You can't use lookbehind with CF's regex engine (uses Apache Jakarta ORO).
However, you can use Java's regex though, which does support them, and I've created a wrapper CFC that makes this even easier. Available from:
http://www.hybridchill.com/projects/jre-utils.html
(Update: The wrapper CFC mentioned above has evolved into a full project. See cfregex.net for details.)
Also, the /.../s stuff isn't required/relevant here.
So, from your example, but with improved regex:
<cfset jrex = createObject('component','jre-utils').init()/>
<cfset reg = jrex.match( "(?<=<)[^<>]+(?=>)" , "Joe <joe@domain.com>" ) />
A quick note, since I've updated that regex a few times; hopefully it's at its best now...
(?<=<) # positive lookbehind - start matching at `<` but don't capture it.
[^<>]+ # any char except `<` or `>`, the `+` meaning one-or-more greedy.
(?=>) # positive lookahead - only succeed if there's a `>` but don't capture it.
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