problem in grep 2.4.2?

[Mickey Ferguson]

I want to do a grep on all files matching *.rul in the current directory and
all subdirectories.  I thought grep -r string *.rul would work, but it
doesn't.  Why is it that file specifications only work in the current
directory, while recursion searches only work with no file specifications?
Shouldn't it just go through each subdirectory, then look at each file that
matches the file specification(s), and perform its search on that list of
files?