Regex, Number or Empty

Question

Can someone show me how to match 4 digits or nothing in Regex? I have been using RegexBuddy and so far i have come up with this but the help text is saying 'Or match regular expression number 2 below - the entire match attempt fails if this one fails to match' in regards to the pattern after the pipe.

I would like either of the two to provide a positive match.

^\d{4}|$

Answer 1

无论是4位还是全无:

^(?:\d{4}|)$

这与你想要做的最接近.

我会选择以下稍微更短的一个:

^(?:\d{4})?$

^\d{4}|$

正则表达式意味着:

^\d{4} 要么 $

第一个匹配任何以4位开头的字符串,而第二个将报告所有内容的匹配(它匹配字符串的结尾,因为没有指定字符,它将匹配所有字符串的结尾).

当你使用一个小组时,你会得到:

或者^(?:\d{4})$或^(?:)$ (注意,OR运算的"限制"改变),然后将其等同于任一^\d{4}$或^$