how to separate the first character of the pattern from the rest?

I am making a form and in my form there is a field called username. my username must follow a specific pattern:

• it must be between 8 to 12 characters

• it can be comprised of "uppercase letters", "lowercase letters" or "numbers" "_" "-" ".".

• the starting character shouldn't be a number

how should I write the pattern format?

pattern="[a-zA-Z0-9-_.]{8,12}

i don't know the rest

please help

thanks in advance