[O] capture template sexp

[Skip Collins]

I have not been able to find good examples of how to use the %(sexp)
construct in capture templates. I have a use case in which I would
like to retrieve some information from the capture target file to
include in the template. Can someone suggest a way construct a sexp
that returns a regular expression found in the target file? Say I need
to find the first headline that begins with the string "* Topic: " and
return the remainder of the line.