Fix optional token elision in naming patterns

[krejko]

Summary

Naming patterns support optional tokens by emitting a sentinel for empty values and stripping the surrounding punctuation afterwards. That works for a bracket group holding a single token, but not for one holding several — so a pattern like [{Series} {SeriesNumber}] leaks a literal sentinel into folder and file names for every book without a series.

With {Author}/[{Series} {SeriesNumber}] {Title} {{Narrator}} ({Year}):

Case	Before	After
Standalone book	J.K. Rowling/[__EMPTY_VAR_] The Ickabog {Stephen Fry} (2020)	J.K. Rowling/The Ickabog {Stephen Fry} (2020)
Series without a number	Cory Doctorow/[Radicalized /] Radicalized (2019)	Cory Doctorow/[Radicalized] Radicalized (2019)
Series with everything	unchanged	unchanged

Two root causes:

1. The sentinel was __EMPTY_VAR__, and _ is part of the separator character class used by the cleanup regexes — so cleanup partially consumed the sentinel itself, leaving __EMPTY_VAR_ behind.
2. The bracket rule only matched a group containing exactly one sentinel. With two, the group survived long enough for the slash rule to rewrite the sentinel as / inside the brackets.

Changes

Fixed

• Replace the __EMPTY_VAR__ sentinel with U+E000, which shares no characters with the separator class.
• Resolve bracket groups before separator and slash handling, so a surviving sentinel can no longer inject a stray / into a name.
• Collapse bracket groups containing any number of empty tokens: drop the group when nothing meaningful survives, otherwise keep it with the empty tokens removed.

Added

• FileNamingService_OptionalTokenElisionTests covering all-tokens-present, empty series, empty series + narrator, series without a number, trailing optional tokens, the filename variant, and a guard that the sentinel never reaches output.

Testing

dotnet test filtered to the naming suites: 28 passed, 1 failed.

The single failure is DownloadNaming_AudiobookMetadataTests.ProcessCompletedDownload_UsesAudiobookMetadata_ForNaming, which fails identically on unmodified canary — verified by stashing this change and re-running that test alone. It is pre-existing and unrelated.

Existing DownloadNaming_PatternCollapseTests and FileNamingService_PatternSelectionTests still pass, so single-token groups and duplicate-component collapsing are unaffected.

Notes

Delimiters are escaped explicitly rather than through Regex.Escape, which does not escape ] — building the negated class with it produces [^\[]], closing the class early. That bug was caught by the new tests.

Behaviour only changes where the old output was already broken; patterns whose groups hold a single token render exactly as before.

🤖 Generated with Claude Code