Is there a grammar for Maven/Plexus property expression syntax

Question

I've been looking for documentation on the maven expression syntax.

I managed to find the plexus-interpolation module which seems to do the work, but its documentation is pretty sparse.

Is there any documentation on the expression format better than the unittests?

https://codehaus-plexus.github.io/plexus-interpolation/ has some usage information, which suggests that the Regex based interpolator as the preferred one.