Hello, We've been here before, but it is still a problem. I formatted this one using Boost.Format, "%1$.20f", especially dealing with 64-bit floating point values: "-6454866677007257221799838306103363329054216765979693126687924767750184064408021645577150612734391912932504859271476290909819606665305867922002041825515395649967939044362235953753132737391831714484437364695236622971838510661738713863719312223765488747173369566663518205119012003640541309414736378162701139968.00000000000000000000" Grammar is defined: numeric_sign %= ( ('-' >> attr(numeric_sign_minus)) | ('+' >> attr(numeric_sign_plus)) ) ; float_lit %= real_parser<float_t, strict_ureal_policies<float_t>>{}; floating_point %= -numeric_sign >> float_lit; With rules defined as: qi::rule<iterator_type, numeric_sign_t()> numeric_sign; qi::rule<iterator_type, float_t()> float_lit; qi::rule<iterator_type, floating_point_number_t()> floating_point; Where float_t is a defined part of the AST: using float_t = double; And floating_point maps to the floating_point_number_t, basically mapping the Sign and Value. Seems like we've been here before with inadequate response, however, reading the Boost documentation, I do not see any reason any Qi should be rejecting this value: https://www.boost.org/doc/libs/1_69_0/libs/spirit/doc/html/spirit/qi/referen... Particularly the RealPolicies: sign = lit('+') | '-' ; nan = -lit("1.0#") >> no_case["nan"] >> -('(' >> *(char_ - ')') >> ')') ; inf = no_case[lit("inf") >> -lit("inity")] ; floating_literal = -sign >> ( nan | inf | fractional_constant >> -exponent_part | +digit >> exponent_part ) ; fractional_constant = *digit >> '.' >> +digit | +digit >> -lit('.') ; exponent_part = (lit('e') | 'E') >> -sign >> +digit ; Although the concern I have is that the RealPolicy grammar defined above is not implemented as Qi Rules, per se, but rather brute force. In other words, error prone in the ways that a brute force approach is error prone. Best regards, Michael Powell