ParameterRegex.cs

const string placeholderPattern = @"
    \[
        \[
            ([^\]]+)  # Description of whatever is in here
        \]

        \[
            ([^\]]+)  # Description of whatever is in here
        \]
    \]";

string input = "[[key][value]]";

MatchCollection matches = Regex.Matches(input,
    placeholderPattern, RegexOptions.IgnorePatternWhitespace);

PerformantParameterRegexPattern.cs

const string placeholderPattern = @"
    \[
        \[
            ((?>
                [^\]]+  # Description of whatever is in here
            ))
        \]

        \[
            ((?>
                [^\]]+  # Description of whatever is in here
            ))
        \]
    \]";

string input = "[[key][value]]";

MatchCollection matches = Regex.Matches(input,
    placeholderPattern, RegexOptions.IgnorePatternWhitespace);