Regex replace string function not working as expected

Question

I'm trying to implement a hashtag function in a web app to easily embed search links into a page. The issue is that I'm trying to do a replace on the hash marks so they don't appear in the HTML output. Since I'm also wanting to be able to also have hash marks in the output I can't just do a final Replace on the entire string at the end of processing. I'm going to want to be able to escape some hash marks like so \\#1 is my answer and I'd find and replace the \\# with just # but that is another problem that I'm not even ready for (but still thinking of).

This is what I have so far mocked up in a console app,

static void Main(string[] args)
{
    Regex _regex = new Regex(@"(#([a-z0-9]+))");
    string link = _regex.Replace("<p>this is #my hash #tag.</p>", MakeLink("$1"));
}
public static string MakeLink(string tag)
{
    return string.Format("<a href=\"/Search?t={0}\">{1}</a>", tag.Replace("#", ""), tag);
}

The output being:
<p>this is <a href="/Search?t=#my">#my</a> hash <a href="/Search?t=#tag">#tag</a>.</p>

But when I run it with breaks while it's running MakeLink() it's string is displayed at "$1" in the debugger output and it's not replacing the hash's as expected.

Is there a better tool for the job than regex? Or can I do something else to get this working correctly?

Answer 1

Note that you're passing a literal "$1" into MakeLink, not the first captured group. Thus your .Replace("#", "") is doing nothing. The regular expression then replaces the two occurrences of "$1" in the output of MakeLink with the first capture group.

If you replace "$1" with "$2" then I think you get the result you want, just not quite in the manner you're expecting.

Answer 2

To not replace your escaped hashtags, just modify your current regex to not match anything that starts with an escape:

Regex _regex = new Regex(@"[^\\](#([a-z0-9]+))");

And then apply a new regex to find only escaped hashtags and replace them with unescaped ones:

Regex _escape = new Regex(@"\\(#([a-z0-9]+))");
_escape.Replace(input, "$1");