See my reply to graff: Re^4: Likely trivial regex question. To clarify your examples with "cat", use the /g modifier in list context to understand: 
perl -E 'say for "cat:dog" =~ /(cat)*/g;say "=";say for "dog:cat" =~ /
+(cat)*/g'

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In reply to Re^5: Likely trivial regex question by choroba
in thread Likely trivial regex question by moodywoody

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