Use a numbered group instead of a named group : Group « Regular Expressions « VB.Net Tutorial

1. VB.Net Tutorial
2. Regular Expressions
3. Group

Imports System.Text.RegularExpressions

Module Example
   Public Sub Main()
      Dim pattern As String = "\b(\d{2}-)*(?(1)\d{7}|\d{3}-\d{2}-\d{4})\b"
      Dim input As String = "01-9999999 000-333333 777-88-9999"
      Console.WriteLine("Matches for {0}:", pattern)
      For Each match As Match In Regex.Matches(input, pattern)
         Console.WriteLine("   {0} at position {1}", match.Value, match.Index)
      Next   
   End Sub
End Module