regex to exclude if preceded by string?

Question

I haven't used regex much before but found something useful on the net that I'm using:

private string ConvertUrlsToLinks(string msg)
{
    string regex = @"((www\.|(http|https|ftp|news|file)+\:\/\/)[&#95;.a-z0-9-]+\.[a-z0-9\/&#95;:@=.+?,##%&~-]*[^.|\'|\\||\# |!|\(|?|\[|,| |>|<|;|\)])";
    Regex r = new Regex(regex, RegexOptions.IgnoreCase);
    return r.Replace(msg, "<a href=\"$1\" title=\"Click to open in a new window or tab\" target=\"&#95;blank\">$1</a>").Replace("href=\"www", "href=\"http://www").Replace(@"\r\n", "<br />").Replace(@"\n", "<br />").Replace(@"\r", "<br />");
}

It does a good job but now I want it to exclude urls that already have a "a href=" in front. There's the ending "/a" to consider too.

Can that be done with regex or have to use totally different approach, like coding?

Answer 1

Try this:

((?<!href=')(?<!href=")(www\.|(http|https|ftp|news|file)+\:\/\/)[&#95;.a-z0-9-]+\.[a-z0-9\/&#95;:@=.+?,##%&~-]*[^.|\'|\\||\# |!|\(|?|\[|,| |>|<|;|\)])

I tested on regex101.com

With the following sample set:

www.google.com
http://hi.com
http://www.fishy.com
href='www.ignore.com'
www.ouch.com

Answer 2

Using your existing regex pattern you could make a few simple changes to handle additional text being prepended or appended to your string:

`.+` <- pattern -> `(.+)?`

Which would give you:

.+((www\.|(http|https|ftp|news|file)+\:\/\/)[&#95;.a-z0-9-]+\.[a-z0-9\/&#95;:@=.+?,##%&~-]*[^.|\'|\\||\# |!|\(|?|\[|,| |>|<|;|\)])(.+)?

So passing the string of either:

<a href='http://www.test.com'>http://www.test.com</a>

...or...

http://www.test.com

Would result in:

<a href="http://www.test.com" title="Click to open in a new window or tab" target="&#95;blank">www.test.com</a>

Examples:

https://regex101.com/r/bO0cW6/1

http://ideone.com/suVw3I

Answer 3

I think it would be a little ToNy tHe pOny to do that in regex after all, so wrote the code, in case anyone is interested here it is:

private string handleatag(string msg, string tagbegin, string tagend)
{
    ArrayList tags = new ArrayList();
    int tagbeginpos = msg.IndexOf(tagbegin);
    int tagendpos;

    string hash = tagbegin.GetHashCode().ToString();

    while (tagbeginpos != -1)
    {
        tagendpos = msg.IndexOf(tagend, tagbeginpos);

        if (tagendpos != -1)
        {
            string atag = msg.Substring(tagbeginpos, tagendpos - tagbeginpos + tagend.Length);
            msg = msg.Replace(atag, hash + tags.Count.ToString());
            tags.Add(atag);
        }
        else
            msg = msg.Remove(tagbeginpos, tagbegin.Length);

        tagbeginpos = msg.IndexOf(tagbegin, tagbeginpos);
    }

    msg = ConvertUrlsToLinks(msg);

    for (int i = 0; i < tags.Count; i++)
        msg = msg.Replace(hash + i.ToString(), tags[i].ToString());

    return msg;
}

private string ConvertUrlsToLinks(string msg)
{
    if (msg.IndexOf("<a href=") != -1)
        return handleatag(msg, "<a href=", "</a>");

    string regex = @"((www\.|(http|https|ftp|news|file)+\:\/\/)[&#95;.a-z0-9-]+\.[a-z0-9\/&#95;:@=.+?,##%&~-]*[^.|\'|\\||\# |!|\(|?|\[|,| |>|<|;|\)])";
    Regex r = new Regex(regex, RegexOptions.IgnoreCase);
    return r.Replace(msg, "<a href=\"$1\" title=\"Click to open in a new window or tab\" target=\"&#95;blank\">$1</a>").Replace("href=\"www", "href=\"http://www").Replace(@"\r\n", "<br />").Replace(@"\n", "<br />").Replace(@"\r", "<br />");
}