Option Explicit
Sub test()
    Dim rCell As Range
    With ActiveSheet
        
        'for testing
        .Cells(1, 1).Value = "[sick leave]"
        .Cells(2, 1).Value = "[SICK leave]"
        .Cells(3, 1).Value = "Sick leave"
        .Cells(4, 1).Value = "[not sick leave]"
        .Cells(5, 1).Value = "[sick leave"
        .Cells(6, 1).Value = "sick leave]"
        .Cells(7, 1).Value = "[something else]"

        .Cells(1, 1).CurrentRegion.Select
        'example
        If Not TypeOf Selection Is Range Then Exit Sub
        
        For Each rCell In Selection.Cells
            'To match the special characters left bracket ([), question mark (?), number sign (#), and asterisk (*),
            'enclose them in brackets. The right bracket (]) can't be used within a group to match itself, but it
            'can be used outside a group as an individual character.
            If rCell.Value Like "[[]*]" Then
                rCell.Value = Mid(rCell.Value, 2, Len(rCell.Value) - 2)
            End If
        Next
    End With
End Sub
This approach is a little brute force, but seems to work as I understand the requirements

Paul