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正则表达式
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<h2 id="正则表达式">正则表达式</h2>
<p>正则表达式可以用<strong>字符串</strong>来描述规则，并用来匹配字符串。</p>
<p>正则表达式是一套标准，它可以用于任何语言。</p>
<p>正则表达式是一串字符串。</p>
<p>在JAVA中使用正则表达式匹配：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> String regex = <span class="string">"20\\d\\d"</span>;<span class="comment">//对应的正则表达式是20\d\d，匹配20##年</span></span><br><span class="line"> System.out.println(<span class="string">"2019"</span>.matches(regex)); <span class="comment">// true</span></span><br><span class="line"> System.out.println(<span class="string">"2100"</span>.matches(regex)); <span class="comment">// false</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<a id="more"></a>
<h3 id="匹配规则">匹配规则</h3>
<p>如何使用正则表达式来做精确匹配：</p>
<ol type="1">
<li><p>对于正则表达式<code>abc</code>来说，它只能精确地匹配字符串<code>"abc"</code>，不能匹配<code>"ab"</code>，<code>"Abc"</code>，<code>"abcd"</code>等其他任何字符串。</p></li>
<li><p>如果正则表达式有特殊字符，那就需要用<code>\</code>转义。例如，正则表达式<code>a\&c</code>，其中<code>\&</code>是用来匹配特殊字符<code>&</code>的，它能精确匹配字符串<code>"a&c"</code>，但不能匹配<code>"ac"</code>、<code>"a-c"</code>、<code>"a&&c"</code>等。</p></li>
</ol>
<blockquote>
<p>要注意正则表达式在Java代码中也是一个字符串，所以，对于正则表达式<code>a\&c</code>来说，对应的Java字符串是<code>"a\\&c"</code>，因为<code>\</code>也是Java字符串的转义字符，两个<code>\\</code>实际上表示的是一个<code>\</code>。</p>
</blockquote>
<ol start="3" type="1">
<li>如果想匹配非ASCII字符，例如中文，那就用<code>\u####</code>的十六进制表示，例如：<code>a\u548cc</code>匹配字符串<code>"a和c"</code>，中文字符<code>和</code>的Unicode编码是<code>548c</code>。</li>
</ol>
<h4 id="总结">总结</h4>
<table>
<thead>
<tr class="header">
<th></th>
<th>匹配规则</th>
<th></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>匹配任意字符</td>
<td><code>.</code></td>
<td></td>
</tr>
<tr class="even">
<td>匹配数字</td>
<td><code>\d</code></td>
<td></td>
</tr>
<tr class="odd">
<td>匹配常用字符</td>
<td><code>\w</code></td>
<td>字母、数字或下划线</td>
</tr>
<tr class="even">
<td>匹配空格字符</td>
<td><code>\s</code></td>
<td>包括空格，还包括tab字符</td>
</tr>
<tr class="odd">
<td>匹配任意个字符</td>
<td><code>*</code></td>
<td></td>
</tr>
<tr class="even">
<td>匹配至少一个字符</td>
<td><code>+</code></td>
<td></td>
</tr>
<tr class="odd">
<td>匹配0个或一个字符</td>
<td><code>?</code></td>
<td></td>
</tr>
<tr class="even">
<td>精确指定n个字符</td>
<td><code>{n}</code></td>
<td></td>
</tr>
<tr class="odd">
<td>指定匹配n~m个字符</td>
<td><code>{n,m}</code></td>
<td>如果没有上限，那么修饰符<br><code>{n,}</code>就可以匹配至少n个字符</td>
</tr>
<tr class="even">
<td>匹配开头和结尾</td>
<td><code>^</code>表示开头<br><code>$</code>表示结尾</td>
<td><code>^A\d{3}$</code>，可以匹配<br><code>"A001"</code>、<code>"A380"</code></td>
</tr>
<tr class="odd">
<td>匹配范围内字符</td>
<td><code>[...]</code></td>
<td></td>
</tr>
</tbody>
</table>
<ul>
<li><code>\d</code>可以匹配一个数字，而<code>\D</code>则匹配一个非数字，<code>\W</code>可以匹配<code>\w</code>不能匹配的字符，<code>\S</code>可以匹配<code>\s</code>不能匹配的字符，这几个正好是反着来的。</li>
</ul>
<h3 id="分组匹配">分组匹配</h3>
<p>如果要从匹配的字符串中提取部分匹配的子串，就需要引入<code>java.util.regex</code>包，用<code>Pattern</code>对象，来进行匹配，匹配后获得一个Matcher对象，具体用法如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> java.util.regex.*;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> Pattern p = Pattern.compile(<span class="string">"(\\d{3,4})\\-(\\d{7,8})"</span>);</span><br><span class="line"> Matcher m = p.matcher(<span class="string">"010-12345678"</span>);</span><br><span class="line"> <span class="keyword">if</span> (m.matches()) {</span><br><span class="line"> String g1 = m.group(<span class="number">1</span>);</span><br><span class="line"> String g2 = m.group(<span class="number">2</span>);</span><br><span class="line"> System.out.println(g1);</span><br><span class="line"> System.out.println(g2);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> System.out.println(<span class="string">"匹配失败!"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<ul>
<li><p><code>Matcher.group(index)</code>方法的参数用1表示第一个子串，2表示第二个子串。如果我们传入0会得到什么呢？答案是<code>010-12345678</code>，即整个正则匹配到的字符串。</p></li>
<li><p><code>String.matches()</code>方法内部调用的就是<code>Pattern</code>和<code>Matcher</code>类的方法。但是反复使用<code>String.matches()</code>对同一个正则表达式进行多次匹配效率较低，因为每次都会创建出一样的<code>Pattern</code>对象。完全可以先创建出一个<code>Pattern</code>对象，然后反复使用，就可以实现编译一次，多次匹配：</p></li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> java.util.regex.*;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> Pattern pattern = Pattern.compile(<span class="string">"(\\d{3,4})\\-(\\d{7,8})"</span>);</span><br><span class="line"> pattern.matcher(<span class="string">"010-12345678"</span>).matches(); <span class="comment">// true</span></span><br><span class="line"> pattern.matcher(<span class="string">"021-123456"</span>).matches(); <span class="comment">// true</span></span><br><span class="line"> pattern.matcher(<span class="string">"022#1234567"</span>).matches(); <span class="comment">// false</span></span><br><span class="line"> <span class="comment">// 获得Matcher对象:</span></span><br><span class="line"> Matcher matcher = pattern.matcher(<span class="string">"010-12345678"</span>);</span><br><span class="line"> <span class="keyword">if</span> (matcher.matches()) {</span><br><span class="line"> String whole = matcher.group(<span class="number">0</span>); <span class="comment">// "010-12345678", 0表示匹配的整个字符串</span></span><br><span class="line"> String area = matcher.group(<span class="number">1</span>); <span class="comment">// "010", 1表示匹配的第1个子串</span></span><br><span class="line"> String tel = matcher.group(<span class="number">2</span>); <span class="comment">// "12345678", 2表示匹配的第2个子串</span></span><br><span class="line"> System.out.println(area);</span><br><span class="line"> System.out.println(tel);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p><strong>贪婪匹配</strong></p>
<ul>
<li><p>正则表达式匹配默认使用贪婪匹配，可以使用<code>?</code>表示对某一规则进行非贪婪匹配。</p></li>
<li><p>注意区分<code>?</code>的含义：<code>\d??</code>。</p></li>
</ul>
<h3 id="使用正则表达式可以干嘛">使用正则表达式可以干嘛</h3>
<p><strong>分割字符串：</strong><code>String.split()</code>方法传入的是正则表达式</p>
<p><strong>搜索字符串：</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> String s = <span class="string">"the quick brown fox jumps over the lazy dog."</span>;</span><br><span class="line"> Pattern p = Pattern.compile(<span class="string">"\\wo\\w"</span>);</span><br><span class="line"> Matcher m = p.matcher(s);</span><br><span class="line"> <span class="keyword">while</span> (m.find()) {</span><br><span class="line"> String sub = s.substring(m.start(), m.end());</span><br><span class="line"> System.out.println(sub);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<blockquote>
<p>row fox dog</p>
</blockquote>
<p><strong>替换字符串：</strong>使用正则表达式替换字符串可以直接调用<code>String.replaceAll()</code>，它的第一个参数是正则表达式，第二个参数是待替换的字符串。</p>
<p><strong>反向引用：</strong>如果我们要引用匹配到的字符串，比如前后各加一个<code><b>xxxx</b></code>，这个时候，使用<code>replaceAll()</code>的时候，我们传入的第二个参数可以使用<code>$1</code>、<code>$2</code>来反向引用匹配到的子串。</p>
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