Why isn't this Bash string substitution working?

Question

I'm trying to do a string substitution in bash to escape the dots in a version number to ultimately pass to grep. When I run

echo ${3.9.1//./\\.}

Expected output is 3\.9\.1. I get a bad substitution error instead. I don't understand how this isn't correct.

Answer 1

Put your string in a variable then you can use Parameter Expansion:

s="3.9.1"
echo "${s//./\\.}"

Output:

3\.9\.1