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I worked in a bank for 5 years, and one thing I can tell you from that is: Never rely on verifyable card details, nor the 1 digit prefix. Some vendors can have different length card numbers, and some cards can be "hybrid" mastercard and visa (for example) with a different prefix.
Though things are becoming more standardised, you should give a pretty wide berth to allowable digit counts, and only take the first digit as a guide, not an absolute.
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