c:\test>expr_parser.pl
'1 + 2' = 3
'2 - 1' = 1
'2 * 1' = 2
'1 / 2' = 0.5
'(((7 + 5) * (9 + 13)) / ((4 + 3) * (17 - 2 + 3)))' = 2.0952380952381
'23 ** 2' = 529
'1.1e10 ** -10' = 3.85543289429532e-101
'1 + 2 * 3' = 7
'3 + 2 ** 2 ** 2 * 3' = 51