regex.texi

正则表达式库

@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).

@item
Otherwise, @samp{*} is ordinary.

@end enumerate

@cindex backtracking
The matcher processes a match-zero-or-more operator by first matching as
many repetitions of the smallest preceding regular expression as it can.
Then it continues to match the rest of the pattern.  

If it can't match the rest of the pattern, it backtracks (as many times
as necessary), each time discarding one of the matches until it can
either match the entire pattern or be certain that it cannot get a
match.  For example, when matching @samp{ca*ar} against @samp{caaar},
the matcher first matches all three @samp{a}s of the string with the
@samp{a*} of the regular expression.  However, it cannot then match the
final @samp{ar} of the regular expression against the final @samp{r} of
the string.  So it backtracks, discarding the match of the last @samp{a}
in the string.  It can then match the remaining @samp{ar}.


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

@cindex @samp{+} 

If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't recognize
this operator.  Otherwise, if the syntax bit @code{RE_BK_PLUS_QM} isn't
set, then @samp{+} represents this operator; if it is, then @samp{\+}
does.

This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression at least once;
@pxref{Match-zero-or-more Operator}, for what it operates on, how some
syntax bits affect it, and how Regex backtracks to match it.

For example, supposing that @samp{+} represents the match-one-or-more
operator; then @samp{ca+r} matches, e.g., @samp{car} and
@samp{caaaar}, but not @samp{cr}.

@node Match-zero-or-one Operator, Interval Operators, Match-one-or-more Operator, Repetition Operators
@subsection The Match-zero-or-one Operator (@code{?} or @code{\?})
@cindex @samp{?}

If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't
recognize this operator.  Otherwise, if the syntax bit
@code{RE_BK_PLUS_QM} isn't set, then @samp{?} represents this operator;
if it is, then @samp{\?} does.

This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression once or not at all;
@pxref{Match-zero-or-more Operator}, to see what it operates on, how
some syntax bits affect it, and how Regex backtracks to match it.

For example, supposing that @samp{?} represents the match-zero-or-one
operator; then @samp{ca?r} matches both @samp{car} and @samp{cr}, but
nothing else.

@node Interval Operators,  , Match-zero-or-one Operator, Repetition Operators
@subsection Interval Operators (@code{@{} @dots{} @code{@}} or @code{\@{} @dots{} @code{\@}})

@cindex interval expression
@cindex @samp{@{}
@cindex @samp{@}}
@cindex @samp{\@{}
@cindex @samp{\@}}

If the syntax bit @code{RE_INTERVALS} is set, then Regex recognizes
@dfn{interval expressions}.  They repeat the smallest possible preceding
regular expression a specified number of times.

If the syntax bit @code{RE_NO_BK_BRACES} is set, @samp{@{} represents
the @dfn{open-interval operator} and @samp{@}} represents the
@dfn{close-interval operator} ; otherwise, @samp{\@{} and @samp{\@}} do.

Specifically, supposing that @samp{@{} and @samp{@}} represent the
open-interval and close-interval operators; then:

@table @code
@item  @{@var{count}@}
matches exactly @var{count} occurrences of the preceding regular
expression.

@item @{@var{min,}@}
matches @var{min} or more occurrences of the preceding regular
expression.

@item  @{@var{min, max}@}
matches at least @var{min} but no more than @var{max} occurrences of
the preceding regular expression.

@end table

The interval expression (but not necessarily the regular expression that
contains it) is invalid if:

@itemize @bullet
@item
@var{min} is greater than @var{max}, or 

@item
any of @var{count}, @var{min}, or @var{max} are outside the range
zero to @code{RE_DUP_MAX} (which symbol @file{regex.h}
defines).

@end itemize

If the interval expression is invalid and the syntax bit
@code{RE_NO_BK_BRACES} is set, then Regex considers all the
characters in the would-be interval to be ordinary.  If that bit
isn't set, then the regular expression is invalid.

If the interval expression is valid but there is no preceding regular
expression on which to operate, then if the syntax bit
@code{RE_CONTEXT_INVALID_OPS} is set, the regular expression is invalid.
If that bit isn't set, then Regex considers all the characters---other
than backslashes, which it ignores---in the would-be interval to be
ordinary.


@node Alternation Operator, List Operators, Repetition Operators, Common Operators
@section The Alternation Operator (@code{|} or @code{\|})

@kindex |
@kindex \|
@cindex alternation operator
@cindex or operator

If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't
recognize this operator.  Otherwise, if the syntax bit
@code{RE_NO_BK_VBAR} is set, then @samp{|} represents this operator;
otherwise, @samp{\|} does.

Alternatives match one of a choice of regular expressions:
if you put the character(s) representing the alternation operator between
any two regular expressions @var{a} and @var{b}, the result matches
the union of the strings that @var{a} and @var{b} match.  For
example, supposing that @samp{|} is the alternation operator, then
@samp{foo|bar|quux} would match any of @samp{foo}, @samp{bar} or
@samp{quux}.

@ignore
@c Nobody needs to disallow empty alternatives any more.
If the syntax bit @code{RE_NO_EMPTY_ALTS} is set, then if either of the regular
expressions @var{a} or @var{b} is empty, the
regular expression is invalid.  More precisely, if this syntax bit is
set, then the alternation operator can't:

@itemize @bullet
@item
be first or last in a regular expression;

@item
follow either another alternation operator or an open-group operator
(@pxref{Grouping Operators}); or

@item
precede a close-group operator.

@end itemize

@noindent
For example, supposing @samp{(} and @samp{)} represent the open and
close-group operators, then @samp{|foo}, @samp{foo|}, @samp{foo||bar},
@samp{foo(|bar)}, and @samp{(foo|)bar} would all be invalid.
@end ignore

The alternation operator operates on the @emph{largest} possible
surrounding regular expressions.  (Put another way, it has the lowest
precedence of any regular expression operator.)
Thus, the only way you can
delimit its arguments is to use grouping.  For example, if @samp{(} and
@samp{)} are the open and close-group operators, then @samp{fo(o|b)ar}
would match either @samp{fooar} or @samp{fobar}.  (@samp{foo|bar} would
match @samp{foo} or @samp{bar}.)

@cindex backtracking
The matcher usually tries all combinations of alternatives so as to 
match the longest possible string.  For example, when matching
@samp{(fooq|foo)*(qbarquux|bar)} against @samp{fooqbarquux}, it cannot
take, say, the first (``depth-first'') combination it could match, since
then it would be content to match just @samp{fooqbar}.  

@comment xx something about leftmost-longest


@node List Operators, Grouping Operators, Alternation Operator, Common Operators
@section List Operators (@code{[} @dots{} @code{]} and @code{[^} @dots{} @code{]})

@cindex matching list
@cindex @samp{[}
@cindex @samp{]}
@cindex @samp{^}
@cindex @samp{-}
@cindex @samp{\}
@cindex @samp{[^}
@cindex nonmatching list
@cindex matching newline
@cindex bracket expression

@dfn{Lists}, also called @dfn{bracket expressions}, are a set of one or
more items.  An @dfn{item} is a character,
@ignore
(These get added when they get implemented.)
a collating symbol, an equivalence class expression, 
@end ignore
a character class expression, or a range expression.  The syntax bits
affect which kinds of items you can put in a list.  We explain the last
two items in subsections below.  Empty lists are invalid.

A @dfn{matching list} matches a single character represented by one of
the list items.  You form a matching list by enclosing one or more items
within an @dfn{open-matching-list operator} (represented by @samp{[})
and a @dfn{close-list operator} (represented by @samp{]}).  

For example, @samp{[ab]} matches either @samp{a} or @samp{b}.
@samp{[ad]*} matches the empty string and any string composed of just
@samp{a}s and @samp{d}s in any order.  Regex considers invalid a regular
expression with a @samp{[} but no matching
@samp{]}.

@dfn{Nonmatching lists} are similar to matching lists except that they
match a single character @emph{not} represented by one of the list
items.  You use an @dfn{open-nonmatching-list operator} (represented by
@samp{[^}@footnote{Regex therefore doesn't consider the @samp{^} to be
the first character in the list.  If you put a @samp{^} character first
in (what you think is) a matching list, you'll turn it into a
nonmatching list.}) instead of an open-matching-list operator to start a
nonmatching list.  

For example, @samp{[^ab]} matches any character except @samp{a} or
@samp{b}.  

If the @code{posix_newline} field in the pattern buffer (@pxref{GNU
Pattern Buffers} is set, then nonmatching lists do not match a newline.

Most characters lose any special meaning inside a list.  The special
characters inside a list follow.

@table @samp
@item ]
ends the list if it's not the first list item.  So, if you want to make
the @samp{]} character a list item, you must put it first.

@item \
quotes the next character if the syntax bit @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is
set.

@ignore
Put these in if they get implemented.

@item [.
represents the open-collating-symbol operator (@pxref{Collating Symbol
Operators}).

@item .]
represents the close-collating-symbol operator.

@item [=
represents the open-equivalence-class operator (@pxref{Equivalence Class
Operators}).

@item =]
represents the close-equivalence-class operator.

@end ignore

@item [:
represents the open-character-class operator (@pxref{Character Class
Operators}) if the syntax bit @code{RE_CHAR_CLASSES} is set and what
follows is a valid character class expression.

@item :]
represents the close-character-class operator if the syntax bit
@code{RE_CHAR_CLASSES} is set and what precedes it is an
open-character-class operator followed by a valid character class name.

@item - 
represents the range operator (@pxref{Range Operator}) if it's
not first or last in a list or the ending point of a range.

@end table

@noindent
All other characters are ordinary.  For example, @samp{[.*]} matches 
@samp{.} and @samp{*}.