java正则表达式如何定量“？ “工作？ [英] How does java regex quantifier “? ” work?

问题描述

System.out.println(Pattern.matches("[amn]?", "a"));

此声明返回true。



但是

This statement returns true.

But

System.out.println(Pattern.matches("[amn]?", "amn"));

System.out.println(Pattern.matches("[amn]?", "adef"));

这些陈述返回false。



为什么？



我尝试了什么：



我对正则表达量词的理解？是这个。



正则表达式： X？

描述： X出现一次或者完全没有



所以语句[amn]？ amn应该返回true，因为a，m，n出现一次。同样在[amn]？ adef只出现一次而且m和n根本不会发生。



我哪里错了？

These statements returns false.

Why ?

What I have tried:

My understanding about regex quantifier "?" is this.

Regex: X?
Description: X occurs once or not at all

So the statement "[amn]?" "amn" should return true because a,m,n occurs once. And similarly in "[amn]?" "adef" a occurs only once and m and n do not occur at all.

Where am I going wrong ?

推荐答案

问题是？意思是：0或1重复前面的项目

这意味着它是可选的。

取整数正则表达式：

The problem is that "?" means just that: "0 or 1 repetition of the preceding item"
Which means it's optional.
Take the "integer number" regex:

^[+-]?\d+

说的是整个字符串由至少一个数字组成，可以在前面加上一个加号或者字母

所以匹配：

What that says is "the whole string consists of at least one digit, which may be preceded by a single plus or minus"
So it matches:

0
1
123456789
-1
+0

但它赢了; t匹配

But it won;t match

A
++0
+-999

你的正则表达式匹配a'，'m'或'n'中的零个或一个字符 - 这意味着它匹配任何东西！

如果写成

Your regex matches "zero or one character from 'a', 'm', or 'n'" - which means that it matches anything at all!
if it was written as

^[amn]?

那么它匹配

Then it would match

a
n
m

但不匹配

But wouldn't match

b
c
9
Hello!

获取 Expresso 的副本[ ^ ] - 它是免费的，它会检查并生成正则表达式。

Get a copy of Expresso[^] - it's free, and it examines and generates Regular expressions.

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