[#boost-urls-grammar-Rule]
= xref:boost.adoc[boost]::xref:boost/urls.adoc[urls]::xref:boost/urls/grammar.adoc[grammar]::Rule
:relfileprefix: ../../../
:mrdocs:


Concept for a grammar Rule

== Synopsis


Declared in `<https://www.github.com/boostorg/url/blob/develop/include/boost/url/grammar/type_traits.hpp#L119[boost/url/grammar/type_traits.hpp]>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class T>
concept Rule = requires (T t, char const*& it, char const* end)
    {
        typename T::value_type;
        { t.parse(it, end) } ‐> std::same_as<system::result<typename T::value_type>>;
    };
----

== Description


This concept is satisfied if `T` is a    valid grammar Rule

A `Rule` defines an algorithm used to match an input    buffer of ASCII characters against a set of syntactical    specifications.

Each rule represents either a terminal symbol or a    composition in the represented grammar.

The library comes with a set of rules for productions    typically found in RFC documents.    Rules are not invoked directly; instead, rule variables are    used with overloads of xref:boost/urls/grammar/parse-02.adoc[parse] which provide a convenient,    uniform front end.


=== Exemplar

For best results, it is suggested that all constructors for    rules be marked `constexpr`.

[,cpp]
----
struct Rule
{
    struct value_type;

    constexpr Rule( Rule const& ) noexcept = default;

    auto parse( char const*& it, char const* end ) const -> result< value_type >;
};

// Declare a variable of type Rule for notational convenience
constexpr Rule rule{};
----

=== Model

* dec&lowbar;octet&lowbar;rule
    
* delim&lowbar;rule
    
* not&lowbar;empty&lowbar;rule
    
* optional&lowbar;rule
    
* range&lowbar;rule
    
* token&lowbar;rule
    
* tuple&lowbar;rule
    
* unsigned&lowbar;rule
    
* variant&lowbar;rule



== See Also


xref:boost/urls/grammar/parse-02.adoc[parse],        xref:boost/urls/grammar/is_rule.adoc[is&lowbar;rule]&period;






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