How to extract a substring from a .NET RegEx?

Question

I have an XML file containing one (or more) key/value pairs. For each of these pairs I want to extract the value which is a two-byte hex value.

So the XML contains this snippet:

<key>LibID</key><val>A67A</val>

Which I can match using the following expression, with the ID in parenthesis.

Match match = Regex.Match(content, @"<key>LibID</key><val>([a-fA-F0-9]{4})</val>");

if (match.Success)
{
  Console.WriteLine("Found Match for {0}\n", match.Value);
  Console.WriteLine("ID was {0}\n", "Help me SO!");
}

How can I change the last part so it returns the ID from the match?

Jon Skeet · Accepted Answer · 2009-04-10 19:08:16Z

81

I think you want

match.Groups[1].Value

(As Dillie-O points out in the comments, it's group 1 because group 0 is always the whole match.)

Short but complete test program:

using System;
using System.Text.RegularExpressions;

class Program
{
  static void Main()
  {
    Regex regex = new Regex("<key>LibID</key><val>([a-fA-F0-9]{4})</val>");
    Match match = regex.Match("Before<key>LibID</key><val>A67A</val>After");

    if (match.Success)
    {
      Console.WriteLine("Found Match for {0}", match.Value);
      Console.WriteLine("ID was {0}", match.Groups[1].Value);
    }      
  }
}

Output:

Found Match for <key>LibID</key><val>A67A</val>
ID was A67A

answered Apr 10, 2009 at 19:08

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Dillie-O Over a year ago

Beat me to it. You're already using a grouped match there Andrew, so you can easily extract the grouped value. Oh, and for the record, index 0 contains the entire string, that's why you check index 1.

Andrew Grant Over a year ago

Super thanks. I knew it was in there somewhere, I just didn't know where :)

CularBytes Over a year ago

Hmm not sure why but my Match Groups contains only 1 value at index 0 which is the one I need. Anyway thanks

Jon Skeet Over a year ago

@CularBytes: Without knowing what pattern you're using or what you're matching against, there's no way we can explain the behavior to you. If you're interested in finding out more, please post a new question with a minimal reproducible example.

JP Alioto · Accepted Answer · 2009-04-10 19:12:42Z

3

Add a grouping construct to your expression ...

<key>(?<id>LibID)</key><val>([a-fA-F0-9]{4})</val>

That will capture the ID. But, you need to put the correct format in your expression for the actual ID, because your regex will only capture "LibID" litterally.

answered Apr 10, 2009 at 19:12

JP Alioto

45.2k6 gold badges91 silver badges112 bronze badges

1 Comment

Alan Moore Over a year ago

I think by "ID" Andrew means the contents of the <val> element, not the <key> element.

Collectives™ on Stack Overflow

How to extract a substring from a .NET RegEx?

2 Answers 2

4 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related