Extract GUID from line in C#

Question

I try to simplify some legacy code using IndexOf to retrieve a GUID from lines. Can I further simplify the code below to get rid of using guids.Any and guids.First?

// Code using regular expression
private static string RetrieveGUID2(string[] lines)
{
    string guid = null;
    foreach (var line in lines)
    {
        var guids = Regex.Matches(line, @"[{(]?[0-9A-F]{8}[-]?([0-9A-F]{4}[-]?){3}[0-9A-F]{12}[)}]?")
            .Cast<Match>().Select(m => m.Value);
        if (guids.Any())
        {
            guid = guids.First();
            break;
        }
    }
    return guid;
}

Below the legacy code given in a sample that compiles:

using System;
using System.Linq;
using System.Text.RegularExpressions;

namespace ConsoleApplication2
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var lines = new[]
            {
                "</ItemGroup>\n",
                "<PropertyGroup\n",
                "Label = \"Globals\">\n",
                "<ProjectGuid>{A68615F1-E672-4B3F-B5E3-607D9C18D1AB}</ProjectGuid>\n",
                "</PropertyGroup>\n"
            };

            Console.WriteLine(RetrieveGUID(lines));
            Console.WriteLine(RetrieveGUID2(lines));
        }

        // Legacy code
        private static string RetrieveGUID(string[] lines)
        {
            string guid = null;
            foreach (var line in lines)
            {
                var startIdx = line.IndexOf("<ProjectGuid>{", StringComparison.Ordinal);
                if (startIdx < 0) continue;
                var endIdx = line.IndexOf("</ProjectGuid>", StringComparison.Ordinal);
                if (endIdx < 0) continue;
                startIdx += "<ProjectGuid>".Length;
                var guidLen = endIdx - startIdx;
                guid = line.Substring(startIdx, guidLen);
                break;
            }
            return guid;
        }

        // Code using regular expression
        private static string RetrieveGUID2(string[] lines)
        {
            string guid = null;
            foreach (var line in lines)
            {
                var guids = Regex.Matches(line, @"[{(]?[0-9A-F]{8}[-]?([0-9A-F]{4}[-]?){3}[0-9A-F]{12}[)}]?")
                    .Cast<Match>().Select(m => m.Value);
                if (guids.Any())
                {
                    guid = guids.First();
                    break;
                }
            }
            return guid;
        }
    }
}

[`Guid.TryParse()`](https://msdn.microsoft.com/en-us/library/system.guid.tryparse(v=vs.110).aspx) would be much better than trying to use your own regex to parse the GUID — Liam, May 30 '17 at 14:25
Possible duplicate of [C#: Test if string is a guid without throwing exceptions?](https://stackoverflow.com/questions/104850/c-test-if-string-is-a-guid-without-throwing-exceptions) — Liam, May 30 '17 at 14:26
CTRL+A, CTRL+C, Edit\Paste Special\Paste XML as Classes, Deserialize Xml, do your stuff in civilized manner. Or, if you are not civilized enough, you can use XPath's as well. — PTwr, May 30 '17 at 14:27

score 11 · Accepted Answer · answered May 30 '17 at 14:32

Yes, you can. Cause you return only the first match of regex, you can use Regex.Match instead of Regex.Matches.

private static string RetrieveGUID2(string[] lines)
{
    foreach (var line in lines)
    {
        var match = Regex.Match(line, @"[{(]?[0-9A-F]{8}[-]?([0-9A-F]{4}[-]?){3}[0-9A-F]{12}[)}]?");
        if (match.Success)
          return match.Value;
    }

    return null;
}

score 1 · Answer 2 · answered May 30 '17 at 14:30

use a foreach loop which breaks out on first iteration - this is actually how First and FirstOrDefault are implemented

foreach(var nextGuid in guids) { guid = nextGuid; break; }

Simplifying it further, you could use FirstOrDefault, which will not throw an error if there are no objects

return Regex
    .Matches(line, @"[{(]?[0-9A-F]{8}[-]?([0-9A-F]{4}[-]?){3}[0-9A-F]{12}[)}]?")
    .Cast<Match>()
    .Select(m => m.Value)
    .FirstOrDefault();

score 1 · Answer 3 · answered May 30 '17 at 14:49

Just to offer an alternative using Guid.TryParse():

public static Guid? RetrieveGuid(IEnumerable<string> lines)
{
    Guid? parseGuid(string text) => Guid.TryParse(text, out Guid guid) ? (Guid?) guid : null;
    return lines.Select(parseGuid).FirstOrDefault(guid => guid != null);
}

Or equivalently:

public static Guid? RetrieveGuid(IEnumerable<string> lines)
{
    return lines.Select(line => Guid.TryParse(line, out Guid guid) ? (Guid?)guid : null)
        .FirstOrDefault(guid => guid != null);
}

This returns a Guid? rather than a string, and a result of null means that no valid Guid was parsed.

Extract GUID from line in C#

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