CLJS-471: prevent empty regexps from causing compiler errors

Depending on the JS vm being used, empty regexes will print as
either #"" or #"(?:)" (the latter being emitted by those with a
webkit heritage AFAICT).

src/clj/cljs/compiler.clj

@@ -151,8 +151,10 @@
   (emits (wrap-in-double-quotes (escape-char x))))
 
 (defmethod emit-constant java.util.regex.Pattern [x]
-  (let [[_ flags pattern] (re-find #"^(?:\(\?([idmsux]*)\))?(.*)" (str x))]
-    (emits \/ (.replaceAll (re-matcher #"/" pattern) "\\\\/") \/ flags)))
+  (if (= "" (str x))
+    (emits "(new RegExp(\"\"))")
+    (let [[_ flags pattern] (re-find #"^(?:\(\?([idmsux]*)\))?(.*)" (str x))]
+      (emits \/ (.replaceAll (re-matcher #"/" pattern) "\\\\/") \/ flags))))
 
 (def ^:const goog-hash-max 0x100000000)
 

test/cljs/cljs/core_test.cljs

@@ -628,6 +628,8 @@
   (assert (= (re-seq (re-pattern "foo") "foo bar foo baz foo zot") (list "foo" "foo" "foo")))
   (assert (= (re-seq (re-pattern "f(.)o") "foo bar foo baz foo zot") (list ["foo" "o"] ["foo" "o"] ["foo" "o"])))
   (assert (= (re-matches (re-pattern "(?i)foo") "Foo") "Foo"))
+  ; new RegExp("").source => "(?:)" on webkit-family envs, "" elsewhere
+  (assert (#{"#\"\"" "#\"(?:)\""} (pr-str #"")))
 
   ;; destructuring
   (assert (= [2 1] (let [[a b] [1 2]] [b a])))