Occurrence refers to the first, second, third, etc., occurrence of the pattern in S. The default is 1 (first). : REGEXP_INSTR « Regular Expressions Functions « Oracle PL/SQL Tutorial

Searching for the second "a" starting at position 1.

SQL> SELECT REGEXP_INSTR('This is a test for print a','a',1,2) position
  2  FROM dual;

  POSITION
----------
        26

SQL>

18.2.REGEXP_INSTR
	18.2.1.	Regexp_Instr returns the location (beginning) of a pattern in a given string
	18.2.2.	REGEXP_INSTR(x, pattern [, start [, occurrence [, return_option [, match_option]]]]) searches for pattern in x.
	18.2.3.	Regexp_Instr
	18.2.4.	Parameters is a field that may be used to define how one wants the search to proceed:
	18.2.5.	Find the 's' and ignore case.
	18.2.6.	The simplest regular expression matches letters, letter for letter
	18.2.7.	SELECT REGEXP_INSTR('Two is bigger than One','One') where_it_is
	18.2.8.	Occurrence refers to the first, second, third, etc., occurrence of the pattern in S. The default is 1 (first).
	18.2.9.	Return-option returns the position of the start or end of the matched string.
	18.2.10.	The Return-option is set to 1 to indicate the end of the found pattern
	18.2.11.	Returns the position of the second occurrence that matches the regular expression s[[:alpha:]]{3} starting at position 1:
	18.2.12.	Returns the position of the second occurrence that matches the letter o starting at position 10 using REGEXP_INSTR()
	18.2.13.	Specify any series of letters and find matches, just like INSTR
	18.2.14.	regexp_instr(comments, '[^ ]+', 1, 9) > 0