C# regex repeating group of the digit in pattern

Question

i use regex pattern

    pattern = "ID\\d+.*?ID\\d+";
    input="ID1...sometxt1...ID1...sometxt2...ID3...sometxt3...ID50"
    input=Regex.Replace(input, pattern, "");
    Console.WriteLine(input);

Output will = "...sometxt2..."
but i need Output
...sometxt2...ID3...sometxt3...ID50,

i need that regex find groups with equal digit after ID. ID3 != ID50, this group must remain, ID1==ID1 - this group must be replaced

Thank!

Answer 1

If you need to replace the whole substrings from ID having the same digits after them, you need to use a capturing group with a backreference:

var pattern = @"\bID(\d+).*?\bID\1\b";

See the regex demo

Explanation:

• \\bID - a whole word "ID"
• (\\d+) - one or more digits captured into Group 1
• .*? - any characters but a newline, as few as possible up to the closest
• \\bID - whole word "ID" followed with....
• \\1 - backreference to the matched digits in Group 1
• \\b - followed with a word boundary (so that we do not match 10 if we have 1 in Group 1).

Note that you will need RegexOptions.Singleline modifier if you have newline characters in your input strings.

Also, do not forget to assign the replacement result to a variable:

var res = Regex.Replace(input, pattern, string.Empty);