Certain characters cause search to crash

Question

I realized that if you type [user:1234] in the search box, you get a white blank page. The site just errors out (It returns a 400 Bad Request)

https://meta.stackexchange.com/questions/tagged/user%3a1234

Also: I just noticed that it strips off the last ] in the query.

Also: I know the search term should be user:1234 I just thought it would be better to provide some sort of error page, rather than just dumping a blank page.

Update: Here seems to be the list (so far) of what bricks it:

• [test] - works fine (redirects to tagged as test)
• [anything:anything] - bricks
• [test:] - bricks
• [:test] - bricks
• [:] - bricks

It seems that using the colon makes it explode. (giggity)

Answer 1

It appears that the colon has some unusual behavior outside of just tags.

• If you do a search using a required term syntax with a colon (+foo:bar), this term behaves differently in two situations. If it is grouped with at least one non-tag term, it is treated as a literal term +foo:bar instead of a required term (so you'll look for things that actually contain the string +foo:bar). I believe enclosing it in quotes like "foo:bar" also treats the quotes as a literal component of the term.
• If the above search is not part of a mixed search (only tag terms or no other terms), then it is skipped and ignored. For example, [foobar] +foo:bar just yields all questions tagged [foobar]. If it is the only term, then it's a blank search and you go to Search Options.
• A mixed search with one tag search term that has a colon (say, foobar [foo:bar]) will be a proper mixed search: it will search for questions that have foobar and are tagged [foo:bar]. However, the presence of this colon will cause the behavior reported in the first scenario: + terms will be treated as literal terms. This means +foobar [foo:bar] returns questions with +foobar in them, and most interestingly +[foo:bar] returns all questions with + in them.

The conclusion is that the 400 error only shows up in "pure tag searches" which include a colon in at least one of the terms. However, there is some strange behavior induced by the first two cases. For purposes one reason or another, a term with a colon that is surrounded by quotes or prefixed by a + is treated as a tag search, even though legal advanced searches traditionally aren't. So the presence of +foo:bar or "foo:bar" will trigger a 400 error if it is grouped with a tag term with a colon, and a combination of just those searches will be treated as no search at all.