このようなものが合うかもしれません:
Function getnum(sText)
''A11, A21A23, A25, A23A21
Dim sNum As String
''Skip the first letter
For i = 2 To Len(sText)
If IsNumeric(Mid(sText, i, 1)) Then
sNum = sNum & Mid(sText, i, 1)
Else
Exit For
End If
Next
getnum = sNum
End Function