#!/usr/bin/perl
our @v;   # external values stack
our ($allnouns, %nouns) = ( qr/\d+(?{'number'})|[-(]/,
  'number' => sub { push @v, pop },
  '-' => sub { expr( qr/\*{2}/ ); $v[-1] = -$v[-1] },
  '(' => sub { expr(); /\G\s*\)/gcx or err("no )") },
  );
our ($allverbs, %verbs) = ( qr/[-+\/]|\*{1,2}/,
  '+' => sub { expr( qr/\*{1,2}|\// ); $v[-2] += pop @v },
  '-' => sub { expr( qr/\*{1,2}|\// ); $v[-2] -= pop @v },
  '*' => sub { expr( qr/\*{2}/ ); $v[-2] *= pop @v },
  '/' => sub { expr( qr/\*{2}/ ); $v[-2] /= pop @v },
  '**' => sub { expr( qr/\*{2}/ ); $v[-2] **= pop @v },
  );
sub expr   # takes regex of verbs that will shift
  {
  (my $shifters, $^R) = pop // $allverbs;
  /\G\s*($allnouns)/gcx ? $nouns{$^R // $1}->($1) : err('bad noun');
  $verbs{$1}->() while /\G\s*($shifters)/gcx;
  }
while(<>)
  {
  eval { @v = (); expr(); /\G\s*\z/gcx or err('incomplete') } or err($@);
  print s/\s*\z/ = @v\n/r;
  }
sub err { exit print "ERROR  ", s/\G/ <@_> /r, " \n" }

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MiAqIDMKMiszCjIqMy00CigyLTMqNCkKLTIqKjQKMSsyKzMrNCs1KzYrNys4KzkrMTA=

2 * 3
2+3
2*3-4
(2-3*4)
-2**4
1+2+3+4+5+6+7+8+9+10