in src/detail/skip_value.cpp [38:45]
void skip_unicode_escape(decode_context &context) {
require_bytes<4>(context, "\\u must be followed by 4 hex digits");
const bool h0 = is_hex_digit(*(context.position++));
const bool h1 = is_hex_digit(*(context.position++));
const bool h2 = is_hex_digit(*(context.position++));
const bool h3 = is_hex_digit(*(context.position++));
fail_if(context, !(h0 && h1 && h2 && h3), "\\u must be followed by 4 hex digits");
}