[bugfix] fail fast if regex is incorrectly specified using capturing groups. (#218)

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.
+
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