Frequently I come across posts which include colons. So far, so good: a colon is a valid punctuation mark in the English language (see the beginning of this sentence).

However, in about half of these cases the colon is followed — for reasons entirely unknown to me — by a hyphen. Resulting in something that looks like this:

:-

I’m not entirely sure what this is supposed to be. From the logic of the posts in question, it is probably supposed to be a colon. From its look, it seems like it should have been a smiley but lost its mouth.

Neither the Wikipedia page colon (punctuation) nor the corresponding section of the Wikipedia style guide note anything about this usage of colon + hyphen. Hence it is pretty clear that this type of usage should be avoided.

If you come across any posts of this kind and the post in question does not violate any be judicious-policy as outlined in the hidden points of editing, please be so kind and remove the extraneous hyphens.

In short:

:- → :

And if anybody can shed any light on why this is done, please answer here accordingly.

Thank you!

• Since we're trying to be pedantic here, should I remove the "Thank you!" in the end? – M.A.R. Aug 22 '16 at 18:31
• @zaq I really wouldn’t have guessed to check ELU O___O – Jan Aug 22 '16 at 18:35
• I always figured that was Big Bird looking in to try and learn some chemistry... – Jon Custer Aug 22 '16 at 19:35

I'm afraid I cannot answer distinctively why people are using :- these days. English is my second language, but I don't think I've ever seen this punctuation mark in any textbooks I used to study English (old Soviet textbooks from 1970s and 1990s Longman's Blueprint series), so my guess would be the writer used really ancient textbooks when they studied English.
Since use of :- makes the sentence hard to parse and may cause confusion with the clauses from logic programming, I always convert :- into a regular colon : when I edit something.