I stumbled on some (c++) code like this:
The only legal characters according to the standard are alphanumerics and the underscore. The standard does require that just about anything Unicode considers alphabetic is acceptable (but only as single code-point characters). In practice, implementations offer extensions (i.e. some do accept a $) and restrictions (most don't accept all of the required Unicode characters). If you want your code to be portable, restrict symbols to the 26 unaccented letters, upper or lower case, the ten digits, and the '_'.