Java 正则表达式的捕获组(capture group)

捕获组分为：

• 普通捕获组(Expression)
• 命名捕获组(?Expression)

普通捕获组

从正则表达式左侧开始，每出现一个左括号"("记做一个分组，分组编号从 1 开始。0 代表整个表达式。

对于时间字符串：2017-04-25，表达式如下

(\\d{4})-((\\d{2})-(\\d{2}))

有 4 个左括号，所以有 4 个分组:

public static final String DATE_STRING = "2017-04-25";
public static final String P_COMM = "(\\d{4})-((\\d{2})-(\\d{2}))";
 
Pattern pattern = Pattern.compile(P_COMM);
Matcher matcher = pattern.matcher(DATE_STRING);
matcher.find();//必须要有这句
System.out.printf("\nmatcher.group(0) value:%s", matcher.group(0));
System.out.printf("\nmatcher.group(1) value:%s", matcher.group(1));
System.out.printf("\nmatcher.group(2) value:%s", matcher.group(2));
System.out.printf("\nmatcher.group(3) value:%s", matcher.group(3));
System.out.printf("\nmatcher.group(4) value:%s", matcher.group(4));

命名捕获组

每个以左括号开始的捕获组，都紧跟着?，而后才是正则表达式。

对于时间字符串：2017-04-25，表达式如下:

(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))

有 4 个命名的捕获组，分别是:

命名的捕获组同样也可以使用编号获取相应值。

public static final String P_NAMED = "(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))";
public static final String DATE_STRING = "2017-04-25";
 
Pattern pattern = Pattern.compile(P_NAMED);
Matcher matcher = pattern.matcher(DATE_STRING);
matcher.find();
System.out.printf("\n===========使用名称获取=============");
System.out.printf("\nmatcher.group(0) value:%s", matcher.group(0));
System.out.printf("\n matcher.group('year') value:%s", matcher.group("year"));
System.out.printf("\nmatcher.group('md') value:%s", matcher.group("md"));
System.out.printf("\nmatcher.group('month') value:%s", matcher.group("month"));
System.out.printf("\nmatcher.group('date') value:%s", matcher.group("date"));
matcher.reset();
System.out.printf("\n===========使用编号获取=============");
matcher.find();
System.out.printf("\nmatcher.group(0) value:%s", matcher.group(0));
System.out.printf("\nmatcher.group(1) value:%s", matcher.group(1));
System.out.printf("\nmatcher.group(2) value:%s", matcher.group(2));
System.out.printf("\nmatcher.group(3) value:%s", matcher.group(3));
System.out.printf("\nmatcher.group(4) value:%s", matcher.group(4));

PS:非捕获组

在左括号后紧跟?:，而后再加上正则表达式，构成非捕获组(?:Expression)。

对于时间字符串：2017-04-25，表达式如下:

(?:\\d{4})-((\\d{2})-(\\d{2}))

这个正则表达式虽然有四个左括号，理论上有 4 个捕获组。但是第一组(?:\d{4})，其实是被忽略的。当使用 matcher.group(4) 时，系统会报错。

public static final String P_UNCAP = "(?:\\d{4})-((\\d{2})-(\\d{2}))";
public static final String DATE_STRING = "2017-04-25";
 
Pattern pattern = Pattern.compile(P_UNCAP);
Matcher matcher = pattern.matcher(DATE_STRING);
matcher.find();
System.out.printf("\nmatcher.group(0) value:%s", matcher.group(0));
System.out.printf("\nmatcher.group(1) value:%s", matcher.group(1));
System.out.printf("\nmatcher.group(2) value:%s", matcher.group(2));
System.out.printf("\nmatcher.group(3) value:%s", matcher.group(3));
 
// Exception in thread "main" java.lang.IndexOutOfBoundsException: No group 4
System.out.printf("\nmatcher.group(4) value:%s", matcher.group(4));

总结

• 普通捕获组使用方便；
• 命名捕获组使用清晰；
• 非捕获组目前在项目中还没有用武之地。