Question

For example I only the second character in each line to be an x, while the 3nd to 10th character must be a hex digit.
At the moment I use a Select Case, then check the position of the caret (using textbox.selectionstart) and see if the key being pressed is a “legal” character.

Is there a better way of doing this as it slows down on large amounts of text.

This is the code I have at the moment:

 Select Case TextBox1.SelectionStart
     Case TextBox1.GetFirstCharIndexOfCurrentLine + 1 
         If Not e.KeyChar = "x" Then
             e.Handled = True
         End If
     Case (TextBox1.GetFirstCharIndexOfCurrentLine + 2) To (TextBox1.GetFirstCharIndexOfCurrentLine + 9) 
         Dim allowedchars As String = "abcdefABCDEF0123456789" & vbCrLf & Chr(Keys.Back)
         If allowedchars.Contains(e.KeyChar) Then
             e.Handled = False
         Else
             e.Handled = True
         End If
 End Select

Answer 1

You say it slows down on large amounts of text; maybe it’s the TextBox1.GetFirstCharIndexOfCurrentLine call that takes longer with large amounts of text. You call it three times, but you could only call it once:

Dim firstChar As Integer = TextBox1.GetFirstCharIndexOfCurrentLine()

Select Case TextBox1.SelectionStart
    Case firstChar + 1 
        ...
    Case firstChar + 2 To firstChar + 9
        ...
 End Select

Code looks cleaner too!