check content of string input

Question

How can I check if my input is a particular kind of string. So no numeric, no "/",...

Do you by any chance mean alphabetic (letters only) instead of string? — BoltClock, Jul 20 '10 at 18:43
Do you mean, "How can I check if a string contains only characters?" — Justin Niessner, Jul 20 '10 at 18:44
Do you mean a string consisting only of alpha-numeric characters? Can you define which characters exactly? — Pekka, Jul 20 '10 at 18:45
Are you wanting validation on a UI control, or on some backend process? If a UI, are you talking about WinForms, WebForms, Silverlight, WPF...? — Pedro, Jul 20 '10 at 18:46
if your input control is a textbox it will always be a string =) — Luiscencio, Jul 20 '10 at 18:46
There was little to no effort put in this question, or effort to rephrase it when others asked. — , May 05 '16 at 02:34

score 15 · Accepted Answer · answered Jul 20 '10 at 18:48

Well, to check that an input is actually an object of type System.String, you can simply do:

bool IsString(object value)
{
    return value is string;
}

To check that a string contains only letters, you could do something like this:

bool IsAllAlphabetic(string value)
{
    foreach (char c in value)
    {
        if (!char.IsLetter(c))
            return false;
    }

    return true;
}

If you wanted to combine these, you could do so:

bool IsAlphabeticString(object value)
{
    string str = value as string;
    return str != null && IsAllAlphabetic(str);
}

Good example of both, being that the title and question don't exactly match. — corsiKa, Jul 20 '10 at 18:49

score 10 · Answer 2 · answered Jul 20 '10 at 18:48

10

If you mean "is the string completely letters", you could do:

string myString = "RandomStringOfLetters";
bool allLetters = myString.All( c => Char.IsLetter(c) );

This is based on LINQ and the Char.IsLetter method.

answered Jul 20 '10 at 18:48

Reed Copsey

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@ahsteele: It actually System.Globalization.CharUnicodeInfo to handle this for non latin characters. – Reed Copsey Jul 20 '10 at 18:59

Mark Byers · Answer 3 · 2010-07-20T18:55:00.763

It's not entirely clear what you want, but you can probably do it with a regular expression. For example to check that your string contains only letters in a-z or A-Z you can do this:

string s = "dasglakgsklg";
if (Regex.IsMatch(s, "^[a-z]+$", RegexOptions.IgnoreCase))
{
    Console.WriteLine("Only letters in a-z.");
}
else
{
    // Not only letters in a-z.
}

If you also want to allow spaces, underscores, or other characters simply add them between the square brackets in the regular expression. Note that some characters have a special meaning inside regular expression character classes and need to be escaped with a backslash.

You can also use \p{L} instead of [a-z] to match any Unicode character that is considered to be a letter, including letters in foreign alphabets.

score 2 · Answer 4 · answered Jul 20 '10 at 18:47

2

using System.Linq;
...

bool onlyAlphas = s.All(c => (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'));

answered Jul 20 '10 at 18:47

StriplingWarrior

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Don't get me wrong: regex has its uses. But for a simple, single-pass algorithm like this, it's overkill. I like Reed's answer more, though. Didn't know about `Char.IsLetter`. – StriplingWarrior Jul 20 '10 at 18:55
@Luiscencio: How am I being brutal? – StriplingWarrior Jul 20 '10 at 18:59
I think Luiscencio meant that you used brute force to solve it. I would have written a for loop. You solution using LINQ is far less brute force than that. – aaaa bbbb Jul 20 '10 at 19:03
Huh. I always thought of brute force as having to do with performance. Brute force would have been throwing all valid characters into an array and doing a doubly-nested loop to make sure that all the characters in that string are one of those characters. Not having known about Char.IsLetter, the approach I took seemed pretty elegant to me. To each his own, though. – StriplingWarrior Jul 20 '10 at 19:36

score 1 · Answer 5 · answered Jul 20 '10 at 18:47

Something like this (have not tested) may fit your (vague) requirement.

if (input is string)
{
    // test for legal characters?
    string pattern = "^[A-Za-z]+$";
    if (Regex.IsMatch(input, pattern))
    {
         // legal string? do something 
    }

    // or
    if (input.Any(c => !char.IsLetter(c)))
    {
         // NOT legal string 
    }
}

check content of string input

5 Answers5

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