isValid = True
if not value:
isValid = False
elif value[0] not in 'abcdefghjiklmnopqrstuvwxyz_ABCDEFGHIJKLMNOPQRSTUVWXYZ':
isValid = False
else:
for cc in value[1:]:
if cc not in 'abcdefghjiklmnopqrstuvwxyz_ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789':
isValid = False
break