How to escape the hyphen char in set by char parser?

Yi Leong <[email protected]> Tue, 17 Jul 2018 00:37:32 +0800
Newsgroups gmane.comp.parsers.spirit.general
Message-ID <CAMFq=pYVq4wpht+c63uoBOfjc7D+-o+OMyA0uttLxPzZ4ac9mw@mail.gmail.com>
As following:
qi::char_("abcd-+x")
what i need just matches the char 'a' | 'b' | 'c' | 'd' | ‘-’ | ‘+’ | ‘x’,
and the char parser takes  hyphen '-' as the range matching from 'd' to
'+'....
and how to escape the hyphen char?
I read this doc :
https://www.boost.org/doc/libs/1_46_0/libs/spirit/doc/html/spirit/qi/reference/char/char.html
and got nothing....

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