* string.c (rb_str_aset_m): Documentation for String#[]= fix
  Raises an IndexError if Regexp match is out of range.
  Github fixes #243 Patch by Dmtiriy Budnik


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38996 b2dd03c8-39d4-4d8f-98ff-823fe69b080e