背景

工作中遇到正则表达式匹配失误的问题,虽最终解决了问题。但讨论过程中提到了贪婪和非贪婪，
大家都不是讲的十分清楚。随后做了一些了解，记录如下。

贪婪？非贪婪？

简言之贪婪就是尽量找最长的匹配字符串，非贪婪尽量找最短的匹配字符串。

正则表达式中的 “？” 作为量词使用时，代表的是匹配“0次或1次”。但“？”跟在量词后面就表示以非贪婪模式匹配。

​ 正则表达式常用量词如下：

* 匹配0次或多次
? 匹配0次或1次
+ 匹配1次或多次
{m,n} 匹配大于等于m次,小于等于n次 (n>m)
{m,} 匹配大于等于m次

Example

我们用 regexp:abcd123 这个字符串来做一下例子

Example1:

{m,n} 与 {m,n}?

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]{3,5}";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: regex

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]{3,5}?";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: reg

Example2:

{m,}与{m,}?

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]{3,}";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: regexp

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]{3,}?";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: reg

Example3:

与?

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]*";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: regexp

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]*?";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: 

Example4:

+与+?

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]+";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: regex

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]+?";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: r

Example4:

?与??

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]?";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: r

public class RegExpTester{
    public static void main(String[] args){
        String str = "regexp:abcd123efg";
        String regexp = "[a-z]??";
        Pattern pattern = Pattern.compile(regexp);
        Matcher matcher = pattern.matcher(str);
        if(matcher.find()){
            System.out.println(matcher.group());
        }
    }
}

//打印结果为: