https://press.rebus.community/programmingfundamentals/chapter/nested-if-then-else/
Nested If Then Else – Programming Fundamentals
Programming Fundamentals - A Modular Structured Approach, 2nd Edition
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https://www.regular-expressions.info/conditional.html
Regex Tutorial: If-Then-Else Conditionals
In Perl’s regex flavor, and compatible flavors, you can use a special construct to create conditional if-then-else regular expressions.
if then elseregextutorialconditionals