C# Regex for a match outside a specific region

You might try splitting the string on occurrences of BEGIN or END so that you can insure that there is only one BEGIN and one END in the string that you apply your regex to. Also, if you are looking for occurrences of SOME that are outside your BEGIN/END braces then I think you'd want to look behind for END and lookahead for BEGIN (positive lookahead/behind), the opposite of what you have.

Hope this helps.

What if you just process the entire haystack and ignore the hay that is in between the braces (am I pushing the metaphor too far?)

For example, look through all the tokens (or characters, if you need to go to that level) and look for your braces. When the opening one is found, you loop through until you find the closing brace. At that point, you start looking for your needles until you find another opening brace. It's a bit more code than a Regex, but might be more readible and easier to troubleshoot.

Would something like this work for you:

(?:^|END)((?!BEGIN).*?)(some)(.*?)(?:BEGIN|$)

This appears to match your text, as I tested using RegExDesigner.NET.

One simple option is to skip the parts you don't want to match, and capture only the needles you need:

MatchCollection matches = Regex.Matches(input, "BEGIN.*?END|(?<Needle>some)");

You'll get the two "some"s you're after by taking the successful "Needle" groups out of all matches:

IEnumerable<Group> needles = matches.Cast<Match>()
                                    .Select(m => m.Groups["Needle"])
                                    .Where(g => g.Success);