Using RegEx to Validate User Input

Question

RESOLUTION - Provided By Wiktor - I have implemented this into my question.

Validating user input while they're typing to match the following format - 11A11

    Private Sub RegexFormatterExample(sender As Object, e As EventArgs)

    Dim txt As TextBox = sender
    Dim pattern As String = ""

    Select Case txt.Name
        Case "txt_example"
            pattern = "^\d{1,2}(?:(?<=^\d{2})[A-Z]\d{0,2})?$"
        Case Else
            pattern = "^\d{1,2}(?:(?<=^\d{2})[A-Z]\d{0,2})?$"
    End Select

    Dim strText As String = txt.Text

    While Not Regex.IsMatch(strText, pattern) AndAlso Not strText = ""
        strText = strText.Substring(0, strText.Length - 1)
    End While

    txt.Text = strText
    txt.Select(txt.Text.Length, 0)
End Sub

I have also attached a .TextChanged handler to the specified txtbox.

The provided answer only allows the user to type in the following format which is what I asked for.

Thanks Wiktor!

Edit

Further to this scenario there is a case where the user has the ability to enter only a minus symbol rather than passing an empty string for where they don't know the value.

To implement this I amended the RegEx pattern to the following. Regex Demo

I am not sure if this is the most efficient way but it seems to be working for me.

What is your Problem? Where is the fail. Provide more information. — Superluminal
– Superluminal, Commented Mar 9, 2020 at 8:56
btw: you dont need the {1} quantifier. [0-9]{2}[A-Z]{1}[0-9]{2}\z. == [0-9]{2}[A-Z][0-9]{2}\z — Superluminal
– Superluminal, Commented Mar 9, 2020 at 8:57
Try ^\d{1,2}(?:(?<=^\d{2})[A-Z]\d{0,2})?$. See the regex demo — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Mar 9, 2020 at 9:41

Wiktor Stribiżew · Accepted Answer · 2020-03-09 09:45:42Z

1

You may use

^\d{1,2}(?:(?<=^\d{2})[A-Z]\d{0,2})?\z

See the .NET regex demo online (\z is replaced with \r?$ since the demo is run against a single multiline string).

Details

^ - start of string
\d{1,2} - one or two digits
(?: - start of a non-capturing group:
- (?<=^\d{2}) - a positive lookbehind check: there must be two digits at the start of the string immediately to the left of the current position
- [A-Z] - an uppercase letter
- \d{0,2} - 0, 1 or 2 digits
)? - end of the non-capturing group, repeat 1 or 0 times (i.e. make it optional)
(?:[A-Z]\d{0,2})? - an optional sequence of an uppercase letter and then 0, 1 or 2 digits
\z - the very end of string (no \n is allowed at the end of the string).

answered Mar 9, 2020 at 9:45

Wiktor Stribiżew

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1 Comment

Oliver Over a year ago

I have implemented this into my question, thankyou!

juha · Accepted Answer · 2020-03-09 09:32:04Z

0

Maybe you can trim \n off (if any) and use

"^[0-9]{2}[A-Z][0-9]{2}$"

instead?

answered Mar 9, 2020 at 9:32

juha

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