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Issue: datepattern does not support regex metacharacters (grouping, lookaheads, etc.)

Issue: datepattern does not support regex metacharacters (grouping, lookaheads, etc.): a task in LegoFlow-SWE (Harbor dataset). Description The datepattern configuration option currently treats everything except % format codes as literal text. Any regex syntax (e.g., non-capturing groups…

The task

**Description** The `datepattern` configuration option currently treats everything except `%` format codes as literal text. Any regex syntax (e.g., non-capturing groups, lookahead/lookbehind assertions, quantifiers with braces, alternation) is escaped, preventing users from writing patterns that use these features.

Part of Lego-X/LegoFlow-SWE.