正则表达式-捕获组和反向引用
作者：互联网
一、捕获组
捕获组是正则中分组的一个概念，若是要对一段字符进行重复,就须要有用到分组,分组在正则中用"()"表示.而后后面能够对这个组进行重复引用。
捕获组分为两类：普通捕获组和命名捕获组。
我们可以通过以下两个简单的demo来体会：
1. 普通捕获组
从正则表达式左侧开始，每出现一个左括号“(”记作一个分组，分组编号从1开始。0表明整个表达式。
public void test8() {
String DATE_STRING = "2021-07-01";
String P_COMM = "(\\d{4})-((\\d{2})-(\\d{2}))";
Pattern pattern = Pattern.compile(P_COMM);
Matcher matcher = pattern.matcher(DATE_STRING);
matcher.find();//必需要有这句
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));
}
打印结果：
matcher.group(0) value:2021-07-01 matcher.group(1) value:2021 matcher.group(2) value:07-01 matcher.group(3) value:07 matcher.group(4) value:01 Process finished with exit code 0
2. 命名捕获组
每一个以左括号开始的捕获组，都紧跟着“?”，然后才是正则表达式。
public void test9() {
String P_NAMED = "(?<year>\\d{4})-(?<md>(?<month>\\d{2})-(?<date>\\d{2}))";
String DATE_STRING = "2021-07-01";
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));
}
程序结果：
===========使用名称获取=============
matcher.group(0) value:2021-07-01
matcher.group('year') value:2021
matcher.group('md') value:07-01
matcher.group('month') value:07
matcher.group('date') value:01
===========使用编号获取=============
matcher.group(0) value:2021-07-01
matcher.group(1) value:2021
matcher.group(2) value:07-01
matcher.group(3) value:07
matcher.group(4) value:01
3. 非捕获组
在左括号后紧跟“?:”，然后再加上正则表达式，构成非捕获组(?:Expression)
public void test10() {
String P_UNCAP = "(?:\\d{4})-((\\d{2})-(\\d{2}))";
String DATE_STRING = "2021-07-01";
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));
}
运行结果：
matcher.group(0) value:2021-07-01 matcher.group(1) value:07-01 matcher.group(2) value:07 matcher.group(3) value:01 java.lang.IndexOutOfBoundsException: No group 4
二、反向引用
1.反向引用须要使用到分组，分组就是使用()括起来的部分为一个总体，在进行分组匹配时的原则是：由外向内，由左向右3d
2.反向引用如:\1,\2等 \1：表示的是引用第一次匹配到的()括起来的部分 \2：表示的是引用第二次匹配到的()括起来的部分 例：String regex = "^(\\d)\\1$"; 首先这里是匹配两位，\d一位,\1又引用\d一位 这里的\1会去引用（\d）匹配到的内容，由于(\d)是第一次匹配到的内容。 如：str = "22"时，(\\d)匹配到2，因此\1引用(\\d)的值也为2，因此str="22"能匹配 str = "23"时，(\\d)匹配到2，由于\1引用(\\d)的值2,而这里是3，因此str="23"不能匹配 下面通过一些demo来体会下： @Test
public void test1() {
String reg = "([a-z]{3}[1-9]{3})[a-z]{3}[1-9]{3}";
String str = "asd123asd123";
//常规写法 true
System.out.println(Pattern.matches(reg, str));
}
@Test
public void test2() {
String reg = "([a-z]{3}[1-9]{3})\\1";
String str = "asd123asd123";
//使用到反向引用 true
System.out.println(Pattern.matches(reg, str));
}
@Test
public void test3() {
String str = "1234567123123123";
// 只能匹配“123123”
Pattern p = Pattern.compile("(\\d\\d\\d)\\1");
Matcher m = p.matcher(str);
// 1
System.out.println(m.groupCount());
while (m.find()) {
String word = m.group();
// 123123 7 13
System.out.println(word + " " + m.start() + " " + m.end());
}
}
@Test
public void test4() {
String pattern = "\\b(\\w+)\\b[\\w\\W]*\\b\\1\\b";
Pattern p = Pattern.compile(pattern, Pattern.CASE_INSENSITIVE);
String phrase = "unique is not duplicate but unique, Duplicate is duplicate.";
Matcher m = p.matcher(phrase);
while (m.find()) {
String val = m.group();
System.out.println("Matching subsequence is \"" + val + "\"");
System.out.println("Duplicate word: " + m.group(1) + "\n");
}
}
@Test
public void test5() {
String reg = "(\\w)(\\w)\\2\\1";
String str = "abba";
// true
System.out.println(Pattern.matches(reg, str));
}
@Test
public void test6() {
String reg = "(\\w)(\\w)\\2\\1";
String str = "abba";
// true
System.out.println(Pattern.matches(reg, str));
}
@Test
public void test7() {
String reg = "([a-z]{3})([1-9]{3})\\1\\2";
String str = "asd123asd123";
// true
System.out.println(Pattern.matches(reg, str));
String reg1 = "([a-z]{3})([1-9]{3})\\2\\1";
String str1 = "asd123123asd";
// true
System.out.println(Pattern.matches(reg1, str1));
}
标签：组和,group,String,正则表达式,matcher,System,value,反向,out 来源： https://www.cnblogs.com/leijisong/p/14956037.html