bstack(?!.overflow)

var res = Regex.Replace(input, @"(?i)bstack(?!.overflow)", "STACK");

System.Text.RegularExpressions.Regex.Replace("This is a stack of pages on stack overflow. Sometimes also referred to as stack.overflow.com"
    ,"stack(?!.overflow)"
    ,"STACK")

(?<=s|^)stack(?=s|.$)