Match a rule, or the empty string
Synopsis
Declared in <boost/url/grammar/optional_rule.hpp>
template<Rule R>
constexpr
/* implementation-defined */
optional_rule(R 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.
Value Type
using value_type = optional< typename Rule::value_type >;Example
Rules are used with the function grammar::parse.
system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );BNF
optional = [ rule ]Specification
Return Value
The adapted rule
Parameters
Name |
Description |
r |
The rule to match |
See Also
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