I'm using ANTLR4 for a class I'm taking right now and I seem to understand most of it, but I can't figure out what '+' does. All I can tell is that it's usually after a set of characters in brackets.
The plus is one of the BNF operators in ANTLR that allow to determine the cardinality of an expression. There are 3 of them: plus, star (aka. kleene operator) and question mark. The meaning is easily understandable:
Such an operator applies to the expression that directly preceeds it, e.g. ab+ (one a and one or more b), [AB]? (zero or one of either A or B) or a (b | c | d)* (a followed by zero or more occurences of either b, c or d).
ANTLR4 also uses a special construct to denote ungreedy matches. The syntax is one of the BNF operators plus a question mark (+?, *?, ??). This is useful when you have: an introducer match, any content, and then match a closing token. Take for instance a string (quote, any char, quote). With a greedy match ANTLR4 would match multiple strings as one (until the final quote). An ungreedy match however only matches until the first found end token (here the quote char).
Side note: I don't know what ?? could be useful for, since it matches a single entry, hence greedyness doesn't play a role here.
Actually, these operators are not part of traditional BNF, but rather of the Extended Backus-Naur Form. These are one of the reasons it's easier (or even possible) to document certain grammars in EBNF than in old-school BNF, which lacks many of these operators.