Can I test a substring in a textBox for isLetter?

Question

I want to make sure the first 4 letters in a eight character code is a letter. Is there a function for that, or do I have to make my own.

Answer 1

You should use a regular expression validator to achieve this.

Answer 2

you can add javascript/regex validation

regex

  ^[a-zA-Z]{4}.*

Answer 3

AFAIK there's no built-in function. A letter might mean different things according to user culture. If you mean ASCII letter you could test like this:

bool IsASCIILetter(char letter) {
    return letter >= 65 && letter <= 90 && letter >= 97 && letter <= 122;
}

Answer 4

If it's the contents of an input control you want to validate, use a RegularExpressionValidator.

This case is probably simple enough to just use a RegEx, but for more complex validation I try to avoid RegExes as they are just too complex to debug. As somebody smarter than me said 'I had a problem which I solved with regular expressions. Now I have two problems'. Even if you are comfortable with regular expressions, the person who maintains your code might not be.

More explicit code is a lot easier to read and maintain:

bool IsValid (string code) {
    if (code.Length != 8)
        return false;

    for (int i = 0; i < 4; ++i) {
        if (!Char.IsLetter(code[i]))
            return false;
    }
    return true;
}

Answer 5

How about Char.IsLetter?

It has 2 overrides: one where you pass it a char and one where you pass it a string and index.

Answer 6

2 options:

Option 1 - Check for it in your code-behind:

    if (txtBox1.Text.Length >= 4 && txtBox1.Text.ToCharArray().Take(4).All(c => char.IsLetter(c)))
    {
        // success
    }
    else
    {
        // validation failed
    }

You didn't state whether you're enforcing a length of 8 or it's optional and upto 8. You can add this check fairly easily.

Option 2 - Use a RegularExpressionValidator with an optional RequiredFieldValidator. If you decide to go this route you really should know exactly what you want to permit for the last 4 characters as this example below allows anything and only restricts the first 4 to letters. Also, again, this will differ if you wanted to enforce a length of 8 or not. This one below makes it mandatory; to make it optional to have 8 characters you would use "[A-Za-z]{4}(.{4})?" instead.

<asp:TextBox ID="txtBox1" runat="server"></asp:TextBox>

<asp:RequiredFieldValidator ID="rfvtxtBox1" runat="server" ControlToValidate="txtBox1" ErrorMessage="txtBox1 is required!" Display="Dynamic">*</asp:RequiredFieldValidator>

<asp:RegularExpressionValidator ID="revtxtBox1" runat="server" ControlToValidate="txtBox1" ValidationExpression="[A-Za-z]{4}.{4}" ErrorMessage="First 4 characters must be a letter!" Display="Dynamic" />

A RequiredFieldValidator is needed if you don't want to allow a blank entry since the RegularExpressionValidator alone won't prevent blank/whitespace entries. For more info see my answer here.