The MID$ function allows you the option of either using MID$ on the  right
  of  the  equals  sign or MID$ on the left of the equals sign.  MID$ on the
  right extracts a specified substring while MID$ on  the  left  replaces  a
  specified substring.

  Example

  New_string$ = MID$(Old_string$, 5%, 8%)

  MID$(old_string$,1%,3%) = replace_string$