Use of '()' in a regexp

[Ed Morton]

Someone just pointed this out to me (gawk 5.1.0):

$ printf 'foo\n' | awk '{gsub(/()/,"x")} 1'
xfxoxox

$ printf 'foo\n' | awk -v RS='()' -v ORS='x\n' '1'
foox

Obviously that's a pretty ridiculous regexp but it still has me 
wondering - why does `gsub()` treat the regexp `()` as matching a null 
string around every character while `RS` treats it as if I'd asked it to 
match the `\n` at the end of the input:

$ printf 'foo\n' | awk -v RS='\n$' -v ORS='x\n' '1'
foox

I could just file this under "don't write stupid regexps" but I was 
wondering if there's a more concrete, satisfying explanation of the 
behavior.

    Ed.