Only use regex for numbers.

Problem

I’ve never worked with regular expressions before, so I’m having issues debugging. I want the regex to match only when the contained string consists entirely of numbers; but, in the two examples below, it matches a string that consists entirely of numbers plus an equals sign, such as “1234=4321.” I’m sure there’s a method to adjust this behavior, but as I previously stated, I’ve never worked with regular expressions very much.

string compare = "1234=4321";
Regex regex = new Regex(@"[\d]");

if (regex.IsMatch(compare))
{ 
    //true
}

regex = new Regex("[0-9]");

if (regex.IsMatch(compare))
{ 
    //true
}

I’m using C# and.NET2.0, in case that matters.

Asked by Timothy Carter

Solution #1

Use the anchors at the start and conclusion of your sentence.

Regex regex = new Regex(@"^\d$");

Use “^\d+$” if you need to match more than one digit.

It’s worth noting that “d” will match [0-9] and other digit characters, such as Eastern Arabic numerals. To limit matches to to the Arabic digits 0 – 9, use “[0-9]+$.”

See @tchrist’s full guide to parsing numbers with regular expressions if you need to include any numeric representations other than digits (for example, decimal values).

Answered by Bill the Lizard

Solution #2

Because your regex will match anything with a number, you’ll want to use anchors to match the entire string before matching one or more numbers:

regex = new Regex("^[0-9]+$");

The & will anchor the string’s beginning, the $ will anchor the string’s end, and the + will match one or more of the characters before it (a number in this case).

Answered by Robert Gamble

Solution #3

If you can live with a decimal point and a thousand marker,

var regex = new Regex(@"^-?[0-9][0-9,\.]+$");

If the number may go negative, you’ll need a “-.”

Answered by Andrew Chaa

Solution #4

It’s matching because it’s looking for “a match,” not a full string match. You can address this by altering your regexp to look for the beginning and end of the string specifically.

^\d+$

Answered by kasperjj

Solution #5

Perhaps my way will be of assistance to you.

    public static bool IsNumber(string s)
    {
        return s.All(char.IsDigit);
    }

Answered by Raz Megrelidze

Post is based on https://stackoverflow.com/questions/273141/regex-for-numbers-only