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.