ch05_01.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

(like <tt class="literal">\t</tt>) eat up some of the string as they match, and others (like
<tt class="literal">\b</tt>) don't.  But we usually reserve the term "assertion" for the
zero-width assertions.  To avoid confusion, we'll call the thing with
width an <em class="emphasis">atom</em>.  (If you're a physicist, you can think of
nonzero-width atoms as massive, in contrast to the zero-width
assertions, which are massless like photons.)</p>

<p>
<a name="INDEX-1285"></a><a name="INDEX-1286"></a>
You'll also see some metacharacters that aren't assertions; rather,
they're structural (just as braces and semicolons define the structure
of ordinary Perl code, but don't really do anything).  These structural
metacharacters are in some ways the most important ones because the
crucial first step in learning to read regular 

expressions is to teach

your eyes to pick out the structural metacharacters.  Once you've
learned that, reading regular expressions is a breeze.<a href="#FOOTNOTE-1">[1]</a>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-1"></a>
<p>[1]Admittedly, a stiff breeze at times, but not something that will blow
you away.</p>

</blockquote>

<p>
<a name="INDEX-1287"></a><a name="INDEX-1288"></a><a name="INDEX-1289"></a>
One such structural metacharacter is the vertical bar, which indicates
<em class="emphasis">alternation</em>:
<blockquote>
<pre class="programlisting">/Frodo|Pippin|Merry|Sam/</pre>
</blockquote>
<a name="INDEX-1290"></a><a name="INDEX-1291"></a>
</p>

<p>That means that any of those strings can trigger a match; this is
covered in <a href="ch05_08.htm#ch05-sect-alt">Section 5.8, "Alternation"</a> later in the chapter.  And in <a href="ch05_07.htm#ch05-sect-candc">Section 5.7, "Capturing and Clustering"</a> after that, we'll show you how to use parentheses around
portions of your pattern to do <em class="emphasis">grouping</em>:
<blockquote>
<pre class="programlisting">/(Frodo|Drogo|Bilbo) Baggins/</pre>
</blockquote>

or even:
<blockquote>
<pre class="programlisting">/(Frod|Drog|Bilb)o Baggins/</pre>
</blockquote>
</p>

<p>
<a name="INDEX-1292"></a><a name="INDEX-1293"></a>
Another thing you'll see are what we call <em class="emphasis">quantifiers</em>, which say how
many of the previous thing should match in a row.
Quantifiers look like this:
<blockquote>
<pre class="programlisting">*  +  ?  *?  {3}  {2,5}</pre>
</blockquote>
<a name="INDEX-1294"></a><a name="INDEX-1295"></a>
You'll never see them in isolation like that, though.  Quantifiers
only make sense when attached to atoms--that is, to assertions that
have width.<a href="#FOOTNOTE-2">[2]</a> Quantifiers attach to
the previous atom only, which in human terms means they normally
quantify only one character.  If you want to match three copies of
"<tt class="literal">bar</tt>" in a row, you need to group the individual
characters of "<tt class="literal">bar</tt>" into a single "molecule" with
parentheses, like this:
<blockquote>
<pre class="programlisting">/(bar){3}/</pre>
</blockquote>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-2"></a>
<p>[2] Quantifiers are a bit like the statement
modifiers in
<a href="ch04_01.htm">Chapter 4, "Statements and Declarations"</a>, which can only
attach to a single statement.  Attaching a quantifier to a zero-width
assertion would be like trying to attach a <tt class="literal">while</tt>
modifier to a declaration--either of which makes about as much sense
as asking your local apothecary for a pound of photons.  Apothecaries
only deal in atoms and such.</p>

</blockquote>

<p>That will match "<tt class="literal">barbarbar</tt>".  If you'd said <tt class="literal">/bar{3}/</tt>, that
would match "<tt class="literal">barrr</tt>"--which might qualify you as Scottish but
disqualify you as barbarbaric.  (Then again, maybe not.  Some of our
favorite metacharacters are Scottish.)  For more on quantifiers,
see "Quantifiers" later.</p>

<p>Now that you've seen a few of the beasties that inhabit regular
expressions, you're probably anxious to start taming them.  However,
before we discuss regular expressions in earnest, we need to backtrack
a little and talk about the pattern-matching operators that make use of
regular expressions.  (And if you happen to spot a few more regex
beasties along the way, just leave a decent tip for the tour guide.)</p>

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