Работа с литерными переменными

var a:string; i:byte; correct:boolean;{Правильна ли строка}lon:byte;////Letter Or Not 1=letter 0=not
begin
readln(a);
correct:=True;
if a[1] in ['0'..'9'] then lon:=0 else lon:=1;
for i:=1 to length(a) do
begin
  if (a[i] in ['0'..'9']) and (lon=0) then correct:=True
  else
  if ((a[i] in ['a'..'z']) or (a[i] in ['A'..'Z'])) and (lon=1) then correct:=True
  else correct:=False;
  lon:=1-lon;
  if not correct then break;
end;
  writeln(correct);
end.

const D=['0'..'9'];
var s: String; i: Integer;
begin
  Write('Строка: '); ReadLn(s); i:=Length(s);
  if Odd(i) and ((s[1] in D)=(s[i] in D)) then begin
    Dec(i); while (i>0) and ((s[i] in D)<>(s[i+1] in D)) do Dec(i);
  end;
  WriteLn(i=0);
end.

const D=['0'..'9'];
var s:string;i:integer;
begin
    write('Строка: '); readln(s); i:=length(s);
    if odd(i) 
     then while (i>1)and(not((s[i]in D)xor(not(s[i-1]in D))))do dec(i);
    write(i=1) 
end.

const D=['0'..'9'];
var s:string;i:integer;
begin
    write('Строка: '); readLn(s); i:=length(s);
    if odd(i) 
     then while (i>1)and((s[i]in D)xor(s[i-1]in D))do dec(i);
    write(i=1) 
end.