"aab" =~ /(*SKIP:go)(.)(?!\1(*MARK:go))/

##</code><code>##

"aab"=~/^.*?(*SKIP:go)(.)(?!\1(*MARK:go))/

##</code><code>##

"aab"=~/(*SKIP:go)(.)(?(?=\1)\1(*MARK:go)(*FAIL)|)/