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A Quick Tip About Using RegexOptions.Compiled

Since Silverlight doesn't support RegexOptions.Compiled, it's the one option that's missing from Regex Hero.

In a full .NET application there may be times when it's worth it to use this option.  It adds significant cost to initializing the Regex instance, but it can also double the speed of the regular expression itself in some cases.  Therefore it's best to use when you can instantiate the Regex object once, and reuse it many times.  Jeff Atwood talks about this very point in his article, To Compile or Not to Compile.

So if you've determined in your case that RegexOptions.Compiled is worth the initial cost of compilation, then I'm going to offer one simple solution that we as programmers often forget about.

Lazy Loading
The advantage with lazy loading is that the object we're concerned about is only instantiated when it's first accessed.  Therefore, we can use RegexOptions.Compiled without necessarily hurting the initial start-up time of our application. In some cases we can even avoid needlessly instantiating objects...
private static Regex _LibraryTagsRegex = null;
public static Regex LibraryTagsRegex
{
 get
 {
  if (_LibraryTagsRegex == null)
  {
   _LibraryTagsRegex = new Regex(@"^library/tagged/(?<Tags>.+)", RegexOptions.Compiled);
  }
  return _LibraryTagsRegex;
 }
}


We can then call the static LibraryTagsRegex in the example above from anywhere in our code, and it's only going to be instantiated the first time it's accessed.

By the way, in my test I was able to perform 2 million LibraryTagsRegex.IsMatch() calls in 256 ms with the Compiled option (as above) vs 461 ms without it. The compilation time for a Regex this simple is very small and this scenario completely justifies the use of RegexOptions.Compiled.

You can also pull this off with the Lazy type new to C# 4.0...
public static Lazy<Regex> LibraryTagsRegex = 
    new Lazy<Regex>(() => new Regex(@"^library/tagged/(?<Tags>.+)", RegexOptions.Compiled));
Then you'd just reference LibraryTagsRegex.Value in your code.
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