java 正则表达式数量

- 日理万妓 2022-10-27 15:59 158阅读 0赞

# 表示数量的符号 #

X?	X, once or zero
    X*	X, zero or more times
    X+	X, one or more times
    X{ n}	X, exactly n times
    X{ n,}	X, at least n times
    X{ n,m}	X, at least n but not more than m times

# 三种模式 #

贪婪匹配模式下会尽可能的多匹配字符,所以在匹配上``之后,仍然向后寻找,直到`标签`

Pattern p = Pattern.compile("<.+>");
 Matcher m = p.matcher("标签");
 m.find();
 System.out.println(m.group());

懒惰匹配模式下,只要匹配到``就会停止

Pattern p = Pattern.compile("<.+?>");
 Matcher m = p.matcher("标签");
 m.find();
 System.out.println(m.group());

占有模式:  
在贪婪模式下，正则表达式 a.*b 在匹配 acbac时，在匹配成功第一个字符a后，匹配算法会继续往后读取字符a之后的字符来匹配.* , 这时贪婪模式就起作用了，a后面的cbac都能匹配.*，但当匹配算法继续往后读取字符时，发现已经没有字符可读了，无法匹配正则表达式中的b了，于是就回溯一个字符，看c是否能匹配b，不能，继续回溯，直到回溯到b时，才匹配到了acb。 而a.*\+b在匹配时是不会回溯的，也就是说当贪婪地读到最后一个字符时发现无有字符与b匹配了，就认为匹配失败了。

参考:  
https://www.cnblogs.com/01picker/p/4842182.html

# Greedy quantifiers #

X?	X, once or not at all
    X*	X, zero or more times
    X+	X, one or more times
    X{ n}	X, exactly n times
    X{ n,}	X, at least n times
    X{ n,m}	X, at least n but not more than m times

# Reluctant quantifiers #

X??	X, once or not at all
    X*?	X, zero or more times
    X+?	X, one or more times
    X{ n}?	X, exactly n times
    X{ n,}?	X, at least n times
    X{ n,m}?	X, at least n but not more than m times

# Possessive quantifiers #

X?+	X, once or not at all
    X*+	X, zero or more times
    X++	X, one or more times
    X{ n}+	X, exactly n times
    X{ n,}+	X, at least n times
    X{ n,m}+	X, at least n but not more than m times