`Regexp` in `MatchData` can be `nil`

`String#sub` with a string pattern defers creating a `Regexp`
until `MatchData#regexp` creates a `Regexp` from the matched
string.  `Regexp#last_match(group_name)` accessed its content
without creating the `Regexp` though.  [Bug #16508]

1 files changed, 4 insertions, 0 deletions

diff --git a/test/ruby/test_regexp.rb b/test/ruby/test_regexp.rb
index 39577bd75d..231fd392d1 100644
--- a/test/ruby/test_regexp.rb
+++ b/test/ruby/test_regexp.rb
@@ -161,6 +161,10 @@ class TestRegexp < Test::Unit::TestCase
     s = "foo"
     s[/(?<bar>o)/, "bar"] = "baz"
     assert_equal("fbazo", s)
+
+    /.*/ =~ "abc"
+    "a".sub("a", "")
+    assert_raise(IndexError) {Regexp.last_match(:_id)}
   end
 
   def test_named_capture_with_nul