Verifying that a string contains only letters in C#

Only letters:

Regex.IsMatch(input, @"^[a-zA-Z]+$");
Regex.IsMatch(input, @"^[a-zA-Z]+$");

1. <code>
2. Regex.IsMatch(input, @"^[a-zA-Z]+$");
3. </code>

<code>
Regex.IsMatch(input, @"^[a-zA-Z]+$");
</code>

Only letters and numbers:

Regex.IsMatch(input, @"^[a-zA-Z0-9]+$");
Regex.IsMatch(input, @"^[a-zA-Z0-9]+$");

1. <code>
2. Regex.IsMatch(input, @"^[a-zA-Z0-9]+$");
3. </code>

<code>
Regex.IsMatch(input, @"^[a-zA-Z0-9]+$");
</code>

Only letters, numbers and underscore:

Regex.IsMatch(input, @"^[a-zA-Z0-9_]+$");
Regex.IsMatch(input, @"^[a-zA-Z0-9_]+$");

1. <code>
2. Regex.IsMatch(input, @"^[a-zA-Z0-9_]+$");
3. </code>

<code>
Regex.IsMatch(input, @"^[a-zA-Z0-9_]+$");
</code>