Fix(clickhouse): map RegexpILike to match

[georgesittas]

Fixes #2406

Couldn't find another variant for case-insensitive matching. There's ILIKE but I don't think it works with regex patterns?

Reference: https://clickhouse.com/docs/en/sql-reference/functions/string-search-functions

[tobymao]

don't you need to modify the regex to add case sensitivy?

[georgesittas]

Ah I missed that, thanks. I can fix it a bit later unless you wanna take this to the finish line.

[tobymao]

https://github.com/google/re2/wiki/Syntax maybe regex on the regx to see if there's a section (?flag)

[georgesittas]

I was playing around in https://play.clickhouse.com/play trying to understand how ?i flag works and realized that it needs to be prepended instead of appended. This is also demonstrated in the test cases here: https://github.com/ClickHouse/ClickHouse/pull/11649/files. Thus I decided to solve this by always concatenating '(?i)' with the haystack. The two test cases I added produce equivalent results in Postgres and ClickHouse. I also checked the case where there are two (?i) flags in the regex and it seems like the second one is a no-op.

cc @samuelcolvin

[samuelcolvin]

Awesome, thanks for catching that.