diff --git a/source/utilities.tex b/source/utilities.tex
index f856a423b4..d3f5015061 100644
--- a/source/utilities.tex
+++ b/source/utilities.tex
@@ -17713,12 +17713,13 @@
   // Parses a width argument id in the format \tcode{\{} \fmtgrammarterm{digit} \tcode{\}}.
   constexpr auto parse(format_parse_context& ctx) {
     auto iter = ctx.begin();
+    auto is_digit = [](auto c) { return c >= '0' && c <= '9'; };
     auto get_char = [&]() { return iter != ctx.end() ? *iter : 0; };
     if (get_char() != '{')
       return iter;
     ++iter;
     char c = get_char();
-    if (!isdigit(c) || (++iter, get_char()) != '}')
+    if (!is_digit(c) || (++iter, get_char()) != '}')
       throw format_error("invalid format");
     width_arg_id = c - '0';
     ctx.check_arg_id(width_arg_id);
@@ -17735,7 +17736,7 @@
       else
         return value;
       });
-    return format_to(ctx.out(), "{0:x<{1}}", s.value, width);
+    return format_to(ctx.out(), "{0:x>{1}}", s.value, width);
   }
 };