You can do it. You just need to select the text you want with .SelectionStart and .SelectionLength. Then you can format the selection with properties such as .SelectionFontUnderline, .SelectionFontBold, etc, or even .SelectedText.

Here's some example code that adjusts a RichTextBox so that pasted HTML or a typed URL is formatted correctly. (I use this as part of an email body editor that allows people to create scheduled mail merges with information from a database. It is triggered by the OnKeyDown event or pressing a paste button.)

Code:

Function FixURL(Optional $txtBody)

	; This regex fixes pasted URL's so they are converted back to the way they are displayed in HTML
	; Richard.von.Mallesch@csiro.au <mailto:Richard.von.Mallesch@csiro.au>  -> Richard.von.Mallesch@csiro.au
	; www.csiro.au <http://www.csiro.au/>  -> www.csiro.au
	; https://www.csiro.au:443/foo/bar.htm <https://www.csiro.au:443/foo/bar.htm>  -> https://www.csiro.au:443/foo/bar.htm
	; <\\server1-syd\share$\file.text>  -> \\server1-syd\share$\file.text
	; It also checks for URLS and formats them as such.

	Dim $KeyCode, $Paste, $regEx, $regEx2, $ss, $match, $diff, $text, $matched

	If VarType($txtBody)
		$Paste = 1
	Else
		$txtBody = $System.Sender
		$KeyCode = $txtBody.KeyCode
	EndIf

	$ss = $txtBody.SelectionStart
	$text = Replace($txtBody.Text, @CRLF, Chr(10))
	If Len($text) = $ss
		$text = $text + " "
	Else
		$text = Left($text, $ss + 1)
	EndIf
	If $KeyCode = 32 Or $KeyCode = 13 Or ($KeyCode = 86 And $txtBody.LastKey = 17) Or $Paste ;Space, Enter, Ctrl+V, or pasted
		$regEx = CreateObject("VBscript.RegExp")
		$regEx.Pattern = "<?((\\\\[\w-.\\$]+)|((([\w.%+-]+@)|(\w{3,6}:(//)?)|(\w+\.))\w+(\.\w+)+(:\d+)?[^\s>]*))>?( <\w{0,6}:?(//)?\1/?>)?"
		$regEx.Global = 1
		$regEx2 = CreateObject("VBscript.RegExp")
		$regEx2.Pattern = "^\w+(\.\w+){2,}$"
		If $KeyCode = 32 Or $KeyCode = 13 $txtBody.LastSelectionStart = InStrRev(Replace(Left($text, -2), Chr(10), " "), " ") EndIf
		For Each $match in $regEx.Execute(SubStr($text, $txtBody.LastSelectionStart + 1, $ss - $txtBody.LastSelectionStart))
			If Not ($match.Value = $match.SubMatches(0) And $RegEx2.Test($match.Value))
				$txtBody.SelectionStart = $txtBody.LastSelectionStart + $match.FirstIndex + $diff
				$txtBody.SelectionLength = $match.Length
				$txtBody.SelectionColor = "Blue"
				$txtBody.SelectionFontUnderline = 1
				$txtBody.SelectedText = $match.SubMatches(0)
				$diff = $diff + Len($match.SubMatches(0)) - $match.Length
			EndIf
			$matched = 1
		Next
		If $matched And $ss + $diff = $txtBody.SelectionStart + $txtBody.SelectionLength
			$txtBody.SelectionStart = $ss + $diff
			$txtBody.SelectionColor = $txtBody.LastColour
			$txtBody.SelectionFontUnderLine = 0
			$txtBody.SelectedText = $txtBody.SelectedText + " "
			$txtBody.SelectionStart = $ss + $diff + 1
		Else
			$txtBody.SelectionStart = $ss + $diff
		EndIf
	Else
		If Right($text, 1) = " " And $KeyCode = 8 ; BackSpace
			$regEx = CreateObject("VBScript.RegExp")
			$regEx.Pattern = "((\\\\[\w-.\\$]+)|((([\w.%+-]+@)|(\w{3,6}:(//)?)|(\w+\.))\w+(\.\w+)+(:\d+)?[^\s>]*))"
			If $regEx.Test($txtBody.LastWord) And $txtBody.SelectionFontUnderLine = 1
				$txtBody.SelectionStart = $txtBody.LastSelectionStart - Len($txtBody.LastWord)
				$txtBody.SelectionLength = Len($txtBody.LastWord) - 1
				$txtBody.SelectedText = $txtBody.LastWord
				$txtBody.SelectionColor = $txtBody.LastColour
				$txtBody.SelectionStart = $txtBody.LastSelectionStart
				$text = $text + " "
			EndIf
		Else
			$txtBody.LastKey = $KeyCode
			$txtBody.LastColour = $txtBody.SelectionColor
		EndIf
	EndIf
	$text = Left($text, -1)
	$ = InStrRev(Replace($text, Chr(10), " "), " ")
	$txtBody.LastWord = SubStr($text, IIf($, $, 1))
	$txtBody.LastSelectionStart = $txtBody.SelectionStart

EndFunction

If you can't get what you want that way, you could also generate the RichText code and set .rtf to it.

Cheers,

Richard