Regular Expression Syntax
NAME
re_format - 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, sepa
rated 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 fol
lowed 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-) inclu
sive, 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 contain
ing 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), `^' (match
ing 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 `\' fol
lowed 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 signifi
cance (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 sepa
rated 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 lit
eral `-', make it the first or last character, or the sec
ond 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 excep
tion of these and some combinations using `[' (see next
paragraphs), all other special characters, including `\',
lose their special significance within a bracket expres
sion.
Within a bracket expression, a collating element (a char
acter, 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 characters of that collating element. The sequence is
a single element of the bracket expression's list. A
bracket expression 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 equiva
lent 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 syn
onymous. 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 char
acters 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
caution in software intended to be portable to other sys
tems.
In the event that an RE could match more than one sub
string 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 ear
lier 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, e.g. `x' becomes `[xX]'.
When it appears inside a bracket expression, all case
counterparts 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-.
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. `|' is an ordinary character and there is no
equivalent for its functionality. `+' and `?' are ordi
nary characters, and their functionality can be expressed
using bounds ({1,} or {0,1} respectively). Also note that
`x+' in modern REs is equivalent to `xx*'. The delimiters
for bounds are `\{' and `\}', with `{' and `}' by them
selves ordinary characters. The parentheses for nested
subexpressions are `\(' and `\)', with `(' and `)' by
themselves ordinary characters. `^' is an ordinary char
acter except at the beginning of the RE or- the beginning
of a parenthesized subexpression, `$' is an ordinary char
acter except at the end of the RE or- the end of a paren
thesized 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 ref
erence: `\' followed by a non-zero decimal digit d matches
the same sequence of characters matched by the dth paren
thesized 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
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 char
acter 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 prob
lems for efficient implementations. They are also some
what 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.