xregex.texi

正则表达式库

isn't set) @samp{+} and @samp{?} (or @samp{\+} and @samp{\?}, depending
on the syntax bit @code{RE_BK_PLUS_QM}) represent repetition operators
only if they're not first in a regular expression or just after an
open-group or alternation operator.  The same holds for @samp{@{} (or
@samp{\@{}, depending on the syntax bit @code{RE_NO_BK_BRACES}) if
it is the beginning of a valid interval and the syntax bit
@code{RE_INTERVALS} is set.

@cnindex RE_CONTEXT_INVALID_OPS
@item RE_CONTEXT_INVALID_OPS
If this bit is set, then repetition and alternation operators can't be
in certain positions within a regular expression.  Specifically, the
regular expression is invalid if it has:

@itemize @bullet

@item
a repetition operator first in the regular expression or just after a
match-beginning-of-line, open-group, or alternation operator; or

@item
an alternation operator first or last in the regular expression, just
before a match-end-of-line operator, or just after an alternation or
open-group operator.

@end itemize

If this bit isn't set, then you can put the characters representing the
repetition and alternation characters anywhere in a regular expression.
Whether or not they will in fact be operators in certain positions
depends on other syntax bits.

@cnindex RE_DOT_NEWLINE
@item RE_DOT_NEWLINE
If this bit is set, then the match-any-character operator matches
a newline; if this bit isn't set, then it doesn't.

@cnindex RE_DOT_NOT_NULL
@item RE_DOT_NOT_NULL
If this bit is set, then the match-any-character operator doesn't match
a null character; if this bit isn't set, then it does.

@cnindex RE_INTERVALS
@item RE_INTERVALS
If this bit is set, then Regex recognizes interval operators; if this bit
isn't set, then it doesn't.

@cnindex RE_LIMITED_OPS
@item RE_LIMITED_OPS
If this bit is set, then Regex doesn't recognize the match-one-or-more,
match-zero-or-one or alternation operators; if this bit isn't set, then
it does.

@cnindex RE_NEWLINE_ALT
@item RE_NEWLINE_ALT
If this bit is set, then newline represents the alternation operator; if
this bit isn't set, then newline is ordinary.

@cnindex RE_NO_BK_BRACES
@item RE_NO_BK_BRACES
If this bit is set, then @samp{@{} represents the open-interval operator
and @samp{@}} represents the close-interval operator; if this bit isn't
set, then @samp{\@{} represents the open-interval operator and
@samp{\@}} represents the close-interval operator.  This bit is relevant
only if @code{RE_INTERVALS} is set.

@cnindex RE_NO_BK_PARENS
@item RE_NO_BK_PARENS
If this bit is set, then @samp{(} represents the open-group operator and
@samp{)} represents the close-group operator; if this bit isn't set, then
@samp{\(} represents the open-group operator and @samp{\)} represents
the close-group operator.

@cnindex RE_NO_BK_REFS
@item RE_NO_BK_REFS
If this bit is set, then Regex doesn't recognize @samp{\}@var{digit} as
the back reference operator; if this bit isn't set, then it does.

@cnindex RE_NO_BK_VBAR
@item RE_NO_BK_VBAR
If this bit is set, then @samp{|} represents the alternation operator;
if this bit isn't set, then @samp{\|} represents the alternation
operator.  This bit is irrelevant if @code{RE_LIMITED_OPS} is set.

@cnindex RE_NO_EMPTY_RANGES
@item RE_NO_EMPTY_RANGES
If this bit is set, then a regular expression with a range whose ending
point collates lower than its starting point is invalid; if this bit
isn't set, then Regex considers such a range to be empty.

@cnindex RE_UNMATCHED_RIGHT_PAREN_ORD
@item RE_UNMATCHED_RIGHT_PAREN_ORD
If this bit is set and the regular expression has no matching open-group
operator, then Regex considers what would otherwise be a close-group
operator (based on how @code{RE_NO_BK_PARENS} is set) to match @samp{)}.

@end table


@node Predefined Syntaxes, Collating Elements vs. Characters, Syntax Bits, Regular Expression Syntax
@section Predefined Syntaxes    

If you're programming with Regex, you can set a pattern buffer's
(@pxref{GNU Pattern Buffers}, and @ref{POSIX Pattern Buffers})
@code{syntax} field either to an arbitrary combination of syntax bits
(@pxref{Syntax Bits}) or else to the configurations defined by Regex.
These configurations define the syntaxes used by certain
programs---@sc{gnu} Emacs,
@cindex Emacs 
@sc{posix} Awk,
@cindex POSIX Awk
traditional Awk, 
@cindex Awk
Grep,
@cindex Grep
@cindex Egrep
Egrep---in addition to syntaxes for @sc{posix} basic and extended
regular expressions.

The predefined syntaxes--taken directly from @file{regex.h}---are:

@example
[[[ syntaxes ]]]
@end example

@node Collating Elements vs. Characters, The Backslash Character, Predefined Syntaxes, Regular Expression Syntax
@section Collating Elements vs.@: Characters    

@sc{posix} generalizes the notion of a character to that of a
collating element.  It defines a @dfn{collating element} to be ``a
sequence of one or more bytes defined in the current collating sequence
as a unit of collation.''

This generalizes the notion of a character in
two ways.  First, a single character can map into two or more collating
elements.  For example, the German
@tex
`\ss'
@end tex
@ifinfo
``es-zet''
@end ifinfo
collates as the collating element @samp{s} followed by another collating
element @samp{s}.  Second, two or more characters can map into one
collating element.  For example, the Spanish @samp{ll} collates after
@samp{l} and before @samp{m}.

Since @sc{posix}'s ``collating element'' preserves the essential idea of
a ``character,'' we use the latter, more familiar, term in this document.

@node The Backslash Character,  , Collating Elements vs. Characters, Regular Expression Syntax
@section The Backslash Character

@cindex \
The @samp{\} character has one of four different meanings, depending on
the context in which you use it and what syntax bits are set
(@pxref{Syntax Bits}).  It can: 1) stand for itself, 2) quote the next
character, 3) introduce an operator, or 4) do nothing.

@enumerate
@item
It stands for itself inside a list
(@pxref{List Operators}) if the syntax bit
@code{RE_BACKSLASH_ESCAPE_IN_LISTS} is not set.  For example, @samp{[\]}
would match @samp{\}.

@item
It quotes (makes ordinary, if it's special) the next character when you
use it either:

@itemize @bullet
@item
outside a list,@footnote{Sometimes
you don't have to explicitly quote special characters to make
them ordinary.  For instance, most characters lose any special meaning
inside a list (@pxref{List Operators}).  In addition, if the syntax bits
@code{RE_CONTEXT_INVALID_OPS} and @code{RE_CONTEXT_INDEP_OPS}
aren't set, then (for historical reasons) the matcher considers special
characters ordinary if they are in contexts where the operations they
represent make no sense; for example, then the match-zero-or-more
operator (represented by @samp{*}) matches itself in the regular
expression @samp{*foo} because there is no preceding expression on which
it can operate.  It is poor practice, however, to depend on this
behavior; if you want a special character to be ordinary outside a list,
it's better to always quote it, regardless.} or

@item
inside a list and the syntax bit @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is set.

@end itemize

@item
It introduces an operator when followed by certain ordinary
characters---sometimes only when certain syntax bits are set.  See the
cases @code{RE_BK_PLUS_QM}, @code{RE_NO_BK_BRACES}, @code{RE_NO_BK_VAR},
@code{RE_NO_BK_PARENS}, @code{RE_NO_BK_REF} in @ref{Syntax Bits}.  Also:

@itemize @bullet
@item
@samp{\b} represents the match-word-boundary operator
(@pxref{Match-word-boundary Operator}).

@item
@samp{\B} represents the match-within-word operator
(@pxref{Match-within-word Operator}).

@item
@samp{\<} represents the match-beginning-of-word operator @*
(@pxref{Match-beginning-of-word Operator}).

@item
@samp{\>} represents the match-end-of-word operator
(@pxref{Match-end-of-word Operator}).

@item
@samp{\w} represents the match-word-constituent operator
(@pxref{Match-word-constituent Operator}).

@item
@samp{\W} represents the match-non-word-constituent operator
(@pxref{Match-non-word-constituent Operator}).

@item
@samp{\`} represents the match-beginning-of-buffer
operator and @samp{\'} represents the match-end-of-buffer operator
(@pxref{Buffer Operators}).

@item
If Regex was compiled with the C preprocessor symbol @code{emacs}
defined, then @samp{\s@var{class}} represents the match-syntactic-class
operator and @samp{\S@var{class}} represents the
match-not-syntactic-class operator (@pxref{Syntactic Class Operators}).

@end itemize

@item
In all other cases, Regex ignores @samp{\}.  For example,
@samp{\n} matches @samp{n}.

@end enumerate

@node Common Operators, GNU Operators, Regular Expression Syntax, Top
@chapter Common Operators

You compose regular expressions from operators.  In the following
sections, we describe the regular expression operators specified by
@sc{posix}; @sc{gnu} also uses these.  Most operators have more than one
representation as characters.  @xref{Regular Expression Syntax}, for
what characters represent what operators under what circumstances.

For most operators that can be represented in two ways, one
representation is a single character and the other is that character
preceded by @samp{\}.  For example, either @samp{(} or @samp{\(}
represents the open-group operator.  Which one does depends on the
setting of a syntax bit, in this case @code{RE_NO_BK_PARENS}.  Why is
this so?  Historical reasons dictate some of the varying
representations, while @sc{posix} dictates others.  

Finally, almost all characters lose any special meaning inside a list
(@pxref{List Operators}).

@menu
* Match-self Operator::			Ordinary characters.
* Match-any-character Operator::	.
* Concatenation Operator::		Juxtaposition.
* Repetition Operators::		*  +  ? @{@}
* Alternation Operator::		|
* List Operators::			[...]  [^...]
* Grouping Operators::			(...)
* Back-reference Operator::		\digit
* Anchoring Operators::			^  $
@end menu

@node Match-self Operator, Match-any-character Operator,  , Common Operators
@section The Match-self Operator (@var{ordinary character})

This operator matches the character itself.  All ordinary characters
(@pxref{Regular Expression Syntax}) represent this operator.  For
example, @samp{f} is always an ordinary character, so the regular
expression @samp{f} matches only the string @samp{f}.  In
particular, it does @emph{not} match the string @samp{ff}.

@node Match-any-character Operator, Concatenation Operator, Match-self Operator, Common Operators
@section The Match-any-character Operator (@code{.})

@cindex @samp{.}

This operator matches any single printing or nonprinting character
except it won't match a:

@table @asis
@item newline
if the syntax bit @code{RE_DOT_NEWLINE} isn't set.

@item null
if the syntax bit @code{RE_DOT_NOT_NULL} is set.

@end table

The @samp{.} (period) character represents this operator.  For example,
@samp{a.b} matches any three-character string beginning with @samp{a}
and ending with @samp{b}.

@node Concatenation Operator, Repetition Operators, Match-any-character Operator, Common Operators
@section The Concatenation Operator

This operator concatenates two regular expressions @var{a} and @var{b}.
No character represents this operator; you simply put @var{b} after
@var{a}.  The result is a regular expression that will match a string if
@var{a} matches its first part and @var{b} matches the rest.  For
example, @samp{xy} (two match-self operators) matches @samp{xy}.

@node Repetition Operators, Alternation Operator, Concatenation Operator, Common Operators
@section Repetition Operators    

Repetition operators repeat the preceding regular expression a specified
number of times.

@menu
* Match-zero-or-more Operator::  *
* Match-one-or-more Operator::   +
* Match-zero-or-one Operator::   ?
* Interval Operators::           @{@}
@end menu

@node Match-zero-or-more Operator, Match-one-or-more Operator,  , Repetition Operators
@subsection The Match-zero-or-more Operator (@code{*})

@cindex @samp{*}

This operator repeats the smallest possible preceding regular expression
as many times as necessary (including zero) to match the pattern.
@samp{*} represents this operator.  For example, @samp{o*}
matches any string made up of zero or more @samp{o}s.  Since this
operator operates on the smallest preceding regular expression,
@samp{fo*} has a repeating @samp{o}, not a repeating @samp{fo}.  So,
@samp{fo*} matches @samp{f}, @samp{fo}, @samp{foo}, and so on.

Since the match-zero-or-more operator is a suffix operator, it may be
useless as such when no regular expression precedes it.  This is the
case when it:

@itemize @bullet
@item 
is first in a regular expression, or

@item 
follows a match-beginning-of-line, open-group, or alternation
operator.

@end itemize

@noindent
Three different things can happen in these cases:

@enumerate
@item
If the syntax bit @code{RE_CONTEXT_INVALID_OPS} is set, then the
regular expression is invalid.

@item
If @code{RE_CONTEXT_INVALID_OPS} isn't set, but
@code{RE_CONTEXT_INDEP_OPS} is, then @samp{*} represents the
match-zero-or-more operator (which then operates on the empty string).