Checking Credit Card number is valid

I need to check whether is credit card number is a valid format for the type of credit card that is giving to me. I am passing the number to other place that will do the actual check.  I had found this
for each card when I tested with actual number for the particular type for card I got errors. Am I missing something in the regular expression?

This is be coded in VBNET on the back end.

  Case "AX"
                    RegEx = New System.Text.RegularExpressions.Regex("3[4,7]\d{13}")
                Case "DS"
                    RegEx = New System.Text.RegularExpressions.Regex("6011-?\d{4}-?\d{4}-?\d{4}")
                Case "MC"
                    RegEx = New System.Text.RegularExpressions.Regex("5[1-5]\d{2}-?\d{4}-?\d{4}-?\d{4}")
                Case "VA"
                    RegEx = New System.Text.RegularExpressions.Regex("4\d{3}-?\d{4}-?\d{4}-?\d{4}")