Lessons · Regex · a match that is not the whole value

A match that is not the whole value

A search that succeeds says the pattern OCCURS somewhere. It never says the value is that thing.

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What it is for

It is how junk ends up stored in the same column as good data: the check passed, and it was answering a different question.

Validating means both ends nailed down. fullmatch, or \A and \Z, or ^ and $ if a trailing newline is acceptable.

re.search(r'\d{4}-\d{2}-\d{2}', '2026-09-09xx').group()

A date is in there, so search succeeds.

re.fullmatch(r'\d{4}-\d{2}-\d{2}', '2026-09-09xx')

But the value is not a date.

re.match(r'[0-9]{3}', '123456789').group()

And match only covers the front, so {3} lets the rest through.

Require three digits and nothing else, both ends.

\A\d{3}

Two half-measures leave a hole: match covers the start, a fixed count stops counting, and nothing at all covers the end.

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