Regex to match a digit two or four times

Question

It\'s a simple question about regular expressions, but I\'m not finding the answer.

I want to determine whether a number appears in sequence exactly two

Answer 1

(?

This is the correct way to do it. The accepted answer is wrong.

It would match 3 digits (or 5). So that is wrong in my eyes.

1) Check there is no digit before a sequence of 2, or 4 digits, or after a sequence of two or four digits.

• ( syntax is negative lookbehind

• (?!) syntax is negative lookahead.

The above would work for mid string:

If your search string has no content around it you could use the ^ and $ start and end of string anchors:

^\d{4}$|^\d{2}$