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<A HREF="#toc">Table of Contents</A><P>
<H2><A NAME="sect0" HREF="#toc0">Name</A></H2>
regex - POSIX 1003.2 regular expressions
<H2><A NAME="sect1" HREF="#toc1">Description</A></H2>
Regular expressions
(``RE''s), as defined in POSIX 1003.2, come in two forms: modern REs (roughly
those of <I>egrep</I>; 1003.2 calls these ``extended'' REs) and obsolete REs (roughly
those of <I>ed</I>; 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. <P>
A (modern)
RE is one* or more non-empty* <I>branches</I>, separated by `|'. It matches anything
that matches one of the branches. <P>
A branch is one* or more <I>pieces</I>, concatenated.
It matches a match for the first, followed by a match for the second, etc.
<P>
A piece is an <I>atom</I> possibly followed by a single* `*', `+', `?', or <I>bound</I>. 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. <P>
A <I>bound</I>
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>i</I> and no comma matches a sequence of exactly
<I>i</I> matches of the atom. An atom followed by a bound containing one integer
<I>i</I> and a comma matches a sequence of <I>i</I> or more matches of the atom. An atom
followed by a bound containing two integers <I>i</I> and <I>j</I> matches a sequence
of <I>i</I> through <I>j</I> (inclusive) matches of the atom. <P>
An atom is a regular expression
enclosed in `()' (matching a match for the regular expression), an empty
set of `()' (matching the null string)*, a <I>bracket expression</I> (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 `\'. <P>
A <I>bracket
expression</I> 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) <I>not</I> from the rest of
the list. If two characters in the list are separated by `-', this is shorthand
for the full <I>range</I> 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. <P>
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. <P>
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'. <P>
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 `[oo'o^']' are all synonymous. An equivalence class
may not* be an endpoint of a range. <P>
Within a bracket expression, the name
of a <I>character class</I> enclosed in `[:' and `:]' stands for the list of all characters
belonging to that class. Standard character class names are: <P>
<blockquote><BR>
<PRE>alnum<tt> </tt> <tt> </tt> digit<tt> </tt> <tt> </tt> punct
alpha<tt> </tt> <tt> </tt> graph<tt> </tt> <tt> </tt> space
blank<tt> </tt> <tt> </tt> lower<tt> </tt> <tt> </tt> upper
cntrl<tt> </tt> <tt> </tt> print<tt> </tt> <tt> </tt> xdigit
</PRE></blockquote>
<P>
These stand for the character classes defined in <I><A HREF="ctype.3.html">ctype</I>(3)</A>
. A locale may
provide others. A character class may not be used as an endpoint of a range.
<P>
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
<I>alnum</I> character (as defined by <I><A HREF="ctype.3.html">ctype</I>(3)</A>
) 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. <P>
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.
<P>
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. <P>
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]'. <P>
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. <P>
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 <I>back reference</I>: `\' followed by
a non-zero decimal digit <I>d</I> matches the same sequence of characters matched
by the <I>d</I>th 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'.
<H2><A NAME="sect2" HREF="#toc2">See Also</A></H2>
<A HREF="regex.3.html">regex(3)</A>
<P>
POSIX 1003.2, section 2.8 (Regular
Expression Notation).
<H2><A NAME="sect3" HREF="#toc3">History</A></H2>
Written by Henry Spencer, based on the 1003.2
spec.
<H2><A NAME="sect4" HREF="#toc4">Bugs</A></H2>
Having two kinds of REs is a botch. <P>
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. <P>
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. <P>
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.
<P>
The syntax for word boundaries is incredibly ugly. <P>
<HR><P>
<A NAME="toc"><B>Table of Contents</B></A><P>
<UL>
<LI><A NAME="toc0" HREF="#sect0">Name</A></LI>
<LI><A NAME="toc1" HREF="#sect1">Description</A></LI>
<LI><A NAME="toc2" HREF="#sect2">See Also</A></LI>
<LI><A NAME="toc3" HREF="#sect3">History</A></LI>
<LI><A NAME="toc4" HREF="#sect4">Bugs</A></LI>
</UL>
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