The main weakness

[DavidHaslam]

The main weakness is that markers within a line of text do not get highlighted unless there is a preceding space. Yet within the rules for USFM, there is no requirement for such a space.

The issue is that there is no flexibility within the "Define your language" GUI to overcome this limitation.

[DavidHaslam]

Notepad++ uses the underlying Scintilla library which is by a variety of editing tools.
My syntax highlighting file would likely have wider uses.

But because of the main weakness it's not really ready submit it to Scintilla.