Domain names followed by full stops create invalid IDN domain names

Question

A comment of mine contains the text See also: http://languagelearning.stackexchange.com‌​. However, this appears as See also: languagelearning.stackexchange.com‌​;. (where did that semi-colon come from?), and it links to http://languagelearning.stackexchange.xn--com-cn0a/, which doesn't exist.

Answer 1

This has been fixed and will be out in the next deployment.

TL;DR: Regex.

Long version: We introduce a non-visible, optional separator entity every 20 characters to make comments wrap. This has to be removed and taken into account when we linkify URLs. The Regex did not match the last ; as it's a valid separator for URLs: if you write http://example.com; the matched URL should be http://example.com. Unfortunately in some rare cases these separators would be introduced just at the right (wrong) spot. In the case linked, the URL with separators inserted ended with a separator and thus a semicolon like so

See also: http://languagelearn‌​ing.stackexchange.com‌​.

and so the URL parsed was

http://languagelearn‌​ing.stackexchange.com‌&#8203

without the trailing ;. Once we remove the separator this gives

http://languagelearning.stackexchange.com‌&#8203

which is what you found the link was pointing to. The semicolon that appears is also because of this.

I've improved muchly complicated the regex so it matches the separator at the end but not a normal semicolon.

---

Update: The new regex had other problems. I'm reverting for now, pending a real fix tomorrow. The battle of the regex rages on. I shall prevail.

---

Done.