in reply to
Automatically generate regular expression from source data
An example might be helpful here.
But still, there must be some method to how you determine what regexes to apply that you could codify? Perhaps you need a parser like Parse::Yapp, Regexp::Grammars, Parser::MGC or Parse::RecDescent instead of a regex.
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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