diff --git a/README.md b/README.md
index b535a36..a36eff7 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,26 @@ It can take a string with a math expression and parses it so it can be evaluated
 
 The packages supports operations like addition, subtraction, multiplication and division.
 
+Changes in comparison to original project
+-----------------------------------------
+
+- allow multi-character variable names
+- allow reuse of formula object:
+```php
+$formula = new Formula('a + b');
+$formula->setVariable('a', 1);
+$formula->setVariable('b', 41);
+$c = $formula->calculate();     // 42
+$formula->setVariable('a', 3);
+$formula->setVariable('b', 1334);
+$d = $formula->calculate();     // 1337    
+```
+- remove try-catch to hide exceptions from stdout (e.g. website)
+- bugfix for formulas that use brackets like `(a + b) * c`
+- remove `round(..., 2)` for divisions to avoid minor calculation errors
+- changed division-by-zero behavior from PHP's error to giving result `NAN` (a PHP constant) because I prefer silent error handling in this case ;-)
+- added exception if a variable is undefined (missing `setVariable(...)` statement)
+
 Installation
 ------------
 
diff --git a/src/ExpressionFactory.php b/src/ExpressionFactory.php
index ffd1eb4..0955b04 100644
--- a/src/ExpressionFactory.php
+++ b/src/ExpressionFactory.php
@@ -15,7 +15,7 @@ public static function factory(string $expression, $variables = [])
             return new Double( $expression );
         } else if ( strpos( $expression, '%' ) !== false ) {
             return new Percent( $expression );
-        } else if ( ( int ) $expression > 0 ) {
+        } else if ( is_numeric($expression) ) {
             return new Integer( $expression );
         } else {
             $variable = new Variable( $expression );
diff --git a/src/Formula.php b/src/Formula.php
index c9ce60f..24e7a1d 100644
--- a/src/Formula.php
+++ b/src/Formula.php
@@ -27,6 +27,11 @@ class Formula
      */
     protected $source = '';
 
+    /**
+     * @var string
+     */
+    protected $parsed = '';
+    
     /**
      * @var Operator
      */
@@ -64,7 +69,7 @@ public function getSource(): string
      */
     public function getExpression(): ?Operator
     {
-        $key = substr($this->source, 1, -1);
+        $key = substr($this->parsed, 1, -1);
 
         if (array_key_exists($key, $this->expressions)) {
             return $this->expressions[$key];
@@ -128,10 +133,13 @@ private function getExpressionObject(string $expression): Expression
      */
     public function parse()
     {
+        $this->parsed = $this->source;
+        $this->expressions = [];
+        
+        $operandPattern = '([\d\.,%]+|[^\{\}\.,%\(\)\*\/\+-]+|\{[\w\d]+\})';
         $patterns = [
-            '/(([\d\.,%]+|[^\{\}]|[\{\w\d\}]+)(\*)([\d\.,%]+|[^\{\}]|[\{\w\d\}]+))/i',
-            '/(([\d\.,%]+|[^\{\}]|[\{\w\d\}]+)(\/)([\d\.,%]+|[^\{\}]|[\{\w\d\}]+))/i',
-            '/(([\d\.,%]+|[^\{\}]|[\{\w\d\}]+)([\+|-])([\d\.,%]+|[^\{\}]|[\{\w\d\}]+))/i',
+            '/('.$operandPattern.'(\*|\/)'.$operandPattern.')/i',
+            '/('.$operandPattern.'(\+|-)'.$operandPattern.')/i'
         ];
 
         $operators = [
@@ -142,34 +150,30 @@ public function parse()
         ];
 
         // Выражение в скобках
-        while (preg_match('/\(((?:(?>[^()]+)|(?R))*)\)/i', $this->source, $results)) {
+        while (preg_match('/\(((?:(?>[^()]+)|(?R))*)\)/i', $this->parsed, $results)) {
             $key = $this->generateKey();
             $formula = new static($results[1]);
-            foreach ($this->variables as $key => $var) {
-                $formula->setVariable($key, $var);
+            foreach ($this->variables as $varKey => $var) {
+                $formula->setVariable($varKey, $var);
             }
             $formula->parse();
 
             $this->expressions[$key] = $formula->getExpression();
-            $this->source = str_replace($results[0], '{' . $key . '}', $this->source);
+            $this->parsed = str_replace($results[0], '{' . $key . '}', $this->parsed);
         }
 
         foreach ($patterns as $pattern) {
-            while (preg_match($pattern, $this->source, $results)) {
-                try {
-                    $left = $this->getExpressionObject( $results[2] );
-                    $right = $this->getExpressionObject( $results[4] );
-                    $key = $this->generateKey();
-
-                    $this->expressions[ $key ] = new $operators[$results[3]]( $left, $right );
-
-                    $this->source = str_replace( $results[1], '{' . $key . '}', $this->source );
-                } catch(\Exception $e) {
-                    echo $e->getMessage();
-                }
+            while (preg_match($pattern, $this->parsed, $results)) {
+                $left = $this->getExpressionObject( $results[2] );
+                $right = $this->getExpressionObject( $results[4] );
+                $key = $this->generateKey();
+
+                $this->expressions[ $key ] = new $operators[$results[3]]( $left, $right );
+
+                $this->parsed = str_replace( $results[1], '{' . $key . '}', $this->parsed );
             }
         }
 
-        $this->result = $this->getExpressionObject( $this->source );
+        $this->result = $this->getExpressionObject( $this->parsed );
     }
 }
\ No newline at end of file
diff --git a/src/expression/Division.php b/src/expression/Division.php
index 63fc788..a2f101f 100644
--- a/src/expression/Division.php
+++ b/src/expression/Division.php
@@ -9,6 +9,6 @@ class Division extends Operator
      */
     public function doCalculate(float $left, float $right): float
     {
-        return round($left / $right, 2);
+        return $right == 0 ? NAN : $left / $right;
     }
 }
\ No newline at end of file
diff --git a/src/operator/Variable.php b/src/operator/Variable.php
index 87bc9a7..1817617 100644
--- a/src/operator/Variable.php
+++ b/src/operator/Variable.php
@@ -7,7 +7,7 @@ class Variable extends Expression
 {
     protected $key;
 
-    public function __construct(string $key, string $value = '0')
+    public function __construct(string $key, string $value = '')
     {
         $this->key = $key;
         parent::__construct($value);
@@ -26,6 +26,9 @@ public function setValue(string $value)
      */
     public function calculate(Operator $operator): float
     {
+        if($this->value === ''){
+            throw new \BadMethodCallException('undefined variable: '.$this->key);
+        }
         $this->value = str_replace(',', '.', $this->value);
         return (float) $this->value;
     }