正则表达式允许数字范围，或为null

Question

I have the following Regular Expression, how can I modify it to also allow null?

[0-9]{5}|[0-9]{10}

I would like it to allow a 5 digit number, a 10 digit number, or null

Thanks

Answer 1

Just append |null :

[0-9]{5}|[0-9]{10}|null

As you probably know, | is the "or" operator, and the string of characters null match the word null. Thus it can be read out as <your previous pattern> or null .

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If you want the pattern to match the null-string, the answer is that it's impossible. That is, there is no way you can make, for instance, Matcher.matches() return true for a null input string. If that's what you're after, you could get away with using the above regexp and matching not on str but on ""+str which would result in "null" if str actually equals null .

Answer 2

如果为null，则表示空字符串，您需要：

^(?:[0-9]{5}|[0-9]{10}|)$

Answer 3

[0-9]{5}|[0-9]{10}|null

should do it. Depending on how you are using the regex, you might need to anchor it in order to be sure that it will always match the entire string and not just a five-digit substring inside an eight-digit string:

^(?:[0-9]{5}|[0-9]{10}|null)$

^ and $ anchor the regex, (?:...) is a non-capturing group that contains the alternation.

Edit: If you mean null =="empty string", then use

^(?:[0-9]{5}|[0-9]{10}|)$