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On Mon, 2023-07-10 at 09:33 +0200, bil til wrote:
> > var =
> > if (string.match(var, "%a+_%a+")) then -- In this example, _ will
> > be normal
> >     print("Valid variable name !")
> > else
> >     print("Invalid variable name")
> > end

Thischecks for presence of a sequence of alphabetical characters
followed by an underscore followed by more alphabetical characters. So
"myvar" won't pass while "my_var" and "*/*my_var+++--" will. Not sure
that's what you want.
> ... also probably most fast like this, and also this for sure will be
> necessary/useful to check (I then want to use this further in my Lua
> user program as _G[var], as described in Roberto's PiL Book Chapter
> 22 ("Global VAriables with Dynamic Names")). 
> (if this is NOT necessary / useful, please correct me...).

Variable names are restricted only when it comes to accessing them via
usual syntax. _G is a table like any other, so any key (even non-string
one!) can be used in it with the sole consequence being your inability
to access such a value without explicitly indexing _G.

But IMO if you're going after variables with dynamic names you probably
want to have them in a separate table from _G to prevent trouble from
potential collisions. Just my 5c.
v <>