validate textbox that can contain set of 3 digits separated by a space.

I have a textbox that can contain any number of space delimited strings.
Each string must be 3 digits.
The first 2 digits must be letters.  The third must be a number.

AA4 BE9 II4                   Valid
AA4BE9                          Invalid as there is no space
AA4 BEK                        Invalid as the last digit is a K not a number
AA9 PO5 WW3  QQ4    invalid as there are 2 spaces between WW3 and QQ4
ZZ3                                Valid

I'm using vb 2008 with asp.net 3.5.
AlHal2Asked:
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AlHal2Author Commented:
This seems to work.  What do people think?

If txtDatatypes.Text.Length Mod 4 = 3 Then
            For RegexCount = 0 To txtDatatypes.Text.Length - 1
                If RegexCount Mod 4 = 0 Or RegexCount Mod 4 = 1 Then
                    If Not Regex.Match(txtDatatypes.Text.Substring(RegexCount, 1), "^[A-Z][a-z]*$", RegexOptions.IgnoreCase).Success Then
                        lblErrs.Text = "The " + (RegexCount + 1).ToString + "th character must be a letter"
                        Exit Sub
                    End If
                End If
                If RegexCount Mod 4 = 2 Then
                    If Not Regex.Match(txtDatatypes.Text.Substring(RegexCount, 1), "^[0-9]*$", RegexOptions.IgnoreCase).Success Then
                        lblErrs.Text = "The " + (RegexCount + 1).ToString + "th character must be a number"
                        Exit Sub
                    End If
                End If
                If RegexCount Mod 4 = 3 Then
                    If txtDatatypes.Text.Substring(RegexCount, 1) <> " " Then
                        lblErrs.Text = "The " + (RegexCount + 1).ToString + "th character must be a space"
                        Exit Sub
                    End If
                End If
            Next
        Else
            lblErrs.Text = "Datatypes box must be a set 3 letter datatypes followed by a space"
            Exit Sub
        End If
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ElrondCTCommented:
It looks like it would work, but to be honest it's not very nice-looking code, at least to my eyes.

Personally, I wouldn't bother with Regex for checking a single character for being alphabetic or numeric; I'd use .IsLetter and .IsDigit instead, which can actually work directly on a specific character in a string without needing to do a .Substring. Rather than having three IFs in succession, when only one of them can be valid, I'd use either ElseIf or Select Case. Using "th" after any number is going to give poor results for numbers like 2 or 21, so I'd rewrite that. Finally, it's preferred to use & rather than + when putting strings together.
        If txtDatatypes.Text.Length Mod 4 = 3 Then
            For CharCount = 0 To txtDatatypes.Text.Length - 1
                Select Case CharCount Mod 4
                    Case 0, 1
                        If Not Char.IsLetter(txtDatatypes.Text, CharCount) Then
                            lblErrs.Text = "Character " & (CharCount + 1).ToString & " must be a letter"
                            Exit Sub
                        End If
                    Case 2
                        If Not Char.IsDigit(txtDatatypes.Text, CharCount) Then
                            lblErrs.Text = "Character " & (CharCount + 1).ToString & " must be a number"
                            Exit Sub
                        End If
                    Case 3
                        If txtDatatypes.Text.Substring(CharCount, 1) <> " " Then
                            lblErrs.Text = "Character " & (CharCount + 1).ToString & " must be a space"
                            Exit Sub
                        End If
                End Select
            Next
        Else
            lblErrs.Text = "Datatypes box must be a set 3 letter datatypes followed by a space"
            Exit Sub
        End If

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Dennis AriesCEO @ Arkro ITCommented:
Why don't you use regex to check if the string consists of two characters and a digiti, followed byzero or more sets of 'a space followed by two characters and a digit'?

From the top of my head something like:
[A-Za-z][A-Za-z][0-9](\ [A-Za-z][A-Za-z][0-9])*
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ElrondCTCommented:
You'd need to add \Z at the end to make sure that you don't match on entries that start correctly, but have errors in later codes. But otherwise, I think Dennis's idea would work, though it wouldn't provide as much information about where a problem lies.
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Dennis AriesCEO @ Arkro ITCommented:
ElrondCT, you are correct.
If you want to provide more information, you will have to do additional checks. Personally, I don't see the benefit of telling that the 49th position should be a digit (or not) since that would be of no help to the user. By retrieving the matches through Regex, you can tell in which set the error occurrs, which will be more helpful, I think.
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AlHal2Author Commented:
Thanks.
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