You need to create a macro. In Excel 2010, press View, Macros, View Macros. Type the name "Dummy" and press Create. In the new window that appears, look at the Project pane on the left. Double-click the sheet you want to apply the formatting to. Copy and paste the following code.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim Text As String
Dim Index1 As Long
Dim Index2 As Long
Text = Target.Text
Index2 = 1
Index1 = InStr(Index2, Text, "[")
If Index1 = 0 Then Exit Do
Index2 = InStr(Index1, Text, "]")
If Index2 = 0 Then Exit Do
Target.Characters(Index1, Index2 - Index1 + 1).Font.Color = &HFF
If you want the formatting to only apply to certain cells, you can insert one of the following lines before the
Dim Text As String line.
If Target.Row <> 12 Then Exit Sub ' Only format row "$12"
If Target.Column <> 12 Then Exit Sub ' Only format column "$L"
Now, whenever you finish editing a cell on the worksheet, it will change the square brackets and the text inside to red. It also [works] if there are [multiple] sets of [square] brackets, but [nested [brackets] will] fail.