String Tests
- DigitQ — test whether all the characters are digits
- LetterQ — test whether all the characters are letters
- StringFreeQ — test whether a string is free of substrings matching a pattern
- StringMatchQ — test whether a string matches a pattern
- StringQ — test whether an expression is a string
- SyntaxQ — test whether a string is a syntactically-correct a Mathics3 expression