Add case insensitive and dot-all modes to RegExpTree dictionary

[johanngan]

Fixes #50905.

The new per-dictionary settings control regex match semantics around case sensitivity and the '.' wildcard with newlines. They must be set at the dictionary level since they're applied to regex engines at pattern-compile-time.

• regexp_dict_flag_case_insensitive: case insensitive matching
• regexp_dict_flag_dotall: '.' matches all characters including newlines

They correspond to HS_FLAG_CASELESS and HS_FLAG_DOTALL in Vectorscan and case_sensitive and dot_nl in RE2. These are the most useful options compatible with the internal behavior of RegExpTreeDictionary around splitting up simple and complex patterns between Vectorscan and RE2.

The alternative is to use (?i) and/or (?s) for all patterns. However, (?s) isn't handled properly by OptimizedRegularExpression::analyze(). And while (?i) is, it still causes the dictionary to treat the pattern as "complex" for sequential scanning with RE2 rather than multi-matching with Vectorscan, even though Vectorscan supports case insensitive literal matching. Setting dictionary-wide flags is both more convenient, and circumvents these problems.

Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Support case-insensitive and dot-all matching modes in RegExpTree dictionaries.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

[robot-ch-test-poll3]

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[johanngan]

(No substantive changes, just updating with upstream. Still ready for review, pending automated test results.)

[johanngan]

Sorry to bother you @hanfei1991, but it seems this PR may have been missed when I put it up 3 months ago (it hasn't even been cleared for testing). I'm sure you're busy, but if you have time, do you have any thoughts here? It seems like a relatively minimal change that could still be pretty useful in specific cases.