Delphi极速字符串替换函数

//此极速字符串替换函数为[盒子论坛hq200306兄]所作，在此感谢！亲测原本48秒的长文本替换操作，现在只要几十毫秒不到！

function PosX(const SubStr, Str: string; Offset: Integer): Integer;
var
  I, LIterCnt, L, J: Integer;
  PSubStr, PS: PChar;
begin
  L := Length(SubStr);
  { Calculate the number of possible iterations. Not valid if Offset < 1. }
  LIterCnt := Length(Str) - Offset - L + 1;

  { Only continue if the number of iterations is positive or zero (there is space to check) }
  if (Offset > 0) and (LIterCnt >= 0) and (L > 0) then
  begin
    PSubStr := PChar(SubStr);
    PS := PChar(Str);
    Inc(PS, Offset - 1);

    for I := 0 to LIterCnt do
    begin
      J := 0;
      while (J >= 0) and (J < L) do
      begin
        if UpCase(PS[I + J]) = UpCase(PSubStr[J]) then
          Inc(J)
        else
          J := -1;
      end;
      if J >= L then
        Exit(I + Offset);
    end;
  end;

  Result := 0;
end;

function StringReplaceEx(const st, oldSubstr, newSubStr: string): string;
var
  idx, len: Integer;
  iStart: Integer;
  sb: TStringBuilder;
begin
  len := Length(oldSubstr);
  iStart := 1;
  sb := TStringBuilder.Create;
  try
    repeat
      idx := posX(oldSubstr, st, iStart);
      if idx > 0 then
      begin
        sb.Append(Copy(st, iStart, idx - iStart));
        sb.Append(newSubStr);
        iStart := idx + len;
      end;
    until idx <= 0;
    sb.Append(Copy(st, iStart, length(st)));
    Result := sb.ToString;
  finally
    sb.Free;
  end;
end;