How to hgignore all files of a particular extension except in one directory and its subdirectories?

How to hgignore all files of a particular extension except in one
directory and its subdirectories?

I would like to use the .hgignore file of Mercurial to ignore all files
with file extension .tex, except those .tex files in one particular
directory and whatever subdirectory of this directory.
I presume syntax: regexp will be required for this.