This code is kind of long and some things are left out.
It just gives and idea of how to do syntax highlighting
and certainly you would want to adapt much of it. There
are three kinds of situations handled by the listed 
code. The first deals with loading an entire flie. It
basically reads in a file, adds RTF (Rich Text Format)
symbols to it, saves it to a file and then loads it
into a Rich Edit. The first three function deal with
this idea. The other two situations are color coding
the word at a given cursor position and color coding
specified lines of text.

This syntax highlighting is based on the idea that
different lines can be color coded independently, i.e.
it cannot color code things that span lines such as
block comments. Each line is parsed with functions and
data structures in the Reserved Words Unit.



Here are some of the variables and funcitons used in
the code, all of which are members of MainForm:

Edit : TRichEdit
LangWordsList : TLangReservedWordsList//from the Reserved Words Unit

//if > 0 then no color coding happens. Used to temporarely turn it off.
PuaseColorCoding : integer

//saves and restores Edit's SelStart, SelLength, and the Scroll Position
procedure(s) Save/LoadEditAttributes

//calls Save/LoadEditAttributes and does Modified as well
procedure(s) Save/LoadAllEditAttributes



//does color coding for an entire file
procedure TMainForm.ColorCodeFromStringList(strList : TStringList);

  function MakeColorTableEntry(color : TColor) : string;
  var
    long : LongInt;
    r, g, b : byte;
  begin
    long := ColorToRGB(color);
    r := long mod $100;
    long := long div $100;
    g := long mod $100;
    long := long div $100;
    b := long mod $100;
    result := '\red' + IntToStr(r) + '\green' + IntToStr(g) + '\blue' + IntToStr(b) + ';';
  end;

  //in RTF the symbols {, }, and / are preceded by a /
  procedure FixRTFSymbols(var str : string);
  var i, len : integer;
  begin
    len := length(str);
    i := 1;
    while i <= len do begin
      if (str[i] = '{') or (str[i] = '}') or (str[i] = '\') then begin
        System.Insert('\', str, i);
        inc(len);
        inc(i);
      end;
      inc(i);
    end;
  end;

var
  i, j : integer;
  str1, str2 : string;
begin
  if (PauseColorCoding > 0) or
     not ColorCodingAvaliable then
    with Edit do begin
      SetRedraw(false);
      //to make new text the color of DefAttributes
      SelStart := 0; SelLength := 1;
      SelAttributes := DefAttributes;
      SetRedraw(true);
      Text := strList.Text;
      exit;
    end;

  if strList.Count > 0 then
    for j := 0 to strList.Count - 1 do
      with LangWordsList do
        with Parses do begin
          ParseLine(strList[j]);
          str1 := '';
          for i := 0 to Items.Count - 1 do begin
            case Items[i]^.Kind of
              tkPlainText : str1 := str1 + '\cf0 ';
              tkSymbol : str1 := str1 + '\cf1 ';
              tkKeyWord : str1 := str1 + '\cf2 ';
            end;
            str2 := copy(strList[j], Items[i].Start, Items[i].Length);
            FixRTFSymbols(str2);
            str1 := str1 + str2;
          end;

          if j > 0 then str1 := '\par ' + str1;
          strList[j] := str1;
        end;

  strList.Insert(0, '{\rtf1\ansi\ansicpg1252\deff0\deftab720{\fonttbl{\f0\fmodern\fcharset1 ' + Edit.Font.Name + ';}}');
  {color table must contain colors for plain text, symbols,
   and key words IN THAT ORDER}
  strList.Insert(1, '{\colortbl' + MakeColorTableEntry(Edit.Font.Color) + MakeColorTableEntry(LangWordsList.SymbolsColor) + MakeColorTableEntry(LangWordsList.KeyWordsColor) + '}');
  strList.Insert(2, '\deflang1033\pard\plain\f0\fs' + IntToStr(2*Edit.Font.Size));

  //RTF files end with a }
  strList.Add('}');

  strList.SaveToFile(SourceDir + FN_TEMP_RTF);
  Edit.PlainText := false;
  inc(PauseColorCoding);
  Edit.Lines.LoadFromFile(SourceDir + FN_TEMP_RTF);
  dec(PauseColorCoding);
  Edit.PlainText := true;
end;

procedure TMainForm.LoadAndColorCode(const fName : TFileName);
var strList : TStringList;
begin
  strList := TStringList.Create;
  strList.LoadFromFile(fName);
  ColorCodeFromStringList(strList);
  strList.Free;
end;

procedure TMainForm.AssignAndColorCode(const str : string);
var strList : TStringList;
begin
  strList := TStringList.Create;
  strList.Text := str;
  ColorCodeFromStringList(strList);
  strList.Free;
end;



procedure TMainForm.InvalidateChars(pos : integer);
var
  rect : TRect;
  point : TPoint;
begin
  with Edit do begin
    Perform(EM_POSFROMCHAR, LongInt(@point), pos);
    rect.Top := point.y; rect.Left := point.x;

    pos := Perform(EM_LINEFROMCHAR, pos, 0) + 1;
    if pos >= Lines.Count then
      rect.Bottom := Height
    else begin
      pos := Perform(EM_LINEINDEX, pos, 0);
      Perform(EM_POSFROMCHAR, LongInt(@point), pos);
      rect.Bottom := point.y;
    end;

    rect.Right := Width;
    InvalidateRect(Handle, @rect, true);
  end;
end;

procedure TMainForm.ColorCodeWord(caretPos : integer);
var j, start, stop : integer;
begin
  if (PauseColorCoding > 0) or not ColorCodingAvaliable then exit;
  assert(caretPos > 0);
  assert(caretPos <= Edit.GetTextLen);

  {to prevent this function from calling itself
   through EditChange}
  inc(PauseColorCoding);

  with Edit do begin
    SetRedraw(false);
    SaveAllEditAttributes;

    start := caretPos; stop := caretPos;
    while (start > 1) and not IsWhiteSpace(Text[start - 1]) do
      dec(start);
    while (stop < GetTextLen) and not IsWhiteSpace(Text[stop + 1]) do
      inc(stop);

    with LangWordsList do
        with Parses do begin
          ParseLine(copy(Text, start, stop - start + 1));
          if Items.Count > 0 then
            for j := 0 to Items.Count - 1 do begin
              //- 1 since Items[x].Start is 1 based
              SelStart := start - 1 + Items[j].Start - 1;
              SelLength := Items[j].Length;
              case Items[j].Kind of
                tkPlainText : if not (caColor in SelAttributes.ConsistentAttributes) or (SelAttributes.Color <> DefAttributes.Color) then SelAttributes.Color := DefAttributes.Color;
                tkSymbol : if not (caColor in SelAttributes.ConsistentAttributes) or (SelAttributes.Color <> SymbolsColor) then SelAttributes.Color := SymbolsColor;
                tkKeyWord : if not (caColor in SelAttributes.ConsistentAttributes) or (SelAttributes.Color <> KeyWordsColor) then SelAttributes.Color := KeyWordsColor;
              end;
            end;
        end;

    LoadAllEditAttributes;

    SetRedraw(true);
    InvalidateChars(max(Perform(EM_LINEINDEX, Perform(EM_LINEFROMCHAR, start, 0), 0), start - 1));
    dec(PauseColorCoding);
  end;//with Edit
end;

procedure TMainForm.InvalidateLines(start, stop : integer);
var
  rect : TRect;
  point : TPoint;
begin
  with Edit do begin
    start := Perform(EM_LINEINDEX, start, 0);
    Perform(EM_POSFROMCHAR, LongInt(@point), start);
    rect.Top := point.y;

    stop := Perform(EM_LINEINDEX, stop + 1, 0);
    Perform(EM_POSFROMCHAR, LongInt(@point), stop);
    rect.Bottom := point.y;
    rect.Left := 0;
    rect.Right := Width;
    InvalidateRect(Handle, @rect, true);
  end;
end;

procedure TMainForm.ColorCodeLines(start, stop : integer; DoInvalidate : boolean);
var i, j, lineIndex : integer;
begin
  if (PauseColorCoding > 0) or not ColorCodingAvaliable then exit;
  assert(start <= stop);
  assert(start >= 0);
  assert(stop <= Edit.Lines.Count);

  if (stop - start > 30) then begin
    ColorCodeAll;
    exit;
  end;

  with Edit do begin
    if start = Lines.Count - 1 then dec(start);
    if stop = Lines.Count - 1 then dec(stop);
    if stop < 0 then exit;

    {to prevent this function from calling itself
     through EditChange}
    inc(PauseColorCoding);

    SetRedraw(false);
    SaveAllEditAttributes;

    with LangWordsList do
      for i := start to stop do
        with Parses do begin
          ParseLine(Lines[i]);
          //- 1 since Items[x].Start is 1 based
          lineIndex := Perform(EM_LINEINDEX, i, 0) - 1;
          if Items.Count > 0 then
            for j := 0 to Items.Count - 1 do begin
              SelStart := lineIndex + Items[j].Start;
              SelLength := Items[j].Length;
              case Items[j].Kind of
                tkPlainText : if not (caColor in SelAttributes.ConsistentAttributes) or (SelAttributes.Color <> DefAttributes.Color) then SelAttributes.Color := DefAttributes.Color;
                tkSymbol : if not (caColor in SelAttributes.ConsistentAttributes) or (SelAttributes.Color <> SymbolsColor) then SelAttributes.Color := SymbolsColor;
                tkKeyWord : if not (caColor in SelAttributes.ConsistentAttributes) or (SelAttributes.Color <> KeyWordsColor) then SelAttributes.Color := KeyWordsColor;
              end;
            end;
        end;

    LoadAllEditAttributes;

    SetRedraw(true);
    if DoInvalidate then InvalidateLines(start, stop);
    dec(PauseColorCoding);
  end;//with Edit
end;