捕获组分类
- 普通捕获组(Expression)
- 命名捕获组(?<name>Expression)
普通捕获组
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
有4个命名的捕获组,分别是(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))
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(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}))
1
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(3)); // Exception in thread "main" java.lang.indexoutofboundsexception: No group 4 System.out.printf("\nmatcher.group(4) value:%s",matcher.group(4));