substring

s = 'http://forum.pcdvd.com.tw/search.php?searchid=7218092&pp=25&page=11'

//find string from 'searchid='
s.substring(s.indexOf('searchid='))
//"searchid=7218092&pp=25&page=11"
s.substring(0, s.indexOf('searchid=')) 
//"http://forum.pcdvd.com.tw/search.php?"

//find string from 'searchid=' but not include 'searchid='
s.substring(s.indexOf('searchid=') + ('searchid=').length)
//"7218092&pp=25&page=11"