执行下面的编码以获得单独的组替换。

new_bib = Regex.Replace(new_bib, @"(?s)(\\bibitem\[[^\]]+\]\{" + pat4 + @"\})[\s\n\v]*([\\\{\}a-zA-Z\.\s\,\;\\\#\\\$\\\%\\\&\*\@\\\!\\\^+\-\\\=\\\~\\\:\\\" + dblqt + @"\\\;\\\`\\\']{20,70})", delegate(Match mts)
                    {
                           var fg = mts.Groups[0].Value.ToString(); 
                           var fs = mts.Groups[1].Value.ToString();
                           var fss = mts.Groups[2].Value.ToString();
                               fss = Regex.Replace(fss, @"[\\\{\}\\\#\\\$\\\%\\\&\*\@\\\!\\\^+\-\\\=\\\~\\\:\\\" + dblqt + @"\\\;\\\`\\\']+", "");
                           return "<augroup>" + fss + "</augroup>" + fs;
                    }, RegexOptions.IgnoreCase);

如果你不想改变你的模式，你可以使用匹配组的Group Index和Length属性。

var text = "example-123-example";
var pattern = @"-(\d+)-";
var regex = new RegEx(pattern);
var match = regex.Match(text);

var firstPart = text.Substring(0,match.Groups[1].Index);    
var secondPart = text.Substring(match.Groups[1].Index + match.Groups[1].Length);
var fullReplace = firstPart + "AA" + secondPart;

这是一个类似于丹尼尔的版本，但替换了多个匹配项:

public static string ReplaceGroup(string input, string pattern, RegexOptions options, string groupName, string replacement)
{
    Match match;
    while ((match = Regex.Match(input, pattern, options)).Success)
    {
        var group = match.Groups[groupName];

        var sb = new StringBuilder();

        // Anything before the match
        if (match.Index > 0)
            sb.Append(input.Substring(0, match.Index));

        // The match itself
        var startIndex = group.Index - match.Index;
        var length = group.Length;
        var original = match.Value;
        var prior = original.Substring(0, startIndex);
        var trailing = original.Substring(startIndex + length);
        sb.Append(prior);
        sb.Append(replacement);
        sb.Append(trailing);

        // Anything after the match
        if (match.Index + match.Length < input.Length)
            sb.Append(input.Substring(match.Index + match.Length));

        input = sb.ToString();
    }

    return input;

这是另一个很好的干净的选择，不需要改变你的模式。

        var text = "example-123-example";
        var pattern = @"-(\d+)-";

        var replaced = Regex.Replace(text, pattern, (_match) =>
        {
            Group group = _match.Groups[1];
            string replace = "AA";
            return String.Format("{0}{1}{2}", _match.Value.Substring(0, group.Index - _match.Index), replace, _match.Value.Substring(group.Index - _match.Index + group.Length));
        });

替换代码:

var text = "example-123-example";
var pattern = @"-(\d+)-";
var replaced = Regex.ReplaceGroupValue(text, pattern, 1, "AA");

扩展类:

public static class RegexExtensions
{
    [Pure]
    public static string ReplaceGroupValue(this Regex source, string input, string groupName, string destinationValue)
    {
        return ReplaceGroupValue(
            source,
            input,
            m => m.Groups[groupName],
            p => destinationValue);
    }

    [Pure]
    public static string ReplaceGroupValue(this Regex source, string input, int groupIdx, string destinationValue)
    {
        return ReplaceGroupValue(
            source,
            input,
            m => m.Groups[groupIdx],
            p => destinationValue);
    }

    [Pure]
    public static string ReplaceGroupValue(this Regex source, string input, string groupName, Func<string, string> destinationValueSelector)
    {
        return ReplaceGroupValue(
            source,
            input,
            m => m.Groups[groupName],
            destinationValueSelector);
    }

    [Pure]
    public static string ReplaceGroupValue(this Regex source, string input, int groupIdx, Func<string, string> destinationValueSelector)
    {
        return ReplaceGroupValue(
            source,
            input,
            m => m.Groups[groupIdx],
            destinationValueSelector);
    }

    [Pure]
    private static string ReplaceGroupValue(
        Regex source,
        string input,
        Func<Match, Group> groupSelector,
        Func<string, string> destinationValueSelector)
    {
        var matchResult = source.Matches(input);

        if (matchResult.Count <= 0)
        {
            return input;
        }

        var text = input;

        foreach (var group in matchResult.OfType<Match>().Select(groupSelector).OrderByDescending(p => p.Index))
        {
            var begin = group.Index > 0 ? text.Substring(0, group.Index) : string.Empty;
            var end = group.Index + group.Length < text.Length
                ? text.Substring(group.Index + group.Length)
                : string.Empty;
            var destinationValue = destinationValueSelector.Invoke(group.Value);
            text = $"{begin}{destinationValue}{end}";
        }

        return text;
    }
}

一个好主意是将所有内容封装在组中，不管是否需要标识它们。这样你就可以在替换字符串中使用它们。例如:

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

或者使用匹配评估器:

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

另一种稍微混乱的方法是使用lookbehind/lookahead:

(?<=-)（\d+）(?=-)