diff --git a/lib/index.js b/lib/index.js
index 31ce7ff..ee2aada 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -49,16 +49,30 @@
       return tag
     }
 
-    return str.replace(/\[(\d+;)?(\d+)*m/g, function(match, b1, b2) {
-      var i, code, res = ''
-      if (b2 == '' || b2 == null) b2 = '0'
-      for (i = 1; i < arguments.length - 2; i++) {
-        if (!arguments[i]) continue
-        code = parseInt(arguments[i])
-        res += tag(code)
+    function validateString(string) {
+      var strArray = string.split('[')
+      if (strArray.length === 0) return false
+      for (var item in strArray) {
+        if (strArray[item].length > 10) {
+          return false
+        }
       }
-      return res
-    }) + tag()
+      return true
+    }
+
+    if (validateString(str)) {
+      return str.replace(/\[(\d+;)?(\d+)*m/g, function(match, b1, b2) {
+        var i, code, res = ''
+        if (b2 == '' || b2 == null) b2 = '0'
+        for (i = 1; i < arguments.length - 2; i++) {
+          if (!arguments[i]) continue
+          code = parseInt(arguments[i])
+          res += tag(code)
+        }
+        return res
+      }) + tag()
+    }
+    return null
   }
 
   /* not implemented: