Is there anyway to have C# Regex.Matches return all the valid entries?

Question

I need to find all the occurrences of _X_ in a string (where X is any upper case English letter, that is A-Z

Consider this string: "_A_B_C_D_"

The occurrences I am looking for are _A_ _B_ _C_ _D_

Using C# Regex.Matches:

var input = "_A_B_C_D_";
var pattern = "_[A-Z]_";
var matches = Regex.Matches(input, pattern);

The result is _A_ _C_

so, obviously my pattern is wrong.

What is the correct pattern?

Update: Thanks folks. This works. (?=(_[A-Z]_))