C#.NET regex not working as expected

Question

Maybe It's because I'm totally fried right now, but this code:

static void Main(string[] args)
    {
        Regex regx = new Regex(@"^.*(vdi([0-9]+\.[0-9]+)\.exe).*$");
        MatchCollection results = regx.Matches("vdi1.0.exe");
        Console.WriteLine(results.Count);

        if (results.Count > 0)
        {
            foreach (Match r in results)
            {
                Console.WriteLine(r.ToString());
            }
        }
    }

ought to produce the output:

2
vdi1.0.exe
1.0

if I'm not crazy. Instead, it's just producing:

1
vdi1.0.exe

What am I missing?

I think you have to use the Groups property to access the subgroups. — Mike Christensen
– Mike Christensen, Commented Jul 16, 2013 at 21:59
As a side point, how would the Regex ever group 1.1? That doesn't event occur in vdi1.0.exe. Did you mean 1.0? — voithos
– voithos, Commented Jul 16, 2013 at 22:01
@MikeChristensen right - I forgot about that. Am I the only one that doesn't find that intuitive at all? — stjns
– stjns, Commented Jul 16, 2013 at 22:04

Mike Christensen · Accepted Answer · 2013-07-16 22:24:09Z

8

Your Regular Expression will only return one Match object with 2 subgroups. You can access those groups using the Groups collection of the Match object.

Try something like:

foreach (Match r in results) // In your case, there will only be 1 match here
{
   foreach(Group group in r.Groups) // Loop through the groups within your match
   {
      Console.WriteLine(group.Value);
   }
}

This allows you to match multiple filenames in a single string, then loop through those matches, and grab each individual group from within the parent match. This makes a bit more sense than returning a single, flattened out array like some languages. Also, I'd consider giving your groups names:

Regex regx = new Regex(@"^.*(?<filename>vdi(?<version>[0-9]+\.[0-9]+)\.exe).*$");

Then, you can refer to groups by name:

string file = r.Groups["filename"].Value;
string ver = r.Groups["version"].Value;

This makes the code a bit more readable, and allows group offsets to change without breaking things.

Also, if you're always parsing only a single filename, there's no reason to loop through a MatchCollection at all. You can change:

MatchCollection results = regx.Matches("vdi1.0.exe");

To:

Match result = regx.Match("vdi1.0.exe");

To obtain a single Match object, and access each Group by name or index.

edited Jul 16, 2013 at 22:24

answered Jul 16, 2013 at 22:03

Mike Christensen

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1 Comment

stjns Over a year ago

Thanks Mike - I didn't even know about naming groups. I'm doing it just like that now.

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