is ArrayIndexOutOfBoundsException possible here?
what if newLimit=0 (i.e. numChars()=0, limit=-1)

is ArrayIndexOutOfBoundsException possible here? what if newLimit=0 (i.e. numChars()=0, limit=-1)

no, this code block will only be entered when the following conditions are met.

if (numBytes() != 0 && pattern.numBytes() == 0)

I would also prefer to see:

SELECT split('', '', -1);
SELECT split('', '', 0);
SELECT split('', '', 1);

here, for more complete testing

Thanks, already added.

sorry, I meant to request using collation (other than UTF8_BINARY) here

In the current situation, if UTF8_LCASE is applied to an empty string, the condition here will not be met because the value of pattern after being collated by collationAwareRegex is(?ui), meaning that #37631 does not support truncating the trailing empty string when the pattern is (?ui).

public UTF8String[] split(UTF8String pattern, int limit) {
    // For the empty `pattern` a `split` function ignores trailing empty strings unless original
    // string is empty.
    if (numBytes() != 0 && pattern.numBytes() == 0) {

Therefore, it seems that the result is not what we want when limit <= 0.

select split('1A2B3C', '(?ui)', -1);    // result is ["1", "A", "2", "B", "3", "C", ""]
select split('1A2B3C', '(?ui)', 0);    // result is ["1", "A", "2", "B", "3", "C", ""]

select split('1A2B3C', '(?ui)', 1);    // result is ["1A2B3C"]
select split('1A2B3C', '(?ui)', 3);    // result is ["1", "A", "2B3C"]
select split('1A2B3C', '(?ui)', 6);     // result is ["1", "A", "2", "B", "3", "C"]
select split('1A2B3C', '(?ui)', 100);    // result is ["1", "A", "2", "B", "3", "C"]

When the pattern is "(?ui)", a simple and direct approach can be taken to correct the result.

public UTF8String[] split(UTF8String pattern, int limit) {
    // For the empty `pattern` a `split` function ignores trailing empty strings unless original
    // string is empty.
    if (numBytes() != 0 && (pattern.numBytes() == 0 || lowercaseRegexPrefix.equals(pattern))) {

However, when the pattern is "(?ui)(?ui)" or "(?ui)(?ui)(?ui)", the result still contains a trailing empty string, and I haven't thought of an efficient way to match and resolve it. Should we consider this a reasonable situation?
Additionally, do you think it is necessary to check in the CollationSupport.lowercaseRegex method whether the regex already has a (?ui) prefix?

how about: instead of checking whether the pattern equals to the (?ui) prefix, we modify the collation implementation (prefixing logic) to avoid appending the prefix at all in the case that pattern is an empty string

I agree with what you're saying, but should we consider that the user's pattern itself might be (?ui) and is unrelated to prefixing logic?

that is an interesting observation, although in that case I don't see why the user's pattern can't be any other flag modifier combination, such as: (?m), (?s), (?x), (?a)

taking this into consideration, there is really nothing special about lowercaseRegexPrefix. instead, you should look for a library method that can discern whether a pattern is "functionally" empty, instead of doing a manual check against lowercaseRegexPrefix

Thank you for your explanation. It looks like there’s no way to validate this "weird" situation without losing performance. I made changes according to your advice. Thanks again.