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

[Bug #20421]

The bug was fixed in Ruby 3.3 via 9dcdffb

spec/ruby/core/string/byteindex_spec.rb

@@ -0,0 +1,14 @@
+# -*- encoding: utf-8 -*-
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+describe "String#byteindex with Regexp" do
+  it "always clear $~" do
+    "a".byteindex(/a/)
+    $~.should_not == nil
+
+    string = "blablabla"
+    string.byteindex(/bla/, string.bytesize + 1)
+    $~.should == nil
+  end
+end

spec/ruby/core/string/index_spec.rb

@@ -224,6 +224,15 @@
     $~.should == nil
   end
 
+  it "always clear $~" do
+    "a".index(/a/)
+    $~.should_not == nil
+
+    string = "blablabla"
+    string.index(/bla/, string.length + 1)
+    $~.should == nil
+  end
+
   it "starts the search at the given offset" do
     "blablabla".index(/.{0}/, 5).should == 5
     "blablabla".index(/.{1}/, 5).should == 5

spec/ruby/core/string/rindex_spec.rb

@@ -265,6 +265,15 @@ def obj.method_missing(*args) 5 end
     $~.should == nil
   end
 
+  it "always clear $~" do
+    "a".rindex(/a/)
+    $~.should_not == nil
+
+    string = "blablabla"
+    string.rindex(/bla/, -(string.length + 1))
+    $~.should == nil
+  end
+
   it "starts the search at the given offset" do
     "blablabla".rindex(/.{0}/, 5).should == 5
     "blablabla".rindex(/.{1}/, 5).should == 5

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));