Question

I need some help with regular expression. I need check login, login can have letters, numbers and underscore. It must have at least one letter and can have a underscore in center.

Now I have this:
^([a-z0-9_])+$/iu
But it allow to use all in any order.

Answer 1

Try this one here:

^(?<=[a-z])(?<=[0-9])[a-z0-9]+(_[a-z0-9]+|)$/iu

So there must be a letter or a number at the beginning at least one time. That can optional come one _ with minimum of one more letters/numbers or nothing instad.

The (?<=[a-z]) is a positive look-behind assertion which means that a letter must be inside.