How can I find numbers in a string

Function RemoveAlphas(ByVal AlphaNum As Variant)
Dim Clean As String
Dim Pos, A_Char$

Pos = 1
If IsNull(AlphaNum) Then Exit Function

For Pos = 1 To Len(AlphaNum)

A_Char$ = Mid(AlphaNum, Pos, 1)
If A_Char$ >= "0" And A_Char$ <= "9" Then
Clean$ = Clean$ + A_Char$
End If
Next Pos

RemoveAlphas = Clean$

End Function