#!/usr/bin/perl

# http://perlmonks.org/?node=678124

use strict;   # browseruk.pl - modified Pratt parser by tybalt89
use warnings; # https://en.wikipedia.org/wiki/Pratt_parser
sub err { die "ERROR  ", s/\G/ <@_> /r, "\n" }

sub expr # two statement parser - precedences: (3 **) (2 * /) (1 + -)
  {
  my $answer =
    /\G\s* - /gcx    ? -expr(3) :  # unary minus
    /\G\s* \+ /gcx   ? +expr(3) :  # unary plus
    /\G\s* \( /gcx   ? ( expr(0), /\G\s* \) /gcx || err 'missing )' )[0] :
    /\G\s* ((?:\d+(?:\.\d*)?|\.\d+)(e[+-]?\d+)?) /gcx ? $1 :
    err 'bad operand';
  $answer =
    /\G\s* \*\* /gcx ? $answer ** expr(3) :
    $_[0] > 2        ? return $answer     :
    /\G\s* \* /gcx   ? $answer * expr(3)  :
    /\G\s* \/ /gcx   ? $answer / expr(3)  :
    $_[0] > 1        ? return $answer     :
    /\G\s* \+ /gcx   ? $answer + expr(2)  :
    /\G\s* \- /gcx   ? $answer - expr(2)  : return $answer while 1;
  }

for ( @ARGV ? @ARGV : <DATA> ) # source as commandline args or DATA
  {
  my $answer = expr(0);
  /\G\s*\z/gc ? print s/\s*\z/ = $answer\n/r : err 'incomplete parse';
  $answer == eval or print "MISMATCH with perl @{[ eval ]}\n";
  }

__DATA__
1 + 2
2 - 1
2 * 1
1 / 2
(((7 + 5) * (9 + 13)) / ((4 + 3) * (17 - 2 + 3)))
23 ** 2
1.1e10 ** -10
1 + 2 * 3
3 + 2 ** 2 ** 2 * 3