vb.net string contains only 4 digit numbers(or a year) - vb.net

Question

how can i check if a string only contains 4 digit numbers ( or a year )
i tried this
Dim rgx As New Regex("^/d{4}")
Dim number As String = "0000"
Console.WriteLine(rgx.IsMatch(number)) // true
number = "000a"
Console.WriteLine(rgx.IsMatch(number)) // false
number = "000"
Console.WriteLine(rgx.IsMatch(number)) //false
number = "00000"
Console.WriteLine(rgx.IsMatch(number)) // true <<< :(
this returns false when its less than 4 or at characters but not at more than 4
thanks!

Answer 1

I actually wouldn't use a regex for this. The expression is deceptively simple (^\d{4}$), until you realize that you also need to evaluate that numeric value to determine a valid year range... unless you want years like 0013 or 9015. You're most likely going to want the value as an integer in the end, anyway. Given that, the best validation is probably just to actually try to convert it to an integer right off the bat:
Dim numbers() As String = {"0000", "000a", "000", "00000"}
For Each number As String In numbers
Dim n As Integer
If Integer.TryParse(number, n) AndAlso number.Length = 4 Then
'It's a number. Now look at other criteria
End If
Next

Answer 2

Use LINQ to check if All characters IsDigit:
Dim result As Boolean = ((Not number Is Nothing) AndAlso ((number.Length = 4) AndAlso number.All(Function(c) Char.IsDigit(c))))

Answer 3

You should use the .NET string manipulation functions.
Firstly the requirements, the string must:
Contain exactly four characters, no more, no less;
Must consist of a numeric value
However your aim is to validate a Date:
Function isKnownGoodDate(ByVal input As String) As Boolean 'Define the function and its return value.
Try 'Try..Catch statement (error handling). Means an input with a space (for example ` ` won't cause a crash)
If (IsNumeric(input)) Then 'Checks if the input is a number
If (input.Length = 4) Then
Dim MyDate As String = "#01/01/" + input + "#"
If (IsDate(MyDate)) Then
Return True
End If
End If
End If
Catch
Return False
End Try
End Function
You may experience a warning:
Function isKnownGoodDate does not return a value on all code
paths. Are you missing a Return statement?
this can be safely ignored.