FORMAT_QMARK_REGEX in sqlite backend does not work as expected

[anonymous]

FORMAT_QMARK_REGEX is not correct (in django/db/backends/sqlite3/base.py).

It uses a lookahead assertion trying to not match if the char before the "%s" is not "%" but lookahead assertion can't be used to do that (it is for looking ahead not behind).

Example:

In the regex '(?!..)b' applied on 'abc' the '..' will match 'bc' so:
 - re.search(r'(?!a)b','abc') => match 'b'
 - re.search(r'(?!b)b','abc') => no match

I don't think this regex could be writen with lookhead assertions.

A possible solution could be:

re.compile(r'(^|[^%])%s').sub(r'\1?','%s %%s %s')

[Aymeric Augustin]

Indeed, the current regex r'(?![^%])%s' is equivalent to r'%s'. The first part r'(?![^%])' will match if and only if it is not followed by r'[^%]', which is equivalent to being followed by %. This is always true since % is the next character in the regex.

This is much clearer with an example; I'm attaching a failing test case.

[Aymeric Augustin]

It's actually a duplicate of #15155 and #13648.