Regex and lookahead to specific number of chars

If I have a string such as
“I love ruby, i really do love ruby, oh yes i do blah blah blah”

and i want to insert an asterix at every 10th char whether its a
whitespace, digit whatever how do i go about it

i was trying

string.gsub!(/[.]{10}/, “*”)

but it doesnt work

any hints???