Проверка строки на наличии символа.

[Azy]

var mystring:String = "split is Shit? Yes, split is SHIT! And substring method is also ShIT!";
var oldChar:RegExp = /shit/gi;
var newChar:String = "good";
mystring  = mystring.replace(oldChar, newChar);
trace(mystring); //split is good? Yes, spit is good! And substring method is also good!