Find sequence of character in word/String and replace with the its occurrence number

Case 1:

String str = "Test something Test";

I have to find Test and replace with it's occurrence number. Here Test appears two time. So the first occurrence shall be replaced with 1 and second occurrence replace with 2.

Expected output

"1 something 2"

This is just small string. It may contain more occurrence of Test in string/word.

Case 2:

String str = "TestsomethingTest";

Expected output

"1something2"

I tried with replace but it replace all occurrence with same number.