Added documentation, changed error message.

Author: aeijdenberg

doc.go

@@ -57,6 +57,11 @@ calling mux.Vars():
 	vars := mux.Vars(request)
 	category := vars["category"]
 
+Note that if any capturing groups are present, mux will panic() during parsing. To prevent
+this, convert any capturing groups to non-capturing, e.g. change "/{sort:(asc|desc)}" to
+"/{sort:(?:asc|desc)}". This is a change from prior versions which behaved unpredictably
+when capturing groups were present.
+
 And this is all you need to know about the basic usage. More advanced options
 are explained below.
 

regexp.go

@@ -112,7 +112,8 @@ func newRouteRegexp(tpl string, matchHost, matchPrefix, matchQuery, strictSlash,
 
 	// Check for capturing groups which used to work in older versions
 	if reg.NumSubexp() != len(idxs)/2 {
-		panic("Regexp appears to be using capturing groups. To fix, see: https://github.com/gorilla/mux/issues/200")
+		panic(fmt.Sprintf("route %s contains capture groups in its regexp. ", template) +
+			"Only non-capturing groups are accepted: e.g. (?:pattern) instead of (pattern)")
 	}
 
 	// Done!