Javascript conditional regular expression if-then-else

问题

I'm trying to limit the entries to a specific format.

If the entry has 5500 or 5100 such as 01\01-5500-000-00 then I want to have this:

^[0-9]{2,}\\[0-9]{2}\-[0-9]{4}\-[0-9]{3}\-$

But if the entry has anything other than 5500 or 5100 I want to have this:

^[0-9]{2,}\\[0-9]{2}\-[0-9]{4}\-[0-9]{3}\-[0-9]{2}$

How can this be accomplished with the if then else idea?

回答1:

Conditional regex syntax is not supported by JavaScript regex engine, but it can be worked around with a non-capturing group containing 2 alternatives:

One with the positive look-ahead and
The second with the reversed, negative look-ahead.

This regex meets your criteria and is JavaScript compatible:

^(?:(?=.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}-[0-9]{4}-[0-9]{3}-|(?!.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}-[0-9]{4}-[0-9]{3}-[0-9]{2})$

See regex demo

Let me break it down:

^ - Start of string
(?:
- (?=.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}-[0-9]{4}-[0-9]{3}- - First alternative with the (?=.*\b5[15]00\b) look-ahead that requires a whole word 5500 or 5100 inside the string, and the first pattern you have
- | - alternation operator
- (?!.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}-[0-9]{4}-[0-9]{3}-[0-9]{2}) - Second alternative that is prepended with the (?!.*\b5[15]00\b) negative look-ahead that makes sure there is no 5100 or 5500 inside the string, and only then matches your second pattern.
$ - end of string.

回答2:

Replace this part of the pattern [0-9]{4} with the literal values for 5100/5500:

/^\d{2,}\\\d{2}-(?:5[15]00-\d{3}-|(?!5[15]00)\d{4}-\d{3}-\d{2})$/

[0-9] is the same thing as \d in JavaScript
For the else part, use the negative lookahead (?!5[15]00)\d{4} to guarantee it's not 5100/5500.

回答3:

Use conditionals, Eg:

(?(?=regex)then|else): [0-9]{2,}\\[0-9]{2}\-(?(?=5[15]00)[0-9]{4}\-[0-9]{3}\-|[0-9]{4}\-[0-9]{3}\-[0-9]{2})

Regex conditionals example: conditionals@regex101.com

In case that you're using a regex engine that is not PCRE based, you can be able to mimic the functionality by doing the following:

((?=positive-regex-statement)then|(?!negavite-regex-statement)then)

Eg.

^[0-9]{2,}\\[0-9]{2}\-((?=5[15]00)[0-9]{4}\-[0-9]{3}\-|[0-9]{4}\-|(?!5[15]00)[0-9]{4}\-[0-9]{3}\-[0-9]{2})$

Mimic regex conditionals example: mimic@regex101.com

来源：https://stackoverflow.com/questions/32852857/javascript-conditional-regular-expression-if-then-else

标签

javascript

regex

ecmascript-5