This is by design.
The Stack Exchange sites use fonts that have varying support for Unicode characters, so what you are seeing is the usage of some of the more esoteric character renderings in that particular font.
If you read through the entire answer, what you're seeing was done intentionally by the author as part of a (humorous) attempt to indicate how futile the author believes it is to try and parse HTML with regular expressions.
If you take the time to read it, you'll notice that it starts out as someone who is trying to reason with the idea of doing such a thing, but then descends into madness, which the esoteric characters are used to emphasize.
That post is so popular that it's become a meme (and almost just as often, a point of contention) on Stack Overflow:
Additionally, you'll notice that the some of these characters in the fonts are used used to render things such as the moderator diamond (♦) as and other interesting characters in user profile names.