C# Remove unwanted characters from a string

Question

I have looked into other posts, and all of them have known unwanted characters. In my case, I have a bunch of characters that I want, and I only want to keep those.

My code is way too messy:

private string RemoveUnwantedChar(string input)
{
    string correctString = "";

    for (int i = 0; i < input.Length; i++)
    {
        if (char.IsDigit(input[i]) || input[i] == '.' || input[i] == '-' || input[i] == 'n'
                || input[i] == 'u' || input[i] == 'm' || input[i] == 'k' || input[i] == 'M'
                || input[i] == 'G' || input[i] == 'H' || input[i] == 'z' || input[i] == 'V'
                || input[i] == 's' || input[i] == '%')
            correctString += input[i];
    }
    return correctString;
}

Characters that I want: 0123456789 and numkMGHzVs%-.

@JeroenvanLangen Oh I didnt know about that community. Thanks! — Liren Yeo
– Liren Yeo, Commented Nov 2, 2015 at 9:54

w.b · Accepted Answer · 2015-11-02 12:54:04Z

8

You can use LINQ:

var allowedChars = "0123456789numkMGHzVs";
var result = String.Join("", input.Where(c => allowedChars.Any(x => x == c)));

Another option:

var result = String.Join("", str.Where(c => allowedChars.Contains(c)));

edited Nov 2, 2015 at 12:54

answered Nov 2, 2015 at 10:10

w.b

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Comments

Tim Schmelter · Accepted Answer · 2015-11-02 10:17:19Z

3

You can use String.Concat + Enumerable.Where with HashSet<T>.Contains:

HashSet<char> AllowedChars = new HashSet<char>("0123456789numkMGHzVs%-.");
private string RemoveUnwantedChar(string input)
{
    return string.Concat(input.Where(AllowedChars.Contains));
}

Here's another efficient aproach using a StringBuilder and a HashSet<T>:

HashSet<char> AllowedChars = new HashSet<char>("0123456789numkMGHzVs%-.");
private string RemoveUnwantedChar(string input)
{
    StringBuilder sb = new StringBuilder(input.Length);
    foreach (char c in input)
        if (AllowedChars.Contains(c))
            sb.Append(c);
    return sb.ToString();
}

answered Nov 2, 2015 at 10:17

Tim Schmelter

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1 Comment

Jeroen van Langen Over a year ago

I like the first method. passing the Contains method to the Where statement.

Community · Accepted Answer · 2015-11-03 13:33:37Z

2

You could do something like this:

// create a lookup hashset
private static HashSet<char> _allowedChars = new HashSet<char>("0123456789numkMGHzVs%-.".ToArray());

private string FilterString(string str)
{
    // tempbuffer
    char[] buffer = new char[str.Length];
    int index = 0;

    // check each character
    foreach (var ch in str)
        if (_allowedChars.Contains(ch))
            buffer[index++] = ch;

    // return the new string.
    return new String(buffer, 0, index);
}

So the trick is, create a hashset to validate each character. The 'messy' way, like you said, is creating new strings and will fragement memory. Also try to avoid many nested if statements. (like you want to avoid)

If you like linq, you could do something like:

// create a lookup hashset
private static HashSet<char> _allowedChars = new HashSet<char>("0123456789numkMGHzVs%-.".ToArray());

private string FilterString2(string str)
{
    return new String(
        str.Where(ch => _allowedChars.Contains(ch)).ToArray());
}

But this will make it less readable..

edited Nov 3, 2015 at 13:33

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answered Nov 2, 2015 at 10:02

Jeroen van Langen

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4 Comments

Liren Yeo Over a year ago

Thanks! That is sort of what I want, an char array of all the wanted characters, which has better readability. Btw, why HashSet? Will a normal 1D char array work?

Jeroen van Langen Over a year ago

Sure, with these few characters, a normal array will work fine.

Ivan Stoev Over a year ago

@Liren Because arrays and lists in general are not for searching, while sets and dictionaries are specifically for searching.

Tim Schmelter Over a year ago

++ but the LINQ approach doesn't need the first str.ToArray()

Dhunt · Accepted Answer · 2015-11-02 10:15:00Z

1

If you are using LINQ you could do this:

char[] validChars = "0123456789numkMGHzVs%-.".ToArray();
var newString = "Teststring012";

string filtered = string.Join("", newString.Where(x => validChars.Contains(x)));

answered Nov 2, 2015 at 10:15

Dhunt

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Comments

VSDekar · Accepted Answer · 2015-11-02 15:59:51Z

1

I like this clear and readable Regex solution.

public string RemoveUnwantedChar(string input) {
    return Regex.Replace(input, "[^0-9numkMGHzVs%\\-.]", "");
}

edited Nov 2, 2015 at 15:59

answered Nov 2, 2015 at 12:44

VSDekar

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