diff --git a/asm.js b/asm.js
index 55165e2..b58bca9 100644
--- a/asm.js
+++ b/asm.js
@@ -280,15 +280,14 @@ class Parser {
 	}
 
     integer(token) {
-        let base;
+        let base, regex;
         switch (token.slice(-1)) {
-        case "b": base = 2; break;
-        case "o": base = 8; break;
-        case "h": base = 16; break;
-        default:  base = 10; break;
+        case "b": base = 2;  regex = /^-?[01]+b$/;     break;
+        case "o": base = 8;  regex = /^-?[0-7]+o$/;    break;
+        case "h": base = 16; regex = /^-?[0-9A-F]+h$/; break;
+        default:  base = 10; regex = /^-?[0-9]+d?$/;   break;
         }
-        const x = parseInt(token, base);
-        return Number.isNaN(x) ? null : x;
+		return regex.test(token) ? parseInt(token, base) : null;
     }
 
     translate_code(token) {