qr{
    (?{ @stack  = ();
        $result = undef;
    })
    \A \s*
    (?<start_number> \d+ (?: [.] \d* )? | [.] \d+ )
    (?{ push @stack, $+{start_number} })
    (?>
        \s+
        (?:
            (?<number> \d+ (?: [.] \d* )? | [.] \d+ )
            (?{ push @stack, $+{number} })
        |
            (?<unary_op> chs | abs | sqrt | sqr | sin | cos | tan )
            (?(?{ @stack < 1 }) (*FAIL) )
            (?{ $operators{ $+{unary_op} }->($stack[-1]) })
        |
            (?<binary_op> [-+/*^] )
            (?(?{ @stack < 2 }) (*FAIL) )
            (?{ $operators{ $+{binary_op} }->($stack[-2], pop @stack) })
        )
    )*
    \s* \z
    (?(?{ @stack != 1 }) (*FAIL) )
    (?{ $result = $stack[0] })
}msx,