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  1. using System;
  2. using System.Text.RegularExpressions;
  3.  
  4. public class Test
  5. {
  6. public static void Main()
  7. {
  8. string pattern = @"^\s*(\d{1,2}-\d{1,2}-\d{2,4})((?:\r?\n(?:(?=\s*\1)|(?!\s*\d{1,2}-\d{1,2}-\d{2,4})).*)*)";
  9. string input = @" 13-08-2020
  10. data day1
  11. 13-08-2020
  12. data day1
  13. 13-08-2020
  14. data day1
  15. 14-08-2020
  16. data day2";
  17. string result = Regex.Replace(
  18. input, pattern, match =>
  19. match.Groups[1].Value + Regex.Replace(
  20. match.Groups[2].Value, @"^[^\S\r\n]*(?:\d{1,2}-\d{1,2}-\d{2,4}\s*)?", ""
  21. , RegexOptions.Multiline
  22. )
  23. , RegexOptions.Multiline
  24. );
  25. Console.WriteLine(result);
  26. }
  27. }
Success #stdin #stdout 0.06s 21236KB
stdin
Standard input is empty
stdout
13-08-2020
data day1 
data day1  
data day1 
14-08-2020
data day2