Allow only a decimal point after an input of three digits in a textbox in VB.NET?

Question

How do I allow only a decimal point after an input of three digits in a textbox in VB.NET?

Let's say I inputed "123" after that I can only put a decimal else it wont allow any other input. So the result would be "123."

    Dim KeyAscii As Integer
    KeyAscii = Asc(myE.KeyChar)

    Select Case KeyAscii
        Case Asc("0") To Asc("9"), Asc(ControlChars.Back)

            myE.Handled = False
        Case Asc(".")

            If InStr(myTextbox.Text, ".") = 0 Then
                myE.Handled = False
            Else : myE.Handled = True
            End If

        Case myE.KeyChar = Chr(127)
            myE.Handled = False
        Case Else
            myE.Handled = True
    End Select

Answer 1

In WinForms you could accomplish this by using the TextChanged-Event of the Textbox and RegularExpressions:

Example:

Imports System.Text.RegularExpressions



Public Class Form1

   Private Sub TextBox1_TextChanged(sender As System.Object, e As System.EventArgs) Handles TextBox1.TextChanged
      '** Regex Pattern
      Dim pattern As String = "^(([0-9]{1,3})|([0-9]{1,3}(\.){1,1}([0-9]){0,3}))$"
      '** Copy of the Textbox Content
      Dim strText As String = TextBox1.Text
      '** Remove chars at the end of the string until the Textbox is empty or the contained chars are valid
      While Not Regex.IsMatch(strText, pattern) AndAlso Not strText = ""
         strText = strText.Substring(0, strText.Length - 1)
      End While
      '** Set the new text
      TextBox1.Text = strText
      '** Set the caret to the end of the string in the textbox
      TextBox1.Select(TextBox1.Text.Length, 0)
   End Sub
End Class

This example lets you write 123 , 345. , 12. , 123.1 , 123.123 and so on...

To improve the number of digits before or after the decimal you can edit the {0,3} in the pattern (The first 2 times are for the digits before the decimal and the third time for the digits after the decimal). Just set instead of 3 the number of digits you like (or replace it with a * or {0,} for unlimited)

Hope this helps.

EDIT 1:

• Changed pattern from "^[0-9]{0,3}(\\.){0,1}$" to "^(([0-9]{1,3})|([0-9]{1,3}(\\.){1,1}([0-9]){0,3}))$" to allow digits after the decimal
• Corrected the While Loop Condition from Textbox1.Text = "" to strText = ""

Answer 2

Try it with:

Select Case myE.KeyChar
    Case "0"c To "9"c, "."c
        myE.Handled = InStr(myTextbox.Text, ".") > 0
    Case ControlChars.Back, Convert.ToChar(127)
        myE.Handled = False
    Case Else
        myE.Handled = True
End Select

Note: There is no point in converting the KeyChar to Integer and then use Asc() to compare.

EDIT : According to your comment the decimal point must be placed after the third digit and 2 or 3 more digits can follow.

Select Case myE.KeyChar
    Case "0"c To "9"c
        myE.Handled = myTextbox.Text.Length = 3 OrElse myTextbox.Text.Length >= 7
    Case "."c
        myE.Handled = myTextbox.Text.Length <> 3
    Case ControlChars.Back, Convert.ToChar(127)
        myE.Handled = False
    Case Else
        myE.Handled = True
End Select

Answer 3

Try this:

Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged Dim wherePointIs As Integer = TextBox1.Text.IndexOf(".") If wherePointIs <> 3 Then 'Whats supposed to happen End If End Sub

This will only check if there is a point at the point of three. You can change it so it checks if there is only ONE decimal point, etc.