re.sub with a count limit

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Accepted Answer

re.sub() accepts an optional 'count' argument that caps the number of replacements made. With count=2, only the first two matches are replaced. The pattern finds '1' first (replaced), then '22' (replaced), and stops before reaching '3'. So the result is 'aNUMbNUMc3' - the third digit run is left untouched. The count parameter is useful when you want to sanitize just the first occurrence, or when you know the leading instances are the ones that matter and the rest should remain as-is.