Originální popis anglicky:
regex - 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; 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 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 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
wctype(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
wctype(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 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 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'.
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
`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.
This page was taken from Henry Spencer's regex package.
