String := "-+-++---+--+-+-"
SubString := "-+-"

MsgBox, % "Без перекрытия — " . Occurrence(String, SubString, 0)
       . "`nС перекрытием — " . Occurrence(String, SubString, 1)

Occurrence(String, SubString, overlap := 0)
{
   Occurrence := start := 0
   While start := InStr(String, SubString, 0, (overlap || A_Index = 1) ? ++start : start + StrLen(SubString))
      Occurrence++
   Return Occurrence
}

-+-++---+--+-+-