| RE_FORMAT(7) | Miscellaneous Information Manual | RE_FORMAT(7) |
re_format — POSIX
1003.2 regular expressions
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.
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 | 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.
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. ‘|’ 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’.
When the REG_ENHANCED flag is passed to
one of the
regcomp()
variants, additional features are activated. Like the enhanced
regex implementations in scripting languages such as
perl(1) and python(1), these additional
features may conflict with the IEEE Std 1003.2
(“POSIX.2”) standards in some ways. Use this with care
in situations which require portability (including to past versions of the
Mac OS X using the previous regex
implementation).
For enhanced basic REs, ‘+’,
‘?’ and
‘|’ remain regular characters, but
‘\+’,
‘\?’ and
‘\|’ have the same special meaning as
the unescaped characters do for extended REs, i.e., one or more matches,
zero or one matches and alteration, respectively. For enhanced extended REs,
back references are available. Additional enhanced features are listed
below.
Within a bracket expression, most characters lose their magic. This also applies to the additional enhanced features, which don't operate inside a bracket expression.
In addition to ‘^’ and
‘$’ (the assertions that match the
null string at the beginning and end of line, respectively), the following
assertions become available:
[[:<:]]’.[[:>:]]’.\b’.The following shortcuts can be used to replace more complicated bracket expressions.
[[:digit:]]’.[^[:digit:]]’.[[:space:]]’.[^[:space:]]’.[[:alnum:]_]’.[^[:alnum:]_]’.Literals are normally just ordinary characters that are matched directly. Under enhanced mode, certain character sequences are converted to specific literals.
Literals can also be specified directly, using their wide character values. Note that when matching a multibyte character string, the string's bytes are converted to wide character before comparing. This means that a single literal wide character value may match more than one string byte, depending on the locale's wide character encoding.
A ‘\Q’ sequence causes
literal (“quote”) mode to be entered, while
‘\E’ ends literal mode, and returns to
normal regular expression processing. This is similar to specifying the
REG_NOSPEC (or REG_LITERAL)
option to
regcomp(),
except that rather than applying to the whole RE string, it only applies to
the part between the ‘\Q’ and
‘\E’. Note that it is not possible to
have a ‘\E’ in the middle of an inline
literal range, as that would terminate literal mode prematurely.
By default, the repetition operators,
‘*’, bound,
‘?’ and
‘+’ are
greedy;
they try to match as many times as possible. In enhanced mode, appending a
‘?’ to a repetition operator makes it
minimal (or
ungreedy);
it tries to match the fewest number of times (including zero times, as
appropriate).
For example, against the string
‘aaa’, the RE
‘a*’ would match the entire string,
while ‘a*?’ would match the null
string at the beginning of the line (matches zero times). Likewise, against
the string ‘ababab’, the RE
‘.*b’, would also match the entire
string, while ‘.*?b’ would only match
the first two characters.
The
regcomp()
flag REG_UNGREEDY will make the regular (greedy)
repetition operators ungreedy by default. Appending
‘?’ makes them greedy again.
Note that minimal repetitions are not specified by an official
standard, so there may be differences between different implementations. In
the current implementation, minimal repetitions have a high precedence, and
can cause other standards requirements to be violated. For instance, on the
string ‘aaaaa’, the RE
‘(aaa??)*’ will only match the first
four characters, violating the rules that the longest possible match is made
and the longest subexpressions are matched. Using
‘(aaa??)*$’ forces the entire string
to be matched.
Normally, the match offsets to parenthesized subexpressions are
recorded in the pmatch array (that is, when
REG_NOSUB is not specified, and
nmatch is large enough to encompass the parenthesized
subexpression in question). In enhanced mode, if the first two characters
following the left parenthesis are
‘?:’, grouping of the remaining
contents is done, but the corresponding offsets are not recorded in the
pmatch array. For example, against the string
‘fubar’, the RE
‘(fu)(bar)’ would have two
subexpression matches in pmatch; the first for
‘fu’ and the second for
‘bar’. But with the RE
‘(?:fu)(bar)’, there would only be one
subexpression match, that of ‘bar’.
Furthermore, against the string
‘fufubar’, the RE
‘(?fu)*(bar)’ would again match the
entire string, but only ‘bar’ would be
recorded in pmatch.
Like the inline literal mode mentioned above, other options can be
switched on and off for part of a RE.
‘(?o..)’ will turn
on the options specified in o.. (one or more options
characters; see below), while
‘(?-o..)’ will turn
off the specified options, and
‘(?o1..-o2..)’
will turn on the first set of options, and turn off the second set.
The available options are:
REG_ICASE
was specified to
regcomp(),
this option can be use to turn that off.REG_NEWLINE was specified to
regcomp(), this option can be use to turn that
off.REG_UNGREEDY was specified to
regcomp(), this option can be use to turn that
off.The scope of the option change begins immediately following the
right parenthesis, but up to the end of the enclosing subexpression (if
any). Thus, for example, given the RE
‘(fu(?i)bar)baz’, the
‘fu’ portion matches case sensitively,
‘bar’ matches case insensitively, and
‘baz’ matches case sensitively again
(since is it outside the scope of the subexpression in which the inline
option was specified).
The inline options syntax can be combined with the non-capturing
parenthesized subexpression to limit the option scope to just that of the
subexpression. Then, for example,
‘fu(?i:bar)baz’ is similar to the
previous example, except for the parenthesize subexpression around
‘fu(?i)bar’ in the previous
example.
The syntax
‘(?#comment)’ can be
used to embed comments within a RE. Note that comment can
not contain a right parenthesis. Also note that while syntactically, option
characters can be added before the ‘#’
character, they will be ignored.
Regular Expression Notation, IEEE Std, 1003.2, section 2.8.
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 bracket syntax for word boundaries is incredibly ugly.
| Sept 29, 2011 | Mac OS X 12 |