Why RegexOptions are compiled to RegexOptions.None in MSIL?

Question

This code

  Regex regex = new Regex("blah", RegexOptions.Singleline & RegexOptions.IgnoreCase);

after compilation looks like this in ILSpy:

  Regex regex = new Regex("blah", RegexOptions.None);

Why does it happen and can it be the reason of regex not matching in .Net 3.5? On 4.5 it works.

Answer 1

RegexOptions.Singleline & RegexOptions.IgnoreCase

is a bitwise AND, and resolves to 0 (i.e. RegexOptions.None).

The RegexOptions enum looks like this:

[Flags]
public enum RegexOptions
{
    None = 0,
    IgnoreCase = 1,
    Multiline = 2,
    ExplicitCapture = 4,
    Compiled = 8,
    Singleline = 16,
    IgnorePatternWhitespace = 32,
    RightToLeft = 64,
    ECMAScript = 256,
    CultureInvariant = 512,
}

So, in binary, we have:

RegexOptions.SingleLine == 10000 
RegexOptions.IngoreCase == 00001

When applying a bitwise AND, we get :

    10000 
AND 00001
    -----
    00000

Replace with

RegexOptions.Singleline | RegexOptions.IgnoreCase

which gives:

    10000 
 OR 00001
    -----
    10001

That ILSpy will decompile in:

Regex regex = new Regex("blah", RegexOptions.Singleline | RegexOptions.IgnoreCase);

But I don't know what "works" in .Net 4.5. I just compiled your code, and ILSpy also outputs:

Regex regex = new Regex("blah", RegexOptions.None);

as intended.