ch05_03.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

<tr>
<td><tt class="literal">\w</tt></td>
<td>Yes</td>
<td>
<p>Match any "word" character (alphanumerics plus "_").</p>
</td>
</tr>

<tr>
<td><tt class="literal">\W</tt></td>
<td>Yes</td>
<td>
<p>Match any nonword character.</p>
</td>
</tr>

<tr>
<td><tt class="literal">\x{</tt><em class="replaceable">abcd</em><tt class="literal">}</tt></td>
<td>Yes</td>
<td>
<p>Match the character given in hexadecimal.</p>
</td>
</tr>

<tr>
<td><tt class="literal">\X</tt></td>
<td>Yes</td>
<td>
<p>Match Unicode "combining character sequence" string.</p>
</td>
</tr>

<tr>
<td><tt class="literal">\z</tt></td>
<td>No</td>
<td>
<p>True at end of string only.</p>
</td>
</tr>

<tr>
<td><tt class="literal">\Z</tt></td>
<td>No</td>
<td>
<p>True at end of string or before optional newline.</p>
</td>
</tr>


</table>

<p>The braces are optional on <tt class="literal">\p</tt> and
<tt class="literal">\P</tt> if the property name is one character.  The
braces are optional on <tt class="literal">\x</tt> if the hexadecimal number
is two digits or less.  The braces are never optional on
<tt class="literal">\N</tt>.</p>

<p>
<a name="INDEX-1498"></a><a name="INDEX-1499"></a>
Only metasymbols with "Match the..." or
"Match any..." descriptions may be used
within character classes (square brackets).  That is, character
classes are limited to containing specific sets of characters, so
within them you may only use metasymbols that describe other specific
sets of characters, or that describe specific individual characters.
Of course, these metasymbols may also be used outside character
classes, along with all the other nonclassificatory metasymbols.  Note
however that <tt class="literal">\b</tt> is two entirely different beasties:
it's a backspace character inside the character class, but a word
boundary assertion outside.</p>

<p>There is some amount of overlap between the characters that a pattern
can match and the characters an ordinary double-quoted string can
interpolate.  Since regexes undergo two passes, it is sometimes
ambiguous which pass should process a given character.  When there is
ambiguity, the variable interpolation pass defers the interpretation of
such characters to the regular expression parser.</p>

<p>But the variable interpolation pass can only defer to the regex parser
when it knows it is parsing a regex.  You can specify regular
expressions as ordinary double-quoted strings, but then you must
follow normal double-quote rules.  Any of the previous metasymbols
that happen to map to actual characters will still work, even though
they're not being deferred to the regex parser.  But you can't use any
of the other metasymbols in ordinary double quotes (or in any similar
constructs such as <tt class="literal">`...`</tt>,
<tt class="literal">qq(...)</tt>, <tt class="literal">qx(...)</tt>, or
the equivalent here documents).  If you want your string to be parsed
as a regular expression without doing any matching, you should be
using the <tt class="literal">qr//</tt> (quote regex) operator.</p>

<p>
<a name="INDEX-1500"></a>
Note that the case and metaquote translation escapes
(<tt class="literal">\U</tt> and friends) must be processed during the
variable interpolation pass because the purpose of those metasymbols
is to influence how variables are interpolated.  If you suppress
variable interpolation with single quotes, you don't get the
translation escapes either.  Neither variables nor translation escapes
(<tt class="literal">\U</tt>, etc.) are expanded in any single quoted
string, nor in single-quoted <tt class="literal">m'...'</tt> or
<tt class="literal">qr'...'</tt> operators.  Even when you do
interpolation, these translation escapes are ignored if they show up
as the <em class="emphasis">result</em> of variable interpolation, since by
then it's too late to influence variable interpolation.</p>

<p>Although the transliteration operator doesn't take regular expressions,
any metasymbol we've discussed that matches a single specific character also
works in a <tt class="literal">tr///</tt> operation.  The rest do not (except for backslash,
which continues to work in the backward way it always works.)</p>






<h3 class="sect2">5.3.2. Specific Characters</h3>

<p>
<a name="INDEX-1501"></a><a name="INDEX-1502"></a>
As mentioned before, everything that's not special in a pattern
matches itself.  That means an <tt class="literal">/a/</tt> matches an "a", an <tt class="literal">/=/</tt>
matches an "=", and so on.  Some characters, though, aren't very
easy to type in from the keyboard or, even if you manage that, don't
show up on a printout; control characters are notorious for this.
In a regular expression, Perl recognizes the following double-quotish
character aliases:</p>

<a name="perl3-tab-re-dblquotechars"></a>
<table border="1">

<tr>
<th>Escape</th>
<th>Meaning</th>
</tr>




<tr>
<td><tt class="literal">\0</tt></td>
<td>Null character (ASCII NUL)</td>
</tr>

<tr>
<td><tt class="literal">\a</tt></td>
<td>Alarm (BEL)</td>
</tr>

<tr>
<td><tt class="literal">\e</tt></td>
<td>Escape (ESC)</td>
</tr>

<tr>
<td><tt class="literal">\f</tt></td>
<td>Form feed (FF)</td>
</tr>

<tr>
<td><tt class="literal">\n</tt></td>
<td>Newline (NL, CR on Mac)</td>
</tr>

<tr>
<td><tt class="literal">\r</tt></td>
<td>Return (CR, NL on Mac)</td>
</tr>

<tr>
<td><tt class="literal">\t</tt></td>
<td>Tab (HT)</td>
</tr>


</table>
<p>
<a name="INDEX-1503"></a><a name="INDEX-1504"></a>
Just as in double-quoted strings, Perl also honors the following four
metasymbols in patterns:</p>

<dl>
<dt>
<b><tt class="literal">\c</tt><em class="replaceable">X</em></b>
</dt>
<dd>
<p>A named control character, like <tt class="literal">\cC</tt> for Control-C, <tt class="literal">\cZ</tt> for
Control-Z, <tt class="literal">\c[</tt> for ESC, and <tt class="literal">\c?</tt> for DEL.
<a name="INDEX-1505"></a>
</p>
</dd>


<dt>
<b><tt class="literal">\</tt><em class="replaceable">NNN</em></b>
</dt>
<dd>
<p> A character specified using its two- or three-digit
octal code.  The leading <tt class="literal">0</tt> is optional, except for
values less than <tt class="literal">010</tt> (8 decimal) since (unlike in
double-quoted strings) the single-digit versions are always considered
to be backreferences to captured strings within a pattern.  Multiple
digits are interpreted as the <em class="emphasis">n</em>th
backreference if you've captured at least <em class="emphasis">n</em>
substrings earlier in the pattern (where <em class="emphasis">n</em> is
considered as a decimal number).  Otherwise, they are interpreted as a
character specified in octal.<a name="INDEX-1506"></a><a name="INDEX-1507"></a>
</p>
</dd>


<dt>
<b><tt class="literal">\x{</tt><em class="replaceable">LONGHEX</em><tt class="literal">}</tt></b>
</dt>
<dt>
<b><tt class="literal">\x</tt><em class="replaceable">HEX</em></b>
</dt>
<dd>
<p>A character number specified as one or two hex digits (<tt class="literal">[0-9a-fA-F]</tt>), as in
<tt class="literal">\x1B</tt>.  The one-digit form is usable only if the character following
it is not a hex digit.  If braces are used, you may use as many digits
as you'd like, which may result in a Unicode character.  For example,
<tt class="literal">\x{262f}</tt> matches a Unicode YIN YANG.
<a name="INDEX-1508"></a>
</p>
</dd>


<dt>
<b><tt class="literal">\N{</tt><em class="replaceable">NAME</em><tt class="literal">}</tt></b>
</dt>
<dd>
<p> A named character, such <tt class="literal">\N{GREEK SMALL
LETTER EPSILON}</tt>, <tt class="literal">\N{greek:epsilon}</tt>, or
<tt class="literal">\N{epsilon}</tt>.  This requires the <tt class="literal">use
charnames</tt> pragma described in <a href="ch31_01.htm">Chapter 31, "Pragmatic Modules"</a>, which also determines which
flavors of those names you may use (<tt class="literal">":long"</tt>,
<tt class="literal">":full"</tt>, <tt class="literal">":short"</tt> respectively,
corresponding to the three styles just shown).
<a name="INDEX-1509"></a>
</p>

<p>A list of all Unicode character names can be found in your closest
Unicode standards document, or in
<em class="replaceable">PATH_TO_PERLLIB</em><em class="emphasis">/unicode/Names.txt</em>.</p>
</dd>

</dl>






<h3 class="sect2">5.3.3. Wildcard Metasymbols</h3>

<p>
<a name="INDEX-1510"></a><a name="INDEX-1511"></a><a name="INDEX-1512"></a><a name="INDEX-1513"></a><a name="INDEX-1514"></a><a name="INDEX-1515"></a><a name="INDEX-1516"></a>
Three special metasymbols serve as generic wildcards, each of them
matching "any" character (for certain values of "any").  These are the
dot ("<tt class="literal">.</tt>"), <tt class="literal">\C</tt>, and
<tt class="literal">\X</tt>.  None of these may be used in a character
class.  You can't use the dot there because it would match (nearly)
any character in existence, so it's something of a universal character
class in its own right. If you're going to include or exclude
everything, there's not much point in having a character class.  The
special wildcards <tt class="literal">\C</tt> and <tt class="literal">\X</tt> have
special structural meanings that don't map well to the notion of
choosing a single Unicode character, which is the level at which
character classes work.</p>

<p>
<a name="INDEX-1517"></a>
The dot metacharacter matches any one character other than a
newline.  (And with the <tt class="literal">/s</tt> modifier, it matches
that, too.)  Like any of the dozen special characters in a pattern, to
match a dot literally, you must escape it with a backslash.  For
example, this checks whether a filename ends with a dot followed by a
one-character extension:
<blockquote>
<pre class="programlisting">if ($pathname =~ /\.(.)\z/s) {
    print "Ends in $1\n";
}</pre>
</blockquote>

The first dot, the escaped one, is the literal character, and the
second says "match any character".  The <tt class="literal">\z</tt> says to
match only at the end of the string, and the <tt class="literal">/s</tt>
modifier lets the dot match a newline as well. (Yes, using a newline
as a file extension Isn't Very Nice, but that doesn't mean it can't
happen.)</p>

<p>
<a name="INDEX-1518"></a>
The dot metacharacter is most often used with a quantifier.  A
<tt class="literal">.*</tt> matches a maximal number of characters, while a
<tt class="literal">.*?</tt> matches a minimal number of characters.  But
it's also sometimes used without a quantifier for its width:
<tt class="literal">/(..):(..):(..)/</tt> matches three colon-separated
fields, each of which is two characters long.</p>

<p>
<a name="INDEX-1519"></a><a name="INDEX-1520"></a>
If you use a dot in a pattern compiled under the lexically scoped
<tt class="literal">use utf8</tt> pragma, then it will match any Unicode
character.  (You're not supposed to need a <tt class="literal">use utf8</tt>
for that, but accidents will happen.  The pragma may not be necessary
by the time you read this.)
<blockquote>
<pre class="programlisting">use utf8;
use charnames qw/:full/;
$BWV[887] = "G\N{MUSIC SHARP SIGN} minor";
($note, $black, $mode) = $BWV[887] =~ /^([A-G])(.)\s+(\S+)/;
print "That's lookin' sharp!\n" if $black eq chr(9839);</pre>
</blockquote>
</p>

<p>
<a name="INDEX-1521"></a>
The <tt class="literal">\X</tt> metasymbol matches a character in a more
extended sense.  It really matches a string of one or more Unicode
characters known as a "combining character sequence".  Such a sequence
consists of a base character followed by any "mark" characters
(diacritical markings like cedillas or diereses) that combine with
that base character to form one logical unit.  <tt class="literal">\X</tt>
is exactly equivalent to <tt class="literal">(?:\PM\pM*)</tt>.  This allows
it to match one logical character, even when that really comprises
several separate characters.  The length of the match in
<tt class="literal">/\X/</tt> would exceed one character if it matched any
combining characters.  (And that's character length, which has little
to do with byte length).</p>

<p>If you are using Unicode and really want to get at a single byte
instead of a single character, you can use the <tt class="literal">\C</tt>
metasymbol.  This will always match one byte (specifically, one C
language <tt class="literal">char</tt> type), even if this gets you out of
sync with your Unicode character stream.  See the appropriate warnings
about doing this in <a href="ch15_01.htm">Chapter 15, "Unicode"</a>.</p>

<a name="INDEX-1522"></a><a name="INDEX-1523"></a><a name="INDEX-1524"></a>



<!-- BOTTOM NAV BAR -->

<hr width="515" align="left">
<div class="navbar">
<table width="515" border="0">
<tr>
<td align="left" valign="top" width="172"><a href="ch05_02.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0"></a></td><td align="right" valign="top" width="172"><a href="ch05_04.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td>
</tr>
<tr>
<td align="left" valign="top" width="172">5.2. Pattern-Matching Operators</td><td align="center" valign="top" width="171"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0"></a></td><td align="right" valign="top" width="172">5.4. Character Classes</td>
</tr>
</table>
</div>
<hr width="515" align="left">

<!-- LIBRARY NAV BAR -->

<img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p>
<font size="-1"><a href="copyrght.htm">Copyright &copy; 2001</a> O'Reilly &amp; Associates. All rights reserved.</font>
</p>
<map name="library-map"> <area shape="rect" coords="2,-1,79,99" href="../index.htm"><area shape="rect" coords="84,1,157,108" href="../perlnut/index.htm"><area shape="rect" coords="162,2,248,125" href="../prog/index.htm"><area shape="rect" coords="253,2,326,130" href="../advprog/index.htm"><area shape="rect" coords="332,1,407,112" href="../cookbook/index.htm"><area shape="rect" coords="414,2,523,103" href="../sysadmin/index.htm">
</map>

<!-- END OF BODY -->

</body>
</html>