The regular expression in the FILTER option of gcovr

I use gcovr to read coverage information for program foo.c. The syntax is

gcovr -r . -b --filter=FILE_PATTERN

where the '-r .' part indicates the current directory as the search root, '-b' is for branch coverage, and the FILE_PATTERN part is to keep only the data files that match this regular expression, according to gcovr's user guide http://ift.tt/1HnhkFK.

So I use this following command line

gcovr -r . -b --filter='foo\\.c'

But this command finds no information regarding foo.c retrieved. It seems I use a wrong regular expression. What do you think?