$_ = '[S [NP This NP] [VP is [NP [NP the turning point NP] [PP to [NP 
+the left NP] PP] NP] VP] . S]';
#$_ = '[VP This stuff is [NP the left NP] [NP other thing NP] VP]';
@( = ('(','');
@) = (')','');
($re=$_)=~s/((\[\w+\s*)|(\s*\w+\])|.)/$)[!$3]\Q$1\E$([!$2]/gs;
$re = join'|',map{quotemeta}eval{/$re/};
die $@ if $@=~/unmatched/;
$_ = (eval{/($re)/})[0];
print join"\n\n",/\s*(\[\w+\s*(?:$re)\s*\w+\]|[^][]*)/g,"";
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