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Build, test, and debug regular expressions
Matches 'Chr.I' or 'Chr.III' while excluding 'Chr.II' and longer runs like 'Chr.IIII'. The negative lookahead (?!I) ensures the matched Roman numeral is not followed by another 'I', so exactly chromosomes I and III are captured. Useful in genomics/bioinformatics contexts, such as filtering references to yeast chromosomes I and III but skipping chromosome II. The g flag finds all occurrences and m treats input line-by-line.