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Java 正则表达式的捕获组(capture group)

综合 hailen 8℃

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:

编号 捕获组 匹配
0 (d{4})-((d{2})-(d{2})) 2017-04-25
1 (d{4}) 2017
2 ((d{2})-(d{2})) 04-25
3 (d{2}) 04
4 (d{2}) 25
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("
matcher.group(0) value:%s", matcher.group(0));
System.out.printf("
matcher.group(1) value:%s", matcher.group(1));
System.out.printf("
matcher.group(2) value:%s", matcher.group(2));
System.out.printf("
matcher.group(3) value:%s", matcher.group(3));
System.out.printf("
matcher.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:

编号 名称 捕获组 匹配
0 0 (?d{4})-(?(?d{2})-(?d{2})) 2017-04-25
1 year (?d{4})- 2017
2 md (?(?d{2})-(?d{2})) 04-25
3 month (?d{2}) 04
4 date (?d{2}) 25

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("
===========使用名称获取=============");
System.out.printf("
matcher.group(0) value:%s", matcher.group(0));
System.out.printf("
matcher.group(‘year’) value:%s", matcher.group("year"));
System.out.printf("
matcher.group(‘md’) value:%s", matcher.group("md"));
System.out.printf("
matcher.group(‘month’) value:%s", matcher.group("month"));
System.out.printf("
matcher.group(‘date’) value:%s", matcher.group("date"));
matcher.reset();
System.out.printf("
===========使用编号获取=============");
matcher.find();
System.out.printf("
matcher.group(0) value:%s", matcher.group(0));
System.out.printf("
matcher.group(1) value:%s", matcher.group(1));
System.out.printf("
matcher.group(2) value:%s", matcher.group(2));
System.out.printf("
matcher.group(3) value:%s", matcher.group(3));
System.out.printf("
matcher.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。

编号 捕获组 匹配
0 (d{4})-((d{2})-(d{2})) 2017-04-25
1 ((d{2})-(d{2})) 04-25
2 (d{2}) 04
3 (d{2}) 25
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("
matcher.group(0) value:%s", matcher.group(0));
System.out.printf("
matcher.group(1) value:%s", matcher.group(1));
System.out.printf("
matcher.group(2) value:%s", matcher.group(2));
System.out.printf("
matcher.group(3) value:%s", matcher.group(3));
// Exception in thread "main" java.lang.IndexOutOfBoundsException: No group 4
System.out.printf("
matcher.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。

原文地址:https://blog.csdn.net/just4you/article/details/70767928

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