:relfileprefix: ../../../
[#boost-urls-grammar-optional_rule]

== xref:boost.adoc[pass:[boost]]::xref:boost/urls.adoc[pass:[urls]]::xref:boost/urls/grammar.adoc[pass:[grammar]]::optional_rule


Match a rule, or the empty string


=== Synopsis

Declared in header `<https://www.github.com/boostorg/url/blob/develop/include/boost/url/grammar/optional_rule.hpp#L124[boost/url/grammar/optional_rule.hpp,window=blank_]>`

[source,cpp,subs="verbatim,macros,-callouts"]
----
template<class Rule>
constexpr
pass:q[_implementation-defined_]
optional_rule(Rule const& r);
----

=== Description


Optional BNF elements are denoted with
square brackets. If the specified rule
returns any error it is treated as if
the rule did not match.
[,cpp]
----
using value_type = optional< typename Rule::value_type >;
----

=== Rules are used with the function

xref:boost/urls/grammar/parse-02.adoc[grammar::parse]
.
[,cpp]
----
system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );
----
[,cpp]
----
optional     = [ rule ]
----

* link:https://datatracker.ietf.org/doc/html/rfc5234#section-3.8[3.8.  Optional Sequence (rfc5234)]





=== Parameters

|===
| Name | Description 

| *r*
| 
The rule to match

|===




=== See Also


xref:boost/urls/grammar/alpha_chars.adoc[alpha_chars]
,
xref:boost/urls/grammar/parse-02.adoc[parse]
,
xref:boost/urls/optional.adoc[optional]
,
xref:boost/urls/grammar/token_rule.adoc[token_rule]
.