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?

Rawling · Accepted Answer · 2011-03-18 14:43:19Z

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.

answered Mar 18, 2011 at 14:43

Rawling

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7 Comments

Tim Meers Over a year ago

PERFECT!!! MakeLink("$2"). This is what I wanted 100% with the expected/desired output of <p>this is <a href="/Search?t=my-tag">my-tag</a> hash <a href="/Search?t=tag">tag</a>.</p>

Tim Meers Over a year ago

I also updated the regex to include a dash between words so (#([a-z0-9\-]+)). Might you have any idea on the escaping portion so that if they have something like my \#1 fan it won't turn the #1 into a link?

Rawling Over a year ago

I'm a bit rusty on this, but I think it's called "negative lookbehind" and you're looking for something like (?<!\\) just in front of your hash: ((?<!\\)#([a-z0-9]+)) Note that (a) I'm not sure if these brackets count as a capture group, in which case you'll need to use $3 instead of $2, and (b) I'm not sure if this works in .NET. Also note that you'll then have to replace \# with #.

Tim Meers Over a year ago

I really wish I could upvote this twice. It only needed a little tweak using my handy dandy RegexBuddy thats helping me in all this, but here is the upcated, working regex, ((?<!\\)#([a-z0-9\-]+)).

Rawling Over a year ago

Yeah, I had issues with my slashes. I still can't figure out quite what the rule for multiple slashes is, but I definitely lost a few along the way.

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ceyko · Accepted Answer · 2011-03-18 15:09:49Z

0

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");

answered Mar 18, 2011 at 15:09

ceyko

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3 Comments

Rawling Over a year ago

What will this do if the very start of the input is a hashtag?

Tim Meers Over a year ago

ceykooo, working code: Regex _regex = new Regex(@"((?<!\\)#([a-z0-9\-]+))"); Then it's a simple replace to replace \# with #

ceyko Over a year ago

We could check for the start of a line @"([^\\]|^)|(#([a-z0-9]+))" and use the subsequent match group, but we'd still have a space matched in the normal case and need some changes in subsequent code. For a pure regex approach, lookbacks are a much better option; I didn't see your comment before my post.

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