Example for @isene on #ruby IRC

str.rb

without = %w[b c]
without_regexp = Regexp.union(without)
split_index = 7
dropped = split_index.pred

str = 'abcdaba123abcd'
pos = str
  .each_grapheme_cluster
  .with_index
  .lazy
  .grep_v(without_regexp)
  .drop(dropped)
  .first
  &.last
index = pos + dropped

[str[...index], str[index..]]
#=> ["abcdaba123ab", "cd"]