Java正则表达式初学者指南(转)

作者:duketian 日期:2008-07-11 10:20:29

字体大小: 小中大

本文将介绍如何在Java中使用正则表达式来处理文本数据。正则表达式就是一个字符串，但和普通的字符串不同的是，正则表达式是对一组相似字符串的抽象，如下面的几个字符串：

a98b c0912d c10b a12345678d ab

我们仔细分析上面五个字符串，可以看出它们有一个共同特征，就是第一个字符必须是'a'或'c'，最后一个字符必须是'b'或'd'，而中间的字符是任意多个数字组成（包括0个数字）。因此，我们可以将这五个字符串的共同特点抽象出来，这就产生了一个正则表达式：[ac]\\d*[bd]。而根据这个正则表达式，我们可以写出无穷多个满足条件的字符串。

在Java中使用正则表达式的方法非常多，最简单的就是和字符串一起使用。在String中有四个方法可以使用正则表达式，它们是matches、split、replaceAll和replaceFirst。

一、matches方法

matches方法可以判断当前的字符串是否匹配给定的正则表达式。如果匹配，返回true，否则，返回false。matches方法的定义如下：

public boolean matches(String regex)

如上面给出的正则表达式我们可以用如下程序验证。

1
2
3

String[] ss = new String[]{"a98b", "c0912d",  "c10b",  "a12345678d",  "ab"};
for(String s: ss)
    System.out.println(s.matches("[ac]\\d*[bd]"));

输出结果：

true
true
true
true
true

下面简单解释一下这个正则表达式的含义。如果我们学过编译原理的词法分析，就会很容易理解上面的正则表达式（因为正则表达式的表示方法和词法分析中的表达式类似）。如在 [...]中的相当于或"|"，如[abcd]相当于a|b|c|d，也就是a或b或c或d。如上面的正则表达式的开头部分是[ac]，就代表着字符串的开头只能是a或c。[bd]表达字符串结尾只能是b或d。而中间的\d表达0-9的数字，由于\在正则表达式中有特殊含义，所以用\\来表示\。而*表示有0或无穷多个（这在词法分析中叫*闭包），由于*跟在\d后面，因此表达有0或无穷多个数字。

二、split方法

split方法使用正则表达式来分割字符串，并以String数组的形式返回分割结果。split有两种重载形式，它们定义如下：

public String[] split(String regex)
public String[] split(String regex, int limit)
如下面的代码将使用split的第一种重载形式来分割HTTP请求头的第一行，代码如下：

 
String s = "GET /index.html HTTP/1.1";
String ss[] = s.split(" +");
for(String str: ss)
System.out.println(str);

输出结果：
GET
/index.html
HTTP/1.1

在使用split的第一种重载形式时应注意，如果分割后的字符串最后有空串，将被忽略。如使用正则表达式\d来分割字符串a0b1c3456时，得到的数组的长度为3，而不是7。
在split的第二种重载形式中有一个limit参数，要分三种情况讨论：

1. 大于0：如limit的值为n，那么将对正则表达式使用n-1次，下面的代码：

String s = "a0b1c3456";
String ss[] = s.split("\\d", 3);
for(String str: ss)
    System.out.println(str);

输出结果:

a
b
c3456

从输出结果可以看出，程序只对" a0b1c3456"使用了两次正则表达式，也就是在少扫描完字符'1'后，不管后面有没有满足条件的字符串，都将后面的字符串作为一个整体来作为返回数组的最后一个值。

2. 小于0：不忽略结尾的空串。也就是上面的例子返回数组的长度应该是7，而不是3。
3. 等于0：这是默认值，相当于split的第一种重载形式。

三、replaceAll 和 replaceFirst方法

为两个方法的定义如下：
public String replaceAll(String regex, String replacement)
public String replaceFirst(String regex, String replacement)

这两个方法用replacement替换当前字符串中和regex匹配的字符串。使用方法很简单，这里不再详述，感兴趣的读者可以参考相关的文档。

对于Java中正则表达式的详细内容，请参考JDK文档。

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