REGEXP_INSTR function returns a number for the position of matched pattern.
Unlike INSTR, REGEXP_INSTR cannot work from the end of the string backward.
The arguments for REGEXP_INSTR are:
REGEXP_INSTR(String to search, Pattern, [Position,[Occurrence, [Return-option, [Parameters]]]])
Pattern is the sought string, which will be expressed as an RE.
These first two arguments are not optional.
Position is the place in 'String to search' to begin the search for Pattern. The default is 1.
SQL> SELECT REGEXP_INSTR('This is a test','a') position FROM dual;
POSITION
----------
9
SQL>