So, as I was saying since early this year, I was working on an update of the documentation of [expr]. This reflects all the new and old stuff that were never fully documented in the help file. I made a Pull Request at
https://github.com/pure-data/pure-data/pull/132
But I also have the help file attached here for anyone to check it out
Basically, I kept all the original information, but reorganized things and added a LOT of missing information. Now we have every single function documented and separated into groups, that's the main addition! I also added a version number and some useful examples/information on how to use [expr] with value/arrays and deal with "$0".
cheers