Odd behaviour of char_ parser?

Hi!

I am hoping that I just don't understand the internals of the char_ 
parser and not that this is some sort of bug (unlikely).

Anyway, I want to parse a string "`foo`" (backticks surrounding the word 
foo). Simple enough, right?

I have a parser that looks like this:

auto const backtick_ascii_foo = lexeme[
     '`' >> *char_("fo") >> '`'
];

The above works perfectly fine and passing the string "`foo`" to 
x3::phrase_parse() along with the above grammar indeed succeeds in parsing.

However, if I change the char_("fo") to contain the code points for "f" 
and "o" like so:

auto const backtick_code_point_foo = lexeme[
     '`' >> *char_("\x60\x6f") >> '`'
];

the string "`foo`" no longer parses.

I'm not sure why and it has me a bit stumped :) Hoping someone can shed 
some light on this.

I've pastebin'd some code that shows the issue here:

http://pastebin.com/5pcrgpx6

$ g++ -I. -I/usr/local/include -std=gnu++14 -o testing.cc.o -c testing.cc
$ g++ testing.cc.o -o testing
$ ./testing
Parsing `foo` with name_code_point parser: 0
Parsing `foo` with name_ascii parser: 1

Any help or insights would be greatly appreciated. Thanks in advance!
-jay

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