|sString||(String) Source string expression.|
|nStart||(Long) Index in the string where the requested substring is to start.
0 = from beginning of the string.
If this argument is negative, then it specifies the position from the end of the string. It means that the number -1 indicates the last character, -2 the one before last character, etc.
|nEnd||[optional](Long) Index in the string that follows right after the substring ends.
If not set, then the substring contains all characters up to the end of the string.
If the argument is negative, then the cgaracters are counted from the end of the string.
var sSub = Pm.StringSub("x100abc", 1, 4); // sSub contains "100"