( returns negative value on error )
: _ ( c -- n )
( '0' is ASCII 48 )
48 -
DUP 0< ( bad ) OVER 10 < ( good ) OR IF EXIT THEN
( 'a' is ASCII 97. 59 = 97 - 48 )
49 -
DUP 0< IF EXIT THEN ( bad )
DUP 6 < IF 10 + EXIT THEN ( good )
( bad )
255 -
;