diff --git a/spec/std/regex/match_data_spec.cr b/spec/std/regex/match_data_spec.cr
index 1079dff145e1..7fc9c3219faa 100644
--- a/spec/std/regex/match_data_spec.cr
+++ b/spec/std/regex/match_data_spec.cr
@@ -65,12 +65,33 @@ describe "Regex::MatchData" do
     end
 
     it "with capture" do
-      matchdata(/f(o)o/, "foo").begin.should eq 0
-      matchdata(/f(o)o/, "foo").begin(1).should eq 1
-      matchdata(/f(o)o/, "foo").begin(-1).should eq 1
-      matchdata(/f(o)o/, ".foo.").begin.should eq 1
-      matchdata(/f(o)o/, ".foo.").begin(1).should eq 2
-      matchdata(/f(o)o/, ".foo.").begin(-1).should eq 2
+      md = matchdata(/f(o)o/, "foo")
+      md.begin.should eq 0
+      md.begin(1).should eq 1
+      md.begin(-1).should eq 1
+
+      md = matchdata(/f(o)o/, ".foo.")
+      md.begin.should eq 1
+      md.begin(1).should eq 2
+      md.begin(-1).should eq 2
+    end
+
+    it "with unmatched capture" do
+      md = matchdata(/f(x)?o/, "foo")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.begin(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.begin(-1)
+      end
+
+      md = matchdata(/f(x)?o/, ".foo.")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.begin(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.begin(-1)
+      end
     end
 
     it "char index" do
@@ -82,6 +103,24 @@ describe "Regex::MatchData" do
     it "char index" do
       matchdata(/foo/, "öfoo").byte_begin.should eq 2
     end
+
+    it "with unmatched capture" do
+      md = matchdata(/f(x)?o/, "foo")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_begin(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_begin(-1)
+      end
+
+      md = matchdata(/f(x)?o/, ".foo.")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_begin(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_begin(-1)
+      end
+    end
   end
 
   describe "#end" do
@@ -99,12 +138,33 @@ describe "Regex::MatchData" do
     end
 
     it "with capture" do
-      matchdata(/f(o)o/, "foo").end.should eq 3
-      matchdata(/f(o)o/, "foo").end(1).should eq 2
-      matchdata(/f(o)o/, "foo").end(-1).should eq 2
-      matchdata(/f(o)o/, ".foo.").end.should eq 4
-      matchdata(/f(o)o/, ".foo.").end(1).should eq 3
-      matchdata(/f(o)o/, ".foo.").end(-1).should eq 3
+      md = matchdata(/f(o)o/, "foo")
+      md.end.should eq 3
+      md.end(1).should eq 2
+      md.end(-1).should eq 2
+
+      md = matchdata(/f(o)o/, ".foo.")
+      md.end.should eq 4
+      md.end(1).should eq 3
+      md.end(-1).should eq 3
+    end
+
+    it "with unmatched capture" do
+      md = matchdata(/f(x)?o/, "foo")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.end(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.end(-1)
+      end
+
+      md = matchdata(/f(x)?o/, ".foo.")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.end(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.end(-1)
+      end
     end
 
     it "char index" do
@@ -116,6 +176,24 @@ describe "Regex::MatchData" do
     it "char index" do
       matchdata(/foo/, "öfoo").byte_end.should eq 5
     end
+
+    it "with unmatched capture" do
+      md = matchdata(/f(x)?o/, "foo")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_end(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_end(-1)
+      end
+
+      md = matchdata(/f(x)?o/, ".foo.")
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_end(1)
+      end
+      expect_raises(IndexError, "Capture group 1 was not matched") do
+        md.byte_end(-1)
+      end
+    end
   end
 
   describe "#[]" do
diff --git a/src/regex/match_data.cr b/src/regex/match_data.cr
index 89e12204986e..085c829073e8 100644
--- a/src/regex/match_data.cr
+++ b/src/regex/match_data.cr
@@ -59,10 +59,15 @@ class Regex
     # When *n* is `0` or not given, uses the match of the entire `Regex`.
     # Otherwise, uses the match of the *n*th capture group.
     #
+    # Raises `IndexError` if the index is out of range or the respective
+    # subpattern is unused.
+    #
     # ```
     # "Crystal".match(/r/).not_nil!.begin(0)     # => 1
     # "Crystal".match(/r(ys)/).not_nil!.begin(1) # => 2
     # "クリスタル".match(/リ(ス)/).not_nil!.begin(0)    # => 1
+    # "Crystal".match(/r/).not_nil!.begin(1)     # IndexError: Invalid capture group index: 1
+    # "Crystal".match(/r(x)?/).not_nil!.begin(1) # IndexError: Capture group 1 was not matched
     # ```
     def begin(n = 0) : Int32
       @string.byte_index_to_char_index(byte_begin(n)).not_nil!
@@ -73,10 +78,15 @@ class Regex
     # When *n* is `0` or not given, uses the match of the entire `Regex`.
     # Otherwise, uses the match of the *n*th capture group.
     #
+    # Raises `IndexError` if the index is out of range or the respective
+    # subpattern is unused.
+    #
     # ```
     # "Crystal".match(/r/).not_nil!.end(0)     # => 2
     # "Crystal".match(/r(ys)/).not_nil!.end(1) # => 4
     # "クリスタル".match(/リ(ス)/).not_nil!.end(0)    # => 3
+    # "Crystal".match(/r/).not_nil!.end(1)     # IndexError: Invalid capture group index: 1
+    # "Crystal".match(/r(x)?/).not_nil!.end(1) # IndexError: Capture group 1 was not matched
     # ```
     def end(n = 0) : Int32
       @string.byte_index_to_char_index(byte_end(n)).not_nil!
@@ -87,15 +97,22 @@ class Regex
     # When *n* is `0` or not given, uses the match of the entire `Regex`.
     # Otherwise, uses the match of the *n*th capture group.
     #
+    # Raises `IndexError` if the index is out of range or the respective
+    # subpattern is unused.
+    #
     # ```
     # "Crystal".match(/r/).not_nil!.byte_begin(0)     # => 1
     # "Crystal".match(/r(ys)/).not_nil!.byte_begin(1) # => 2
     # "クリスタル".match(/リ(ス)/).not_nil!.byte_begin(0)    # => 3
+    # "Crystal".match(/r/).not_nil!.byte_begin(1)     # IndexError: Invalid capture group index: 1
+    # "Crystal".match(/r(x)?/).not_nil!.byte_begin(1) # IndexError: Capture group 1 was not matched
     # ```
     def byte_begin(n = 0) : Int32
       check_index_out_of_bounds n
       n += size if n < 0
-      @ovector[n * 2]
+      value = @ovector[n * 2]
+      raise_capture_group_was_not_matched(n) if value < 0
+      value
     end
 
     # Returns the position of the next byte after the match.
@@ -103,15 +120,22 @@ class Regex
     # When *n* is `0` or not given, uses the match of the entire `Regex`.
     # Otherwise, uses the match of the *n*th capture group.
     #
+    # Raises `IndexError` if the index is out of range or the respective
+    # subpattern is unused.
+    #
     # ```
     # "Crystal".match(/r/).not_nil!.byte_end(0)     # => 2
     # "Crystal".match(/r(ys)/).not_nil!.byte_end(1) # => 4
     # "クリスタル".match(/リ(ス)/).not_nil!.byte_end(0)    # => 9
+    # "Crystal".match(/r/).not_nil!.byte_end(1)     # IndexError: Invalid capture group index: 1
+    # "Crystal".match(/r(x)?/).not_nil!.byte_end(1) # IndexError: Capture group 1 was not matched
     # ```
     def byte_end(n = 0) : Int32
       check_index_out_of_bounds n
       n += size if n < 0
-      @ovector[n * 2 + 1]
+      value = @ovector[n * 2 + 1]
+      raise_capture_group_was_not_matched(n) if value < 0
+      value
     end
 
     # Returns the match of the *n*th capture group, or `nil` if there isn't