start—Numeric expression that sets the starting position for each search (If the start expression is omitted, the search begins with the first character of a string (InStr), or –1 (InStrRev). If start contains Null, an error occurs. The start argument is required if compare is specified.)
string1—String expression being searched
string2—String expression being searched for
compare—Numeric expression that specifies the kind of comparison to use when substrings are evaluated (If compare is omitted, a binary comparison is performed.)