Verifying that a string contains only letters in C#

I have an input string and I want to verify that it contains:

- Only letters or
- Only letters and numbers or
- Only letters, numbers or underscore

To clarify, I have 3 different cases in the code, each calling for different validation. What's the simplest way to achieve this in C#?

You can loop on the chars of string and check using the Char Method [IsLetter][1]
but you can also do a trick using String method [IndexOfAny][2] to search other charaters that are not suppose to be in the string.


[1]:

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[2]:

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Only letters:

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

Only letters and numbers:

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

Only letters, numbers and underscore:

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

Iterate through strings characters and use functions of 'Char' called 'IsLetter' and 'IsDigit'.

If you need something more specific - use Regex class.

Letters only:

Regex.IsMatch(theString, @"^[\p{L}]+$");

Letters and numbers:

Regex.IsMatch(theString, @"^[\p{L}\p{N}]+$");

Letters, numbers and underscore:

Regex.IsMatch(theString, @"^[\w]+$");

Note, these patterns also match international characters (as opposed to using the `a-z` construct).

I think is a good case to use Regular Expressions:

public bool IsAlpha(string input)
{
return Regex.IsMatch(input, "^[a-zA-Z]+$");
}

public bool IsAlphaNumeric(string input)
{
return Regex.IsMatch(input, "^[a-zA-Z0-9]+$");
}

public bool IsAlphaNumericWithUnderscore(string input)
{
return Regex.IsMatch(input, "^[a-zA-Z0-9_]+$");
}