“Ruby Regex reemplazar el grupo de captura” Código de respuesta

Ruby Regex reemplazar el grupo de captura

You can't. gsub replaces the entire match; it does not do anything with the captured groups. It will not make any difference whether the groups are captured or not.
In order to achieve the result, you need to use lookbehind and lookahead.

"5,214".gsub(/(?<=\d),(?=\d)/, '.')