Replace only some groups with Regex

A good idea could be to encapsulate everything inside groups, no matter if need to identify them or not. That way you can use them in your replacement string. For example:

var pattern = @"(-)(\d+)(-)";
var replaced = Regex.Replace(text, pattern, "$1AA$3"); 

or using a MatchEvaluator:

var replaced = Regex.Replace(text, pattern, m => m.Groups[1].Value + "AA" + m.Groups[3].Value);

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Another way, slightly messy, could be using a lookbehind/lookahead:

(?<=-)(\d+)(?=-)

You can do this using lookahead and lookbehind:

var pattern = @"(?<=-)\d+(?=-)";
var replaced = Regex.Replace(text, pattern, "AA");