Fix return value incorrect when fromIndex parameter is negative

[pengzhiwei2018]

For udf Instr when the fromIndex < 0, current return value is incorrect. It should searches backward from the resulting position.

[lkxdsb]

Hi maintainers, I would like to work on this issue.

Before implementing it, I would like to confirm the expected semantics for a negative from argument. My understanding is:

• a negative from counts backward from the end of the string;
• the search proceeds backward from that position;
• nth > 1 returns the nth occurrence encountered while searching backward;
• return values remain 1-based, and no match returns 0;
• the behavior should be consistent for both String and BinaryString overloads.

For example, for instr("abcabc", "a", -1, 1), I expect 4, and for instr("abcabc", "a", -1, 2), I expect 1.

Could you please confirm whether these semantics are correct? If so, I will add the implementation and unit tests covering negative positions, multiple backward matches, out-of-range positions, and both input types.

[kitalkuyo-gita]

Hi maintainers, I would like to work on this issue.

Before implementing it, I would like to confirm the expected semantics for a negative from argument. My understanding is:

• a negative from counts backward from the end of the string;
• the search proceeds backward from that position;
• nth > 1 returns the nth occurrence encountered while searching backward;
• return values remain 1-based, and no match returns 0;
• the behavior should be consistent for both String and BinaryString overloads.

For example, for instr("abcabc", "a", -1, 1), I expect 4, and for instr("abcabc", "a", -1, 2), I expect 1.

Could you please confirm whether these semantics are correct? If so, I will add the implementation and unit tests covering negative positions, multiple backward matches, out-of-range positions, and both input types.

@lkxdsb Hello! I’m glad to see you contributing to the community. The maintainers are currently facing company performance reviews and are indeed extremely busy, so please accept our apologies for the delay. Please go ahead and complete the POC implementation, then CC me on it; I will make time to review it.