introduce unsafe_regexp

fixes #1964

lib/compress.js

@@ -83,6 +83,7 @@ function Compressor(options, false_by_default) {
         unsafe_comps  : false,
         unsafe_math   : false,
         unsafe_proto  : false,
+        unsafe_regexp : false,
         unused        : !false_by_default,
         warnings      : false,
     }, true);
@@ -3765,7 +3766,7 @@ merge(Compressor.prototype, {
             if (fixed) {
                 if (d.should_replace === undefined) {
                     var init = fixed.evaluate(compressor);
-                    if (init !== fixed) {
+                    if (init !== fixed && (compressor.option("unsafe_regexp") || !(init instanceof RegExp))) {
                         init = make_node_from_constant(init, fixed);
                         var value_length = init.optimize(compressor).print_to_string().length;
                         var fn;

test/compress/evaluate.js

@@ -990,3 +990,50 @@ Infinity_NaN_undefined_LHS: {
         "}",
     ]
 }
+
+issue_1964_1: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unsafe_regexp: false,
+        unused: true,
+    }
+    input: {
+        function f() {
+            var long_variable_name = /\s/;
+            return "a b c".split(long_variable_name)[1];
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            var long_variable_name = /\s/;
+            return "a b c".split(long_variable_name)[1];
+        }
+        console.log(f());
+    }
+    expect_stdout: "b"
+}
+
+issue_1964_2: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unsafe_regexp: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            var long_variable_name = /\s/;
+            return "a b c".split(long_variable_name)[1];
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            return "a b c".split(/\s/)[1];
+        }
+        console.log(f());
+    }
+    expect_stdout: "b"
+}