Regex class and RegexOptions.Compiled option

[Callixte]

I have a question about Regex class and wether it is clerver to put that RegexOptions.Compiled option. Msdn says: To improve performance, the regular expression engine caches all regular expressions in memory. This avoids the need to reparse an expression into high-level byte code each time it is used. Does it mean that the Regex object is cached as itself or that another Regex with same pattern and options will use the cached one and will not need to parse the pattern again? If it is the first case, it means that compiling is useful only when the same object is used several times. If it is the latter, the compiled class will be used every time the same pattern is used. Thx. Callixte.

[Curtis Schlak]

Callixte, I don't think that .NET pools Regex objects. It seems that using the RegexOptions.Compiled will only give you significant benefits if you reuse the same object. I wrote the following code to test for reference and object equality, to see if .NET pools the Regex object like it pools the String object in some internal cache. However, the code seems to imply that each object stands alone.

using System;
using System.Text.RegularExpressions;

namespace ThrowAway
{
public class TMain
{
[STAThread]
public static void Main()
{
Regex compiled1 = new Regex( "(a(b)+)+", RegexOptions.Compiled );
Regex compiled2 = new Regex( "(a(b)+)+", RegexOptions.Compiled );
Regex noncompiled1 = new Regex( "(a(b)+)+" );
Regex noncompiled2 = new Regex( "(a(b)+)+" );
Console.WriteLine( compiled1 == compiled2 ); // Writes False
Console.WriteLine( compiled1.Equals( compiled2 ) ); // Writes False
Console.WriteLine( compiled1 == noncompiled1 ); // Writes False
Console.WriteLine( compiled1.Equals( noncompiled1 ) ); // Writes False
Console.WriteLine( noncompiled1 == noncompiled2 ); // Writes False
Console.WriteLine( noncompiled1.Equals( noncompiled2 ) ); // Writes False
}
}
}

Now, this doesn't really prove anything; however, I feel that, if the .NET runtime cached compiled1 and compiled2 to the same internal pool, then at lease compiled1 == compiled2 should evaluate to true. I hope that helps.

[Callixte]

Thanks Curtis, My guess was that compiled1 and compiled2 would be of the same "class" dynamicly inherited of Regex, so CLR doesn't have to parse twice the expression et re-use the same byte code. Callixte.