ok made find the below code on a forum and changed it according to my need it a little odd but working.
In mouse move envent
Public cursorAt As Integer
Public wordEnd As Integer
Public isEnd As Boolean
Public wordStart As Integer
cursorAt = RichTextBox1.GetCharIndexFromPosition(e.Location)
If cursorAt = 0 Then Return
wordEnd = New Regex("[\s$]{1}").Match(RichTextBox1.Text.Substring(cursorAt)).Index
isEnd = Not New Regex("\s").IsMatch(RichTextBox1.Text.Substring(cursorAt))
wordStart = New Regex("[\s^]{1}", RegexOptions.RightToLeft).Match(RichTextBox1.Text.Substring(0, cursorAt)).Index
If isEnd Then
RichTextBox1.Select(wordStart, RichTextBox1.Text.Length)
Else
RichTextBox1.Select(wordStart, cursorAt - wordStart + wordEnd)
End If
and then simply replace then word
RichTextBox1.SelectedText = myclickeditem