Not-Predicate Parser (!a)

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##### Not-Predicate Parser (`!a`)

###### Description

Syntactic predicates assert a certain conditional syntax to be satisfied before evaluating another production. Similar to semantic predicates, `eps`, syntactic predicates do not consume any input. The not-predicate, `!a`, is a negative syntactic predicate that returns a zero length match only if its predicate fails to match.

###### Header

    // forwards to <boost/spirit/home/qi/operator/not_predicate.hpp>
    #include <boost/spirit/include/qi_not_predicate.hpp>

Also, see Include Structure.

###### Model of

> `UnaryParser`

Notation

`a`

  A `Parser`

###### Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in `UnaryParser`.

Expression | Semantics
--- | ---
`!a` | If the predicate `a` matches, fail. Otherwise, return a zero length match.

###### Attributes

See Compound Attribute Notation.

Expression | Attribute
--- | ---
`!a` | `unused_type`

###### Complexity

> The complexity is defined by the complexity of the predicate, `a`

###### Example

| Note
The test harness for the example(s) below is presented in the Basics Examples section.

Some using declarations:

    using boost::spirit::ascii::char_;
    using boost::spirit::ascii::alpha;
    using boost::spirit::qi::lit;
    using boost::spirit::qi::symbols;

Here's an alternative to the `*(r - x) >> x` idiom using the not-predicate instead. This parses a list of characters terminated by a ';':

    test_parser("abcdef;", *(!lit(';') >> char_) >> ';');

The following parser ensures that we match distinct keywords (stored in a symbol table). To do this, we make sure that the keyword does not follow an alpha or an underscore:

    symbols<char, int> keywords;
    keywords = "begin", "end", "for";

    // This should fail:
    test_parser("beginner", keywords >> !(alpha | '_'));

    // This is ok:
    test_parser("end ", keywords >> !(alpha | '_'), false);

    // This is ok:
    test_parser("for()", keywords >> !(alpha | '_'), false);