This checks whether or not a string belongs to the language described by the regular expression(RE)
Note: A string to be Kleene starred must be put into parentheses even if it is a single symbol such as (a)*
If the string is in the language, the string is returned. Otherwise the first error position is displayed.
For example type in (ab)*bba for RE and ababbba for STRING. You should get "ababbba" back.
Type in (ab)*bba for RE and aabba for STRING. You should get "Invalid token @ 2" back.