escaping backslash in a string

I have it working now, I think. Since it's going into a regular
expression, I have to escape each of the escape characters, resulting
in four backslashes altogether:

sub("[.]", "x", "a.b")

[1] "axb"

sub("[.]", "\.", "a.b")

[1] "a.b"

sub("[.]", "\\.", "a.b")

[1] "a.b"

sub("[.]", "\\\.", "a.b")

[1] "a.b"

sub("[.]", "\\\\.", "a.b")

[1] "a\\.b"

cat(sub("[.]", "\\\\.", "a.b"))

a\.b>

or

cat(sub("\\.", "\\\\.", "a.b"))

a\.b> 

Dan