A "safe string" is composed of one or more alphanumeric characters, with underscore also available, but the first character has to be a letter or a dash. You could define this as a regular expression: /\-?A-Za-z_A-Za-z0-9_*/.

What about letters outside the ASCII range? Perl allows Unicode letters as variable names, for example. Do barewords here get the same treatment? $α{niņa}=5;