Sorry we removed Brief and Unix regular expressions. We try to keep things compatible when it's reasonable. Brief syntax was odd enough that there were actually macro bugs where things weren't escaped correctly and it would have been a mess for us to try to add syntax to provide full syntax of v20 regular expression features. As for Unix regular expressions (a syntax we made up), it's syntax is very close to Perl and users will benefit from the switch since Perl syntax is arguable the most popular syntax.
If you need any help converting your Brief or Unix regular expressions, just post them. Brief regex is quite a bit different than Perl or SlickEdit. As for Unix regular expressions, like Lee said, it's very similar to Perl. Making the switch should be pretty easy. Just make sure you use \R when you want to match whatever the line format is for a particular file and not \n which now means to just match a single 10 character.
Type "help Perl regular expressions" on the SlickEdit command line to get help on Perl regular expressions in the new help system. We tried to improve the on-line help for Perl and SlickEdit regular expressions for v20.
I use SlickEdit syntax because I'm used to it, it is the most compatible with really old versions of SlickEdit, and it has a few advantages over Perl (defaults to minimal matching which helps to avoid buggy regular expressions that are not tested enough, braces for no match group which is easier to read, fewer ambiguities like \1 which can mean multiple things depending on the rest of your regex, [] is a valid empty character match set). Arguably it's few advantages are negated by the fact the Perl regular expressions are the most popular.