check_systemdunit: update doc page

docs/20_Checks/check_systemdunit.md

@@ -112,3 +112,16 @@ CRITICAL: Unit justadummy.service could not be found.
 ### Other units
 
 With ``$ ./check_systemdunit -l`` you get a grouped list of all unit types. check_systemdunit handles all types - not only services.
+
+### Regex examples
+
+Here are a few examples for services with regex:
+
+* different names on different os:
+  * ``check_systemdunit -r '(apache2|httpd)\.service'``
+* Some operateing write a service with "d" - some not. To define the service that can have the "d" or not
+  * ``check_systemdunit -r 'nmb[d]{0,1}.service'``
+  * ``check_systemdunit -r 'smb[d]{0,1}.service'``
+  * ``check_systemdunit -r 'ssh[d]{0,1}.service'``
+* Placeholders for version numbers
+  * ``check_systemdunit -r 'php.*fpm\.service'``