Oracle substitution for charindex function in SQL Server?

Instr (STR, CHR) --- location where CHR appears 1st times in Str

Instr (STR, CHR, 3rd) --- searches for the position where the CHR appears 2nd times in STR starting from the STR character position

Instr (STR, CHR,-3, 2) --- searches for the position where the CHR appears 3rd Times in STR from right to left.

Instr (C1, C2 [, <I> [, J])
C1 and c2 are strings, and I and j are integers. The function returns the position where C2 appears for the nth occurrence of C1, and searches for the position starting from the nth occurrence of C1. If no expected character is found, 0 is returned. If I is a negative number, the search proceeds from right to left, but the position is calculated from left to right, the default values of I and j are 1.
Select instr ('Mississippi ',' I ', 3, 3) from dual
Instr ('Mississippi ',' I ', 3, 3)
11

Select instr ('Mississippi ',' I ',-2, 3) from dual

Instr ('Mississippi ',' I ',-2, 3)
2