`stringToInt` returns `nil` for values like "Vol. 3"

```swift
/(?:\d|[., '])+/
```

matches one-or-more of (digit **or** separator), so a run consisting *only* of separators is a valid match. `firstMatch` on `"Vol. 3"` finds `". "` (from "Vol."), which filters down to an empty string, and `Int("")` is `nil`.

Any track-number tag where a separator appears before the first digit parses as "no track number." Requiring at least one digit in the match (e.g. anchoring on `\d`) would fix it.

Search the project for stringToInt and the shown regular expression, then reproduce the failure with the track-number value "Vol. 3". Verify that values with separators before digits parse their number correctly and that values containing no digits still behave as intended.

Written by the indexing model from the issue text.