diff --git a/parse.y b/parse.y
index 915db67..0a93ead 100644
--- a/parse.y
+++ b/parse.y
@@ -10311,6 +10311,17 @@ rb_intern_str(VALUE str)
     return intern_str(rb_str_dup(str));
 }
 
+ID
+rb_interned_str(VALUE str)
+{
+    st_data_t id;
+
+    if (st_lookup(global_symbols.sym_id, str, &id))
+	return (ID)id;
+
+    return (ID)0;
+}
+
 VALUE
 rb_id2str(ID id)
 {
diff --git a/string.c b/string.c
index 04984f0..12f7487 100644
--- a/string.c
+++ b/string.c
@@ -7196,6 +7196,61 @@ rb_str_intern(VALUE s)
 
 /*
  *  call-seq:
+ *     str.interned   -> symbol
+ *
+ *  Returns the <code>Symbol</code> corresponding to <i>str</i>, returning
+ *  nil if it did not previously exist.  See <code>Symbol#id2name</code>.
+ *
+ *     x = :banana
+ *     "banana".interned          #=> :banana
+ *     "mango".interned           #=> nil
+ *
+ */
+
+VALUE
+rb_str_interned(VALUE s)
+{
+    VALUE str = RB_GC_GUARD(s);
+    ID id;
+
+    id = rb_interned_str(str);
+    if (id == ID(0))
+	return Qnil;
+
+    return ID2SYM(id);
+}
+
+
+/*
+ *  call-seq:
+ *     str.to_existing_sym   -> symbol
+ *
+ *  Returns the <code>Symbol</code> corresponding to <i>str</i>, raising an
+ *  ArgumentError if it did not previously exist.
+ *  See <code>Symbol#id2name</code>.
+ *
+ *     x = :banana
+ *     "banana".to_existing_sym   #=> :banana
+ *     "mango".to_existing_sym    #=> ArgumentError
+ *
+ */
+
+VALUE
+rb_str_to_existing_sym(VALUE s)
+{
+    VALUE str = RB_GC_GUARD(s);
+    ID id;
+
+    id = rb_interned_str(str);
+    if (id == ID(0))
+	rb_raise(rb_eArgError, "undefined symbol :'%s'", StringValuePtr(str));
+
+    return ID2SYM(id);
+}
+
+
+/*
+ *  call-seq:
  *     str.ord   -> integer
  *
  *  Return the <code>Integer</code> ordinal of a one-character string.
@@ -8238,6 +8293,8 @@ Init_String(void)
     rb_define_method(rb_cString, "crypt", rb_str_crypt, 1);
     rb_define_method(rb_cString, "intern", rb_str_intern, 0);
     rb_define_method(rb_cString, "to_sym", rb_str_intern, 0);
+    rb_define_method(rb_cString, "interned", rb_str_interned, 0);
+    rb_define_method(rb_cString, "to_existing_sym", rb_str_interned_err, 0);
     rb_define_method(rb_cString, "ord", rb_str_ord, 0);
 
     rb_define_method(rb_cString, "include?", rb_str_include, 1);