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 Home » Tip Bank » .NET » VB.NET » String Manipulation
Language: VB4,VB5,VB6,VBS
Expertise: Intermediate
Jul 22, 2000

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# IsValidCreditCardNumber - Check whether a credit card number is valid

``````' Validate a credit card numbers
' Returns True if valid, False if invalid
'
' Example:
'  If IsValidCreditCardNumber(Value:="1234-123456-12345", IsRequired:=True)

Function IsValidCreditCardNumber(Value As Variant, Optional ByVal IsRequired As _
Boolean = True) As Boolean
Dim strTemp As String
Dim intCheckSum As Integer
Dim blnDoubleFlag As Boolean
Dim intDigit As Integer
Dim i As Integer

On Error GoTo ErrorHandler

IsValidCreditCardNumber = True
Value = Trim\$(Value)

If IsRequired And Len(Value) = 0 Then
IsValidCreditCardNumber = False
End If

' If after stripping out non-numerics, there is nothing left,
'  they entered junk
For i = 1 To Len(Value)
If IsNumeric(Mid\$(Value, i, 1)) Then strTemp = strTemp & Mid\$(Value, i, _
1)
Next
If IsRequired And Len(strTemp) = 0 Then
IsValidCreditCardNumber = False
End If

'Handle different lengths for different credit card types
Select Case Mid\$(strTemp, 1, 1)
Case "3"    'Amex
If Len(strTemp) <> 15 Then
IsValidCreditCardNumber = False
Else
Value = Mid\$(strTemp, 1, 4) & "-" & Mid\$(strTemp, 5, _
6) & "-" & Mid\$(strTemp, 11, 5)
End If
Case "4"    'Visa
If Len(strTemp) <> 16 Then
IsValidCreditCardNumber = False
Else
Value = Mid\$(strTemp, 1, 4) & "-" & Mid\$(strTemp, 5, _
4) & "-" & Mid\$(strTemp, 9, 4) & "-" & Mid\$(strTemp, 13, 4)
End If
Case "5"    'Mastercard
If Len(strTemp) <> 16 Then
IsValidCreditCardNumber = False
Else
Value = Mid\$(strTemp, 1, 4) & "-" & Mid\$(strTemp, 5, _
4) & "-" & Mid\$(strTemp, 9, 4) & "-" & Mid\$(strTemp, 13, 4)
End If
Case Else      'Discover - Dont know rules yet
If Len(strTemp) > 20 Then
IsValidCreditCardNumber = False
End If
End Select

'Now check for Check Sum (Mod 10)
intCheckSum = 0
blnDoubleFlag = 0
For i = Len(strTemp) To 1 Step -1                   ' Working backwards
intDigit = Asc(Mid\$(strTemp, i, 1))             ' Isolate character
If intDigit > 47 Then                           ' Skip if not a intDigit
If intDigit < 58 Then
intDigit = intDigit - 48                ' Remove ASCII bias
If blnDoubleFlag Then
' If in the "double-add" phase
intDigit = intDigit + intDigit      '   then double first
If intDigit > 9 Then
intDigit = intDigit - 9         ' Cast nines
End If
End If
blnDoubleFlag = Not blnDoubleFlag       ' Flip doubling flag
intCheckSum = intCheckSum + intDigit    ' Add to running sum
If intCheckSum > 9 Then                 ' Cast tens
intCheckSum = intCheckSum - 10      ' (same as MOD 10 but
' faster)
End If
End If
End If
Next

If intCheckSum <> 0 Then                            '  Must sum to zero
IsValidCreditCardNumber = False
End If

ExitMe:
Exit Function
ErrorHandler:
Err.Raise Err.Number, "IsValidCreditCardNumber", Err.Description

End Function

'###########################################################
'#
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'#   Pragmatic Software Co. Inc, the creators of Defect Tracker,
'#   the tool of choice for tracking functional specifications,
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'###########################################################
``````
Steve Miller

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