In the simplified example, there are 2 Regular Expressions, one case sensitive, the other not. The idea would be to efficiently create an IEnumerable collection (see "combined" below) combining the results.

string test = "abcABC";
string regex = "(?<grpa>a)|(?<grpb>b)|(?<grpc>c)]";
Regex regNoCase = new Regex(regex, RegexOptions.IgnoreCase);
Regex regCase = new Regex(regex);

MatchCollection matchNoCase = regNoCase.Matches(test);
MatchCollection matchCase = regCase.Matches(test);

//Combine matchNoCase and matchCase into an IEnumerable
IEnumerable<Match> combined= null;
foreach (Match match in combined)
{
    //Use the Index and (successful) Groups properties 
    //of the match in another operation

}

In practice, the MatchCollections might contain thousands of results and be run frequently using long dynamically created REGEXes, so I'd like to shy away from copying the results to arrays, etc. I am still learning LINQ and am fuzzy on how to go about combining these or what the performance hits to an already sluggish process will be.