如何替换ruby中最后一次出现的子串？

def replace_last_occurrence(string, old_substring, new_substring)
  array = string.split(old_substring)
  last_occurrence = array.pop
  new_string = array.join(old_substring)
  new_string.gsub!(last_occurrence, new_substring)
  return new_string
end

# 示例用法
original_string = "This is a test. This is only a test."
old_substring = "is"
new_substring = "was"
result = replace_last_occurrence(original_string, old_substring, new_substring)
puts result