Logback replace regex to detect empty values - regex

Question

I am trying to use logback's replace feature to not have empty values printed in my MDC log pattern.
http://logback.qos.ch/manual/layouts.html#replace
I am trying to follow an example from here
http://logogin.blogspot.com/2013/04/logback-mdc-and-empty-values.html
Some background
For 90% of the time my log pattern prints
2014-08-28 11:30:27,014 emp:Peter org:IT Expense submitted
For 5% of the time it prints
2014-08-28 11:30:27,014 emp: org: Cleanup jobs.
This is because the emp and org do not need to be supplied on MDC in the latter case.
For these cases, I want the emp: and org: to not be present at all in the log line.
Desired
2014-08-28 11:30:27,014 Cleanup jobs.
Possible solution with replace
Here is my variable and the appender I am using. The idea is that the mdcPattern will resolve to an empty string for no emp and org values.
<variable scope="context" name="mdcPattern" value="%replace( emp:%X{empName} org:%X{orgName} ) {'[a-z]+:( |$)', ''}"/>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d ${mdcPattern} %thread %-5level %logger{25} - %msg%n</pattern>
</encoder>
</appender>
However the replace regex does not work. I see log lines as:
2014-08-28 11:30:27,014 emp: org: {'[a-z]+:( |$
My regex is a bit weak. I can't seem to understand why the replace pattern is appearing as is in my log line. Any help is greatly appreciated.

Answer 1

With help from the author of the original blog post, I've managed to get this to work.
He has provided another example on GitHub.
http://logogin.blogspot.com/2013/04/logback-mdc-and-empty-values.html
https://gist.github.com/logogin/ff44c254f655340b653c
I had extra spaces in the replacement pattern which I've removed.
Eg: {orgName} ) {'[a-z]+
To: {orgName}){'[a-z]+