Try this:

 abc((,(?<delim>\s+))|(?<delim>[-/.]))xyz\k<delim>

Hm... I thought the explanation on the MS page was really quite good. I
can't readily come up with a good sample where you'd actually need the
syntax, though :-) I'm trying, nevertheless.

The syntax (?(expression)yes|no) makes a zero-width assertion of the
expression and if it matches continues to match the yes expression. If
the assertion didn't match, it continues with the no part instead.
Say you have a line that can start with "date" or it might not.
Depending on that, you want to parse the line differently, saving the
date if there's one. You could do this:

  ^(?(date)date (?<date>\d\d-\d\d-\d\d\d\d)|.*)$

This would parse out the date if there was one and match the whole line
if the date wasn't found. As I said, this is not a good sample - you can
do it with a different syntax - but I hope it shows the point.

The second syntax (?(name)yes|no) does just the same thing, only it
doesn't evaluate the "name" as an expression, but accesses a named
capture that has already been found instead.

               Oliver Sturm