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
Do
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
Loop
End Sub
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.