[demindiro]

> A lot of the times, the blue highlighting seems to indicate I've matched all the necessary parts of the input, which should mean the puzzle is solved, but it's Game Over instead.

You need to include punctuation for some regexes.

e.g.

   The brown fox.
             ^^^^

Would be matched with:

  /fox./

[anamexis]

You have to be careful about escaping, in case you do not watch to match "foxy" as well.