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Serious question - is there anything fundamentally wrong with using regex here?

For the specific example cited (credit card numbers) the Luhn check is very standard. So, no there isn't :)

:)

... seriously, they should have broken that down, or used the multi-line with comments pattern, if they really want people to contribute to this.

Yeah, seriously. Are email addresses even regular to the extent that a regex would work 100% of the time?