CODEWITHSUNDEEP
c#regex
Philethius
Philethius

Reputation: 87

Detecting "\" (backslash) using Regex

I have a C# Regex like

[\"\'\\/]+

that I want to use to evaluate and return error if certain special characters are found in a string.

My test string is:

\test

I have a call to this method to validate the string:

public static bool validateComments(string input, out string errorString)
{
    errorString = null;
    bool result;

    result = !Regex.IsMatch(input, "[\"\'\\/]+");  // result is true if no match
                                                   // return an error if match

    if (result == false)
        errorString = "Comments cannot contain quotes (double or single) or slashes.";

    return result;
}

However, I am unable to match the backslash. I have tried several tools such as regexpal and a VS2012 extension that both seem to match this regex just fine, but the C# code itself won't. I do realize that C# is escaping the string as it is coming in from a Javascript Ajax call, so is there another way to match this string?

It does match /test or 'test or "test, just not \test

Upvotes: 3

Views: 15952

Answers (4)

Ibrahim Najjar
Ibrahim Najjar

Reputation: 19423

You can solve this by making the string verbatim like this @:

result = !Regex.IsMatch(input, @"[\""\'\\/]+");

Upvotes: 3

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726539

Since backslashes are used as escapes inside regex themselves, I find it best to use verbatim strings when working with the regex library:

string input = @"\test";
bool result = !Regex.IsMatch(input, @"[""'\\]+");
//                                     ^^
// You need to double the double-quotes when working with verbatim strings;
// All other characters, including backslashes, remain unchanged.
if (!result) {
    Console.WriteLine("Comments cannot contain quotes (double or single) or slashes.");
}

The only issue with that is that you must double your double-quotes (which is ironically what you need to do in your case).

Demo on ideone.

Upvotes: 2

xanatos
xanatos

Reputation: 111850

The \ is used even by Regex(es). Try "[\"\'\\\\/]+" (so double escape the \)

Note that you could have @"[""'\\/]+" and perhaps it would be more readable :-) (by using the @ the only character you have to escape is the ", by the use of a second "")

You don't really need the +, because in the end [...] means "one of", and it's enough for you.

Don't eat what you can't chew... Instead of regexes use 

// result is true if no match
result = input.IndexOfAny(new[] { '"', '\'', '\\', '/' }) == -1;

I don't think anyone ever lost the work because he preferred IndexOf instead of a regex :-)

Upvotes: 8

David W
David W

Reputation: 10184

For the trivial case, I am able to use regexhero.net for your test expression using the simple:

\\

to validate

\test

The code generated by RegExHero:

string strRegex = @"\\";

RegexOptions myRegexOptions = RegexOptions.IgnoreCase;
Regex myRegex = new Regex(strRegex, myRegexOptions);
string strTargetString = @"\test";
foreach (Match myMatch in myRegex.Matches(strTargetString))
{
  if (myMatch.Success)
  {
    // Add your code here
  }
}

Upvotes: 0

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