You would start with searching this site via "Super search" for something like "nth character occurrence". Otherwise, following algorithm comes to mind (untested; would be pleased to know of any errors) ...

keep a counter to count the number of occurrences of the pattern,
initially -1 (assuming 0-based indexing)

keep a marker to note the position of the pattern

run loop from the start of a line to its end

  check if pattern exists
    if not,
      exit out of loop

    else
       check if counter is same the nth-place required
       if yes,
          exit out of loop

       else,
         increment the counter
         set the marker to the start of the pattern
         loop the loop from the marker + the length of the pattern + 1

at the end, equivalence of counter & desired nth occurrence will
give you the result.
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