.NET testing a string for numeric value

Question

Consider the need for a function in C# that will test whether a string is a numeric value.

The requirements:

• must return a boolean.
• function should be able to allow for whole numbers, decimals, and negatives.
• assume no using Microsoft.VisualBasic to call into IsNumeric(). Here's a case of reinventing the wheel, but the exercise is good.

Current implementation:

 //determine whether the input value is a number
    public static bool IsNumeric(string someValue)
    {
        Regex isNumber = new Regex(@"^\d+$");
        try
        {
            Match m = isNumber.Match(someValue);
            return m.Success;                           
        }
        catch (FormatException)
        {return false;}
    }

Question: how can this be improved so that the regex would match negatives and decimals? Any radical improvements that you'd make?

Answer 1

Regex isNumber = new Regex(@"^[-+]?(\d*\.)?\d+$");

Updated to allow either + or - in front of the number.

Edit: Your try block isn't doing anything as none of the methods within it actually throw a FormatException. The entire method could be written:

// Determine whether the input value is a number
public static bool IsNumeric(string someValue)
{
  return new Regex(@"^[-+]?(\d*\.)?\d+$").IsMatch(someValue);
}

Answer 2

Just off of the top of my head - why not just use double.TryParse ? I mean, unless you really want a regexp solution - which I'm not sure you really need in this case :)

Answer 3

Well, for negatives you'd need to include an optional minus sign at the start:

^-?\d+$

For decimals you'd need to account for a decimal point:

^-?\d*\.?\d*$

And possible exponential notation:

^-?\d*\.?\d*(e\d+)?$

Answer 4

Can you just use .TryParse?

int x;
double y;
string spork = "-3.14";

if (int.TryParse(spork, out x))
    Console.WriteLine("Yay it's an int (boy)!");
if (double.TryParse(spork, out y))
    Console.WriteLine("Yay it's an double (girl)!");

Answer 5

Unless you really want to use regex, Noldorin posted a nice extension method in another Q&A.

Update

As Patrick rightly pointed out, the link points to an extension method that check whether the object is a numeric type or not, not whether it represents a numeric value. Then using double.TryParse as suggested by Saulius and yodaj007 is probably the best choice, handling all sorts of quirks with different decimal separators, thousand separators and so on. Just wrap it up in a nice extension method:

public static bool IsNumeric(this string value)
{
    double temp;
    return double.TryParse(value.ToString(), out temp);
}

...and fire away:

string someValue = "89.9";
if (someValue.IsNumeric()) // will be true in the US, but not in Sweden
{
    // wow, it's a number!
]

Answer 6

I can't say that I would use regular expressions to check if a string is a numeric value. Slow and heavy for such a simple process. I would simply run over the string one character at a time until I enter an invalid state:

public static bool IsNumeric(string value)
{
    bool isNumber = true;

    bool afterDecimal = false;
    for (int i=0; i<value.Length; i++)
    {
        char c = value[i];
        if (c == '-' && i == 0) continue;

        if (Char.IsDigit(c))
        {
            continue;
        }

        if (c == '.' && !afterDecimal)
        {
            afterDecimal = true;
            continue;
        }

        isNumber = false;
        break;
    }

    return isNumber;
}

The above example is simple, and should get the job done for most numbers. It is not culturally sensitive, however, but it should be strait-forward enough to make it culturally sensitive.

Answer 7

Also, make sure the resulting code passes the Turkey Test:
http://www.moserware.com/2008/02/does-your-code-pass-turkey-test.html