How can I validate a string to only allow alphanumeric characters in it?

Question

In .NET 4.0 you can use LINQ:

if (yourText.All(char.IsLetterOrDigit))
{
    //just letters and digits.
}

yourText.All will stop execute and return false the first time char.IsLetterOrDigit reports false since the contract of All cannot be fulfilled then.

Note! this answer do not strictly check alphanumerics (which typically is A-Z, a-z and 0-9). This answer allows local characters like åäö.

Update 2018-01-29

The syntax above only works when you use a single method that has a single argument of the correct type (in this case char).

To use multiple conditions, you need to write like this:

if (yourText.All(x => char.IsLetterOrDigit(x) || char.IsWhiteSpace(x)))
{
}

Use the following expression:

^[a-zA-Z0-9]*$

ie:

using System.Text.RegularExpressions;

Regex r = new Regex("^[a-zA-Z0-9]*$");
if (r.IsMatch(SomeString)) {
  ...
}

You could do it easily with an extension function rather than a regex ...

public static bool IsAlphaNum(this string str)
{
    if (string.IsNullOrEmpty(str))
        return false;

    for (int i = 0; i < str.Length; i++)
    {
        if (!(char.IsLetter(str[i])) && (!(char.IsNumber(str[i]))))
            return false;
    }

    return true;
}

Per comment :) ...

public static bool IsAlphaNum(this string str)
{
    if (string.IsNullOrEmpty(str))
        return false;

    return (str.ToCharArray().All(c => Char.IsLetter(c) || Char.IsNumber(c)));
}

While I think the regex-based solution is probably the way I'd go, I'd be tempted to encapsulate this in a type.

public class AlphaNumericString
{
    public AlphaNumericString(string s)
    {
        Regex r = new Regex("^[a-zA-Z0-9]*$");
        if (r.IsMatch(s))
        {
            value = s;                
        }
        else
        {
            throw new ArgumentException("Only alphanumeric characters may be used");
        }
    }

    private string value;
    static public implicit operator string(AlphaNumericString s)
    {
        return s.value;
    }
}

Now, when you need a validated string, you can have the method signature require an AlphaNumericString, and know that if you get one, it is valid (apart from nulls). If someone attempts to pass in a non-validated string, it will generate a compiler error.

You can get fancier and implement all of the equality operators, or an explicit cast to AlphaNumericString from plain ol' string, if you care.

I needed to check for A-Z, a-z, 0-9; without a regex (even though the OP asks for regex).

Blending various answers and comments here, and discussion from https://stackoverflow.com/a/9975693/292060, this tests for letter or digit, avoiding other language letters, and avoiding other numbers such as fraction characters.

if (!String.IsNullOrEmpty(testString)
    && testString.All(c => Char.IsLetterOrDigit(c) && (c < 128)))
{
    // Alphanumeric.
}

^\w+$ will allow a-zA-Z0-9_

Use ^[a-zA-Z0-9]+$ to disallow underscore.

Note that both of these require the string not to be empty. Using * instead of + allows empty strings.