regex in bash

[qjulian]

hello,
I want to check in bash shell if a string on VAR does not begin with "-" and at the same time if the string is built of [-_a-zA-Z0-9] but not longer than specified on LEN variable

LEN=10
[[ $VAR =~ ^[_a-zA-Z0-9][-_a-zA-Z0-9]{1,${LEN}}$ ]]

how to test length of whole marked in red is not greater than $LEN?

 

[qjulian]

this sounds good

^[[:alnum:]][-[:alnum:]]{0,$((LEN-1))}$