CODEWITHSUNDEEP
c#.netregex
Abhishek Kumar
Abhishek Kumar

Reputation: 352

String format Check gives unexpected result (Regex)

I want to check format of a string which is ABC-nnn, where ABC represents alphabets (English) in capital letters. nnn represents triple digit number for example 123 or 001 or 012 A complete example would be FBI-026. I used regex for that and below is my code. 

public bool IsSubstringMatchingFormat(int numberOfLettersBeforeHyphen, int numberOfNumbersAfterHyphen, string stringToMatch)
{
    Regex regex = new Regex($@"^[A-Z]{numberOfLettersBeforeHyphen}-\d{numberOfNumbersAfterHyphen}");
    return regex.IsMatch(stringToMatch);
}

I call it IsSubstringMatchingFormat(3, 3, "SMB-123") but it returns false. Please provide your insight.

Upvotes: 1

Views: 53

Answers (3)

Chris
Chris

Reputation: 27599

Have you actually checked what the string you are passing into the regex looks like? ie evaluate $@"^[A-Z]{numberOfLettersBeforeHyphen}-\d{numberOfNumbersAfterHyphen}"and see if that is the regex you want? I can tell you that it isn't because it will end up being ^[A-Z]3-\d3 which does not do what you want.

What I think you'll want is:

$@"^[A-Z]{{{numberOfLettersBeforeHyphen}}}-\d{{{numberOfNumbersAfterHyphen}}}"

This adds the escaped curly braces back into your regex to give:

^[A-Z]{3}-\d{3}

The equivalent of this using String.Format would be:

String.Format(
    @"^[A-Z]{{{0}}}-\d{{{1}}}",
    numberOfLettersBeforeHyphen, 
    numberOfLettersAfterHyphen);

Upvotes: 4

Kell
Kell

Reputation: 3317

The {} are being removed when you format. Try this:

        public static bool IsSubstringMatchingFormat(int numberOfLettersBeforeHyphen, int numberOfNumbersAfterHyphen, string stringToMatch)
    {
        var expre = @"^[A-Z]{numberOfLettersBeforeHyphen}-\d{numberOfNumbersAfterHyphen}";//
        expre = expre.Replace("numberOfLettersBeforeHyphen", numberOfLettersBeforeHyphen.ToString())
            .Replace("numberOfNumbersAfterHyphen", numberOfNumbersAfterHyphen.ToString());
        Regex regex = new Regex(expre);
        return regex.IsMatch(stringToMatch);
    }

Upvotes: 0

Pedro von Hertwig Batista
Pedro von Hertwig Batista

Reputation: 3002

Your curly braces are being consumed by the string interpolation and are not making it into the regex. If you try to print the regex, you'll see it's something like

^[A-Z]3-\d3

Which is something else entirely.

Upvotes: 0

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