Attempting to create a filter with a regular expression rule does not
work.  Here's what I did:

Chose "Edit/Filters"
Clicked "New" under the list of filters and give the filter a name.
Clicked "New" under the list of conditions.
Changed the pull-down selection from "Simple" to "Regular Expression"
Changed the "Match Fields" to "To"
Typed my regular expression into the bottom text box then click "Add".
Clicked "OK"
Clicked "Edit" under the list of filter conditions.

The regular expression had disappeared.

I re-entered the regular expression and clicked "Add" and "OK" again.
Clicked "Apply" to apply the new filter conditions.
Clicked "Edit" under the list of filter conditions again.

This time not only had the regular expression have disappeared but the
"Match Fields" set of checkboxes had changed from "To" to "Body" and "Cc".

If I quit from balsa and check the config file in ~/.balsa it shows
the filter with an empty condition and when re-starting balsa it warns
about the filter with no condition and deletes it.