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RE_FORMAT(7) Miscellaneous Information Manual RE_FORMAT(7)
NAME
re_format - POSIX 1003.2 regular expressions
DESCRIPTION
Regular expressions ("REs"), as defined in IEEE Std 1003.2 ("POSIX.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. IEEE Std 1003.2
("POSIX.2") leaves some aspects of RE syntax and semantics open; `<*>'
marks decisions on these aspects that may not be fully portable to other
IEEE Std 1003.2 ("POSIX.2") implementations.
Extended regular expressions
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 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 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 `x' and `y'
are the members of an equivalence class, then `[[=x=]]', `[[=y=]]', and
`[xy]' 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.
A bracketed expression like `[[:class:]]' can be used to match a single
character that belongs to a character class. The reverse, matching any
character that does not belong to a specific class, the negation operator
of bracket expressions may be used: `[^[:class:]]'.
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 IEEE Std 1003.2 ("POSIX.2"), and should be used with
caution in software intended to be portable to other systems. The
additional word delimiters `\<' and `\>' are provided to ease
compatibility with traditional SVR4 systems but are not portable and
should be avoided.
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 POSIX-
compliant.
Basic regular expressions
Obsolete ("basic") regular expressions differ in several respects. `|'
is an ordinary character and there is no equivalent for its
functionality. `+' and `?' are ordinary 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
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.) `\([bc]\)\1'
matches `bb' or `cc' but not `bc'.
SEE ALSO
regex(3)
Regular Expression Notation, IEEE Std, 1003.2, section 2.8.
BUGS
Having two kinds of REs is a botch.
The current IEEE Std 1003.2 ("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.
IEEE Std 1003.2 ("POSIX.2") 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.
NetBSD 10.99 February 22, 2021 NetBSD 10.99