What is $1 and $2 in Regular Expressions?

Question

I have simple question regarding regular expressions in C#.

What is $1 and $2 in C# regular expression?

Does both come under groups?

Answer 1

That is values of captured groups by index. $1 is a first captured group, and $2 is a second captured group. As David pointed, these values used in replacement patterns.

string input = "Hello World";
string result = Regex.Replace(input, @"(\w+) (\w+)", "$2 $1");

Output: World Hello