diff --git a/include/fast_float/ascii_number.h b/include/fast_float/ascii_number.h
index 9afcdc4..eba8b9a 100644
--- a/include/fast_float/ascii_number.h
+++ b/include/fast_float/ascii_number.h
@@ -273,6 +273,7 @@ template <typename UC>
 fastfloat_really_inline FASTFLOAT_CONSTEXPR20
 parsed_number_string_t<UC> parse_number_string(UC const *p, UC const * pend, parse_options_t<UC> options) noexcept {
   chars_format const fmt = options.format;
+  parse_rules const rules = options.rules;
   UC const decimal_point = options.decimal_point;
 
   parsed_number_string_t<UC> answer;
@@ -288,8 +289,15 @@ parsed_number_string_t<UC> parse_number_string(UC const *p, UC const * pend, par
     if (p == pend) {
       return answer;
     }
-    if (!is_integer(*p) && (*p != decimal_point)) { // a sign must be followed by an integer or the dot
-      return answer;
+    if (rules == parse_rules::json) {
+      if (!is_integer(*p)) { // a sign must be followed by an integer
+        return answer;
+      }    
+    } else {
+      FASTFLOAT_DEBUG_ASSERT(rules == parse_rules::std);
+      if (!is_integer(*p) && (*p != decimal_point)) { // a sign must be followed by an integer or the dot
+        return answer;
+      }
     }
   }
   UC const * const start_digits = p;
@@ -306,8 +314,14 @@ parsed_number_string_t<UC> parse_number_string(UC const *p, UC const * pend, par
   UC const * const end_of_integer_part = p;
   int64_t digit_count = int64_t(end_of_integer_part - start_digits);
   answer.integer = span<const UC>(start_digits, size_t(digit_count));
+  // disallow leading zeros
+  if (rules == parse_rules::json && start_digits[0] == UC('0') && digit_count > 1) {
+    return answer;
+  }
+
   int64_t exponent = 0;
-  if ((p != pend) && (*p == decimal_point)) {
+  const bool has_decimal_point = (p != pend) && (*p == decimal_point);
+  if (has_decimal_point) {
     ++p;
     UC const * before = p;
     // can occur at most twice without overflowing, but let it occur more, since
@@ -327,6 +341,10 @@ parsed_number_string_t<UC> parse_number_string(UC const *p, UC const * pend, par
   if (digit_count == 0) {
     return answer;
   }
+  // or at least two if a decimal point exists, with json rules
+  else if (rules == parse_rules::json && has_decimal_point && digit_count == 1) {
+    return answer;
+  }
   int64_t exp_number = 0;            // explicit exponential part
   if ((fmt & chars_format::scientific) && (p != pend) && ((UC('e') == *p) || (UC('E') == *p))) {
     UC const * location_of_e = p;
diff --git a/include/fast_float/float_common.h b/include/fast_float/float_common.h
index 4a290f4..466d094 100644
--- a/include/fast_float/float_common.h
+++ b/include/fast_float/float_common.h
@@ -19,6 +19,11 @@ enum chars_format {
   general = fixed | scientific
 };
 
+enum class parse_rules {
+  std,
+  json
+};
+
 template <typename UC>
 struct from_chars_result_t {
   UC const* ptr;
@@ -29,13 +34,15 @@ using from_chars_result = from_chars_result_t<char>;
 template <typename UC>
 struct parse_options_t {
   constexpr explicit parse_options_t(chars_format fmt = chars_format::general,
-    UC dot = UC('.'))
-    : format(fmt), decimal_point(dot) {}
+    UC dot = UC('.'), parse_rules prules = parse_rules::std)
+    : format(fmt), decimal_point(dot), rules(prules) {}
 
   /** Which number formats are accepted */
   chars_format format;
   /** The character used as decimal point */
   UC decimal_point;
+  /** Rules to use for parsing */
+  parse_rules rules;
 };
 using parse_options = parse_options_t<char>;