Re: REgular expression to check the string that allows "a","b" and "c" to occur only once in any order.

print "ok!\n" unless /([abc]).*\1/;
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the RE above checks if one of [abc] is repeated

if you want to declare "abcz" wrong too, just:

print "ok!\n" unless /(([abc]).*\2|[^abc])/;
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which also check if a bad char is present.

Oha

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