Fast Lua code checking, whether user entered a valid variable name?

[bil til <biltil52@...>]

Sorry, for this quite basic question, but I do not find nice answers
for this in Internet - although I think this is quite a common problem
for a Lua user program:

I would like the user to enter some variable name, or just some name
which then I also want to use as name for a Lua global object.

Is there any fast / standard method in my Lua user program to check,
whether the user entered a string which is allowed as a "standard
name" for a variable naming? (start by ascii char, containing only
chars and numbers and '_')? (As I see, there are not such "C style
character checking" functions in basic Lua, like "ischar / islower /
isupper / isdigit...").

Or is it wise / without problems, to allow the user also to use "more
bizarre names" for naming variables dynamically in his Lua code, e. g.
variable names containing spaces, or UTF characters or further things?