Let me start with a short tutorial of directory aliases. There will be more examples to show what is supported and what you can do with it.
No string match functions can be used in aliases. Only the first part of the source is matched with the URI and then the path is replaced accordingly. In Absolute Path Directory Aliases, Destination is an absolute path.
/data/ -> /www/mydata/
If we have a path /data/... on the server, it will be physically loaded from /www/mydata/.... (with Linux path of course).
On Windows you could write
/data/ -> c:\www\data\
So if the user specifies /data/ in the URL it does not have to be in the actual web sites directory but it can be loaded externally from some other location (c:\www\data\). That is what directory aliases is all about.