I turned off stemmming since it adds unexpected results and have been making my own library. This works:

regexp_filter=(?i)b(Process|Switch|Business)b=>1es

as does

regexp_filter=(?i)b(Plan)b=>1ning

In other words a search on Business or Businesses finds the same results as does Plan or Planning.

However single s plurals fails:

regexp_filter=(?i)b(Dog|Word)b=>1s

As only Dogs or Words is found while Dog and Word find nothing. If I reverse the process to:

regexp_filter=(?i)b(Dog|Word)sb=>1

Now it only finds Dog or Word but not Dogs or Words.

So every other suffix modification works for both forms but for some reason the added/removed ‘s’ only finds the modified term.