URL completo Regex -> URL base

URL completo:
http://stackoverflow.com/questions/6038061/regular-expression-to-find-urls-within-a-string

Base:

stackoverflow.com

Regex:
(http (s)?: / /) | ( /.*) {1}

Exemplo em JavaScript:

function toBaseURL(fullURL){
    return fullURL.replace(/(http(s)?://)|(/.*){1}/g, '');
}