For example, a keyword could be defined as such: (from user guide)
Select ${animal} from list
Select "${city}" "${team}"
I execute "${cmd:[^"]+}"
I type ${a:\d+} ${operator:[+-]} ${b:\d+}
Today is ${date:\d{4}-\d{2}-\d{2}}
http://robotframework.googlecode.com/hg/doc/userguide/RobotFrameworkUserGuide.html?r=2.7.5#embedding-arguments-into-keyword-name