It look slike the following expression should be close:
That is, a period, open curly bracket, 1, comma, 18, and closing curly bracket.
The period means any character except line-feed, and the range of numbers in curly brackets means the character can occur that many times (so, effectively a lower and upper bound on length in this scenario).
If the DocuWare help isn't helping, any tutorial on regular expressions should be of assistance. Figuring out regex stuff can be a challenge... Just google for whatever issue you are trying to solve (including keywords "regular expression" or "regex"), and it can help zero in on what you are trying to do.