re_format - POSIX 1003.2 regular expressions
Regular expressions (``RE''s), as defined in POSIX 1003.2,
come in two
forms: modern REs (roughly those of egrep(1); 1003.2 calls
these ``extended''
REs) and obsolete REs (roughly those of ed(1);
1003.2 ``basic''
REs). Obsolete REs mostly exist for backward compatibility
in some old
programs; 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, 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, 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 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 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 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 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 characters
starting and ending with a word character 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 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 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, 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 POSIXcompliant.
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 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.) `c]1' matches `bb' or
`cc'
but not `bc'.
regex(3)
POSIX 1003.2, section 2.8 (Regular Expression Notation).
Having two kinds of REs is a botch.
The current 1003.2 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
`ab*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.
OpenBSD 3.6 March 20, 1994
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