Как на си будет:
case Lexem of
'0'..'9' :Result:=ltNumber;
'a'..'z','_' :Result:=ltVariable;
'+','-','/','*','^':Result:=ltBinOp;
'~' :Result:=ltUnOp;
'(' :Result:=ltOpenBrick;
')' :Result:=ltCloseBrick;
else
...
end

с '0'..'9' не знаю сё делать.

switch (Lexem)
{
case '0'..'9' : return ltNumber;
}