Mastering Regular Expression. Part 7: Full C# Regex Application — Log File Analyzer

What We’ll Build:

• A C# console app that parses .log files using regular expressions.
• Extracts:
• Timestamps
• Error and warning levels
• Stack traces
• Custom events
• Outputs:
• Summary report to the console or file
• Count of each log level
• First and last timestamps
• Pattern search (optional)

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7.1 Log Format Assumption

We’ll assume a log format like:

[2025-08-04 12:01:43] [INFO] Application started.
[2025-08-04 12:01:45] [WARN] Low disk space.
[2025-08-04 12:01:47] [ERROR] System.NullReferenceException: Object reference not set to an instance of an object.
   at MyApp.Program.Main()
[2025-08-04 12:01:49] [INFO] Application ended.

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7.2 Project Structure

/LogAnalyzer

  └── Program.cs

  └── LogParser.cs

  └── LogEntry.cs

  └── Sample.log

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7.3 LogEntry.cs

public class LogEntry {
    public DateTime Timestamp { get; set; }
    public string Level { get; set; }
    public string Message { get; set; }
    public List<string> StackTrace { get; set; } = new();

    public override string ToString() {
        return $"[{Timestamp:yyyy-MM-dd HH:mm:ss}] [{Level}] {Message}" +
               (StackTrace.Any() ? "\n" + string.Join("\n", StackTrace) : "");
    }
}

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7.4 LogParser.cs

using System.Text.RegularExpressions;

public class LogParser {
    private static readonly Regex EntryPattern = new Regex(@"
        ^\[(?<timestamp>[\d\-:\s]+)\]\s
        \[(?<level>[A-Z]+)\]\s
        (?<message>.+)
    ", RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);

    public static List<LogEntry> ParseLog(string[] lines) {
        var entries = new List<LogEntry>();
        LogEntry current = null;

        foreach (string line in lines) {
            Match m = EntryPattern.Match(line);

            if (m.Success) {
                current = new LogEntry {
                    Timestamp = DateTime.Parse(m.Groups["timestamp"].Value),
                    Level = m.Groups["level"].Value,
                    Message = m.Groups["message"].Value
                };
                entries.Add(current);
            }
            else if (line.StartsWith("   at") && current != null) {
                current.StackTrace.Add(line.Trim());
            }
        }

        return entries;
    }
}

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7.5 Program.cs

class Program {
    static void Main(string[] args) {
        string logPath = "Sample.log";

        if (!File.Exists(logPath)) {
            Console.WriteLine($"Log file not found: {logPath}");
            return;
        }

        var lines = File.ReadAllLines(logPath);
        var entries = LogParser.ParseLog(lines);

        // Basic Stats
        var grouped = entries.GroupBy(e => e.Level);
        foreach (var group in grouped)
            Console.WriteLine($"{group.Key}: {group.Count()} entries");

        Console.WriteLine("\nFirst log: " + entries.First().Timestamp);
        Console.WriteLine("Last log: " + entries.Last().Timestamp);

        // Show all errors
        Console.WriteLine("\n--- ERROR Logs ---");
        foreach (var entry in entries.Where(e => e.Level == "ERROR"))
            Console.WriteLine(entry);
    }
}

---

7.6 Sample.log

[2025-08-04 12:01:43] [INFO] Application started.
[2025-08-04 12:01:45] [WARN] Low disk space.
[2025-08-04 12:01:47] [ERROR] System.NullReferenceException: Object reference not set to an instance of an object.
   at MyApp.Program.Main()
[2025-08-04 12:01:49] [INFO] Application ended.

---

7.7 Output Example

INFO: 2 entries
WARN: 1 entries
ERROR: 1 entries

First log: 8/4/2025 12:01:43 PM
Last log: 8/4/2025 12:01:49 PM

--- ERROR Logs ---
[2025-08-04 12:01:47] [ERROR] System.NullReferenceException: Object reference not set...
at MyApp.Program.Main(

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7.8 Optional Features

🧵 Filter by date range

var recent = entries.Where(e => e.Timestamp >= DateTime.Today.AddHours(-1));

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🔍 Pattern Search in Messages

string keyword = "disk";
var matches = entries.Where(e => e.Message.Contains(keyword, StringComparison.OrdinalIgnoreCase));

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📤 Export Report to File

File.WriteAllLines("report.txt", entries.Select(e => e.ToString()));

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🧰 JSON Report (with System.Text.Json)

string json = JsonSerializer.Serialize(entries, new JsonSerializerOptions { WriteIndented = true });
File.WriteAllText("report.json", json);

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Summary

In this part, you:

✅ Built a real-world log analyzer in C# using Regex
✅ Parsed structured logs and extracted stack traces
✅ Counted levels, printed summaries, and filtered messages
✅ Learned modular and reusable design

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Coming Up Next

In Part 8, we’ll explore regex testing, debugging, and tool integrations, including:

• Visual Studio’s built-in regex testing tools
• Regex101 and RegexStorm for .NET
• Writing unit tests for regex
• Debugging complex patterns with explanation tools
• Live regex validation in C# apps