What does the \? (backslash question mark) escape sequence mean?
escaping, nsstring, objective-c
Solution
It specifies a literal question mark. It is needed because of a little-known feature called trigraphs, where you can write a three-character sequence starting with question marks to substitute another character. If you have trigraphs enabled, in order to write "??" in a string, you need to write it as `"?\?"` in order to prevent the preprocessor from trying to read it as the beginning of a trigraph.
(If you're wondering "Why would anybody introduce a feature like this?": Some keyboards or character sets didn't include commonly used symbols like `{`. so they introduced trigraphs so you could write `??<` instead.)
Problem
I'm writing a regular expression in Objective-C. The escape sequence `\w` is illegal and emits a warning, so the regular expression `/\w/` must be written as `@"\\w"`; the escape sequence `\?` is valid, apparently, and doesn't emit a warning, so the regular expression `/\?/` must be written as `@"\\?"` (i.e., the backslash must be escaped). Question marks aren't invisible like `\t` or `\n`, so why is `\?` a valid escape sequence? Edit: To clarify, I'm not asking about the quantifier, I'm asking about a string escape sequence. That is, this doesn't emit a warning: ``` NSString *valid = @"\?"; ``` By contrast, this does emit a warning ("Unknown escape sequence '\w'"): ``` NSString *invalid = @"\w"; ```