Here's a snippet of a first attempt, which does not work: |> Array.map (fun s -> Regex(@"(one|two|three|four|five|six|seven|eight|nine|\d)").Matches(s)) |>

Here's a snippet of a first attempt, which does not work: |> Array.map (fun s -> Regex(@"(one|two|three|four|five|six|seven|eight|nine|\d)").Matches(s)) |> Array.map (fun m -> m |> Seq.map (fun g -> g.Value) |> Seq.toList) And here's the correct code: |> Array.map (fun s -> Regex(@"(?=(one|two|three|four|five|six|seven|eight|nine|\d))").Matches(s)) |> Array.map (fun m -> m |> Seq.map (fun g -> g.Groups.[1].Value) |> Seq.toList) There's no "fundamental" difference between the two, rather, one uses a regex with positive lookahead and the other doesn't. The first approach was not operating under the assumption that there would be strings like "oneighth" at the end of a line. That's a detail and it only applies to one part of a functional pipeline!