.TH RE_FORMAT 7 "7 Feb 1994"
.BY "Henry Spencer"
.SH NAME
re_format \- POSIX 1003.2 regular expressions
.SH DESCRIPTION
Regular expressions (``RE''s),
as defined in POSIX 1003.2, come in two forms:
modern REs (roughly those of
.IR egrep ;
1003.2 calls these ``extended'' REs)
and obsolete REs (roughly those of
.IR ed ;
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;
`\(dg' marks decisions on these aspects that
may not be fully portable to other 1003.2 implementations.
.PP
A (modern) RE is one\(dg or more non-empty\(dg \fIbranches\fR,
separated by `|'.
It matches anything that matches one of the branches.
.PP
A branch is one\(dg or more \fIpieces\fR, concatenated.
It matches a match for the first, followed by a match for the second, etc.
.PP
A piece is an \fIatom\fR possibly followed
by a single\(dg `*', `+', `?', or \fIbound\fR.
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.
.PP
A \fIbound\fR 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\(dg) inclusive,
and if there are two of them, the first may not exceed the second.
An atom followed by a bound containing one integer \fIi\fR
and no comma matches
a sequence of exactly \fIi\fR matches of the atom.
An atom followed by a bound
containing one integer \fIi\fR and a comma matches
a sequence of \fIi\fR or more matches of the atom.
An atom followed by a bound
containing two integers \fIi\fR and \fIj\fR matches
a sequence of \fIi\fR through \fIj\fR (inclusive) matches of the atom.
.PP
An atom is a regular expression enclosed in `()' (matching a match for the
regular expression),
an empty set of `()' (matching the null string)\(dg,
a \fIbracket expression\fR (see below), `.'
(matching any single character), `^' (matching the null string at the
beginning of a line), `$' (matching the null string at the
end of a line), a `\e' followed by one of the characters
`^.[$()|*+?{\e'
(matching that character taken as an ordinary character),
a `\e' followed by any other character\(dg
(matching that character taken as an ordinary character,
as if the `\e' had not been present\(dg),
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\(dg.
It is illegal to end an RE with `\e'.
.PP
A \fIbracket expression\fR 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) \fInot\fR from the rest of the list.
If two characters in the list are separated by `\-', this is shorthand
for the full \fIrange\fR of characters between those two (inclusive) in the
collating sequence,
e.g. `[0-9]' in ASCII matches any decimal digit.
It is illegal\(dg 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.
.PP
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 `\e', lose their
special significance within a bracket expression.
.PP
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'.
.PP
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 \o'o^' are the members of an equivalence class,
then `[[=o=]]', `[[=\o'o^'=]]', and `[o\o'o^']' are all synonymous.
An equivalence class may not\(dg be an endpoint
of a range.
.PP
Within a bracket expression, the name of a \fIcharacter class\fR enclosed
in `[:' and `:]' stands for the list of all characters belonging to that
class.
Standard character class names are:
.PP
.RS
.nf
.ta 3c 6c 9c
alnum	digit	punct
alpha	graph	space
blank	lower	upper
cntrl	print	xdigit
.fi
.RE
.PP
These stand for the character classes defined in
.IR ctype (3).
A locale may provide others.
A character class may not be used as an endpoint of a range.
.PP
There are two special cases\(dg 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
.I alnum
character (as defined by
.IR 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.
.PP
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.
.PP
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.
.PP
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]'.
.PP
No particular limit is imposed on the length of REs\(dg.
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.
.PP
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 `\e{' and `\e}',
with `{' and `}' by themselves ordinary characters.
The parentheses for nested subexpressions are `\e(' and `\e)',
with `(' and `)' by themselves ordinary characters.
`^' is an ordinary character except at the beginning of the
RE or\(dg the beginning of a parenthesized subexpression,
`$' is an ordinary character except at the end of the
RE or\(dg 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 \fIback reference\fR:
`\e' followed by a non-zero decimal digit \fId\fR
matches the same sequence of characters
matched by the \fId\fRth parenthesized subexpression
(numbering subexpressions by the positions of their opening parentheses,
left to right),
so that (e.g.) `\e([bc]\e)\e1' matches `bb' or `cc' but not `bc'.
.SH SEE ALSO
regex(3)
.PP
POSIX 1003.2, section 2.8 (Regular Expression Notation).
.SH BUGS
Having two kinds of REs is a botch.
.PP
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.
.PP
Back references are a dreadful botch,
posing major problems for efficient implementations.
They are also somewhat vaguely defined
(does
`a\e(\e(b\e)*\e2\e)*d' match `abbbd'?).
Avoid using them.
.PP
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.
.PP
The syntax for word boundaries is incredibly ugly.