diff --git a/common/reflection.ys b/common/reflection.ys
index 0edd948da..0d7efa614 100644
--- a/common/reflection.ys
+++ b/common/reflection.ys
@@ -23,6 +23,7 @@ classes:
 - java.lang.String
 - java.lang.System
 - java.lang.Thread
+- java.math.BigInteger
 
 --- !YS-v0:
 
diff --git a/core/src/yamlscript/ast.clj b/core/src/yamlscript/ast.clj
index bd5bbad24..5b4218856 100644
--- a/core/src/yamlscript/ast.clj
+++ b/core/src/yamlscript/ast.clj
@@ -43,14 +43,7 @@
 
 (defn Chr [s] {:Chr (symbol s)})
 
-(defn Int [s]
-  {:Int (cond
-          (re-find #"^0x" s)
-          (Long/parseLong (subs s 2) 16)
-          (re-find #"^0o" s)
-          (Long/parseLong (subs s 2) 8)
-          :else
-          (parse-long s))})
+(defn Int [s] {:Int (bigint s)})
 
 (defn Flt [s] {:Flt (parse-double s)})
 
diff --git a/core/src/yamlscript/re.clj b/core/src/yamlscript/re.clj
index 2c6986f53..583717421 100644
--- a/core/src/yamlscript/re.clj
+++ b/core/src/yamlscript/re.clj
@@ -68,15 +68,8 @@
 (def fnum (re #"(?:$inum\.\d+(?:e$inum)?)"))   ; Floating point literal token
 (def xnum (re #"(?:$fnum|$inum)"))         ; Numeric literal token
                                            ; Maybe Number token
-(def mnum (re #"(?x)
-                (?: $xnum
-                  (?:[-+/*%_] \d* |
-                     \.\d+ |
-                     \.\w+ [\?\!]? (?=[^\(\w\?\!] | $)
-                  )*
-                )
-              "))
-
+(def mnum (re #"(?:$xnum[/0-9A-Za-z\.]*)")); *anything* that starts with a number, could be a number
+                                           ; We check later with `yamlscript.ysreader/is-bad-number?`
 (def xsym #"(?:[=!]~~?)")                  ; Special operator token
 
 ;; Operator symbol token
diff --git a/core/src/yamlscript/runtime.clj b/core/src/yamlscript/runtime.clj
index ec497b0b1..cc4e1cf97 100644
--- a/core/src/yamlscript/runtime.clj
+++ b/core/src/yamlscript/runtime.clj
@@ -260,7 +260,7 @@
         [sci/file file
          ARGS (vec
                 (map #(cond
-                        (re-matches re/inum %1) (parse-long %1)
+                        (re-matches re/inum %1) (bigint %1)
                         (re-matches re/fnum %1) (parse-double %1)
                         (re-matches re/keyw %1) (keyword (subs %1 1))
                         :else %1)
diff --git a/core/src/yamlscript/ysreader.clj b/core/src/yamlscript/ysreader.clj
index a631978c4..58a4bc15c 100644
--- a/core/src/yamlscript/ysreader.clj
+++ b/core/src/yamlscript/ysreader.clj
@@ -302,7 +302,7 @@
          (let [[[key val]] (seq %1)
                val (str val)]
            (if (and (= :Sym key) (re-matches #"_\d+" val))
-             (let [n (parse-long (subs val 1))]
+             (let [n (bigint (subs val 1))]
                (swap! args assoc n true)
                (swap! maxn max n))
              %1))
@@ -398,7 +398,7 @@
     (= "true" token) [(Bln token) tokens]
     (= "false" token) [(Bln token) tokens]
     (is-narg? token) (let [n (subs token 1)
-                           n (parse-long n)
+                           n (bigint n)
                            _ (when (or (<= n 0) (> n 20))
                                (die "Invalid numbered argument: " token))
                            n (str "_" n)]
diff --git a/core/src/ys/std.clj b/core/src/ys/std.clj
index 6db508a88..e1fdf1e9c 100644
--- a/core/src/ys/std.clj
+++ b/core/src/ys/std.clj
@@ -619,7 +619,7 @@
      number? x
      string? (if (re-find #"\." x)
                (parse-double x)
-               (parse-long x))
+               (bigint x))
      nil? (util/die "Can't convert a nil value to a number")
      seqable? (count x)
      char? (int x)
diff --git a/core/test/compiler-stack.yaml b/core/test/compiler-stack.yaml
index efe64bde6..7a0c7924c 100644
--- a/core/test/compiler-stack.yaml
+++ b/core/test/compiler-stack.yaml
@@ -1553,10 +1553,16 @@
   clojure: |
 
 
-- name: Octal and hex literals not loading correctly
+- name: Octal literals aren't supported
   yamlscript: |
     !YS-v0:
     - 0o12
+  error: |
+    Invalid number: 0x12
+
+- name: Hex literals aren't supported
+  yamlscript: |
+    !YS-v0:
     - 0x12
-  build: |
-    {:Vec [{:Int 10} {:Int 18}]}
+  error: |
+    Invalid number: 0x12
diff --git a/core/test/resolver.yaml b/core/test/resolver.yaml
index 23725dbda..faf2b2456 100644
--- a/core/test/resolver.yaml
+++ b/core/test/resolver.yaml
@@ -12,7 +12,7 @@
     - >
       folded
       block scalar
-    - [41, -42, +43, +0, -0, 0o1337, 0xcafebabe]
+    - [41, -42, +43, +0, -0]
     - [.1, -.2, +.3, 4.00, -5.99]
     - [.1e-00, -.2e+3, +.3e11, 4.00E-4, -5.99E99]
     - [true, True, TRUE]
@@ -35,9 +35,7 @@
         {:int "-42"}
         {:int "+43"}
         {:int "+0"}
-        {:int "-0"}
-        {:int "0o1337"}
-        {:int "0xcafebabe"}]}
+        {:int "-0"}]}
       {:seq
        [{:flt ".1"}
         {:flt "-.2"}
diff --git a/yamltest/src/yamltest/core.clj b/yamltest/src/yamltest/core.clj
index d19e777bf..340b2fcd2 100644
--- a/yamltest/src/yamltest/core.clj
+++ b/yamltest/src/yamltest/core.clj
@@ -46,7 +46,7 @@
                 (let [kw (keyword (subs kw 1))]
                   (when (contains? short-keys kw) kw))
                 (when (re-matches #"\d+" opt)
-                  (parse-long opt))))
+                  (bigint opt))))
             opts))
         (repeat true)))))
 
diff --git a/ys/share/ys-0.bash b/ys/share/ys-0.bash
index ffe2fe525..2ce0bcb5b 100755
--- a/ys/share/ys-0.bash
+++ b/ys/share/ys-0.bash
@@ -156,7 +156,7 @@ $program
   (let [argv (vec
               (map
                 #(if (re-matches #"\d+" %)
-                  (parse-long %) %)
+                  (bigint %) %)
                   args))]
     (apply main argv)))
 EOF