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

User · Answer

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

User · Answer

You can do this using lookahead and lookbehind   var pattern        lt  -  d    -    var replaced   Regex Replace text  pattern   AA

User · Answer

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   gt  m Groups 1  Value    AA    m Groups 3  Value       Another way  slightly messy  could be using a lookbehind lookahead      lt  -   d     -

User · Answer

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                    amp               -                   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                           amp               -                   dblqt                                                      return   lt augroup gt     fss     lt  augroup gt     fs                         RegexOptions IgnoreCase

User · Answer

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    gt                        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

User · Answer

Here is a version similar to Daniel s but replacing multiple matches  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  gt  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  lt  input Length              sb Append input Substring match Index   match Length             input   sb ToString               return input

User · Answer

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   gt                                var group   m Groups groupName                   var sb   new StringBuilder                    var previousCaptureEnd   0                  foreach  var capture in group Captures Cast lt Capture gt                                           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       lt version gt   d      4            var result   regex ReplaceGroup input    version    1 2 3      Result    assembly  AssemblyFileVersion  1 2 3    assembly  AssemblyFileVersion  1 2 3

[c#] Replace only some groups with Regex

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