问如何仅当以给定的整数数开始时才解析字符串

EN

#include <boost/spirit/include/qi.hpp>
#include <string>
#include <iostream>

int main(int argc, char* argv[])
{
    std::string s(argv[1]) ;
    int n = 3;//can be runtime parameter
    std::string result;
    auto expr = boost::spirit::qi::omit
        [
            boost::spirit::qi::repeat(n)[boost::spirit::qi::int_] 
        ]   
        >> +boost::spirit::qi::char_("A-Za-z");

    bool b = boost::spirit::qi::phrase_parse(s.begin(), s.end(), expr, boost::spirit::qi::space, result);
    std::cout << std::boolalpha << b << '\n';
    if(b)
    {
        std::cout << result << '\n';
    }
}