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-.TH REGEX 7 "25 Oct 1995"
-.BY "Henry Spencer"
-.SH NAME
-regex \- 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 HISTORY
-Written by Henry Spencer, based on the 1003.2 spec.
-.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.