LibJS: Make RegExp.prototype.source spec-compliant

Basically:
- And edge case for this object being RegExp.prototype.source
- Return "(?:)" for empty pattern
- Escape some things properly

Libraries/LibJS/Runtime/RegExpPrototype.cpp

@@ -30,6 +30,7 @@
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/RegExpObject.h>
 #include <LibJS/Runtime/RegExpPrototype.h>
+#include <LibJS/Token.h>
 
 namespace JS {
 
@@ -84,6 +85,20 @@ static RegExpObject* regexp_object_from(VM& vm, GlobalObject& global_object)
     return static_cast<RegExpObject*>(this_object);
 }
 
+static String escape_regexp_pattern(const RegExpObject& regexp_object)
+{
+    auto pattern = regexp_object.pattern();
+    if (pattern.is_empty())
+        return "(?:)";
+    // FIXME: Check u flag and escape accordingly
+    pattern.replace("\n", "\\n", true);
+    pattern.replace("\r", "\\r", true);
+    pattern.replace(LINE_SEPARATOR, "\\u2028", true);
+    pattern.replace(PARAGRAPH_SEPARATOR, "\\u2029", true);
+    pattern.replace("/", "\\/", true);
+    return pattern;
+}
+
 JS_DEFINE_NATIVE_GETTER(RegExpPrototype::dot_all)
 {
     auto regexp_object = regexp_object_from(vm, global_object);
@@ -142,11 +157,20 @@ JS_DEFINE_NATIVE_GETTER(RegExpPrototype::multiline)
 
 JS_DEFINE_NATIVE_GETTER(RegExpPrototype::source)
 {
+    auto this_object = this_object_from(vm, global_object);
+    if (!this_object)
+        return {};
+
+    // FIXME: This is obnoxious - we should have an easier way of looking up %RegExp.prototype%.
+    auto& regexp_prototype = global_object.get(vm.names.RegExp).as_object().get(vm.names.prototype).as_object();
+    if (this_object == &regexp_prototype)
+        return js_string(vm, "(?:)");
+
     auto regexp_object = regexp_object_from(vm, global_object);
     if (!regexp_object)
         return {};
 
-    return js_string(vm, regexp_object->pattern());
+    return js_string(vm, escape_regexp_pattern(*regexp_object));
 }
 
 JS_DEFINE_NATIVE_GETTER(RegExpPrototype::sticky)

Libraries/LibJS/Tests/builtins/RegExp/RegExp.js

@@ -1,9 +1,8 @@
 test("basic functionality", () => {
-    // FIXME: update when toString is spec-compliant
+    expect(RegExp().toString()).toBe("/(?:)/");
-    expect(RegExp().toString()).toBe("//");
+    expect(RegExp(undefined).toString()).toBe("/(?:)/");
-    expect(RegExp(undefined).toString()).toBe("//");
     expect(RegExp("foo").toString()).toBe("/foo/");
     expect(RegExp("foo", undefined).toString()).toBe("/foo/");
     expect(RegExp("foo", "g").toString()).toBe("/foo/g");
-    expect(RegExp(undefined, "g").toString()).toBe("//g");
+    expect(RegExp(undefined, "g").toString()).toBe("/(?:)/g");
 });

Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.source.js

@@ -0,0 +1,8 @@
+test("basic functionality", () => {
+    expect(RegExp.prototype.source).toBe("(?:)");
+    expect(RegExp().source).toBe("(?:)");
+    expect(/test/.source).toBe("test");
+    expect(/\n/.source).toBe("\\n");
+    // FIXME: RegExp parse doesn't parse \/ :(
+    // expect(/foo\/bar/.source).toBe("foo\\/bar");
+});