Classic++, including classic mistakes. (:

Your regex will match "bbar" despite it containing "bar". "bb" matches b[^a] and then "a" and "r" each match [^b].

I started exploring this idea in depth and hope to write a lengthy node on it one day. Trying to fix the problems leads you down paths similar to:

/^([^b]|b[^a]| ba[^r])+$/
/^([^b]+|b+[^ab]| b+a[^rb])*b*a?$/
/^([^b]+|b+[^ab]|(b+a)+([^rb]|b+[^ab]))*[ba]*$/
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