^+++++ +++++ = t/en
$t- = n/ine
^- = E/OF
^ = e/rror state
^+++++++++++++++++++++++++++++++++++++++++++++++ = z/ero character - 1
$z+++++ +++++ + = T/ '0' - 1 + 11
^ = Z / actual zero

P/arse a number from stdin / -- input
{
    &r / our result :D

    _ (
        &c>c / read into local
        $cE?=_^+=e#; / if EOF then exit setting error

        &s$z~=s/ubtract / put char - ('0' - 1) into s

        &f^=f s( / if s >= 0 then our char is '0' or above
                  / meaning it's possibly a digit

            ^$c$T~~=s/ubtract / put -(char - ('0' - 1 + 11)) into s
            &f^=f s( / if s >= 0 then our char is '9' or below
                      / meaning it's a digit

                / c is a digit here
                $c$z~- / convert character into number
                t[$r]n[*] / shift result one decimal digit left
                *=r / finally append character

            ^+=f#)
            $fZ?=_#;=_
        ^+=f#)
        $fZ?=_^+=e#;=_ / otherwise exit setting error
    )
    $r
}

@P