What is this Regex trying to do?

Question

Im working on an assignment for class and it includes using some checks for input to ensure valid input is given, using a regex expression. The example given for the IsValidNumber check is string regex = @"^-?\\d*(\\.\\d+)?$";

I'm not really sure if this is actually checking for valid numbers because i've never seen that before. Wouldn't something like string regex = @"^[0-9]\d*$ work or am i missing something? Note I do need to be able to pass through a float and not just an int.

UPDATE: Using the regex I was provided, i ran the program only to run into an issue with the error check that didn't allow anything to pass through even if valid

Here is the method that is calling IsNumberValid

private static string GetNumericInput(string c)
    {
        do
        {
            Console.Write(c);
            string var = Console.ReadLine();
            if (!String.IsNullOrEmpty(var) && IsValidNumber(var))
            {
                return var;
            }
            else
            {
                Console.WriteLine("Input was either Null/Empty, or contained non-numeric characters.");
                Console.WriteLine("Retry input.");
                Console.WriteLine();
            }
        }
        while (true);
    }

and here is IsValidNumber

public static bool IsValidNumber(string value)
        {
            string regex = @"^-?\\d*(\\.\\d+)?$";

            return Regex.IsMatch(value, regex);
        }

I'm not entirely sure how to read breakpoints for errors but when i put breakpoints at string regex = @"^-?\\d*(\\.\\d+)?$"; and return Regex.IsMatch(value, regex); it shows value for regex as null and "^-?\\d*(\\.\\d+)?$" respectively.

Like most regexes it is really trying to murder you in your sleep. — Wyatt Barnett, Feb 04 '15 at 19:51
**Tip:** Paste it on [regexr.com](http://www.regexr.com/) and hover over each component to see what it does. — Christian, Feb 04 '15 at 19:52

M A · Accepted Answer · 2015-02-04T21:17:59.827

4

The regex is taking into consideration the decimal point, i.e. it matches not just integers but also floating point numbers. This is what the \\. catches. Also the ^-? matches a possible negative sign. ^[0-9]\d* only matches positive integers.

The statement string regex = @"^-?\\d*(\\.\\d+)?$"; creates a regex pattern that's literally ^-?\\d*(\\.\\d+)?$ because of the @ at the front of the string. You don't need to use double backslashes in this case, simply:

string regex = @"^-?\d*(\.\d+)?$";

edited Feb 04 '15 at 21:17

answered Feb 04 '15 at 19:50

M A

71,713
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174

2

The `^-?` also accounts for the number starting with a dash, which catches negative numbers – Kyeotic Feb 04 '15 at 19:50
OK thanks for the answer. I went ahead and used that but I'm running into an error, telling me that numbers which should be valid are returned as invalid. Any idea what may cause this? – Nick B Feb 04 '15 at 20:50
@NickB Could you update the question? Or post another one? – M A Feb 04 '15 at 20:51
@manouti I just updated the question with more details on whats going on – Nick B Feb 04 '15 at 21:09
That first `*` should be a `+` unless you consider `-` or empty string a valid number – pguardiario Feb 05 '15 at 02:40

score 2 · Answer 2 · answered Feb 04 '15 at 19:50

2

In addition to @manouti answer of this regex takes in consideration floating point numbers as well, it also takes in account for negative numbers

answered Feb 04 '15 at 19:50

austin wernli

1,801
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What is this Regex trying to do?

2 Answers2