A Rule is an object which tries to match the beginning of an input character buffer against a particular syntax. It returns a result containing a value if the match was successful, or an error_code if the match failed. Rules are not invoked directly. Instead they are passed as values to a parse function, along with the input character buffer to process. The first overload requires that the entire input string match, otherwise else an error occurs. The second overload advances the input buffer pointer to the first unconsumed character upon success, allowing a stream of data to be parsed sequentially:

To satisfy the Rule concept, a class or struct must declare the nested type value_type indicating the type of value returned upon success, and a const member function parse with a prescribed signature. In the following code we define a rule that matches a single comma:

Since rules are passed by value, we declare a constexpr variable of the type for syntactical convenience. Variable names for rules are usually suffixed with _rule:

Now we can call parse with the string of input and the rule variable thusly:

Rule expressions can come in several styles. The rule defined above is a compile-time constant. The unsigned_rule matches an unsigned decimal integer. Here we construct the rule at run time and specify the type of unsigned integer used to hold the result with a template parameter:

The function delim_rule returns a rule which matches the passed character literal. This is a more general version of the comma rule which we defined earlier. There is also an overload which matches exactly one character from a character set.