Replace only some groups with Regex

Question

Let's suppose I have the following regex:

-(\d+)-

and I want to replace, using C#, the Group 1 (\d+) with AA, to obtain:

-AA-

Now I'm replacing it using:

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

But I don't really like this, because if I change the pattern to match _(\d+)_ instead, I will have to change the replacement string by _AA_ too, and this is against the DRY principle.

I'm looking for something like:

Keep the matched text exactly how it is, but change Group 1 by this text and Group 2 by another text...

Edit:
That was just an example. I'm just looking for a generic way of doing what I said above.

It should work for:

anything(\d+)more_text and any pattern you can imagine.

All I want to do is replace only groups, and keep the rest of the match.

Answer 1

You can do this using lookahead and lookbehind:

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

Answer 2

I also had need for this and I created the following extension method for it:

public static class RegexExtensions
{
    public static string ReplaceGroup(
        this Regex regex, string input, string groupName, string replacement)
    {
        return regex.Replace(
            input,
            m =>
            {
                var group = m.Groups[groupName];
                var sb = new StringBuilder();
                var previousCaptureEnd = 0;
                foreach (var capture in group.Captures.Cast<Capture>())
                {
                    var currentCaptureEnd =
                        capture.Index + capture.Length - m.Index;
                    var currentCaptureLength =
                        capture.Index - m.Index - previousCaptureEnd;
                    sb.Append(
                        m.Value.Substring(
                            previousCaptureEnd, currentCaptureLength));
                    sb.Append(replacement);
                    previousCaptureEnd = currentCaptureEnd;
                }
                sb.Append(m.Value.Substring(previousCaptureEnd));

                return sb.ToString();
            });
    }
}

Usage:

var input = @"[assembly: AssemblyFileVersion(""2.0.3.0"")][assembly: AssemblyFileVersion(""2.0.3.0"")]";
var regex = new Regex(@"AssemblyFileVersion\(""(?<version>(\d+\.?){4})""\)");


var result = regex.ReplaceGroup(input , "version", "1.2.3");

Result:

[assembly: AssemblyFileVersion("1.2.3")][assembly: AssemblyFileVersion("1.2.3")]

Answer 3

If you don't want to change your pattern you can use the Group Index and Length properties of a matched group.

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;

Answer 4

Here is another nice clean option that does not require changing your pattern.

        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));
        });

Answer 5

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);

---

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

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

Answer 6

go through the below coding to get the separate group replacement.

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);