PHP preg_match添加一个字符

I'm just trying to add an underscore to this preg_match parameter here's what I have :

/^[a-z\d]{2,64}$/i

How would I add an underscore to this regex ?

Thanks!

If you would allow another char in the matching, you should just add it to the maching group

/^[_a-z\d]{2,64}$/i