C# Regex.Match Examples

You want to use the Regex.Match method in the C# programming language. This is to isolate part of a string based on patterns surrounding it. Here we see the Match method and various ways of using it, with sample input and output, using the C# programming language.

Input and output required for examplesInput string: /content/some-page.aspxRequired match: some-pageInput string: /content/alternate-1.aspxRequired match: alternate-1Input string: /images/something.pngRequired match: -

Use Regex.Match

Here we see how you can match the filename in a directory path with Regex. Note that this has more constraints regarding the acceptable characters than many methods do. You can see the char range in the second parameter to Regex.Match.

Program that uses Regex.Match [C#]using System;using System.Text.RegularExpressions;class Program{ static void Main() { // First we see the input string. string input = "/content/alternate-1.aspx"; // Here we call Regex.Match. Match match = Regex.Match(input, @"content/([A-Za-z0-9\-]+)\.aspx$", RegexOptions.IgnoreCase); // Here we check the Match instance. if (match.Success) { // Finally, we get the Group value and display it. string key = match.Groups[1].Value; Console.WriteLine(key); } }}Outputalternate-1

Overview of the example. It uses the verbatim @ string. Pay close attention to the syntax with the @ symbol, which designates the syntax we can use in the pattern. Its pattern starts with "content/". We require that our group, which is in parentheses, is after the "content/" string. The symbols in the [ and ] are ranges of characters, or single characters. These are the allowed characters in our group.

What it captures from the string. It captures a Group. The content in the parentheses, Group, is collected. Then we require that the match succeeds, and then we access the value with Groups[1].

Tip:
It is important to note that the indexing of the Groups collection on Match objects starts at 1. Some computer languages start with 1, but the C# language doesn't usually. It does here, and we must remember this.

Use ToLower instead

Here I found that that by using ToLower instead of RegexOptions.IgnoreCase on the Regex yielded a 10% or higher improvement. Since I needed a lowercase result, calling the C# string ToLower method first was simpler.

Program that also uses Regex.Match [C#]using System;using System.Text.RegularExpressions;class Program{ static void Main() { // This is the input string. string input = "/content/alternate-1.aspx"; // Here we lowercase our input first. input = input.ToLower(); Match match = Regex.Match(input, @"content/([A-Za-z0-9\-]+)\.aspx$"); }}

Static Regex instance

Here we see that using a Regex instance object is faster than using the static Regex.Match. For performance, you should always use an instance object. It can be shared throughout the entire project.

Program that uses static Regex [C#]using System;using System.Text.RegularExpressions;class Program{ static void Main() { // The input string again. string input = "/content/alternate-1.aspx"; // This calls the static method specified. Console.WriteLine(RegexUtil.MatchKey(input)); }}static class RegexUtil{ static Regex _regex = new Regex(@"/content/([a-z0-9\-]+)\.aspx$"); /// <summary> /// This returns the key that is matched within the input. /// </summary> static public string MatchKey(string input) { Match match = _regex.Match(input.ToLower()); if (match.Success) { return match.Groups[1].Value; } else { return null; } }}Outputalternate-1

Explanation. It uses an instance Regex. This static class stores an instance Regex that can be used project-wide. We initialize it inline. The custom method exposes a MatchKey method. This is a useful method I developed to return the string that we want from the input value.

Pattern description. It uses a letter range. In this code I show the Regex with the "A-Z" range removed, because the string is lowercased already. I found that removing as many options from the Regex as possible boosted performance.

Tip:
With this code, I found that using RegexOptions.RightToLeft made the pattern slightly faster as well. The expression engine would have to evaluate fewer characters in this case. This option could slow down or speed up your Regex.

Optimize Regex

You can add the RegexOptions.Compiled flag to your regular expressions for a substantial performance gain at runtime. This will however make your program start up slower. In this example, RegexOptions.Compiled yielded 30% faster performance.

Summary

We saw three examples of the same regular expression in the C# programming language, all of which function similarly and use Regex.Match from the System.Text.RegularExpressions namespace. The final example above was benchmarked as the fastest, although many factors will determine performance with Regex.