ch05_02.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

which would make a rather dull sequel.</p>

<p>
<a name="INDEX-1423"></a><a name="INDEX-1424"></a><a name="INDEX-1425"></a><a name="INDEX-1426"></a><a name="INDEX-1427"></a>
You can't use a <tt class="literal">s///</tt> operator directly on an array.
For that, you need a loop.  By a lucky coincidence, the aliasing
behavior of <tt class="literal">for</tt>/<tt class="literal">foreach</tt>,
combined with its use of <tt class="literal">$_</tt> as the default loop
variable, yields the standard Perl idiom to search and replace each
element in an array:
<blockquote>
<pre class="programlisting">for (@chapters) { s/Bilbo/Frodo/g }  # Do substitutions chapter by chapter.
s/Bilbo/Frodo/g for @chapters;       # Same thing.</pre>
</blockquote>

As with a simple scalar variable, you can combine the substitution
with an assignment if you'd like to keep the original values around,
too:
<blockquote>
<pre class="programlisting">@oldhues = ('bluebird', 'bluegrass',  'bluefish', 'the blues');
for (@newhues = @oldhues) { s/blue/red/ }
print "@newhues\n";           # prints: redbird redgrass redfish the reds</pre>
</blockquote>
<a name="INDEX-1428"></a><a name="INDEX-1429"></a>
The idiomatic way to perform repeated substitutes on the same
variable is to use a once-through loop.   For example, here's how
to canonicalize whitespace in a variable:
<blockquote>
<pre class="programlisting">for ($string) {
    s/^\s+//;       # discard leading whitespace
    s/\s+$//;       # discard trailing whitespace
    s/\s+/ /g;      # collapse internal whitespace
}</pre>
</blockquote>

which just happens to produce the same result as:
<blockquote>
<pre class="programlisting">$string = join(" ", split " ", $string);</pre>
</blockquote>
You can also use such a loop with an assignment, as we did in the
array case:
<blockquote>
<pre class="programlisting">for ($newshow = $oldshow) {
    s/Fred/Homer/g;
    s/Wilma/Marge/g;
    s/Pebbles/Lisa/g;
    s/Dino/Bart/g;
}</pre>
</blockquote>
</p>




<h3 class="sect3">5.2.3.2. When a global substitution just isn't global enough</h3>

<p>
<a name="INDEX-1430"></a>
Occasionally, you can't just use a <tt class="literal">/g</tt> to get all
the changes to occur, either because the substitutions have to happen
right-to-left or because you need the length of <tt class="literal">$`</tt>
to change between matches.  You can usually do what you want by
calling <tt class="literal">s///</tt> repeatedly.  However, you want the
loop to stop when the <tt class="literal">s///</tt> finally fails, so you
have to put it into the conditional, which leaves nothing to do in the
main part of the loop.  So we just write a <tt class="literal">1</tt>, which
is a rather boring thing to do, but bored is the best you can hope for
sometimes.  Here are some examples that use a few more of those odd
regex beasties that keep popping up:
<blockquote>
<pre class="programlisting"># put commas in the right places in an integer
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/;

# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&amp;)*8 - length($`)%8)/e;

# remove (nested (even deeply nested (like this))) remarks
1 while s/\([^()]*\)//g;

# remove duplicate words (and triplicate (and quadruplicate...))
1 while s/\b(\w+) \1\b/$1/gi;</pre>
</blockquote>
That last one needs a loop because otherwise it would turn this:
<blockquote>
<pre class="programlisting">Paris in THE THE THE THE spring.</pre>
</blockquote>
into this:
<blockquote>
<pre class="programlisting">Paris in THE THE spring.</pre>
</blockquote>
which might cause someone who knows a little French to picture Paris
sitting in an artesian well emitting iced tea, since "th&amp;#233;" is
French for "tea".  A Parisian is never fooled, of course.</p>

<a name="INDEX-1431"></a>







<h3 class="sect2">5.2.4. The tr/// Operator (Transliteration)</h3>

<p>
<a name="INDEX-1432"></a>
<blockquote>
<pre class="programlisting">
<em class="replaceable">LVALUE</em> =~ tr/<em class="replaceable">SEARCHLIST</em>/<em class="replaceable">REPLACEMENTLIST</em>/cds
tr/<em class="replaceable">SEARCHLIST</em>/<em class="replaceable">REPLACEMENTLIST</em>/cds</pre>
</blockquote>
<a name="INDEX-1433"></a><a name="INDEX-1434"></a>
For <em class="emphasis">sed</em> devotees, <tt class="literal">y///</tt> is
provided as a synonym for <tt class="literal">tr///</tt>. This is why you
can't call a function named <tt class="literal">y</tt>, any more than you
can call a function named <tt class="literal">q</tt> or
<tt class="literal">m</tt>.  In all other respects, <tt class="literal">y///</tt>
is identical to <tt class="literal">tr///</tt>, and we won't mention it
again.</p>

<p>
<a name="INDEX-1435"></a><a name="INDEX-1436"></a>
This operator might not appear to fit into a chapter on pattern
matching, since it doesn't use patterns.  This operator scans a
string, character by character, and replaces each occurrence of a
character found in <em class="replaceable">SEARCHLIST</em> (which is not
a regular expression) with the corresponding character from
<em class="replaceable">REPLACEMENTLIST</em> (which is not a replacement
string).  It looks a bit like <tt class="literal">m//</tt> and
<tt class="literal">s///</tt>, though, and you can even use the
<tt class="literal">=~</tt> or <tt class="literal">!~</tt> binding operators on
it, so we describe it here.  (<tt class="literal">qr//</tt> and
<tt class="literal">split</tt> are pattern-matching operators, but you don't
use the binding operators on them, so they're elsewhere in the book.
Go figure.)</p>

<p>Transliteration returns the number of characters replaced or deleted.
If no string is specified via the <tt class="literal">=~</tt> or <tt class="literal">!~</tt> operator, the <tt class="literal">$_</tt>
string is altered.  The <em class="replaceable">SEARCHLIST</em> and <em class="replaceable">REPLACEMENTLIST</em> may
define ranges of sequential characters with a dash:</p>

<p>
<a name="INDEX-1437"></a>
<blockquote>
<pre class="programlisting">$message =~ tr/A-Za-z/N-ZA-Mn-za-m/;    # rot13 encryption.</pre>
</blockquote>
<a name="INDEX-1438"></a><a name="INDEX-1439"></a>
Note that a range like <tt class="literal">A-Z</tt> assumes a linear
character set like ASCII.  But each character set has its own ideas of
how characters are ordered and thus of which characters fall in a
particular range.  A sound principle is to use only ranges that begin
from and end at either alphabets of equal case
(<tt class="literal">a-e</tt>, <tt class="literal">A-E</tt>), or digits
(<tt class="literal">0-4</tt>).  Anything else is suspect.  When in doubt,
spell out the character sets in full: <tt class="literal">ABCDE</tt>.</p>

<p>The <em class="replaceable">SEARCHLIST</em> and <em class="replaceable">REPLACEMENTLIST</em> are not variable interpolated as
double-quoted strings; you may, however, use those backslash sequences
that map to a specific character, such as <tt class="literal">\n</tt> or
<tt class="literal">\015</tt>.</p>

<p>
<a name="INDEX-1440"></a><a href="ch05_02.htm#perl3-tab-trmods">Table 5-3</a> lists the modifiers applicable to
the <tt class="literal">tr///</tt> operator.  They're completely different
from those you apply to <tt class="literal">m//</tt>,
<tt class="literal">s///</tt>, or <tt class="literal">qr//</tt>, even if some look
the same.</p>

<a name="perl3-tab-trmods"></a>
<h4 class="objtitle">Table 5.3. tr/// Modifiers</h4>
<table border="1">

<tr>
<th>Modifier</th>
<th>Meaning</th>
</tr>




<tr>
<td><tt class="literal">/c</tt></td>
<td>Complement <em class="replaceable">SEARCHLIST</em>.<a name="INDEX-1441"></a></td>
</tr>

<tr>
<td><tt class="literal">/d</tt></td>
<td>Delete found but unreplaced characters.<a name="INDEX-1442"></a><a name="INDEX-1443"></a></td>
</tr>

<tr>
<td><tt class="literal">/s</tt></td>
<td>Squash duplicate replaced characters.<a name="INDEX-1444"></a><a name="INDEX-1445"></a></td>
</tr>


</table>

<p>If the <tt class="literal">/c</tt> modifier is specified, the character set in <em class="replaceable">SEARCHLIST</em>
is complemented; that is, the effective search list consists of all the
characters <em class="emphasis">not</em> in <em class="replaceable">SEARCHLIST</em>.  In the case of Unicode, this can represent a <em class="emphasis">lot</em> of characters, but since they're stored logically,
not physically, you don't need to worry about running out of memory.</p>

<p>The <tt class="literal">/d</tt> modifier turns <tt class="literal">tr///</tt> into
what might be called the "transobliteration" operator: any characters
specified by <em class="replaceable">SEARCHLIST</em> but not given a
replacement in <em class="replaceable">REPLACEMENTLIST</em> are
deleted. (This is slightly more flexible than the behavior of some
<em class="emphasis">tr</em>(1) programs, which delete anything they
find in <em class="replaceable">SEARCHLIST</em>, period.)</p>

<p>If the <tt class="literal">/s</tt> modifier is specified, sequences of characters converted
to the same character are squashed down to a single instance of the
character.</p>

<p>
<a name="INDEX-1446"></a>
If the <tt class="literal">/d</tt> modifier is used, <em class="replaceable">REPLACEMENTLIST</em> is always interpreted
exactly as specified.  Otherwise, if <em class="replaceable">REPLACEMENTLIST</em> is shorter than
<em class="replaceable">SEARCHLIST</em>, the final character is replicated until it is long
enough.  If <em class="replaceable">REPLACEMENTLIST</em> is null, the <em class="replaceable">SEARCHLIST</em> is
replicated, which is surprisingly useful if you just want to count
characters, not change them.  It's also useful for squashing
characters using <tt class="literal">/s</tt>.
<blockquote>
<pre class="programlisting">tr/aeiou/!/;                 # change any vowel into !
tr{/\\\r\n\b\f. }{_};        # change strange chars into an underscore

tr/A-Z/a-z/ for @ARGV;       # canonicalize to lowercase ASCII

$count = ($para =~ tr/\n//); # count the newlines in $para
$count = tr/0-9//;           # count the digits in $_

$word =~ tr/a-zA-Z//s;       # bookkeeper -&gt; bokeper

tr/@$%*//d;                  # delete any of those
tr#A-Za-z0-9+/##cd;          # remove non-base64 chars

# change en passant
($HOST = $host) =~ tr/a-z/A-Z/;

$pathname =~ tr/a-zA-Z/_/cs; # change non-(ASCII)alphas to single underbar

tr [\200-\377]
   [\000-\177];              # strip 8th bit, bytewise</pre>
</blockquote>

If the same character occurs more than once in <em class="replaceable">SEARCHLIST</em>,
only the first is used.  Therefore, this:
<blockquote>
<pre class="programlisting">tr/AAA/XYZ/</pre>
</blockquote>

will change any single character A to an X (in <tt class="literal">$_</tt>).</p>

<p>
<a name="INDEX-1447"></a><a name="INDEX-1448"></a>
Although variables aren't interpolated into <tt class="literal">tr///</tt>, you can still
get the same effect by using <tt class="literal">eval</tt><em class="replaceable">EXPR</em>:
<blockquote>
<pre class="programlisting">$count = eval "tr/$oldlist/$newlist/";
die if $@;  # propagates exception from illegal eval contents</pre>
</blockquote>
<a name="INDEX-1449"></a><a name="INDEX-1450"></a>
</p>

<p>
<a name="INDEX-1451"></a><a name="INDEX-1452"></a>
One more note: if you want to change your text to uppercase or
lowercase, don't use <tt class="literal">tr///</tt>.  Use the
<tt class="literal">\U</tt> or <tt class="literal">\L</tt> sequences in a
double-quoted string (or the equivalent <tt class="literal">uc</tt> and
<tt class="literal">lc</tt> functions) since they will pay attention to
locale or Unicode information and <tt class="literal">tr/a-z/A-Z/</tt>
won't.  Additionally, in Unicode strings, the <tt class="literal">\u</tt>
sequence and its corresponding <tt class="literal">ucfirst</tt> function
understand the notion of titlecase, which for some languages may be
distinct from simply converting to uppercase.</p>

<a name="INDEX-1453"></a>





<a name="INDEX-1454"></a><a name="INDEX-1455"></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_01.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_03.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td>
</tr>
<tr>
<td align="left" valign="top" width="172">5.1. The Regular Expression Bestiary</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">5.3. Metacharacters and Metasymbols</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>