regex - Double \ in regular expression iOS

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Does anyone understand what this
(([A-Za-z\\s])+)\\? means?
I wonder why it should be
"\\s" and "\\"
?

If I entered "\s",
Xcode just doesn't understand and if I entered "\?", it just
doesn't match the
"?".

I have
googled a lot, but I did not find a solution. Anyone knows?

Answer

The actual regex is
(([A-Za-z\s])+)\?. This matches one or more letters and
whitespace characters followed by an question mark. The \ has
two different meanings here. In the first instance \s has a
fixed meaning and stands for any white space characters. In the second instance the
\? means the literal question mark character. The escaping is
necessary as the question mark means one or none of the previous
otherwise.

You can't type your regex like this
in a string literal in C code though. C also does some escaping using the backslash
character. For example "\n" is translated to a string
containing only a newline character. There are some other escape sequences with special
meanings. If the character after the backslash doesn't have a special meaning the
backslash is just removed. That means if you want to have a single backspace in your
string you have to write two.

So if you wrote
your regex string as you wanted you'd get different results as it would be interpreted
as (([A-Za-zs])+)? which has a completely different meaning. So
when you write a regex in an ObjC (or any other C-based language) string literal you
must double all backslash characters.