Adding a colon as a word boundary for syntax highlighting

[Tim Johnson]

Hi:
I'm using the following regex subexpression:
"\\<\\("
As the left-hand word boundary for syntax highlighting in
a lisp-style programming language.

"\\(" adds the opening parenthesis as a word boundary.

I would like to add the colon (':') as an additional word
boundary character.
The subexpressions:
"\\<\\(:" and "\\<\\(\\:" don't seem to work.
In addition I have the following entry:
(?: . "w") to the syntax table. 
Could someone advise me on how to make this work correctly.

On a related note, I believe that there is an emacs add-on that
allows the user to test elisp regexes, but for the life of me,
I can't remember where to find it.
TIA
Tim