[linux] sed with literal string--not input file

This should be easy: I want to run sed against a literal string, not an input file. If you wonder why, it is to, for example edit values stored in variables, not necessarily text data.

When I do:

sed 's/,/','/g' "A,B,C"

where A,B,C is the literal which I want to change to A','B','C

I get

Can't open A,B,C

As though it thinks A,B,C is a file.

I tried piping it to echo:

echo "A,B,C" | sed 's/,/','/g' 

I get a prompt.

What is the right way to do it?

My version using variables in a bash script:

Find any backslashes and replace with forward slashes:

input="This has a backslash \\"

output=$(echo "$input" | sed 's,\\,/,g')

echo "$output"

Works like you want:

echo "A,B,C" | sed s/,/\',\'/g