Regular expression capturing more than expected

Question

New dad, so my eyes are tired and I'm trying to figure out why this code:

var regex = new Regex(@"(https:)?\/");
Console.WriteLine (regex.Replace("https://foo.com", ""));

Emits:

foo.com

I only have the one forward slash, so why are both being captured in the group for the replacement?

Congrats on entering the domain of fatherhood! :) – Drew Kennedy Jun 29 '15 at 19:45 — Drew Kennedy, Jun 29 '15 at 19:45

score 4 · Accepted Answer · answered Jun 29 '15 at 19:49

4

In a specified input string, replaces all strings that match a regular expression pattern with a specified replacement string.

Every single / matches the regular expression pattern @"(https:)?\/". If you try e.g. "https://foo/./com/", all /s would be removed.

answered Jun 29 '15 at 19:49

AlexD

Criminy - because `https` is optional, all slashes will always be matched. – Mister Epic Jun 29 '15 at 19:59
@MisterEpic I suggest using var regex = new Regex(@"(https?:(\/){2})?"); – iheanyi Jun 29 '15 at 20:00

score 3 · Answer 2 · answered Jun 29 '15 at 19:53

If you check what matches are generated, it becomes clear. Add this to your code:

var matches = regex.Matches("https://foo.com");
foreach (Match match in matches)
{
    Console.WriteLine(match.Value);
}

And you'll see that https:/ is matched and replaced, / is matched and replaced (because https:is optional) and foo.com remains.

2 Answers2