The JSON format rejects leading zeros, which JavaScript also rejects,
but it further requires digits on both sides of the decimal point,
which JavaScript does not: ".5", "5." and "5.e3" are all valid
DecimalLiteral productions. Users lexing JavaScript therefore could not
use chars_format::json and had to re-check the integer span themselves
after a general-mode parse.
chars_format::javascript is general mode plus the leading-zero rule
(reported as leading_zeros_in_integer_part), with no_infnan. It is a
runtime check on the format, like fortran, so it reuses the existing
parse_number_string<false> instantiation. A leading minus sign is
accepted, and a leading plus sign with allow_leading_plus.
Claude-Session: https://claude.ai/code/session_01KbAWRBpE3MPy5cXECc5vWZ
