m = %r!
    (?<element> \g<stag> \g<content>* \g<etag> ){0}
    (?<stag> < \g<name> \s* > ){0}
    (?<name> [a-zA-Z_:]+ ){0}
    (?<content> [^<&]+ (\g<element> | [^<&]+)* ){0}
    (?<etag> </ \k<name+1> >){0}
    \g<element>
      !x.match('<foo>f<bar>bbb</bar>f</foo>')
p m

#<MatchData "<foo>f<bar>bbb</bar>f</foo>" element:"<foo>f<bar>bbb</bar>f</foo>" stag:"<bar>" name:"bar" content:"f<bar>bbb</bar>f" etag:"</foo>">