matches the start of a string with the set's characters
If the match is unsuccessful, then len is std::undefined.
Use the std::alt-function to construct a set of characters.
| std::match | match the grammar object with the start of the specified string |
| std::alt | |
| std::match_back | match the grammar object with the end of the specified string |
| std::search | search from the grammar object from the start or end of the specified string |