Issue with my regular expression?

Question

I'm trying to locate the number of matches in a relative path for directory up references ("..\"). So I have the following pattern : "(\.\.\\)", which works as expected for the path "..\..\a\b" where it will give me 2 successful groups ("..\"), but when I try the path "..\a\b" it will also return 2 when it should return 1. I tried this in a reg ex tool such as Expresso and it seems to work as expected in there but not in .net, any ideas?

Answer 1

Try this instead:

(\.\.\\)

The dots (.) were matching any character, not the literal value. To match the literal value you must escape them with a leading backslash.

Answer 2

I'm getting the correct answer, try the following:

Console.WriteLine(Regex.Matches(@"..\..\a\b", @"(\.\.\\)").Count); //2
Console.WriteLine(Regex.Matches(@"..\a\b", @"(\.\.\\)").Count); //1

Did you escape or use literal strings for the \ in .NET?

Answer 3

Since Expresso runs on .net, your statement "I tried this in a reg ex tool such as Expresso and it seems to work as expected in there but not in .net" doesn't seem to make a lot of sense.
What this suggests me is that it's not the regular expression which is the problem, but your usage of it.
Have a close look at the Regex method you're using to collect results and how you process those results, that might be where the problem lies.

Hope this helps!

Answer 4

Did you escape the backslahes to escape the dots in your Regex? I.e. "\\.\\.\\\\" or @"\.\.\\"?

You could always not use Regex for this task and use

Int32 count = url.Split(new string[] { "\\" }, StringSplitOptions.RemoveEmptyEntries)
                 .Where(s => s == "..")
                 .Count();

instead. =)