不需要垃圾回收器与拆分/合并字符串。替换工作良好，没有内存峰值，快速，接受函数作为参数了：

public class Program
{
    public static void Main()
    {
        var pattern = @"(?<num>\{((?<n1>\d+)?NUM|NUM(?<n2>\d+))\})";
        var regx = new Regex(pattern, RegexOptions.ExplicitCapture | RegexOptions.Multiline);
        var input =
            @"The quick brown fox {NUM} jumped over {211NUM} the lazy dog {NUM}.  The Quick brown fox {NUM10} jumped over the lazy dog. ";

        var output = regx.Replace(input, ToReplaceMatch);
        Console.WriteLine(output);
    }

    private static string ToReplaceMatch(Match match)
    {
        if (match.Groups["num"].Success)
        {
            var num = int.TryParse(match.Groups["n1"].Value ?? match.Groups["n2"].Value, out var res)
                ? res
                : (int?)null;
            return ToReplaceNum(num);
        }

        return match.Value;
    }

    private static string ToReplaceNum(int? number)
    {
        return "FOOBAR" + (number ?? 0);
    }
}

输出：

The quick brown fox FOOBAR0 jumped over FOOBAR211 the lazy dog FOOBAR0.  The Quick brown fox FOOBAR0 jumped over the lazy dog.

您可以尝试使用以下正则表达式模式只匹配您的关键字：(?<g1>{NUM})|(?<g2>{\d*NUM})|(?<g3>{NUM\d*})
然后使用Regex.Replace函数根据您的逻辑替换关键字。

string pattern = @"(?<g1>{NUM})|(?<g2>{\d*NUM})|(?<g3>{NUM\d*})";
string input = "The quick brown fox {NUM} jumped over {211NUM} the lazy dog {NUM}.  The Quick brown fox {NUM10} jumped over the lazy dog. ";
output = Regex.Replace(input, pattern, delegate(Match match)
{
    if (match.Groups["g1"].Success)
    {
       return "{Group 1}";
    }
    if (match.Groups["g2"].Success)
    {
       return "{Group 2}";
    }
    if (match.Groups["g3"].Success)
    {
       return "{Group 3}";
    }
    return match.Value;
});

参见here演示