regular expression for username

I want regul开发者_如何学Car expression for username in asp.net/c# where username should be minimum 6 and maximum 15 characters. Alphanumeric characters are allowed. special characters ._ are allowed. No spaces are acceptable. The starting character should be alphabet.

I want regular expression for password where the conditions are exactly same as above but with only difference that starting character should be either alphanumeric, no special characters allowed.

username

[A-Za-z][A-Za-z0-9._]{5,14}

password

[A-Za-z0-9]{6,15}

but not allowing special chars in passwords is not wise