The purpose of this text is to attempt to answer the incessant queries on coloring text in RichTextBoxes. There is no magic command to do so, unless of course you have purchased a third party control from somewhere or other. There are plenty around, and some are cheap, some are not...

I have demonstrated very simply the two methods I think are the easiest: the first is what everybody tries to begin with, the second is a simulation of Microsofts VB IDE style (Bear in mind, it might not be the same method, but it's the same effect). Method 3 is new and is a more complex and more efficient version of Method 2, with a slight twist. So, without further ado.. onto the explanations.

Method1

RTB.SelStart = 1 ' this is the starting position of the selection in the RTB
RTB.SelLength = 10 ' this is how many characters are selected, and thus how many are colored
RTB.SelColor = vbRed ' this is the color to set the selection to

Now, this method is slightly more complex than that. You will obviously want to have different words as different colors, and some maybe not colored at all. Well, to do this, you must create an array of words, and the colors to be associated to them. You could have two seperate arrays, but this is where I think Types come in quite handy:

Type WORD_TYPE
 Text As String
 Color As Long
End Type
Dim Words() As WORD_TYPE

So, all you have to do now is go through all the text in your textbox, and match all the instances of these words, then use the method above to color them.

 ' go through each item in the Words array
 For i = LBound(Words) To UBound(Words)
 ' find each instance of the word in the rtb
 p1 = InStr(1, Text, Words(i).Text)
 Do While p1 > 0
  ' color it to the appropriate color
  RTB.SelStart = p1 - 1
  RTB.SelLength = Len(Words(i).Text)
  RTB.SelColor = Words(i).Color
  ' go on to the next word
  p1 = InStr(p1 + 1, Text, Words(i).Text)
 Loop
 Next i

 ' store the current cursor pos
 lCursor = RTB.SelStart
 ' do the coloring
 DoColor RTB
 ' reset the cursor
 RTB.SelStart = lCursor
 ' restore the color
 RTB.SelColor = vbBlack

Method2

When a user is typing in your RTB, only one line of text is being altered (normally only one word of text at a time as well, but we'll leave that for now..). As such, you only really need to color that single line and not the entire box as we were doing before.

Also, there is no need to color the line if nothing has changed, so checking which key has been pressed in the KeyDown() event rather than the KeyUp() event will sort that out.

Finally - as Microsoft does in the VB IDE, we don't really need to color the line until the user moves the cursor off it - this means one quick operation once per line, rather than twenty times as the user types it all out.

The actual coloring routine is remarkably similar, this handling is done outside of it really. The only extra bits the coloring routine needs to know are where to start coloring from (the beginning of the current line) and where to stop (the end of the current line). We can find this out with a couple of Instr() calls.

  ' store the current cursor pos
  ' and current selection if there is any
  lCursor = RTB.SelStart
  lSelectLen = RTB.SelLength
  ' get the line start and end
  lStart = InStrRev(RTB.Text, vbCrLf, RTB.SelStart + 1) + 2
  lFinish = InStr(RTB.SelStart + 1, RTB.Text, vbCrLf)
  If lFinish = 0 Then lFinish = Len(RTB.Text)
  ' do the coloring
  DoColor RTB, lStart, lFinish
  ' reset the cursor
  RTB.SelStart = lCursor
  RTB.SelLength = lSelectLen
  ' restore the color
  RTB.SelColor = vbBlack
	 ' flip the flag
  bDirty = False

Lastly, as a little kicker - we're going to emulate MS even more and remove all coloring from a line when it is altered, or made "dirty". This is again, done in the KeyDown() sub, not the coloring routine.

 ' a different key was pressed - and
 ' this will alter the line so it
 ' needs recoloring when we move off it
 bDirty = True
 ' remove all coloring from this line
 ' as a visual reminder to the user
 ' that it has been changed
 ' get the line start and end
 lStart = InStrRev(RTB.Text, vbCrLf, RTB.SelStart + 1) + 2
 lFinish = InStr(RTB.SelStart + 1, RTB.Text, vbCrLf)
 If lFinish = 0 Then lFinish = Len(RTB.Text)
 ' color the line (remembering the cursor position)
 lCursor = RTB.SelStart
 RTB.SelStart = lStart
 RTB.SelLength = lFinish - lStart
 RTB.SelColor = vbBlack
 RTB.SelStart = lCursor

Aside from those differences, it is pretty much the same.

Method 3

To start with, you might think this has little difference, but with a little tweak here, a little tweak there, great advances can be made. The other parts are the same - this only colors one line at a time, and as each line is made 'dirty' it is colored black so that the user knows it's been changed.

Firstly, the array of keywords is sorted alphabetically. This means that rather than looping through the entire array to locate a word, we can perform a standard binary search algorithm on the array to locate the item faster. It has been shown that a binary search is an effective algorithm for locating items in a sorted array.

To further speed up this search, we can build an index on the sorted array. This index will store the begin and end positions of each letter in the array. For example, if words beginning with the letter "B" begin at words array element 2, then the "B" letter continues until words array element 4 and we know these positions, we can greatly reduce the number of elements that the binary search has to compare.

Private Type LETTER_TYPE
 Start As Long
 Finish As Long
End Type
 Words(0) = "And"
 Words(1) = "As"
 Words(2) = "Byte" ' starts at 2
 Words(3) = "ByRef" '
 Words(4) = "ByVal" ' ends at 4
 Words(5) = "Else"
 Words(6) = "End"
' so the LETTER_TYPE for "B" would be as follows:
' Letter.Start = 2
' Letter.Finish = 4

So this is fine, but one other major problem remains. How do we decide which elements in the line to be color are seperate? The obvious method would be to use the Split() function to split the line at each space character (" ", Chr(34)). On closer inspection though, this won't work. For example, it wouldn't recognise "ByVal" in the following line:

 Private Sub Moo(ByVal sParam As String)

The final problem with these selection methods is the flickering of the line whilst it's being colored. On multiple suggestions, I've implemented the LockWindowUpate API, which means that the flicker on each line is reduced, however, on my machine, the whole RTB flickers sometimes. This is a better trade off though.

Conclusion

There are a couple of problems though - at present, Methods 1 and 2 only recognise VB comment lines if they start as a comment line, not if the line is hybrid code and comment. Also, there are only a few keywords in the array. However, since this was not meant as a working drop-in component, I'll leave them to you to fix. Method 3 is seemingly complete, and fixes many many logic bugs in the first two methods (coloring text pasted into the RTB is a major one)... but there is still one problem. Flicker. The flicker that comes with using a selection method. Even with the LockWindowUpdate API, there is still flicker... =(

Method 4 is in production, and involves converting Method 3 to alter the RTF of the box, rather than the selection... also fixing the last logic bugs (Undo, Redo etc)... fingers crossed eh?