Sub RegExPsuedoLookBack()
Dim RegEx As Object, Matches As Object, Match As Object
Dim oRng As Range
'Sample document text: Jack Sprat, Johnny Sprat, Tom Sprat, Mary Sprat and Bill Sprat.
  'With RegEx look ahead, you can find all instances of x = "Sprat" preceded by either y = "Jack " or y = "Tom "
  Set RegEx = CreateObject("VBScript.RegExp")
  With RegEx
    .Global = True
    .Pattern = "(?=Tom|Jack)\S+ Sprat"
  End With
  Set Matches = RegEx.Execute(ActiveDocument.Range.Text)
  For Each Match In Matches
    'Returns all instances of "Sprat" immediately after "Tom " or "Jack ".
    Debug.Print Match.Value 'Notice the match return includes the proceeding y qualifyer.
  Next
  'A RegEx look back will return all instances of x = "Sprat" preceded by y = "Jack " or y = "Tom " but the match return only inclues the x value "Sprat"
  Set RegEx = CreateObject("VBScript.RegExp")
  With RegEx
    .Global = True
    .Pattern = "(?<=Tom |Jack )\Sprat"
  End With
  On Error GoTo lbl_EH
  Set Matches = RegEx.Execute(ActiveDocument.Range.Text)
  For Each Match In Matches
    'If this worked, it would return all instances of "Sprat" immediately after "Tom " or "Jack ".
    Debug.Print Match.Value
  Next
lbl_ER:
  'VBA workaround for RegEx look back.
  With RegEx
    .Global = True
    'Use a modified look ahead as demonstrated previously.
    .Pattern = "((?=Tom|Jack)\S+ )Sprat" 'Note the addition of parens "()" around the y element will result in a submatch.
  End With
  Set Matches = RegEx.Execute(ActiveDocument.Range.Text)
  For Each Match In Matches
    'Returns x = "Jack Sprat" and "Tom Sprat" matches at before.
    'However, also returns the submatches "Jack " from the "Jack Sprat" match and "Tom " from the "Tom Sprat" match.
    'Use these manipulate the returned range to achieve a VBA pseudo lookback method.
    ActiveDocument.Range(Match.FirstIndex + Len(Match.Submatches(0)), Match.FirstIndex + Match.Length).HighlightColorIndex = wdBrightGreen
  Next
lbl_Exit:
  Exit Sub
lbl_EH:
  MsgBox "Unfortunatley RegEx look back is not supported in VBA." & vbCr + vbCr _
       & "Proceding to work around.", vbInformation + vbOKOnly, "INVALID OPERATION"
  Resume lbl_ER
End Sub

Sub test2()
Dim RegEx As Object, Matches As Object, Match As Object, text As String
    text = "Fellow John John is a good student. Fellow Jack Jack also"
    
    Debug.Print text
    
    Set RegEx = CreateObject("VBScript.RegExp")
    With RegEx
        .Global = True
        .Pattern = "([A-Za-z]+) \1"
        If .test(text) Then
            text = .Replace(text, "$1") ' in every place in string Text where a result is found, that result is replaced with its $1
            
            Debug.Print text
            
        End If
    End With
    
    text = "John Smith is a good student. Jack Sprat also"
    With RegEx
        .Global = True
        .Pattern = "([A-Z][a-z]+) ([A-Z][a-z]+)"
        If .test(text) Then
            text = .Replace(text, "$2 $1")
            
            Debug.Print text
            
        End If
    End With
End Sub