Regex expression to match alpha character as first character and the rest alphanumeric

Question

Im trying to write a regex to match the following:

must start with a letter
the rest must be alphanumeric 
at least one character in length
max characters 12

Should me able to match

a
k2jdj
hello
example72812

Answer 1

You can try

[a-zA-Z][a-zA-Z0-9]{0,11}

Answer 2

this?

 [a-zA-Z][a-zA-Z0-9]{0,11}