Need help with validating an alphanumeric string within a textbox control.

Question

Hi everyone,

I am developing a simple WinForms app for a presentation next week and need to validate that a particular string in a particular textbox is alphanumeric.  I have included the code from the button click event handler below:

        // Alphanumeric
        private void btnValidate4_Click(object sender, EventArgs e)
        {
            try
            {
                if (txtAlphanumeric.Text == "")
                    MessageBox.Show("Please enter an alphanumeric string before clicking the 'Validate' button.", "Whoa there, pardner!!", MessageBoxButtons.OK,
                        MessageBoxIcon.Information);

                else
                {
                    Regex rgxAlphanumeric = new Regex("^[a-zA-Z0-9]*$");
                    Match alphaNumericMatch = rgxAlphanumeric.Match(txtAlphanumeric.Text);

                    if (!alphaNumericMatch.Success)
                        MessageBox.Show("The string supplied does NOT match the specified pattern.", "Error!", MessageBoxButtons.OK,
                            MessageBoxIcon.Error);

                    else
                    {
                        MessageBox.Show("The supplied string matches the specified pattern.", "Success!", MessageBoxButtons.OK,
                            MessageBoxIcon.Exclamation);

                    }
                }
            }

            catch (Exception exc)
            {
                MessageBox.Show(exc.ToString(), "Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);

            }

            finally
            {
                ClearControls();

            }
        }

I think the regular expression is written correctly but, when I pass in '111' or 'aaa' (for example), it tells me that string matches the pattern; which it doesn't.  Any elucidation/help is appreciated.

Thanks,

Brian

Answer 1

Your pattern behaves correctly and matches alphanumeric characters. "111" and "aaa" are valid matches. Perhaps you can tell us why you think they shouldn't match, and provide a sample of strings that you think should and should not match. Currently your pattern matches any letter from A-Z (lower- and upper-case) and any number from 0-9, and matches them zero or more times. So "111" matches since "1" is in the valid 0-9 range, and you are allowing zero or more repetitions of the pattern, and each "1" in "111" is valid (same concept for "aaa").

---

Document my code? Why do you think it's called "code"?

Answer 2

I suspected that the regex I used as correct, and you are right, '111' and 'aaa' are valid matches.  I suppose though - and this is just in my mind's eye - that when I hear 'alphanumeric' I think along the lines of 'a234kbkasd2', or something similar. 

But, since this is going to be used for a one-off training session, I suppose I should just leave the code 'as-is' and call it good.

Thanks for your response!

--Brian