[#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-09.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-09.adoc[`parse`], xref:boost/urls/grammar/is_rule.adoc[`is&lowbar;rule`]&period;


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