廖雪峰Java9正则表达式-2正则表达式进阶-5非贪婪匹配

逃离我推掉我的手 2021-11-29 21:56 402阅读 0赞

## 1.贪婪匹配 ##

问题：给定一个字符串表示的数字，判断该数字末尾0的个数?

*  "123000": 3个0
 *  "10100": 2个0
 *  "1001": 0个0

先使用"^(\\d+)(0\*)$"匹配这三个字符串

public class Phone{
        public static void matchForZero(String s){
            Pattern pattern = Pattern.compile("(\\d+)(0*)$");
            Matcher matcher = pattern.matcher(s);
            if (matcher.matches()){
                System.out.print("第一组数："+matcher.group(1)+"\t");
                System.out.println("第二组数："+matcher.group(2));
            }
        }
    }

public class PhoneTest {
       @Test
        public void testMatchForZero(){
            Phone.matchForZero("123000");
            Phone.matchForZero("10100");
            Phone.matchForZero("1001");
       }
    }

![1418970-20190506210607186-828435556.png][]  
结果：0并未匹配到，group1将整个字符串完全匹配  
原因：正则表达式默认使用贪婪匹配，尽可能多的向后匹配

## 2.非贪婪匹配 ##

1中的解决方法：使用？实现非贪婪匹配  
修改方法，再次运行

public class Phone{
        public static void matchForZero(String s){
            Pattern pattern = Pattern.compile("(\\d+?)(0*)$");// \d尽可能少的匹配，0尽可能多的匹配
            Matcher matcher = pattern.matcher(s);
            if (matcher.matches()){
                System.out.print("第一组数："+matcher.group(1)+"\t");
                System.out.println("第二组数："+matcher.group(2));
            }
        }
    }

![1418970-20190506210959395-1320510444.png][]

## 3.区分非贪婪匹配与个数匹配 ##

注意：？既能表示非贪婪匹配，也能表示0个或1个，所以要注意其含义

public class Phone{
        public static void matchForZero(String s){
            Pattern pattern = Pattern.compile("(\\d??)(9*)$");
            //第一个?表示0个或1个，可以匹配0或1个9
            //第二个?表示非贪婪匹配，尽可能少的匹配，两者综合，即匹配0个
            Matcher matcher = pattern.matcher(s);
            if (matcher.matches()){
                System.out.print("第一组数："+matcher.group(1)+"\t");
                System.out.println("第二组数："+matcher.group(2));
            }
        }
    }

public class PhoneTest {
       @Test
        public void testMatchForZero(){
            Phone.matchForZero("9999");
       }
    }

![1418970-20190506211824425-1791262090.png][]

## 4.代码示例 ##

### 4.1贪婪匹配 ###

ZeroCount.java

package com.testList;
    
    import java.util.regex.Matcher;
    import java.util.regex.Pattern;
    
    public class ZeroCount {
        public static int zeros(String s){
            Pattern pattern = Pattern.compile("^\\d+(0*)$");
            Matcher matcher = pattern.matcher(s);
            if(matcher.matches()){
                String zeroStr = matcher.group(1);
                return zeroStr.length();
            }
            throw new IllegalArgumentException("Not a number");
        }
    }

ZeroCountTest.java

package com.testList;
    
    import org.junit.Test;
    
    import static org.junit.Assert.*;
    
    public class ZeroCountTest {
    
        @Test
        public void zeros() {
            assertEquals(0, ZeroCount.zeros("123456"));
            assertEquals(1, ZeroCount.zeros("123450"));
            assertEquals(2, ZeroCount.zeros("123400"));
            assertEquals(3, ZeroCount.zeros("123000"));
            assertEquals(4, ZeroCount.zeros("120000"));
            assertEquals(2, ZeroCount.zeros("100100"));
        }
    }

![1418970-20190506213120922-774055579.png][]

### 4.2非贪婪匹配 ###

package com.testList;
    
    import java.util.regex.Matcher;
    import java.util.regex.Pattern;
    
    public class ZeroCount {
        public static int zeros(String s){
            Pattern pattern = Pattern.compile("^\\d+?(0*)$");
            Matcher matcher = pattern.matcher(s);
            if(matcher.matches()){
                String zeroStr = matcher.group(1);
                return zeroStr.length();
            }
            throw new IllegalArgumentException("Not a number");
        }
    }

![1418970-20190506213303696-1960263558.png][]

## 5.总结 ##

*  正则表达式匹配默认使用贪婪匹配
 *  使用?表示对某一规则进行非贪婪匹配
 *  注意区分?的含义 \\d??

转载于:https://www.cnblogs.com/csj2018/p/10822294.html

[1418970-20190506210607186-828435556.png]: /images/20211129/5b8d0434e8674e7dbc0f412a10a3a4b6.png
[1418970-20190506210959395-1320510444.png]: /images/20211129/b6b7b5663c0a41e4927639cfa9fff9eb.png
[1418970-20190506211824425-1791262090.png]: /images/20211129/cd8a78b4ed864b49a811e918580dbcc9.png
[1418970-20190506213120922-774055579.png]: /images/20211129/23aee47998e441d688073d9b79b7ab80.png
[1418970-20190506213303696-1960263558.png]: /images/20211129/a93737bd19764841a3e72a3fe915a07e.png