Validate a number with regex

Question

I have a WPF form where the user is able to enter width and height to scale an image. I want to validate the number with a regular expression. The user should be able to enter only number greater than zero.

At the moment I use the PreviewTextInput event

<TextBox Name="Height" Width="50" PreviewTextInput="Height_ValidateNumber"></TextBox>

and check the input with this method

    private void Height_ValidateNumber(object sender, TextCompositionEventArgs e)
    {
        Regex regex = new Regex("^[1-9][0-9]*$");
        e.Handled = regex.IsMatch(e.Text);
    }

The regex I use is ^[1-9][0-9]\*$

The problem with this is that I'm actully able to enter everything but no digits except zero...

If I am using [^1-9][0-9]\*$ I am able to enter all digits except zero...

I think that the regex ^[1-9][0-9]\*$ is not wrong. I think it is another problem.

What range do you want the number to be? 1 to 100? 10 to 999? — zx81, May 07 '14 at 08:28
Gerret, I saw that. I was just wondering if when validating, you might want to reject width and height values that make no sense, such as `99999999999`, hence the question. It's a different regex task if you have a range. — zx81, May 07 '14 at 08:32
Please do not extend your question by repeating the same thing over and over. Be more speciffic instead. Also, just use `TryParse` method and work on integers. — Tarec, May 07 '14 at 08:32

Sriram Sakthivel · Accepted Answer · 2014-05-07T08:46:23.187

2

You're filtering all the valid values instead of invalid

Change this

e.Handled = regex.IsMatch(e.Text);

to

e.Handled = !regex.IsMatch(e.Text);

Update1 : e.Text gives newly entered text, you can concatenate TextBox.Text with e.Text to frame full text.

TextBox tb = (TextBox) sender;
Regex regex = new Regex("^[1-9][0-9]*$");
e.Handled = !regex.IsMatch(tb.Text + e.Text);

edited May 07 '14 at 08:46

answered May 07 '14 at 08:29

Sriram Sakthivel

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Yea, but I mainly have to problem that I can not enter a zero also after I have entered any digit. – Gerret May 07 '14 at 08:33
@Gerret What do you mean by that? You mean `50` is invalid number? – Sriram Sakthivel May 07 '14 at 08:35
Yes, 50 should be a valid number. Everything greater than 0 should be a valid number. But if I use my regex I am not able to enter a zero and I do not know why... – Gerret May 07 '14 at 08:37
Thank you very much! Did not know that I only get the changed Text it is working now :D – Gerret May 07 '14 at 08:50

score 1 · Answer 2 · answered May 07 '14 at 08:50

I know you asked for a RegExpr, but why dont you use:

long number;
if (UInt32.TryParse(e.Text, out number)) 
    // You can use ANY .net Number class here 
    //(you want > 0, use the UInt16,UInt32,UInt64 Structs)

Seems easier and more logical to me :)

Validate a number with regex

2 Answers2