This was just my personal experience (which certainly came from trying out a different language than I typically use in my day to day), but I'd argue that day 1 part 2 wasn't _hard_, but improperly specified from the prompt. The examples given are:

  two1nine
  eightwothree
  abcone2threexyz
  xtwone3four
  4nineeightseven2
  zoneight234
  7pqrstsixteen

There is one critical example missing from this set and you can't exactly just figure out how you're meant to substitute the values without an example like:

  oneight