C# - RegexOptions.Compiled

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

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

Regex.Match

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.

Version 1 We use a Regex.Match call that does not include the RegexOptions argument, and thus is not compiled.

Version 2 Here we use a compiled regular expression by passing RegexOptions.Compiled as the third argument to Match().

Result Match() returns the same values with both calls. The performance is the only difference.

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.

But Some programs simply run all day and therefore start up is not as important.

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.

Tip For a Regex that is only executed a few times, it is probably not worth compiling it.

And After all, startup time for a program is part of the total execution time.

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

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This page was last updated on Sep 23, 2023 (new example).

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