in reply to
fourth digit should not match digit "2"
See the answers you (or your cohort) got to Regex for last digit should not be "2" and adapt them. Maybe consider reading your course material, and/or perlre or perlretut.
By rote learning.
Via Genetic memory.
It's provided by my firmware.
I just remember them.
Thirty days hath September
My computer gets it right, usually.
I just ask someone else.
Someone punches me on the first of the month.
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