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<h2 class="title"><a name="soundex.stage1"></a>18.3. 优化正则表达式
</h2>
</div>
</div>
<div></div>
</div>
<div class="abstract">
<p> Soundex 函数的第一件事是检查输入是否是一个空字符串。怎样做是最好的方法？</p>
</div>
<p>如果你回答 “<span class="quote">正则表达式</span>”，坐在角落里反省你糟糕的直觉。正则表达式几乎永远不是最好的答案，而且应该被尽可能避开。这不仅仅是基于性能考虑，而是因为调试和维护都很困难，当然性能也是个原因。
</p>
<p>这是 <tt class="filename">soundex/stage1/soundex1a.py</tt> 检查 <tt class="varname">source</tt> 是否全部由字母构成的一段代码，至少是一个字母 (而不是空字符串)：
</p>
<div class="informalexample"><pre class="programlisting">
allChars = string.uppercase + string.lowercase
<span class='pykeyword'>if</span> <span class='pykeyword'>not</span> re.search(<span class='pystring'>'^[%s]+$'</span> % allChars, source):
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
</pre></div>
<p><tt class="filename">soundex1a.py</tt> 表现如何？为了方便，<tt class="literal">__main__</tt> 部分包含了一段代码：调用 <tt class="filename">timeit</tt> 模块，为三个不同名字分别建立测试，依次测试，并显示每个测试的最短耗时：
</p>
<div class="informalexample"><pre class="programlisting"><span class='pykeyword'>
if</span> __name__ == <span class='pystring'>'__main__'</span>:
<span class='pykeyword'>from</span> timeit <span class='pykeyword'>import</span> Timer
names = (<span class='pystring'>'Woo'</span>, <span class='pystring'>'Pilgrim'</span>, <span class='pystring'>'Flingjingwaller'</span>)
<span class='pykeyword'>for</span> name <span class='pykeyword'>in</span> names:
statement = <span class='pystring'>"soundex('%s')"</span> % name
t = Timer(statement, <span class='pystring'>"from __main__ import soundex"</span>)
<span class='pykeyword'>print</span> name.ljust(15), soundex(name), min(t.repeat())
</pre></div>
<p>那么，应用正则表达式的 <tt class="filename">soundex1a.py</tt> 表现如何呢？
</p>
<div class="informalexample"><pre class="screen">
<tt class="prompt">C:\samples\soundex\stage1></tt><span class="userinput">python soundex1a.py</span>
<span class="computeroutput">Woo W000 19.3356647283
Pilgrim P426 24.0772053431
Flingjingwaller F452 35.0463220884</span>
</pre></div>
<p>正如你预料，名字越长，算法耗时就越长。有几个工作可以令我们减小这个差距 (使函数对于长输入花费较短的相对时间) 但是算法的本质决定它不可能每次运行时间都相同。</p>
<p>另一点应铭记于心的是，我们测试的是有代表性的名字样本。<tt class="literal">Woo</tt> 是个被缩短到单字符并补零的小样本；<tt class="literal">Pilgrim</tt> 是个夹带着特别字符和忽略字符的平均长度的正常样本；<tt class="literal">Flingjingwaller</tt> 是一个包含连续重复字符并且特别长的样本。其它的测试可能同样有帮助，但它们已经很好地代表了不同的样本范围。
</p>
<p>那么那个正则表达式如何呢？嗯，缺乏效率。因为这个表达式测试不止一个范围的字符 (<tt class="literal">A-Z</tt> 的大写范围和 <tt class="literal">a-z</tt> 的小写字母范围)，我们可以使用一个正则表达式的缩写语法。这便是 <tt class="filename">soundex/stage1/soundex1b.py</tt>:
</p>
<div class="informalexample"><pre class="programlisting">
<span class='pykeyword'>if</span> <span class='pykeyword'>not</span> re.search(<span class='pystring'>'^[A-Za-z]+$'</span>, source):
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
</pre></div>
<p><tt class="filename">timeit</tt> 显示 <tt class="filename">soundex1b.py</tt> 比 <tt class="filename">soundex1a.py</tt> 稍微快一些，但是没什么令人激动的变化：
</p>
<div class="informalexample"><pre class="screen">
<tt class="prompt">C:\samples\soundex\stage1></tt><span class="userinput">python soundex1b.py</span>
<span class="computeroutput">Woo W000 17.1361133887
Pilgrim P426 21.8201693232
Flingjingwaller F452 32.7262294509</span>
</pre></div>
<p>在 <a href="../refactoring/refactoring.html" title="15.3. 重构">第 15.3 节 “重构”</a> 中我们看到正则表达式可以被编译并在重用时以更快速度获得结果。因为这个正则表达式在函数中每次被调用时都不变化，我们可以编译它一次并使用被编译的版本。这便是 <tt class="filename">soundex/stage1/soundex1c.py</tt>：
</p>
<div class="informalexample"><pre class="programlisting">
isOnlyChars = re.compile(<span class='pystring'>'^[A-Za-z]+$'</span>).search
<span class='pykeyword'>def</span><span class='pyclass'> soundex</span>(source):
<span class='pykeyword'>if</span> <span class='pykeyword'>not</span> isOnlyChars(source):
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
</pre></div>
<p><tt class="filename">soundex1c.py</tt> 中使用被编译的正则表达式产生了显著的提速：
</p>
<div class="informalexample"><pre class="screen">
<tt class="prompt">C:\samples\soundex\stage1></tt><span class="userinput">python soundex1c.py</span>
<span class="computeroutput">Woo W000 14.5348347346
Pilgrim P426 19.2784703084
Flingjingwaller F452 30.0893873383</span>
</pre></div>
<p>但是这样的优化是正路吗？这里的逻辑很简单：输入 <tt class="varname">source</tt> 应该是非空，并且需要完全由字母构成。如果编写一个循环查看每个字符并且抛弃正则表达式，是否会更快些？
</p>
<p>这便是 <tt class="filename">soundex/stage1/soundex1d.py</tt>：
</p>
<div class="informalexample"><pre class="programlisting">
<span class='pykeyword'>if</span> <span class='pykeyword'>not</span> source:
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
<span class='pykeyword'>for</span> c <span class='pykeyword'>in</span> source:
<span class='pykeyword'>if</span> <span class='pykeyword'>not</span> (<span class='pystring'>'A'</span> <= c <= <span class='pystring'>'Z'</span>) <span class='pykeyword'>and</span> <span class='pykeyword'>not</span> (<span class='pystring'>'a'</span> <= c <= <span class='pystring'>'z'</span>):
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
</pre></div>
<p>这个技术在 <tt class="filename">soundex1d.py</tt> 中恰好<span class="emphasis"><em>不及</em></span> 编译后的正则表达式快 (尽管比使用未编译的正则表达式快<sup>[<a name="d0e39385" href="#ftn.d0e39385">14</a>]</sup>)：
</p>
<div class="informalexample"><pre class="screen">
<tt class="prompt">C:\samples\soundex\stage1></tt><span class="userinput">python soundex1d.py</span>
<span class="computeroutput">Woo W000 15.4065058548
Pilgrim P426 22.2753567842
Flingjingwaller F452 37.5845122774</span>
</pre></div>
<p>为什么 <tt class="filename">soundex1d.py</tt> 没能更快？答案来自 <span class="application">Python</span> 的编译本质。正则表达式引擎以 C 语言编写，被编译后则能本能地在你的计算机上运行。另一方面，循环是以 <span class="application">Python</span> 编写，要通过 <span class="application">Python</span> 解释器。尽管循环相对简单，但没能简单到补偿花在代码解释上的时间。正则表达式永远不是正确答案……但例外还是存在的。
</p>
<p>恰巧 <span class="application">Python</span> 提供了一个晦涩的字符串方法。你有理由不了解它，因为本书未曾提到它。这个方法便是 <tt class="methodname">isalpha()</tt>，它检查一个字符串是否只包含字母。
</p>
<p>这便是 <tt class="filename">soundex/stage1/soundex1e.py</tt>：
</p>
<div class="informalexample"><pre class="programlisting">
<span class='pykeyword'>if</span> (<span class='pykeyword'>not</span> source) <span class='pykeyword'>and</span> (<span class='pykeyword'>not</span> source.isalpha()):
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
</pre></div>
<p>在 <tt class="filename">soundex1e.py</tt> 中应用这个特殊方法我们能得到多少好处? 很多。
</p>
<div class="informalexample"><pre class="screen">
<tt class="prompt">C:\samples\soundex\stage1></tt><span class="userinput">python soundex1e.py</span>
<span class="computeroutput">Woo W000 13.5069504644
Pilgrim P426 18.2199394057
Flingjingwaller F452 28.9975225902</span>
</pre></div>
<div class="example"><a name="d0e39452"></a><h3 class="title">例 18.3. 目前为止最好的结果：<tt class="filename">soundex/stage1/soundex1e.py</tt></h3><pre class="programlisting"><span class='pykeyword'>
import</span> string, re
charToSoundex = {<span class='pystring'>"A"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"B"</span>: <span class='pystring'>"1"</span>,
<span class='pystring'>"C"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"D"</span>: <span class='pystring'>"3"</span>,
<span class='pystring'>"E"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"F"</span>: <span class='pystring'>"1"</span>,
<span class='pystring'>"G"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"H"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"I"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"J"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"K"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"L"</span>: <span class='pystring'>"4"</span>,
<span class='pystring'>"M"</span>: <span class='pystring'>"5"</span>,
<span class='pystring'>"N"</span>: <span class='pystring'>"5"</span>,
<span class='pystring'>"O"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"P"</span>: <span class='pystring'>"1"</span>,
<span class='pystring'>"Q"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"R"</span>: <span class='pystring'>"6"</span>,
<span class='pystring'>"S"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"T"</span>: <span class='pystring'>"3"</span>,
<span class='pystring'>"U"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"V"</span>: <span class='pystring'>"1"</span>,
<span class='pystring'>"W"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"X"</span>: <span class='pystring'>"2"</span>,
<span class='pystring'>"Y"</span>: <span class='pystring'>"9"</span>,
<span class='pystring'>"Z"</span>: <span class='pystring'>"2"</span>}
<span class='pykeyword'>def</span><span class='pyclass'> soundex</span>(source):
<span class='pykeyword'>if</span> (<span class='pykeyword'>not</span> source) <span class='pykeyword'>and</span> (<span class='pykeyword'>not</span> source.isalpha()):
<span class='pykeyword'>return</span> <span class='pystring'>"0000"</span>
source = source[0].upper() + source[1:]
digits = source[0]
<span class='pykeyword'>for</span> s <span class='pykeyword'>in</span> source[1:]:
s = s.upper()
digits += charToSoundex[s]
digits2 = digits[0]
<span class='pykeyword'>for</span> d <span class='pykeyword'>in</span> digits[1:]:
<span class='pykeyword'>if</span> digits2[-1] != d:
digits2 += d
digits3 = re.sub(<span class='pystring'>'9'</span>, <span class='pystring'>''</span>, digits2)
<span class='pykeyword'>while</span> len(digits3) < 4:
digits3 += <span class='pystring'>"0"</span>
<span class='pykeyword'>return</span> digits3[:4]
<span class='pykeyword'>if</span> __name__ == <span class='pystring'>'__main__'</span>:
<span class='pykeyword'>from</span> timeit <span class='pykeyword'>import</span> Timer
names = (<span class='pystring'>'Woo'</span>, <span class='pystring'>'Pilgrim'</span>, <span class='pystring'>'Flingjingwaller'</span>)
<span class='pykeyword'>for</span> name <span class='pykeyword'>in</span> names:
statement = <span class='pystring'>"soundex('%s')"</span> % name
t = Timer(statement, <span class='pystring'>"from __main__ import soundex"</span>)
<span class='pykeyword'>print</span> name.ljust(15), soundex(name), min(t.repeat())
</pre></div>
<div class="footnotes">
<h3 class="footnotetitle">Footnotes</h3>
<div class="footnote">
<p><sup>[<a name="ftn.d0e39385" href="#d0e39385">14</a>] </sup>注意 <tt class="filename">soundex1d.py</tt> 在后两个测试点上都比 <tt class="filename">soundex1b.py</tt> 慢，这点与作者所说的矛盾。本章另还有多处出现了正文与测试结果矛盾的地方，每个地方都会用译注加以说明。这个 bug 将在下个版本中得到修正。――译注
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