regex.texi

正则表达式库

@menu
* Character Class Operators::   [:class:]
* Range Operator::          start-end
@end menu

@ignore
(If collating symbols and equivalence class expressions get implemented,
then add this.)

node Collating Symbol Operators
subsubsection Collating Symbol Operators (@code{[.} @dots{} @code{.]})

If the syntax bit @code{XX} is set, then you can represent
collating symbols inside lists.  You form a @dfn{collating symbol} by
putting a collating element between an @dfn{open-collating-symbol
operator} and an @dfn{close-collating-symbol operator}.  @samp{[.}
represents the open-collating-symbol operator and @samp{.]} represents
the close-collating-symbol operator.  For example, if @samp{ll} is a
collating element, then @samp{[[.ll.]]} would match @samp{ll}.

node Equivalence Class Operators
subsubsection Equivalence Class Operators (@code{[=} @dots{} @code{=]})
@cindex equivalence class expression in regex
@cindex @samp{[=} in regex
@cindex @samp{=]} in regex

If the syntax bit @code{XX} is set, then Regex recognizes equivalence class
expressions inside lists.  A @dfn{equivalence class expression} is a set
of collating elements which all belong to the same equivalence class.
You form an equivalence class expression by putting a collating
element between an @dfn{open-equivalence-class operator} and a
@dfn{close-equivalence-class operator}.  @samp{[=} represents the
open-equivalence-class operator and @samp{=]} represents the
close-equivalence-class operator.  For example, if @samp{a} and @samp{A}
were an equivalence class, then both @samp{[[=a=]]} and @samp{[[=A=]]}
would match both @samp{a} and @samp{A}.  If the collating element in an
equivalence class expression isn't part of an equivalence class, then
the matcher considers the equivalence class expression to be a collating
symbol.

@end ignore

@node Character Class Operators, Range Operator,  , List Operators
@subsection Character Class Operators (@code{[:} @dots{} @code{:]})

@cindex character classes
@cindex @samp{[:} in regex
@cindex @samp{:]} in regex

If the syntax bit @code{RE_CHARACTER_CLASSES} is set, then Regex
recognizes character class expressions inside lists.  A @dfn{character
class expression} matches one character from a given class.  You form a
character class expression by putting a character class name between an
@dfn{open-character-class operator} (represented by @samp{[:}) and a
@dfn{close-character-class operator} (represented by @samp{:]}).  The
character class names and their meanings are:

@table @code

@item alnum 
letters and digits

@item alpha
letters

@item blank
system-dependent; for @sc{gnu}, a space or tab

@item cntrl
control characters (in the @sc{ascii} encoding, code 0177 and codes
less than 040)

@item digit
digits

@item graph
same as @code{print} except omits space

@item lower 
lowercase letters

@item print
printable characters (in the @sc{ascii} encoding, space 
tilde---codes 040 through 0176)

@item punct
neither control nor alphanumeric characters

@item space
space, carriage return, newline, vertical tab, and form feed

@item upper
uppercase letters

@item xdigit
hexadecimal digits: @code{0}--@code{9}, @code{a}--@code{f}, @code{A}--@code{F}

@end table

@noindent
These correspond to the definitions in the C library's @file{<ctype.h>}
facility.  For example, @samp{[:alpha:]} corresponds to the standard
facility @code{isalpha}.  Regex recognizes character class expressions
only inside of lists; so @samp{[[:alpha:]]} matches any letter, but
@samp{[:alpha:]} outside of a bracket expression and not followed by a
repetition operator matches just itself.

@node Range Operator,  , Character Class Operators, List Operators
@subsection The Range Operator (@code{-})

Regex recognizes @dfn{range expressions} inside a list. They represent
those characters
that fall between two elements in the current collating sequence.  You
form a range expression by putting a @dfn{range operator} between two 
@ignore
(If these get implemented, then substitute this for ``characters.'')
of any of the following: characters, collating elements, collating symbols,
and equivalence class expressions.  The starting point of the range and
the ending point of the range don't have to be the same kind of item,
e.g., the starting point could be a collating element and the ending
point could be an equivalence class expression.  If a range's ending
point is an equivalence class, then all the collating elements in that
class will be in the range.
@end ignore
characters.@footnote{You can't use a character class for the starting
or ending point of a range, since a character class is not a single
character.} @samp{-} represents the range operator.  For example,
@samp{a-f} within a list represents all the characters from @samp{a}
through @samp{f}
inclusively.

If the syntax bit @code{RE_NO_EMPTY_RANGES} is set, then if the range's
ending point collates less than its starting point, the range (and the
regular expression containing it) is invalid.  For example, the regular
expression @samp{[z-a]} would be invalid.  If this bit isn't set, then
Regex considers such a range to be empty.

Since @samp{-} represents the range operator, if you want to make a
@samp{-} character itself
a list item, you must do one of the following:

@itemize @bullet
@item
Put the @samp{-} either first or last in the list.

@item
Include a range whose starting point collates strictly lower than
@samp{-} and whose ending point collates equal or higher.  Unless a
range is the first item in a list, a @samp{-} can't be its starting
point, but @emph{can} be its ending point.  That is because Regex
considers @samp{-} to be the range operator unless it is preceded by
another @samp{-}.  For example, in the @sc{ascii} encoding, @samp{)},
@samp{*}, @samp{+}, @samp{,}, @samp{-}, @samp{.}, and @samp{/} are
contiguous characters in the collating sequence.  You might think that
@samp{[)-+--/]} has two ranges: @samp{)-+} and @samp{--/}.  Rather, it
has the ranges @samp{)-+} and @samp{+--}, plus the character @samp{/}, so
it matches, e.g., @samp{,}, not @samp{.}.

@item
Put a range whose starting point is @samp{-} first in the list.

@end itemize

For example, @samp{[-a-z]} matches a lowercase letter or a hyphen (in
English, in @sc{ascii}).


@node Grouping Operators, Back-reference Operator, List Operators, Common Operators
@section Grouping Operators (@code{(} @dots{} @code{)} or @code{\(} @dots{} @code{\)})

@kindex (
@kindex )
@kindex \(
@kindex \)
@cindex grouping
@cindex subexpressions
@cindex parenthesizing

A @dfn{group}, also known as a @dfn{subexpression}, consists of an
@dfn{open-group operator}, any number of other operators, and a
@dfn{close-group operator}.  Regex treats this sequence as a unit, just
as mathematics and programming languages treat a parenthesized
expression as a unit.

Therefore, using @dfn{groups}, you can:

@itemize @bullet
@item
delimit the argument(s) to an alternation operator (@pxref{Alternation
Operator}) or a repetition operator (@pxref{Repetition
Operators}).

@item 
keep track of the indices of the substring that matched a given group.
@xref{Using Registers}, for a precise explanation.
This lets you:

@itemize @bullet
@item
use the back-reference operator (@pxref{Back-reference Operator}).

@item 
use registers (@pxref{Using Registers}).

@end itemize

@end itemize

If the syntax bit @code{RE_NO_BK_PARENS} is set, then @samp{(} represents
the open-group operator and @samp{)} represents the
close-group operator; otherwise, @samp{\(} and @samp{\)} do.

If the syntax bit @code{RE_UNMATCHED_RIGHT_PAREN_ORD} is set and a
close-group operator has no matching open-group operator, then Regex
considers it to match @samp{)}.


@node Back-reference Operator, Anchoring Operators, Grouping Operators, Common Operators
@section The Back-reference Operator (@dfn{\}@var{digit})

@cindex back references

If the syntax bit @code{RE_NO_BK_REF} isn't set, then Regex recognizes
back references.  A back reference matches a specified preceding group.
The back reference operator is represented by @samp{\@var{digit}}
anywhere after the end of a regular expression's @w{@var{digit}-th}
group (@pxref{Grouping Operators}).

@var{digit} must be between @samp{1} and @samp{9}.  The matcher assigns
numbers 1 through 9 to the first nine groups it encounters.  By using
one of @samp{\1} through @samp{\9} after the corresponding group's
close-group operator, you can match a substring identical to the
one that the group does.

Back references match according to the following (in all examples below,
@samp{(} represents the open-group, @samp{)} the close-group, @samp{@{}
the open-interval and @samp{@}} the close-interval operator):

@itemize @bullet
@item
If the group matches a substring, the back reference matches an
identical substring.  For example, @samp{(a)\1} matches @samp{aa} and
@samp{(bana)na\1bo\1} matches @samp{bananabanabobana}.  Likewise,
@samp{(.*)\1} matches any (newline-free if the syntax bit
@code{RE_DOT_NEWLINE} isn't set) string that is composed of two
identical halves; the @samp{(.*)} matches the first half and the
@samp{\1} matches the second half.

@item
If the group matches more than once (as it might if followed
by, e.g., a repetition operator), then the back reference matches the
substring the group @emph{last} matched.  For example,
@samp{((a*)b)*\1\2} matches @samp{aabababa}; first @w{group 1} (the
outer one) matches @samp{aab} and @w{group 2} (the inner one) matches
@samp{aa}.  Then @w{group 1} matches @samp{ab} and @w{group 2} matches
@samp{a}.  So, @samp{\1} matches @samp{ab} and @samp{\2} matches
@samp{a}.

@item
If the group doesn't participate in a match, i.e., it is part of an
alternative not taken or a repetition operator allows zero repetitions
of it, then the back reference makes the whole match fail.  For example,
@samp{(one()|two())-and-(three\2|four\3)} matches @samp{one-and-three}
and @samp{two-and-four}, but not @samp{one-and-four} or
@samp{two-and-three}.  For example, if the pattern matches
@samp{one-and-}, then its @w{group 2} matches the empty string and its
@w{group 3} doesn't participate in the match.  So, if it then matches
@samp{four}, then when it tries to back reference @w{group 3}---which it
will attempt to do because @samp{\3} follows the @samp{four}---the match
will fail because @w{group 3} didn't participate in the match.

@end itemize

You can use a back reference as an argument to a repetition operator.  For
example, @samp{(a(b))\2*} matches @samp{a} followed by two or more
@samp{b}s.  Similarly, @samp{(a(b))\2@{3@}} matches @samp{abbbb}.

If there is no preceding @w{@var{digit}-th} subexpression, the regular
expression is invalid.


@node Anchoring Operators,  , Back-reference Operator, Common Operators
@section Anchoring Operators    

@cindex anchoring
@cindex regexp anchoring

These operators can constrain a pattern to match only at the beginning or
end of the entire string or at the beginning or end of a line.

@menu
* Match-beginning-of-line Operator::  ^
* Match-end-of-line Operator::        $
@end menu


@node Match-beginning-of-line Operator, Match-end-of-line Operator,  , Anchoring Operators
@subsection The Match-beginning-of-line Operator (@code{^})

@kindex ^
@cindex beginning-of-line operator
@cindex anchors

This operator can match the empty string either at the beginning of the
string or after a newline character.  Thus, it is said to @dfn{anchor}
the pattern to the beginning of a line.

In the cases following, @samp{^} represents this operator.  (Otherwise,
@samp{^} is ordinary.)

@itemize @bullet

@item
It (the @samp{^}) is first in the pattern, as in @samp{^foo}.

@cnindex RE_CONTEXT_INDEP_ANCHORS @r{(and @samp{^})}
@item
The syntax bit @code{RE_CONTEXT_INDEP_ANCHORS} is set, and it is outside
a bracket expression.

@cindex open-group operator and @samp{^}
@cindex alternation operator and @samp{^}
@item
It follows an open-group or alternation operator, as in @samp{a\(^b\)}
and @samp{a\|^b}.  @xref{Grouping Operators}, and @ref{Alternation
Operator}.

@end itemize

These rules imply that some valid patterns containing @samp{^} cannot be
matched; for example, @samp{foo^bar} if @code{RE_CONTEXT_INDEP_ANCHORS}
is set.

@vindex not_bol @r{field in pattern buffer}
If the @code{not_bol} field is set in the pattern buffer (@pxref{GNU
Pattern Buffers}), then @samp{^} fails to match at the beginning of the
string.  @xref{POSIX Matching}, for when you might find this useful.

@vindex newline_anchor @r{field in pattern buffer}
If the @code{newline_anchor} field is set in the pattern buffer, then
@samp{^} fails to match after a newline.  This is useful when you do not
regard the string to be matched as broken into lines.


@node Match-end-of-line Operator,  , Match-beginning-of-line Operator, Anchoring Operators
@subsection The Match-end-of-line Operator (@code{$})

@kindex $
@cindex end-of-line operator
@cindex anchors

This operator can match the empty string either at the end of
the string or before a newline character in the string.  Thus, it is
said to @dfn{anchor} the pattern to the end of a line.

It is always represented by @samp{$}.  For example, @samp{foo$} usually