Fix `String#index` to clear MatchData when a regexp is passed

[Bug #20421]

The bug was fixed in Ruby 3.3 via 9dcdffb8bf8a3654fd78bf1a58b30c8e13888a7a

1 files changed, 3 insertions, 1 deletions

diff --git a/string.c b/string.c
index 1f7321a1ce..ed51fa4af7 100644
--- a/string.c
+++ b/string.c
@@ -3993,8 +3993,10 @@ rb_str_index_m(int argc, VALUE *argv, VALUE str)
     }
 
     if (RB_TYPE_P(sub, T_REGEXP)) {
-        if (pos > str_strlen(str, NULL))
+        if (pos > str_strlen(str, NULL)) {
+            rb_backref_set(Qnil);
             return Qnil;
+        }
         pos = str_offset(RSTRING_PTR(str), RSTRING_END(str), pos,
                          rb_enc_check(str, sub), single_byte_optimizable(str));