Validation of textbox input in C#

//for email validation    
System.Text.RegularExpressions.Regex rEMail = new System.Text.RegularExpressions.Regex(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");

if (txt_email.Text.Length > 0)
{
    if (!rEMail.IsMatch(txt_email.Text))
    {
        MessageBox.Show("E-Mail expected", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        txt_email.SelectAll();
        e.Cancel = true;
    }
}

//for mobile validation    
Regex re = new Regex("^9[0-9]{9}");

if (re.IsMatch(txt_mobile.Text.Trim()) == false || txt_mobile.Text.Length > 10)
{
    MessageBox.Show("Invalid Indian Mobile Number !!");
    txt_mobile.Focus();
}

This code will check whether an email address is valid:

string inputText = textBox1.Text;

if (Regex.IsMatch(inputText, 
                  @"^(?("")("".+?""@)|(([0-9a-zA-Z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-zA-Z])@))" + 
                  @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-zA-Z][-\w]*[0-9a-zA-Z]\.)+[a-zA-Z]{2,6}))$"))
{
  MessageBox.Show("yes");
}
else
{
  MessageBox.Show("no");
}

(source: http://msdn.microsoft.com/en-us/library/01escwtf.aspx)

For phone numbers, it's not so simple - the answer depends on where in the world you are, whether you want to allow international numbers, how mobiles are numbered (for example, in the USA, you can't tell from a phone number alone whether it's a mobile number or not). Look up "Telephone numbering plan" on Wikipedia for more information.

In ASP.NET you can use a RegularExpressionValidator control.

To determine the regular expression itself, you can experiment with a tool like Expresso.

Be aware that validating emails with regular expressions is a hard task, if you want to allow all the possibly valid email formats; probably the best thing to do in that case would be to send an email to the entered address with a confirmation link, and when that link is clicked, you assume that the mail is valid.

See Email Address Validation Using Regular Expression (The Code Project) for email validation and see Best practice for parsing and validating mobile number (Stack Overflow) for mobile number validation.

I do the numeric validation this way as shown in the code below.

No need for checking char by char and user culture is respected!

namespace Your_App_Namespace
{
public static class Globals
{
    public static double safeval = 0; // variable to save former value!

    public static bool isPositiveNumeric(string strval, System.Globalization.NumberStyles NumberStyle)
    // checking if string strval contains positive number in USER CULTURE NUMBER FORMAT!
    {
        double result;
        boolean test;
        if (strval.Contains("-")) test = false;
        else test = Double.TryParse(strval, NumberStyle, System.Globalization.CultureInfo.CurrentCulture, out result);
        // if (test == false) MessageBox.Show("Not positive number!");
        return test;
    }

    public static string numstr2string(string strval, string nofdec)
    // conversion from numeric string into string in USER CULTURE NUMBER FORMAT!
    // call example numstr2string("12.3456", "0.00") returns "12.34"
    {
        string retstr = "";
        if (Globals.isPositiveNumeric(strval, System.Globalization.NumberStyles.Number)) retstr = double.Parse(strval).ToString(nofdec);
        else retstr = Globals.safeval.ToString(nofdec);
        return retstr;
    }

    public static string number2string(double numval, string nofdec)
    // conversion from numeric value into string in USER CULTURE NUMBER FORMAT!
    // call example number2string(12.3456, "0.00") returns "12.34"
    {
        string retstr = "";
        if (Globals.isPositiveNumeric(numval.ToString(), System.Globalization.NumberStyles.Number)) retstr = numval.ToString(nofdec);
        else retstr = Globals.safeval.ToString(nofdec);
        return retstr;
    }
}

// Other Your_App_Namespace content

}

// This the way how to use those functions in any of your app pages

    // function to call when TextBox GotFocus

    private void textbox_clear(object sender, System.Windows.RoutedEventArgs e)
    {
        TextBox txtbox = e.OriginalSource as TextBox;
        // save original value
        Globals.safeval = double.Parse(txtbox.Text);
        txtbox.Text = "";
    }

    // function to call when TextBox LostFocus

    private void textbox_change(object sender, System.Windows.RoutedEventArgs e)
    {
        TextBox txtbox = e.OriginalSource as TextBox;
        // text from textbox into sting with checking and string format
        txtbox.Text = Globals.numstr2string(txtbox.Text, "0.00");
    }

For Email Validation use the following Regex Expression as below in Lost Focus event of the Textbox.

Use System.Text.RegularExpression Namespace for Regex.

Regex emailExpression = new Regex(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");

and then check it by using below code

if (emailExpression.IsMatch(textbox.Text))
{
    //Valid E-mail
}

For PhoneNumber Validation use the following code in PreviewTextInput event of the Textbox.

private void PhoneNumbeTextBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
    e.Handled = !AreAllValidNumericChars(e.Text);       
}


private bool AreAllValidNumericChars(string str)
{
    bool ret = true;
    if (str == System.Globalization.NumberFormatInfo.CurrentInfo.CurrencyDecimalSeparator |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.CurrencyGroupSeparator |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.NegativeSign |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.NegativeInfinitySymbol |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.NumberDecimalSeparator |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.NumberGroupSeparator |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.PercentDecimalSeparator |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.PercentGroupSeparator |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.PerMilleSymbol |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.PositiveInfinitySymbol |
            str == System.Globalization.NumberFormatInfo.CurrentInfo.PositiveSign)
            return ret;

    int l = str.Length;
    for (int i = 0; i < l; i++)
    {
        char ch = str[i];
        ret &= Char.IsDigit(ch);
    }

    return ret;
}