Non-capturing groups (?:...)

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Accepted Answer

A capturing group (\d{4}) stores the matched text and makes it accessible via .groups() or .group(N). A non-capturing group (?:\d{4}) uses the grouping purely for structure - alternation, quantifiers, etc. - without allocating a capture slot. In m1, all three date parts are in capturing groups, so .groups() returns all three. In m2, the year part uses (?:...), so it participates in matching but contributes nothing to .groups(). Only the month and day capturing groups remain, giving a two-element tuple. You need (?:...) whenever you want to group something for a quantifier or alternation but do not want that group polluting the index of a later capturing group you actually care about. For example, in re.search(r'(?:foo|bar)(\d+)', text), group 1 is always the digits, not potentially group 2 depending on which branch matched.