Hello, this is probably best done with regexp using our custom validation.
Two regexpressions that popped into my head are: ([A-z,0-9]){20}
and ([.~\w\t\,\.\!\?]){20}
The first is a bit cleaner, while the second is more inclusive. You can find out a lot about regular expressions here: http://www.regular-expressions.info/reference.html and test it here: http://regexpal.com/