$source = 'dasdasdkjasjk asldkas;ldlasd askdjaslkdjasl lasdkj;lasdjl www10.subdomain.com jasdhkjsahd лофырвлофырв лофыр';

preg_match_all('#(www[0-9]{1,2}\.subdomain\.com)#',$source,$m);

echo $m;