The difference operator: the rule lit("policeman") - "police" will always fail ?

[Alex Sekhoy]

Hello, dear spirit devs!

i am curious if there's a way to parse "foobar" while not allowing "foo"? here's an explanation:

//cut here
struct keywords : qi::symbols<char> {
    keywords(){
        add
         ("if")
         ("else")
         //...etc
    }
} keywords_symbols;

rule<Iterator, std::string(), ascii::space> name;
name %= raw[
                        lexeme
                        [
                           ((alpha | char_('_')) >> *(alnum | char_('_')))
                            - keywords_symbols
                        ]
                     ];
//cut here

This rule doesn't allow such inputs as "ifxxx", "else123".

According to the new docs about "The Difference operator, (a -  b)":
Unlike classic Spirit, with Spirit2, the expression will always fail if the RHS is a successful match regardless if the RHS matches less characters. For example, the rule lit("policeman") - "police" will always fail to match. Spirit2 does not count the matching chars while parsing and there is no reliable and fast way to check if the LHS matches more than the RHS.

Could you be so kind to provide me with solutions for my task? Thank you guys!

[Dainis Polis]

Alex Sekhoy wrote:
> Hello, dear spirit devs!
>
> i am curious if there's a way to parse "foobar" while not allowing "foo"?
> here's an explanation:
>  
Only when you not allow   foo >>  ! lit('b')
In another words if we know char , after  one  is  known  ending  of phrase,
then  this ending char must be  after   "foo"  and  after "foobar".
Then only after this char we know who is who. ;-)
 

 
 


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