type 
  TCharSet = set of Char; 

function StripNonConforming(const S: string; 
  const ValidChars: TCharSet): string; 
var 
  DestI: Integer; 
  SourceI: Integer; 
begin 
  SetLength(Result, Length(S)); 
  DestI := 0; 
  for SourceI := 1 to Length(S) do 
    if S[SourceI] in ValidChars then 
    begin 
      Inc(DestI); 
      Result[DestI] := S[SourceI] 
    end; 
  SetLength(Result, DestI) 
end; 

function StripNonNumeric(const S: string): string; 
begin 
  Result := StripNonConforming(S, ['0'..'9']) 
end;