Enter input string to search: 5
No match found.

Enter your regex: [0-4[6-8]]
Enter input string to search: 6
I found the text "6" starting at index 0 and ending at index 1.

Enter your regex: [0-4[6-8]]
Enter input string to search: 8
I found the text "8" starting at index 0 and ending at index 1.

Enter your regex: [0-4[6-8]]
Enter input string to search: 9
No match found.</pre>

<div id="h4"><a name="reg3_1_4"></a>3.1.4　交集<span class="returnContents"><a href="#contents">返回目錄</a></span></div>
　　建一個僅僅匹配自身嵌套類中公共部分字符的字符類時，可以像<code>[0-9&&[345]]</code>中那樣使用<code>&&</code>。這種方式構建出來的<em>交集</em>（intersection）簡單字符類，僅僅以匹配兩個字符類中的 3，4，5 共有部分。

<pre id="console">Enter your regex: [0-9&&[345]]
Enter input string to search: 3
I found the text "3" starting at index 0 and ending at index 1.

Enter your regex: [0-9&&[345]]
Enter input string to search: 4
I found the text "4" starting at index 0 and ending at index 1.

Enter your regex: [0-9&&[345]]
Enter input string to search: 5
I found the text "5" starting at index 0 and ending at index 1.

Enter your regex: [0-9&&[345]]
Enter input string to search: 2
No match found.

Enter your regex: [0-9&&[345]]
Enter input string to search: 6
No match found.</pre>

　　下面演示兩個范圍交集的例子：

<pre id="console">Enter your regex: [2-8&&[4-6]]
Enter input string to search: 3
No match found.

Enter your regex: [2-8&&[4-6]]
Enter input string to search: 4
I found the text "4" starting at index 0 and ending at index 1.

Enter your regex: [2-8&&[4-6]]
Enter input string to search: 5
I found the text "5" starting at index 0 and ending at index 1.

Enter your regex: [2-8&&[4-6]]
Enter input string to search: 6
I found the text "6" starting at index 0 and ending at index 1.

Enter your regex: [2-8&&[4-6]]
Enter input string to search: 7
No match found.</pre>

<div id="h4"><a name="reg3_1_5"></a>3.1.5　差集<span class="returnContents"><a href="#contents">返回目錄</a></span></div>
　　最后，可以使用<em>差集</em>（subtraction）來否定一個或多個嵌套的字符類，比如：<code>[0-9&&[^345]]</code>，這個是構建一個匹配除 3，4，5 之外所有 0 到 9 間數字的簡單字符類。

<pre id="console">Enter your regex: [0-9&&[^345]]
Enter input string to search: 2
I found the text "2" starting at index 0 and ending at index 1.

Enter your regex: [0-9&&[^345]]
Enter input string to search: 3
No match found.

Enter your regex: [0-9&&[^345]]
Enter input string to search: 4
No match found.

Enter your regex: [0-9&&[^345]]
Enter input string to search: 5
No match found.

Enter your regex: [0-9&&[^345]]
Enter input string to search: 6
I found the text "6" starting at index 0 and ending at index 1.

Enter your regex: [0-9&&[^345]]
Enter input string to search: 9
I found the text "9" starting at index 0 and ending at index 1.</pre>

　　到此為止，已經涵蓋了如何建立字符類的部分。在繼續下一節之前，可以試著回想一下那張<a href="#fig1">字符類表</a>。

<div id="h2"><a name="reg4"></a>4　預定義字符類<span class="returnContents"><a href="#contents">返回目錄</a></span></div>
　　Pattern 的 API 包有許多有用的<em>預定義字符類</em>（predefined character classes），提供了常用正則表達式的簡寫形式。<br/>
<table border="0" cellpadding="0" cellspacing="0" class="regTab" align="center">
  <caption>預定義字符類</caption>
  <tr>
    <td class="regCenter"><code>.</code></td>
    <td>任何字符（匹配或者不匹配行結束符）</td>
  </tr>
  <tr>
    <td class="regCenter"><code>\d</code></td>
    <td>數字字符：<code>[0-9]</code></td>
  </tr>
  <tr>
    <td class="regCenter"><code>\D</code></td>
    <td>非數字字符：<code>[^0-9]</code></td>
  </tr>
  <tr>
    <td class="regCenter"><code>\s</code></td>
    <td>空白字符：<code>[\t\n\x0B\f\r]</code></td>
  </tr>
  <tr>
    <td class="regCenter"><code>\S</code></td>
    <td>非空白字符：<code>[^\s]</code></td>
  </tr>
  <tr>
    <td class="regCenter"><code>\w</code></td>
    <td>單詞字符：<code>[a-zA-Z_0-9]</code></td>
  </tr>
  <tr>
    <td class="regCenter"><code>\W</code></td>
    <td>非單詞字符：<code>[^\w]</code></td>
  </tr>
</table>
　　上表中，左列是構造右列字符類的簡寫形式。例如：<code>\d</code>指的是數字范圍（0～9），<code>\w</code>指的是單詞字符（任何大小寫字母、下劃線或者是數字）。無論何時都有可能使用預定義字符類，它可以使代碼更易閱讀，更易從難看的字符類中排除錯誤。<br/>
　　以反斜線（<code>\</code>）開始的構造稱為<em>轉義構造</em>（escaped constructs）。回顧一下在 <a href="#reg2">字符串</a> 一節中的轉義構造，在那里我們提及了使用反斜線，以及用于引用的<code>\Q</code>和<code>\E</code>。在字符串中使用轉義構造，必須在一個反斜線前再增加一個反斜用于字符串的編譯，例如：<br/>
<pre name="java" id="java">private final String REGEX = "\\d";        // 單個數字</pre>

　　這個例子中<code>\d</code>是正則表達式，另外的那個反斜線是用于代碼編譯所必需的。但是測試用具讀取的表達式，是直接從控制臺中輸入的，因此不需要那個多出來的反斜線。<br/>

　　下面的例子說明了預字義字符類的用法：<br/>

<pre id="console">Enter your regex: .
Enter input string to search: @
I found the text "@" starting at index 0 and ending at index 1.

Enter your regex: .
Enter input string to search: 1
I found the text "1" starting at index 0 and ending at index 1.

Enter your regex: .
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.

Enter your regex: \d
Enter input string to search: 1
I found the text "1" starting at index 0 and ending at index 1.

Enter your regex: \d
Enter input string to search: a
No match found.

Enter your regex: \D
Enter input string to search: 1
No match found.

Enter your regex: \D
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.

Enter your regex: \s
Enter input string to search:  
I found the text " " starting at index 0 and ending at index 1.

Enter your regex: \s
Enter input string to search: a
No match found.

Enter your regex: \S
Enter input string to search:  
No match found.

Enter your regex: \S
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.

Enter your regex: \w
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.

Enter your regex: \w
Enter input string to search: !
No match found.

Enter your regex: \W
Enter input string to search: a
No match found.

Enter your regex: \W
Enter input string to search: !
I found the text "!" starting at index 0 and ending at index 1.</pre>

　　在開始的三個例子中，正則表達式是簡單的，<code>.</code>（“點”元字符）表示“任意字符”，因此，在所有的三個例子（隨意地選取了“@”字符，數字和字母）中都是匹配成功的。在接下來的例子中，都使用了預定義字符類表格中的單個正則表達式構造。你應該可以根據這張表指出前面每個匹配的邏輯：<br/>
　　<code>\d</code> 匹配數字字符<br/>
　　<code>\s</code> 匹配空白字符<br/>
　　<code>\w</code> 匹配單詞字符<br/>
　　也可以使用意思正好相反的大寫字母：<br/>
　　<code>\D</code> 匹配非數字字符<br/>
　　<code>\S</code> 匹配非空白字符<br/>
　　<code>\W</code> 匹配非單詞字符<br/>

<div id="h2"><a name="reg5"></a>5　量詞<span class="returnContents"><a href="#contents">返回目錄</a></span></div>
　　這一節我們來看一下貪婪（greedy）、勉強（reluctant）和侵占（possessive）量詞，來匹配指定表達式<code>X</code>的次數。<br/>
　　<em>量詞</em>（quantifiers）允許指定匹配出現的次數，方便起見，當前 Pattern API 規范下，描述了貪婪、勉強和侵占三種量詞。首先粗略地看一下，量詞<code>X?</code>、<code>X??</code>和<code>X?+</code>都允許匹配 X 零次或一次，精確地做同樣的事情，但它們之間有著細微的不同之處，在這節結束前會進行說明。<br/>

<table border="0" cellpadding="0" cellspacing="0" class="regTab" align="center">
  <thead>
    <tr>
      <td colspan="3">量　詞　種　類</td>
      <td rowspan="2">意　　義</td>
    </tr>
    <tr>
      <td>貪婪</td>
      <td>勉強</td>