diff --git a/core/matchdata/offset_spec.rb b/core/matchdata/offset_spec.rb
index 1ccb54b7a7..a03d58aad1 100644
--- a/core/matchdata/offset_spec.rb
+++ b/core/matchdata/offset_spec.rb
@@ -1,30 +1,102 @@
-# -*- encoding: utf-8 -*-
-
 require_relative '../../spec_helper'
 
 describe "MatchData#offset" do
-  it "returns a two element array with the begin and end of the nth match" do
-    match_data = /(.)(.)(\d+)(\d)/.match("THX1138.")
-    match_data.offset(0).should == [1, 7]
-    match_data.offset(4).should == [6, 7]
+  it "returns beginning and ending character offset of whole matched substring for 0 element" do
+    m = /(.)(.)(\d+)(\d)/.match("THX1138.")
+    m.offset(0).should == [1, 7]
+  end
+
+  it "returns beginning and ending character offset of n-th match, all the subsequent elements are capturing groups" do
+    m = /(.)(.)(\d+)(\d)/.match("THX1138.")
+
+    m.offset(2).should == [2, 3]
+    m.offset(3).should == [3, 6]
+    m.offset(4).should == [6, 7]
+  end
+
+  it "accepts String as a reference to a named capture" do
+    m = /(?<f>foo)(?<b>bar)/.match("foobar")
+
+    m.offset("f").should == [0, 3]
+    m.offset("b").should == [3, 6]
+  end
+
+  it "accepts Symbol as a reference to a named capture" do
+    m = /(?<f>foo)(?<b>bar)/.match("foobar")
+
+    m.offset(:f).should == [0, 3]
+    m.offset(:b).should == [3, 6]
   end
 
-  it "returns [nil, nil] when the nth match isn't found" do
-    match_data = /something is( not)? (right)/.match("something is right")
-    match_data.offset(1).should == [nil, nil]
+  it "returns [nil, nil] if a capturing group is optional and doesn't match" do
+    m = /(?<x>q..)?/.match("foobarbaz")
+
+    m.offset("x").should == [nil, nil]
+    m.offset(1).should == [nil, nil]
   end
 
-  it "returns the offset for multi byte strings" do
-    match_data = /(.)(.)(\d+)(\d)/.match("TñX1138.")
-    match_data.offset(0).should == [1, 7]
-    match_data.offset(4).should == [6, 7]
+  it "returns correct beginning and ending character offset for multi-byte strings" do
+    m = /\A\u3042(.)(.)?(.)\z/.match("\u3042\u3043\u3044")
+
+    m.offset(1).should == [1, 2]
+    m.offset(3).should == [2, 3]
   end
 
   not_supported_on :opal do
-    it "returns the offset for multi byte strings with unicode regexp" do
-      match_data = /(.)(.)(\d+)(\d)/u.match("TñX1138.")
-      match_data.offset(0).should == [1, 7]
-      match_data.offset(4).should == [6, 7]
+    it "returns correct character offset for multi-byte strings with unicode regexp" do
+      m = /\A\u3042(.)(.)?(.)\z/u.match("\u3042\u3043\u3044")
+
+      m.offset(1).should == [1, 2]
+      m.offset(3).should == [2, 3]
     end
   end
+
+  it "returns [nil, nil] if a capturing group is optional and doesn't match for multi-byte string" do
+    m = /\A\u3042(.)(.)?(.)\z/.match("\u3042\u3043\u3044")
+
+    m.offset(2).should == [nil, nil]
+  end
+
+  it "converts argument into integer if is not String nor Symbol" do
+    m = /(?<f>foo)(?<b>bar)/.match("foobar")
+
+    obj = Object.new
+    def obj.to_int; 2; end
+
+    m.offset(1r).should == [0, 3]
+    m.offset(1.1).should == [0, 3]
+    m.offset(obj).should == [3, 6]
+  end
+
+  it "raises IndexError if there is no group with the provided name" do
+    m = /(?<f>foo)(?<b>bar)/.match("foobar")
+
+    -> {
+      m.offset("y")
+    }.should raise_error(IndexError, "undefined group name reference: y")
+
+    -> {
+      m.offset(:y)
+    }.should raise_error(IndexError, "undefined group name reference: y")
+  end
+
+  it "raises IndexError if index is out of bounds" do
+    m = /(?<f>foo)(?<b>bar)/.match("foobar")
+
+    -> {
+      m.offset(-1)
+    }.should raise_error(IndexError, "index -1 out of matches")
+
+    -> {
+      m.offset(3)
+    }.should raise_error(IndexError, "index 3 out of matches")
+  end
+
+  it "raises TypeError if can't convert argument into Integer" do
+    m = /(?<f>foo)(?<b>bar)/.match("foobar")
+
+    -> {
+      m.offset([])
+    }.should raise_error(TypeError, "no implicit conversion of Array into Integer")
+  end
 end