Stripping out non-numeric characters in string

Question

There are many ways, but this should do (don't know how it performs with really large strings though):

private static string GetNumbers(string input)
{
    return new string(input.Where(c => char.IsDigit(c)).ToArray());
}

Feels like a good fit for a regular expression.

var s = "40,595 p.a.";
var stripped = Regex.Replace(s, "[^0-9]", "");

"[^0-9]" can be replaced by @"\D" but I like the readability of [^0-9].

An extension method will be a better approach:

public static string GetNumbers(this string text)
    {
        text = text ?? string.Empty;
        return new string(text.Where(p => char.IsDigit(p)).ToArray());
    }

public static string RemoveNonNumeric(string value) => Regex.Replace(value, "[^0-9]", "");

Use either a regular expression that's only capturing 0-9 and throws away the rest. A regular expression is an operation that's going to cost a lot the first time though. Or do something like this:

var sb = new StringBuilder();
var goodChars = "0123456789".ToCharArray();
var input = "40,595";
foreach(var c in input)
{
  if(goodChars.IndexOf(c) >= 0)
    sb.Append(c);
}
var output = sb.ToString();

Something like that I think, I haven't compiled though..

LINQ is, as Fredrik said, also an option