re.html

unix 下的C开发手册,还用详细的例程。

<li>
as the first character of a bracket expression
(see
<xref href=rebrack><a href="#tag_007_003_005">
RE Bracket Expression
</a></xref>).

</ul>

<dt>$<dd>The dollar sign is special when used as an anchor.

</dl>
<h4><a name = "tag_007_003_004">&nbsp;</a>Periods in BREs</h4>
A period
(.),
when used outside a bracket expression, is a BRE that matches any
character in the supported character set except NUL.
<h4><a name = "tag_007_003_005">&nbsp;</a>RE Bracket Expression</h4>
<xref type="3" name="rebrack"></xref>
A bracket expression
(an expression enclosed in square brackets, [])
is an RE that matches a single
collating element contained in the non-empty set of
collating elements represented by the bracket expression.
<p>
The following rules and definitions apply to bracket expressions:
<ol>
<p>
<li>
A
<i>bracket expression</i>
is either a matching list expression
or a non-matching list expression.
It consists of one or more expressions:
collating elements,
collating symbols,
equivalence classes,
character classes
or range expressions.
Portable applications
must not use range expressions, even though all
implementations support them.
The right-bracket (]) loses its special meaning and represents itself
in a bracket expression if it
occurs first in the list (after an initial circumflex (^), if any).
Otherwise, it terminates the bracket expression, unless it
appears in a collating symbol (such as [.].])
or is the ending right-bracket
for a collating symbol, equivalence class or character class.
The special characters:
<code>
<pre>
.  *  [  \
</code>
</pre>
<p>
(period, asterisk, left-bracket and backslash, respectively)
lose their special meaning within a bracket expression.
<p>
The character sequences:
<code>
<pre>
[.   [=    [:
</code>
</pre>
<p>
(left-bracket followed by a period, equals-sign or colon)
are special inside a bracket expression and are used to
delimit collating symbols, equivalence class expressions
and character class expressions.
These symbols must be followed by a valid expression
and the matching terminating sequence .], =] or :],
as described in the following items.
<p>
<li>
A
<i>matching list</i>
expression specifies a list that
matches any one of the expressions
represented in the list.
The first character in the list must not be the circumflex.
For example, [abc] is an RE that matches any of the characters a, b or c.
<p>
<li>
A
<i>non-matching list</i>
expression begins with a circumflex (^),
and specifies a list that matches any character or collating element
except for
the expressions represented in the list after the leading circumflex.
For example, [^abc] is an RE that matches any character
or collating element except the characters a, b or c.
The circumflex
will have this special meaning only when it occurs first in the
list, immediately following the left-bracket.
<p>
<li>
A
<i>collating symbol</i>
is a collating element enclosed within
bracket-period ([. .]) delimiters.
Collating elements are defined as described in
<xref href=collorder><a href="locale.html#tag_005_003_002_004">
Collation Order
</a></xref>.
Multi-character collating elements
must be represented as collating symbols
when it is necessary
to distinguish them from a list of the individual characters that make
up the multi-character collating element.
For example, if the string ch
is a collating element in the current collation sequence with the
associated collating symbol &lt;ch&gt;, the expression [[.ch.]]
will be treated as an RE matching the character sequence ch, while
[ch] will be treated as an RE matching c or h.
Collating symbols will be recognised only inside bracket expressions.
This implies that the RE [[.ch.]]*c
matches the first to fifth character in the string chchch.
If the string is not a collating element in the current collating
sequence definition, or if the collating element has no characters
associated with it (for example, see the symbol &lt;HIGH&gt;
in the example collation definition shown in
<xref href=collorder><a href="locale.html#tag_005_003_002_004">
Collation Order
</a></xref>),
the symbol will be treated as an invalid expression.
<p>
<li>
An
<i>equivalence class expression</i>
represents the set of collating elements
belonging to an equivalence class,
as described in
<b>Collation Order</b>.
Only primary equivalence classes will be recognised.
The class is expressed by enclosing any one
of the collating elements in the equivalence class
within bracket-equal ([= =]) delimiters.
For example, if a, &agrave;  and &acirc;  belong to the same equivalence class, then
[[=a=]b], [[=&agrave;=]b] and [[=&acirc;=]b] will each be equivalent to [a&agrave;&acirc;b].
If the collating element does not belong to an equivalence class,
the equivalence class expression will be treated as a
<i>collating symbol</i>.
<p>
<li>
A
<i>character class expression</i>
represents the set of characters belonging
to a character class, as defined
in the LC_CTYPE category in the current locale.
All character classes specified in
the current locale will be recognised.
A character class expression
is expressed as a character class
name enclosed within bracket-colon ([: :])
delimiters.
<p>
The following character class expressions
are supported in all locales:
<code>
<pre>
[:alnum:]   [:cntrl:]   [:lower:]   [:space:]
[:alpha:]   [:digit:]   [:print:]   [:upper:]
[:blank:]   [:graph:]   [:punct:]   [:xdigit:]
</code>
</pre>
<p>
In addition, character class expressions of the form:
<code>
<pre>
[:<i>name</i>:]
</code>
</pre>
<p>
are recognised in those locales where the
<i>name</i>
keyword has been given a
<b>charclass</b>
definition in the LC_CTYPE category.
<p>
<li>
A
<i>range expression</i>
represents the set of collating elements that
fall between two elements in the current collation sequence,
inclusively.
It is expressed as the starting point
and the ending point separated by a hyphen (-).
<p>
Range expressions must not be used in
portable applications because
their behaviour is dependent on the collating sequence.
Ranges will be treated according to
the current collating sequence, and include such characters that
fall within the range based on that collating sequence, regardless
of character values.
This, however, means that the interpretation
will differ depending on collating sequence.
If, for instance,
one collating sequence defines 
&auml;
as a variant of a, while another defines it as a letter
following z, then the expression [&auml;-z]
is valid in the first language and invalid in the second.
<p>
In the following, all examples assume the collation sequence specified
for the POSIX locale,
unless another collation sequence is specifically defined.
<p>
The starting range point and the ending range point must be a collating
element or collating symbol.
An equivalence class expression used as a
starting or ending point of a range
expression produces unspecified results.
An equivalence class can be used portably within
a bracket expression, but only outside the range.
For example, the unspecified expression [[=e=]-f]
should be given as [[=e=]e-f].
The ending range point must collate
equal to or higher than the starting range point; otherwise, the
expression will be treated as invalid.
The order used is the order in which the collating elements are
specified in the current collation definition.
One-to-many mappings (see the description of
LC_COLLATE
in
<xref href=locale><a href="locale.html#tag_005_003_002">
Locale
</a></xref>)
will not be performed.
For example, assuming that the character eszet (&szlig;) is placed
in the collation sequence after r and s, but before t
and that it maps to the sequence ss for collation purposes,
then the expression [r-s] matches only r and s, but the expression
[s-t] matches s, &szlig;  or t.
<p>
The interpretation of range expressions where the ending range point
is also the starting range point of a subsequent range expression
(for instance [a-m-o]) is undefined.
<p>
The hyphen character will be treated
as itself if it occurs first (after an initial ^,
if any) or last in the list, or as
an ending range point in a range expression.
As examples, the expressions [-ac] and [ac-]
are equivalent and match any of the characters a, c or -;
[^-ac] and [^ac-] are equivalent and match any characters except
a, c or -; the expression [%--]
matches any of the characters between % and - inclusive;
the expression [--@] matches any of the characters between
- and @ inclusive; and the expression [a--@] is invalid,
because the letter a follows the symbol - in the POSIX locale.
To use a hyphen as the starting range point, it must either come first
in the bracket expression or be specified as a collating symbol,
for example: [][.-.]-0], which matches either a right bracket
or any character
or collating element that collates between hyphen and 0, inclusive.
<p>
If a bracket expression must specify both - and ], the ]
must be placed first (after the ^, if any) and the -
last within the bracket expression.
<p>
</ol>
<h4><a name = "tag_007_003_006">&nbsp;</a>BREs Matching Multiple Characters</h4>
<xref type="3" name="bremult"></xref>
The following rules can be used to construct
BREs matching multiple characters from BREs matching a single character:
<ol>
<p>
<li>
The concatenation of BREs matches the concatenation of
the strings matched by each component of the BRE.
<p>
<li>
A
<i>subexpression</i>
can be defined within a BRE by enclosing it between the
character pairs \( and \) .
Such a subexpression matches whatever it would have matched
without the \( and \),
except that anchoring within subexpressions is optional behaviour; see
<xref href=breanc><a href="#tag_007_003_008">
BRE Expression Anchoring
</a></xref>.
Subexpressions can be arbitrarily nested.
<p>
<li>
The
<i>back-reference</i>
expression \<i>n</i> matches the same (possibly empty)
string of characters as was matched by a subexpression enclosed between
\( and \) preceding the \<i>n</i>.
The character <i>n</i> must be a digit
from 1 to 9 inclusive,
<i>n</i>th subexpression (the one that begins with the <i>n</i>th
\( and ends with the corresponding paired \)).
The expression is invalid if less than <i>n</i> subexpressions precede
the \<i>n</i>.
For example, the expression ^\(.*\)\1$
matches a line consisting of two adjacent appearances of the same string,
and the expression \(a\)*\1 fails to match a.
The limit of nine back-references to subexpressions in the RE
is based on the use of a single digit identifier.
This does not imply that only nine subexpressions are allowed in REs.
The following is a valid BRE with ten subexpressions:
<pre>
<code>
\(\(\(ab\)*c\)*d\)\(ef\)*\(gh\)\{2\}\(ij\)*\(kl\)*\(mn\)*\(op\)*\(qr\)*
</code>
</pre>
<p>
<li>
When a BRE matching a single character, a subexpression or a
back-reference is followed by the special character asterisk (*),
together with that asterisk it matches
what zero or more consecutive occurrences of the BRE would match.
For example, [ab]* and [ab][ab] are equivalent when matching the string ab.
<p>
<li>
When a BRE matching a single character, a subexpression or a
back-reference is followed by an
<i>interval expression</i>
of the format
\{<i>m</i>\}, \{<i>m</i>,\} or
\{<i>m</i>,<i>n</i>\},
together with that interval expression it matches
what repeated consecutive occurrences of the BRE would match.
The values of <i>m</i> and <i>n</i> will be decimal integers in the range
0 &lt;= <i>m</i> &lt;=  <i>n</i>  &lt;=
{RE_DUP_MAX},
where <i>m</i> specifies the exact or minimum number of
occurrences and <i>n</i> specifies the maximum number of occurrences.
The expression
\{<i>m</i>\} matches exactly <i>m</i> occurrences of the
preceding BRE,
\{<i>m</i>,\} matches at least <i>m</i> occurrences and
\{<i>m,n</i>\} matches any number of occurrences between
<i>m</i> and <i>n</i>, inclusive.
<p>
For example, in the string
abababccccccd
the BRE
c\{3\}
is matched by characters seven to nine, the BRE
\(ab\)\{4,\}
is not matched at all and the BRE
c\{1,3\}d
is matched by characters ten to thirteen.
<p>
</ol>
<p>
The behaviour of multiple adjacent duplication symbols
(*
and intervals) produces undefined results.
<h4><a name = "tag_007_003_007">&nbsp;</a>BRE Precedence</h4>
The order of precedence is as shown in the following table:
<pre>
<table  bordercolor=#000000 border=1 align=center><tr valign=top><th colspan=2 align=center><b>BRE Precedence (from high to low)</b>
<tr valign=top><td align=left>collation-related bracket symbols
<td align=left>[= =]  [: :]  [. .]
<tr valign=top><td align=left>escaped characters
<td align=left>\&lt;<i>special character</i>&gt;
<tr valign=top><td align=left>bracket expression
<td align=left>[ ]