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HomeBackend DevelopmentPHP Tutorial正则表达式中”+“,”*“,”.“在 []以内和以外有什么区别? 非转义

例如:

<code>[\w.]+和[\w.+*?]

</code>

这些符号在[]里面与外面有什么区别?

回复内容:

例如:

<code>[\w.]+和[\w.+*?]

</code>

这些符号在[]里面与外面有什么区别?

首先正则表达式[]里面的内容代表可选组合的意思,
类似

<code>/[\w.]/.test("a"); // true
/[\w.]/.test("."); // true
/[\w.]/.test("a."); // true
/[\w.]/.test(" "); // false</code>

上面的正则表达式可选组合中可以匹配一个字符,或者一个点号,但是不能匹配空字符;

说说区别:
1..[]里面的时候,它就是为了匹配.的,在外面的时候它用来匹配任意字符的,见下面这个示例:

<code>/./.test(" "); //true
/[.]/.test(" "); //false</code>

2.*+[]外面被称为量词(数量词的量词),是表示要匹配的数量的,
*表示任意数量,集合表示是[0, +00);
+表示至少1次,集合表示是[1, +00);
不得不说的是,作为量词时,*+前面必须跟着匹配含义的正则表达式(部分);
[]内就表示它本身的意义了,为了匹配*或者+

<code>/[+]/.test("+"); //true
/+/.test(""); //Uncaught SyntaxError: Invalid regular expression: /+/: Nothing to repeat(…)
/.+/.test("oops"); //true</code>
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