function subDomain(url) {
 
// IF THERE, REMOVE WHITE SPACE FROM BOTH ENDS
url = url.replace(new RegExp(/^\s+/),""); // START
url = url.replace(new RegExp(/\s+$/),""); // END
 
// IF FOUND, CONVERT BACK SLASHES TO FORWARD SLASHES
url = url.replace(new RegExp(/\\/g),"/");
 
// IF THERE, REMOVES 'http://', 'https://' or 'ftp://' FROM THE START
url = url.replace(new RegExp(/^http\:\/\/|^https\:\/\/|^ftp\:\/\//i),"");
 
// IF THERE, REMOVES 'www.' FROM THE START OF THE STRING
url = url.replace(new RegExp(/^www\./i),"");
 
// REMOVE COMPLETE STRING FROM FIRST FORWARD SLASH ON
url = url.replace(new RegExp(/\/(.*)/),"");
 
// REMOVES '.??.??' OR '.???.??' FROM END - e.g. '.CO.UK', '.COM.AU'
if (url.match(new RegExp(/\.[a-z]{2,3}\.[a-z]{2}$/i))) {
      url = url.replace(new RegExp(/\.[a-z]{2,3}\.[a-z]{2}$/i),"");
 
// REMOVES '.??' or '.???' or '.????' FROM END - e.g. '.US', '.COM', '.INFO'
} else if (url.match(new RegExp(/\.[a-z]{2,4}$/i))) {
      url = url.replace(new RegExp(/\.[a-z]{2,4}$/i),"");
}
 
// CHECK TO SEE IF THERE IS A DOT '.' LEFT IN THE STRING
var subDomain = (url.match(new RegExp(/\./g))) ? true : false;
 
return(subDomain);
 
}

http://www.web-wise-wizard.com/javascript-tutorials/javascript-regular-expressions-regexp.html

Check if a url contains a subdomain

regex, url

JavaScript