Regular expression for 7 digits followed by an optional 3 letters

Question

I'm new to regular expressions, and I'm trying to validate receipt numbers in our database with a regular expression.

Our receipts can come in the following formats:

• 0123456 (Manditory seven digits, no more, no less)
• 0126456a (Manditory seven digits with one letter a-z)
• 0126456ab (Manditory seven digits with two letters a-z)
• 0126456abc (Manditory seven digits with three letters a-z)

I've tried using a bunch of different regex combinations, but none seem to work. Right now I have:

(\d)(\d)(\d)(\d)(\d)(\d)(\d)([a-z])?([a-z])?([a-z])?

But this is allowing for more than seven digits, and is allowing more than 3 letters.

Here is my VBA function within Access 2010 that will validate the expression:

Function ValidateReceiptNumber(ByVal sReceipt As String) As Boolean

    If (Len(sReceipt) = 0) Then
        ValidateReceiptNumber = False
        Exit Function
    End If

    Dim oRegularExpression     As RegExp

'   Sets the regular expression object
    Set oRegularExpression = New RegExp

    With oRegularExpression
'   Sets the regular expression pattern
        .Pattern = "(\d)(\d)(\d)(\d)(\d)(\d)(\d)([a-z])?([a-z])?([a-z])?"

'   Ignores case
        .IgnoreCase = True

'       Test Receipt string
        ValidateReceiptNumber = .Test(sReceipt)

    End With
End Function

Solution 1

You probably need to use anchors at the ends. Further your regex can be simplified to: -

^\d{7}[a-z]{0,3}$

• \d{7} matches exactly 7 digits. You don't need to use \d 7 times for that.
• {0,3} creates a range, and matches 0 to 3 repetition of preceding pattern,
• Caret(^) matches the start of the line
• Dollar($) matches the end of the line.

Solution 2

^(\d){7}[a-z]{0,3}$ might work well. The ^ and $ will match the start and end of line respectively.

Solution 3

You may want to make sure you are matching the entire string, by using anchors.

^(\d)(\d)(\d)(\d)(\d)(\d)(\d)([a-z])?([a-z])?([a-z])?$

You can also simplify the regex. First, you don't need all those parentheses.

^\d\d\d\d\d\d\d[a-z]?[a-z]?[a-z]?$

Also, you can use limited repetition, to prevent repeating yourself.

^\d{7}[a-z]{0,3}$

Where {7} means 'exactly 7 times', and {0,3} means '0-3 times'.