LibJS: Implement RegExp.prototype.replace with RegExpExec abstraction

Also rename the 'rx' variable to 'regexp_object' to match other RegExp
methods.

Userland/Libraries/LibJS/Runtime/RegExpPrototype.cpp

@@ -330,8 +330,8 @@ JS_DEFINE_NATIVE_FUNCTION(RegExpPrototype::symbol_replace)
     auto string_value = vm.argument(0);
     auto replace_value = vm.argument(1);
 
-    auto rx = regexp_object_from(vm, global_object);
-    if (!rx)
+    auto* regexp_object = regexp_object_from(vm, global_object);
+    if (!regexp_object)
         return {};
     auto string = string_value.to_string(global_object);
     if (vm.exception())
@@ -347,26 +347,21 @@ JS_DEFINE_NATIVE_FUNCTION(RegExpPrototype::symbol_replace)
             return {};
     }
 
-    auto global_value = rx->get(vm.names.global);
+    auto global_value = regexp_object->get(vm.names.global);
     if (vm.exception())
         return {};
 
     bool global = global_value.to_boolean();
     if (global) {
-        rx->set(vm.names.lastIndex, Value(0), true);
+        regexp_object->set(vm.names.lastIndex, Value(0), true);
         if (vm.exception())
             return {};
     }
 
-    // FIXME: Implement and use RegExpExec - https://tc39.es/ecma262/#sec-regexpexec
-    auto* exec = Value(rx).get_method(global_object, vm.names.exec);
-    if (!exec)
-        return {};
-
     MarkedValueList results(vm.heap());
 
     while (true) {
-        auto result = vm.call(*exec, rx, string_value);
+        auto result = regexp_exec(global_object, *regexp_object, string);
         if (vm.exception())
             return {};
         if (result.is_null())
@@ -390,13 +385,13 @@ JS_DEFINE_NATIVE_FUNCTION(RegExpPrototype::symbol_replace)
         if (match_str.is_empty()) {
             // FIXME: Implement AdvanceStringIndex to take Unicode code points into account - https://tc39.es/ecma262/#sec-advancestringindex
             //        Once implemented, step (8a) of the @@replace algorithm must also be implemented.
-            auto last_index = rx->get(vm.names.lastIndex);
+            auto last_index = regexp_object->get(vm.names.lastIndex);
             if (vm.exception())
                 return {};
             auto this_index = last_index.to_length(global_object);
             if (vm.exception())
                 return {};
-            rx->set(vm.names.lastIndex, Value(this_index + 1), true);
+            regexp_object->set(vm.names.lastIndex, Value(this_index + 1), true);
             if (vm.exception())
                 return {};
         }

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.replace.js

@@ -197,3 +197,38 @@ test("search value is coerced to a string", () => {
     expect(newString).toBe("abc");
     expect(coerced).toBe("x");
 });
+
+test("override exec with function", () => {
+    let calls = 0;
+
+    let re = /test/;
+    let oldExec = re.exec.bind(re);
+    re.exec = function (...args) {
+        ++calls;
+        return oldExec(...args);
+    };
+
+    expect("test".replace(re, "x")).toBe("x");
+    expect(calls).toBe(1);
+});
+
+test("override exec with bad function", () => {
+    let calls = 0;
+
+    let re = /test/;
+    re.exec = function (...args) {
+        ++calls;
+        return 4;
+    };
+
+    expect(() => {
+        "test".replace(re, "x");
+    }).toThrow(TypeError);
+    expect(calls).toBe(1);
+});
+
+test("override exec with non-function", () => {
+    let re = /test/;
+    re.exec = 3;
+    expect("test".replace(re, "x")).toBe("x");
+});