Regular expression limit string size

Question

How to limit string size for this regular expression?

/^[a-z][a-z0-9]*(?:_[a-z0-9]+)*$/

I just need to add the quantifier {3,16}

Answer 1

Sprinkle in some positive lookahead to test for the total length of the string by adding

(?=.{3,16}$)

at the start of the regex. The final regex is then:

/^(?=.{3,16}$)[a-z][a-z0-9]*(?:_[a-z0-9]+)*$/