Regex.Replace ignoring non-capturing group

Question

I have the following code:

var pattern = @"(?:red).*(\d+)";
var regX = new Regex(pattern);
var input = "this is red with number 111";
var replaced = regX.Replace(input, "666");

The replaced is then: this is 666 instead of: this is red with number 666

Why is this happening?

Answer 1

You need to use positive lookbehind assertion based regex since (?:red).* part of your regex matches characters. So when replacing, all the matched chars got replaced.

var pattern = @"(?<=red.*?)\d+";
var regX = new Regex(pattern);
var input = "this is red with number 111";
var replaced = regX.Replace(input, "666");

OR

Use capturing groups.

var pattern = @"(red.*?)\d+";

Replace the matched chars with $1 or \1 + 666