a bracket expression.
Within a bracket expression, a collating element (a charac-
ter, 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 charac-
ters of that collating element. The sequence is a single
element of the bracket expression's list. A bracket expres-
sion containing a multi-character collating element can thus
match more than one character, e.g. 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 ctype(3).
A locale may provide others. A character class may not be
used as an endpoint of a range.
There are two special cases- of bracket expressions: the
bracket expressions `[[:<:]]' and `[[:>:]]' match the null
string at the beginning and end of a word respectively. A
word is defined as a sequence of word characters which is
neither preceded nor followed by word characters. A word
character is an alnum character (as defined by ctype(3)) or
an underscore. This is an extension, compatible with but
not specified by POSIX 1003.2, and should be used with cau-
tion in software intended to be portable to other systems.
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, e.g. `x' becomes `[xX]'. When
it appears inside a bracket expression, all case counter-
parts of it are added to the bracket expression, so that
(e.g.) `[x]' becomes `[xX]' and `[^x]' becomes `[^xX]'.
No particular limit is imposed on the length of REs-. Pro-
grams 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 delim-
iters for bounds are `\{' and `\}', with `{' and `}' by
themselves ordinary characters. The parentheses for nested
subexpressions are `\(' and `\)', with `(' and `)' by them-
selves 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 non-zero 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 (e.g.)
`\([bc]\)\1' matches `bb' or `cc' but not `bc'.
SEE ALSO
regex(3)
POSIX 1003.2, section 2.8 (Regular Expression Notation).
BUGS
Having two kinds of REs is a botch.
The current 1003.2 spec says that `)' is an ordinary charac-
ter 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.
1003.2'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.
The syntax for word boundaries is incredibly ugly.
AUTHOR
This page was taken from Henry Spencer's regex package.