RegexOptions.Compiled

This option often improves performance with C# Regex code. With it, regular expressions are executed faster.

There are some tradeoffs—with Compiled Regex, startup time will increase. And benefits are insignificant in many programs.

Example

We can use a compiled regular expression in C# by passing RegexOptions.Compiled to methods like Match. This will enhance performance if the Match method is called many times.

using System;
using System.Text.RegularExpressions;

// Use compiled regex to match word starting with c.
var value = "bird cat";
Match match = Regex.Match(value, @"c.+", RegexOptions.Compiled);
Console.WriteLine(match.Value);
cat

Benchmark

Here we test the performance change with RegexOptions.Compiled. By passing RegexOptions.Compiled to Regex.Match, we improve performance significantly in this case.

using System;
using System.Diagnostics;
using System.Text.RegularExpressions;

const int _max = 1000000;
string value = "dot net 777 perls";
// Version 1: not compiled.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    Match match = Regex.Match(value, @"\d+");
}
s1.Stop();
// Version 2: compiled regular expression.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    Match match = Regex.Match(value, @"\d+", RegexOptions.Compiled);
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
232.98 ns    Not compiled
153.67 ns    Compiled

Discussion

When should we use RegexOptions.Compiled? If your program starts up too slowly, using RegexOptions.Compiled will make that problem worse.

Note, startup time

If start up and execution time are both important, you have to make a tradeoff. Consider only using Compiled on Regexes that are run many times.

We saw the performance gain for a simple Regex method call with RegexOptions.Compiled. Further we considered the issue of startup time.