Regex: Using lookahead assertion to check if character exist at most a certain number of times

Question

How do I use lookahead assertion to determine if a certain character exist at most a certain number of times in a string.

For example, let's say I want to check a string that has at least one character to make sure that it contains "@" at most 2 times. Thanks in advance. Using python if that matters.

Answer 1

There are lots of ways to do this, for example:

/^(?=([^@]*@){,2}[^@]*$)./

Answer 2

Using a negative lookahead assertion, you can make sure that @ doesn't occur three times:

(?!.*@.*@.*@.*).*

Answer 3

I believe Mark's answer wont quite work as you need to exclude the @ from being matched at other times. Try this:

^(?=(([^@]*@){0,2}[^@]*$))

Edit: Mark fixed his answer, ours should be the same now. Also, fixed.