Shimon,
It look slike the following expression should be close:
.{1,18}
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.
Thanks,
Joe Kaufman