pl@nereida:~/Lperltesting$ cat calc510withactions3.pl
#!/usr/local/lib/perl/5.10.1/bin//perl5.10.1
use v5.10;

# Infix to postfix translator using 5.10 regexp
# Original grammar:

#  exp ->   exp [-+] term
#         | term
#  term ->   term [*/] digits
#          | digits

# Applying left-recursion elimination we have:

#  exp  ->   term re
#  re   ->   [+-] term re
#          | # empty
#  term ->   digits rt
#  rt   ->   [*/] rt
#          | # empty


my $input;

sub echo {
  my $p = substr($input, 0, pos($input));
  say $p
      .(" " x (length($input)-length($p)))
      ."\t".$_[0];
}

my @stack;
local our $op = '';
my $regexp = qr{
    (?&exp)

    (?(DEFINE)
        (?<exp>    (?&term) (?&re)
                     (?{ echo "exp -> term re" })
        )

        (?<re>     \s* ([+-]) (?&term) \s* (?{ push @stack, $^N }) (?&re)
                     (?{ echo "re -> [+-] term re" })
                 | # empty
                     (?{ echo "re -> empty" })
        )

        (?<term>   ((?&digits))
                      (?{ # intermediate action
                          push @stack, $^N
                      })
                   (?&rt)
                      (?{
                          echo "term-> digits($^N) rt";
                      })
        )

        (?<rt>     \s*([*/])
                           (?{ # intermediate action
                              local $op = $^N;
                           })
                   ((?&digits)) \s*
                           (?{  # intermediate action
                                push @stack, $^N, $op
                            })
                   (?&rt) # end of <rt> definition
                           (?{
                                echo "rt -> [*/] digits($^N) rt"
                            })
                  |  # empty
                     (?{ echo "rt -> empty" })
        )

        (?<digits>  \s* \d+
        )
    )
}xms;

$input = <>;
chomp($input);
if ($input =~ $regexp) {
  say "matches: $&\nStack=(@stack)";
}
else {
  say "does not match";
}

##</code><code>##

pl@nereida:~/Lperltesting$ ./calc510withactions3.pl
2-8*4/2/4-1
2               rt -> empty
2               term-> digits(2) rt
2-8*4/2/4       rt -> empty
2-8*4/2/4       rt -> [*/] digits(4) rt
2-8*4/2/4       rt -> [*/] digits(2) rt
2-8*4/2/4       rt -> [*/] digits(4) rt
2-8*4/2/4       term-> digits(8) rt
2-8*4/2/4-1     rt -> empty
2-8*4/2/4-1     term-> digits(1) rt
2-8*4/2/4-1     re -> empty
2-8*4/2/4-1     re -> [+-] term re
2-8*4/2/4-1     re -> [+-] term re
2-8*4/2/4-1     exp -> term re
matches: 2-8*4/2/4-1
Stack=(2 8 4 * 2 / 4 / - 1 -)