com.struts.action.TestActionServlet looks like a servlet someone at your company wrote. Check the source code for what it does. It's not a good package name for what it is worth unless you work for a company called
Struts.
You are correct that the value is a regular expression. Diagramming it, we have:
.* - zero or more of any characters
[\\/] - slash or backslash
struts-config.*.xml - wildcard for files beginning with struts config that are xml
$ - end of
string
In other words, it finds struts config files with anything extra in the name like struts-config.qa.xml