I take a correct cracked at it.
Remember in Computer Science, an Alphanumeric value means the first character is an alphabet or underscore. Thereafter the character can be 0-9, A-Z, a-z, or underscore (_). Tested under php:
$regex = '/^[A-Za-z_][A-Za-z\d_]*$/' or take this
^[A-Za-z_][A-Za-z\d_]*$ and place it in your development language.
