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

Answer 2

Use regex

/^[a-z](?:[a-z\d]|_(?!_)){1,14}[a-z\d]$/

or

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