function IsTrueFormat(s: string): boolean;
const Letter = ['А','Б','В','Г','Д','Е' {и т.д.}];
      Digit = ['0','1','2','3','4','5','6','7','8','9'];
begin
   Result:= (s[ 1 ] in Letter) and
            (s[ 2 ] in Digit) and
            (s[ 3 ] in Digit) and
            (s[ 4 ] in Digit) and
            (s[ 5 ] in Letter) and
            (s[ 6 ] in Letter);
end;

Table1.First; 
while not Table1.EOF do 
begin 
   if IsTrueFormat(Table1P1.AsString) then
    {изменить значение поля Р2};
   Table1.Next;
end;

function IsTrueFormat(s: string): boolean;
const 
   Letter = 'АБВГДЕ..';
   Digit = '0123456789';
begin
   Result:= (Length(s) =  6 ) and
            (s[ 1 ] in Letter) and
            (s[ 2 ] in Digit) and
            (s[ 3 ] in Digit) and
            (s[ 4 ] in Digit) and
            (s[ 5 ] in Letter) and
            (s[ 6 ] in Letter);
end;