[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.
+
 And this is all you need to know about the basic usage. More advanced options
 are explained below.
 

mux_test.go

@@ -1389,6 +1389,16 @@ func TestSubrouterErrorHandling(t *testing.T) {
 	}
 }
 
+// See: https://github.com/gorilla/mux/issues/200
+func TestPanicOnCapturingGroups(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Errorf("(Test that capturing groups now fail fast) Expected panic, however test completed sucessfully.\n")
+		}
+	}()
+	NewRouter().NewRoute().Path("/{type:(promo|special)}/{promoId}.json")
+}
+
 // ----------------------------------------------------------------------------
 // Helpers
 // ----------------------------------------------------------------------------

regexp.go

@@ -109,6 +109,13 @@ func newRouteRegexp(tpl string, matchHost, matchPrefix, matchQuery, strictSlash,
 	if errCompile != nil {
 		return nil, errCompile
 	}
+
+	// Check for capturing groups which used to work in older versions
+	if reg.NumSubexp() != len(idxs)/2 {
+		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!
 	return &routeRegexp{
 		template:       template,