Indeed, this would fail on some cases. Probably because the test were not tight enough 2 years ago, one of them being

String: ", . pears\nlemons\nbananas ' watermelons = ! watermelons"
Markers: []

I think you are using index 0 . This is OK if there are only one marker in each line ( I mean before \n).
If there are more than marker in ecah line, there are many parts in this list and index 0 is not enough.