in reply to Regexp experts, come to rescue!
Apart for the previous problem you should also anchor your regexps:
/^-[45][15j]$/ # note the ^ and $otherwise you will match the regexp anywhere in the string: "toto-45tata" would match
Then your interpretation is right, otherwise it would be
if any character is '-' followed by 4 or 5 followed by 1 or 5 or j...
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