A numeric argument.
Specifies the number of the occurrence to be returned. For example, if occurrence_arg is 2, the function matches the first occurrence in source_string and starts searching from the character following the first occurrence in source_string for the second occurrence in source_string.
If occurrence_arg is greater than the number of matches found, 0 is returned.
If occurrence_arg is NULL, a NULL result is returned. If occurrence_arg is omitted, the default value (1) is used.