How to compare two strings for similarity or highlight differences in Excel?

Sub highlight()
    Dim xRg1 As Range
    Dim xRg2 As Range
    Dim xTxt As String
    Dim xCell1 As Range
    Dim xCell2 As Range
    Dim I As Long
    Dim J As Integer
    Dim xLen As Integer
    Dim xDiffs As Boolean
    On Error Resume Next
    If ActiveWindow.RangeSelection.Count > 1 Then
      xTxt = ActiveWindow.RangeSelection.AddressLocal
    Else
      xTxt = ActiveSheet.UsedRange.AddressLocal
    End If
lOne:
    Set xRg1 = Application.InputBox("Range A:", "Kutools for Excel", xTxt, , , , , 8)
    If xRg1 Is Nothing Then Exit Sub
    If xRg1.Columns.Count > 1 Or xRg1.Areas.Count > 1 Then
        MsgBox "Multiple ranges or columns have been selected ", vbInformation, "Kutools for Excel"
        GoTo lOne
    End If
lTwo:
    Set xRg2 = Application.InputBox("Range B:", "Kutools for Excel", "", , , , , 8)
    If xRg2 Is Nothing Then Exit Sub
    If xRg2.Columns.Count > 1 Or xRg2.Areas.Count > 1 Then
        MsgBox "Multiple ranges or columns have been selected ", vbInformation, "Kutools for Excel"
        GoTo lTwo
    End If
    If xRg1.CountLarge <> xRg2.CountLarge Then
       MsgBox "Two selected ranges must have the same numbers of cells ", vbInformation, "Kutools for Excel"
       GoTo lTwo
    End If
    xDiffs = (MsgBox("Click Yes to highlight similarities, click No to highlight differences ", vbYesNo + vbQuestion, "Kutools for Excel") = vbNo)
    Application.ScreenUpdating = False
    xRg2.Font.ColorIndex = xlAutomatic
    For I = 1 To xRg1.Count
        Set xCell1 = xRg1.Cells(I)
        Set xCell2 = xRg2.Cells(I)
        If xCell1.Value2 = xCell2.Value2 Then
            If Not xDiffs Then xCell2.Font.Color = vbRed
        Else
            xLen = Len(xCell1.Value2)
            For J = 1 To xLen
                If Not xCell1.Characters(J, 1).Text = xCell2.Characters(J, 1).Text Then Exit For
            Next J
            If Not xDiffs Then
                If J <= Len(xCell2.Value2) And J > 1 Then
                    xCell2.Characters(1, J - 1).Font.Color = vbRed
                End If
            Else
                If J <= Len(xCell2.Value2) Then
                    xCell2.Characters(J, Len(xCell2.Value2) - J + 1).Font.Color = vbRed
                End If
            End If
        End If
    Next
    Application.ScreenUpdating = True
End Sub