diff --git a/src/org/mozilla/javascript/regexp/NativeRegExp.java b/src/org/mozilla/javascript/regexp/NativeRegExp.java
index 07dab9c483..e6d91c25eb 100644
--- a/src/org/mozilla/javascript/regexp/NativeRegExp.java
+++ b/src/org/mozilla/javascript/regexp/NativeRegExp.java
@@ -171,9 +171,9 @@ public static void init(Context cx, Scriptable scope, boolean sealed)
         defineProperty(scope, "RegExp", ctor, ScriptableObject.DONTENUM);
     }
 
-    NativeRegExp(Scriptable scope, Object regexpCompiled)
+    NativeRegExp(Scriptable scope, RECompiled regexpCompiled)
     {
-        this.re = (RECompiled)regexpCompiled;
+        this.re = regexpCompiled;
         this.lastIndex = 0;
         ScriptRuntime.setBuiltinProtoAndParent(this, scope, TopLevel.Builtins.RegExp);
     }
@@ -218,7 +218,7 @@ Scriptable compile(Context cx, Scriptable scope, Object[] args)
             this.lastIndex = thatObj.lastIndex;
             return this;
         }
-        String s = args.length == 0 ? "" : ScriptRuntime.toString(args[0]);
+        String s = args.length == 0 ? "" : escapeRegExp(args[0]);
         String global = args.length > 1 && args[1] != Undefined.instance
             ? ScriptRuntime.toString(args[1])
             : null;
@@ -230,7 +230,7 @@ Scriptable compile(Context cx, Scriptable scope, Object[] args)
     @Override
     public String toString()
     {
-        StringBuffer buf = new StringBuffer();
+        StringBuilder buf = new StringBuilder();
         buf.append('/');
         if (re.source.length != 0) {
             buf.append(re.source);
@@ -255,6 +255,30 @@ private static RegExpImpl getImpl(Context cx)
         return (RegExpImpl) ScriptRuntime.getRegExpProxy(cx);
     }
 
+    private static String escapeRegExp(Object src) {
+        String s = ScriptRuntime.toString(src);
+        // Escape any naked slashes in regexp source, see bug #510265
+        StringBuilder sb = null; // instantiated only if necessary
+        int start = 0;
+        int slash = s.indexOf('/');
+        while (slash > -1) {
+            if (slash == start || s.charAt(slash - 1) != '\\') {
+                if (sb == null) {
+                    sb = new StringBuilder();
+                }
+                sb.append(s, start, slash);
+                sb.append("\\/");
+                start = slash + 1;
+            }
+            slash = s.indexOf('/', slash + 1);
+        }
+        if (sb != null) {
+            sb.append(s, start, s.length());
+            s = sb.toString();
+        }
+        return s;
+    }
+
     private Object execSub(Context cx, Scriptable scopeObj,
                            Object[] args, int matchType)
     {
diff --git a/src/org/mozilla/javascript/regexp/RegExpImpl.java b/src/org/mozilla/javascript/regexp/RegExpImpl.java
index ae87736ec8..202669e65c 100644
--- a/src/org/mozilla/javascript/regexp/RegExpImpl.java
+++ b/src/org/mozilla/javascript/regexp/RegExpImpl.java
@@ -56,7 +56,7 @@ public Object compileRegExp(Context cx, String source, String flags)
     public Scriptable wrapRegExp(Context cx, Scriptable scope,
                                  Object compiled)
     {
-        return new NativeRegExp(scope, compiled);
+        return new NativeRegExp(scope, (RECompiled) compiled);
     }
 
     public Object action(Context cx, Scriptable scope,
@@ -136,7 +136,7 @@ private static Object matchOrReplace(Context cx, Scriptable scope,
         Scriptable topScope = ScriptableObject.getTopLevelScope(scope);
 
         if (args.length == 0) {
-            Object compiled = NativeRegExp.compileRE(cx, "", "", false);
+            RECompiled compiled = NativeRegExp.compileRE(cx, "", "", false);
             re = new NativeRegExp(topScope, compiled);
         } else if (args[0] instanceof NativeRegExp) {
             re = (NativeRegExp) args[0];
@@ -149,7 +149,7 @@ private static Object matchOrReplace(Context cx, Scriptable scope,
             } else {
                 opt = null;
             }
-            Object compiled = NativeRegExp.compileRE(cx, src, opt, forceFlat);
+            RECompiled compiled = NativeRegExp.compileRE(cx, src, opt, forceFlat);
             re = new NativeRegExp(topScope, compiled);
         }
 
diff --git a/testsrc/doctests/regexp-literals.doctest b/testsrc/doctests/regexp.literals.doctest
similarity index 100%
rename from testsrc/doctests/regexp-literals.doctest
rename to testsrc/doctests/regexp.literals.doctest
diff --git a/testsrc/doctests/regexp.source.doctest b/testsrc/doctests/regexp.source.doctest
new file mode 100644
index 0000000000..5a9dde4c30
--- /dev/null
+++ b/testsrc/doctests/regexp.source.doctest
@@ -0,0 +1,90 @@
+js> x = RegExp('')
+/(?:)/
+js> x.source
+
+js> RegExp('a').source
+a
+js> RegExp('\\\\').source
+\\
+js> RegExp('\\\\\\\\').source
+\\\\
+js> x = RegExp('/')
+/\//
+js> x.source
+\/
+js> x.test('/')
+true
+js> x = RegExp('//')
+/\/\//
+js> x.source
+\/\/
+js> x.test('//')
+true
+js> x = RegExp('\/')
+/\//
+js> x.source
+\/
+js> x.test('/')
+true
+js> x = RegExp('\\/')
+/\//
+js> x.source
+\/
+js> x.test('/')
+true
+js> x = RegExp('\\\/')
+/\//
+js> x.source
+\/
+js> x.test('/')
+true
+js> x = RegExp('\\\\/')
+/\\//
+js> x.source
+\\/
+js> x.test('/')
+false
+js> x.test('\\/')
+true
+js> x = RegExp('/abc\/foo\\/bar\\\/xyz/')
+/\/abc\/foo\/bar\/xyz\//
+js> x.source
+\/abc\/foo\/bar\/xyz\/
+js> x.test('/abc/foo/bar/xyz/')
+true
+js> RegExp('[^/]*')
+/[^\/]*/
+js> RegExp('[^\/]*')
+/[^\/]*/
+js> RegExp('[^\\/]*')
+/[^\/]*/
+
+js> /./.compile('')
+/(?:)/
+js> /./.compile('a')
+/a/
+js> /./.compile('\\\\')
+/\\/
+js> /./.compile('\\\\\\\\')
+/\\\\/
+js> /./.compile('/')
+/\//
+js> /./.compile('//')
+/\/\//
+js> /./.compile('\/')
+/\//
+js> /./.compile('\\/')
+/\//
+js> /./.compile('\\\/')
+/\//
+js> /./.compile('\\\\/')
+/\\//
+js> /./.compile('/abc\/foo\\/bar\\\/xyz/')
+/\/abc\/foo\/bar\/xyz\//
+js> /./.compile('[^/]*')
+/[^\/]*/
+js> /./.compile('[^\/]*')
+/[^\/]*/
+js> /./.compile('[^\\/]*')
+/[^\/]*/
+