The regular expression for the inclusion of jobs looks at the full name of the job including folders, however the regular expression for grouping jobs only looks at the name of the job. That means it's impossible to group jobs based on their folder. For example if I have jobs:

folder1/subfolder1/job1
folder1/subfolder1/job2
folder1/subfolder2/job1
folder1/subfolder2/job2
folder2/subfolder1/job1
folder2/subfolder1/job2
folder2/subfolder2/job1
folder2/subfolder2/job2

In this case I can have a view that shows all builds in folder1 by setting the regular expression to '(folder1)\/.\/.$'

If I now want to group the builds by subfolder I would imagine that I need to set the categorization regular expression to: '.\/(subfolder2)\/.$'. However that won't work because the categorization regex only works on the build name (i.e. job1 / job2 etc.)