double backslashes in usage section of .Rd files

I have written a package, where a function definition includes a regexp 
pattern including double backslashes, such as

myfunction <- function (pattern = ".*\\.txt$")

when I R CMD CHECK the corresponding .Rd file, I get warnings 
(code/documentation mismatch), if I enforce two backslashes in the 
documentation print out by

\usage { myfunction (pattern = ".*\\\\.txt$") }

have I to live with this or is their a way to avoid the warnings (apart 
from being satisfied with a wrong manpage ...)?

regards
joerg