I have the following rule file:

rule selectcmd match regex '(?i)SELECT .*'

users %@% match any rules selectcmd

On the second line there is a single space. MariaDB MaxScale refuse to start and in the log I get:
2018-02-24 12:06:34 warning: Firewall rules file lacks a trailing newline.
2018-02-24 12:06:34 error : [dbfwfilter] Error on line 3, syntax error: users
If you remove the single space on line two in the rules file, it works as expected:

rule selectcmd match regex '(?i)SELECT .*'

users %@% match any rules selectcmd

There is the error message about a missing newline, but this is actually not a correct description of the issue and doesn't help in diagnosing the error.