1 Correct answer
Possibly you came across this trick that accidentally worked for others, and so it does did for you 
The thing is: sure, it works. But for other reasons than you might think! The purpose of the question mark is just to have *something* in the character class to negate. It works as well (or, possibly even better!) with another random character that should not appear anywhere in your text. Try it with alpha α, sha ш, or san ϻ and you'll see (unless you happen to use lots of text in Greek, Cyrillic,
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Replies
Possibly you came across this trick that accidentally worked for others, and so it does did for you 
The thing is: sure, it works. But for other reasons than you might think! The purpose of the question mark is just to have *something* in the character class to negate. It works as well (or, possibly even better!) with another random character that should not appear anywhere in your text. Try it with alpha α, sha ш, or san ϻ and you'll see (unless you happen to use lots of text in Greek, Cyrillic, or ancient Greek).
The Question Mark trick "works" because it (ab)uses negation: the sequence [^?] matches any character except the "?" in the list -- and this, in turn, works because you need to match not only regular characters but also a Paragraph Return. A standard GREP expression cannot match the Return character -- it is the exception to the rule that ". matches anything" -- but negating it kind of circumvents the rule.
And it worked until you need to use the "?".
A better option is to use the toggle "(?s)" at the start. This allows the catch-all period to also match a return, and so it will find a match that spans multiple paragraphs:
(?s)\|.+?\|
By the way #1: see how I use .+? instead of a regular .+? (Um. That is "period plus question" instead of "period plus". Plus a question mark at the end because it was stated as a question, but that's not part of the GREP.) A regular period-plus matches as much as possible, and so it will match both the leftmost and rightmost "|" in "|what can|possibly|go wrong|", but I'm certain you'd want to match the in-between pipes as well, like this: "|what can|possibly|go wrong|". Adding a question mark after the "+" makes it match a shortest possible match, instead of a longest possible.
By the way #2: it is very likely that the random use of the question mark in your own GREP originated from this correct use, but only through half-understanding its purpose, after which it was shuffled around until suddenly the expression "magically" worked.
Until you needed it to match a literal question mark, of course.
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