diff --git a/.travis.yml b/.travis.yml
index 842031b..47d1020 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,9 @@
 language: php
 
 php:
-  - 5.3
-  - 5.4
+  - 5.6
+  - 7.2
 
 before_script:
   - wget http://getcomposer.org/composer.phar
-  - php composer.phar install
\ No newline at end of file
+  - php composer.phar install
diff --git a/README.md b/README.md
index 8c611b8..2fdb5c1 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,15 @@ Simple math expressions calculator
 
 ## Install via Composer
 
-All instructions to install here: https://packagist.org/packages/nxp/math-executor
+Stable branch
+```
+composer require "nxp/math-executor" "dev-master"
+```
+
+Dev branch
+```
+composer require "nxp/math-executor" "dev-dev"
+```
 
 ## Sample usage:
 
diff --git a/src/NXP/Classes/Calculator.php b/src/NXP/Classes/Calculator.php
index 41e0d9f..bfa7015 100644
--- a/src/NXP/Classes/Calculator.php
+++ b/src/NXP/Classes/Calculator.php
@@ -13,6 +13,7 @@ namespace NXP\Classes;
 use NXP\Classes\Token\InterfaceOperator;
 use NXP\Classes\Token\TokenFunction;
 use NXP\Classes\Token\TokenNumber;
+use NXP\Classes\Token\TokenString;
 use NXP\Classes\Token\TokenVariable;
 use NXP\Exception\IncorrectExpressionException;
 use NXP\Exception\UnknownVariableException;
@@ -32,15 +33,18 @@ class Calculator
      */
     public function calculate($tokens, $variables)
     {
-        $stack = array();
+        $stack = [];
         foreach ($tokens as $token) {
             if ($token instanceof TokenNumber) {
                 array_push($stack, $token);
             }
+            if ($token instanceof TokenString) {
+                array_push($stack, $token);
+            }
             if ($token instanceof TokenVariable) {
                 $variable = $token->getValue();
                 if (!array_key_exists($variable, $variables)) {
-                    throw new UnknownVariableException();
+                    throw new UnknownVariableException($variable);
                 }
                 $value = $variables[$variable];
                 array_push($stack, new TokenNumber($value));
diff --git a/src/NXP/Classes/Lexer.php b/src/NXP/Classes/Lexer.php
index e541732..82b2c53 100644
--- a/src/NXP/Classes/Lexer.php
+++ b/src/NXP/Classes/Lexer.php
@@ -17,6 +17,7 @@ use NXP\Classes\Token\TokenLeftBracket;
 use NXP\Classes\Token\TokenNumber;
 use NXP\Classes\Token\TokenRightBracket;
 use NXP\Classes\Token\TokenVariable;
+use NXP\Classes\Token\TokenString;
 use NXP\Exception\IncorrectBracketsException;
 use NXP\Exception\IncorrectExpressionException;
 
@@ -42,7 +43,7 @@ class Lexer
      */
     public function stringToTokensStream($input)
     {
-        $matches = array();
+        $matches = [];
         preg_match_all($this->tokenFactory->getTokenParserRegex(), $input, $matches);
         $tokenFactory = $this->tokenFactory;
         $tokensStream = array_map(
@@ -62,10 +63,13 @@ class Lexer
      */
     public function buildReversePolishNotation($tokensStream)
     {
-        $output = array();
-        $stack = array();
+        $output = [];
+        $stack = [];
 
         foreach ($tokensStream as $token) {
+            if ($token instanceof TokenString) {
+                $output[] = $token;
+            }
             if ($token instanceof TokenNumber) {
                 $output[] = $token;
             }
diff --git a/src/NXP/Classes/Token/TokenFunction.php b/src/NXP/Classes/Token/TokenFunction.php
index 23f64bd..2ed8ace 100644
--- a/src/NXP/Classes/Token/TokenFunction.php
+++ b/src/NXP/Classes/Token/TokenFunction.php
@@ -29,7 +29,7 @@ class TokenFunction extends AbstractContainerToken implements InterfaceFunction
      */
     public function execute(&$stack)
     {
-        $args = array();
+        $args = [];
         list($places, $function) = $this->value;
         for ($i = 0; $i < $places; $i++) {
             array_push($args, array_pop($stack)->getValue());
diff --git a/src/NXP/Classes/Token/TokenString.php b/src/NXP/Classes/Token/TokenString.php
new file mode 100644
index 0000000..cab0711
--- /dev/null
+++ b/src/NXP/Classes/Token/TokenString.php
@@ -0,0 +1,25 @@
+<?php
+/**
+ * This file is part of the MathExecutor package
+ *
+ * (c) Alexander Kiryukhin
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code
+ */
+
+namespace NXP\Classes\Token;
+
+/**
+ * @author Bruce Wells <brucekwells@gmail.com>
+ */
+class TokenString extends AbstractContainerToken
+{
+    /**
+     * @return string
+     */
+    public static function getRegex()
+    {
+        return '"([^"]|"")*"';
+    }
+}
diff --git a/src/NXP/Classes/TokenFactory.php b/src/NXP/Classes/TokenFactory.php
index 19ba1cf..d70dd55 100644
--- a/src/NXP/Classes/TokenFactory.php
+++ b/src/NXP/Classes/TokenFactory.php
@@ -17,6 +17,7 @@ use NXP\Classes\Token\TokenLeftBracket;
 use NXP\Classes\Token\TokenNumber;
 use NXP\Classes\Token\TokenRightBracket;
 use NXP\Classes\Token\TokenVariable;
+use NXP\Classes\Token\TokenString;
 use NXP\Exception\UnknownFunctionException;
 use NXP\Exception\UnknownOperatorException;
 use NXP\Exception\UnknownTokenException;
@@ -31,24 +32,35 @@ class TokenFactory
      *
      * @var array
      */
-    protected $operators = array();
+    protected $operators = [];
 
     /**
      * Available functions
      *
      * @var array
      */
-    protected $functions = array();
+    protected $functions = [];
 
     /**
      * Add function
-     * @param $name
-     * @param $function
-     * @param $places
+     * @param string   $name
+     * @param callable $function
+     * @param int      $places
      */
-    public function addFunction($name, $function, $places = 1)
+    public function addFunction($name, callable $function, $places = 1)
     {
-        $this->functions[$name] = array($places, $function);
+        $this->functions[$name] = [$places, $function];
+    }
+
+    /**
+     * get functions
+     *
+     * @return array containing callback and places indexed by
+     *         function name
+     */
+    public function getFunctions()
+    {
+        return $this->functions;
     }
 
     /**
@@ -61,7 +73,7 @@ class TokenFactory
         $class = new \ReflectionClass($operatorClass);
 
         if (!in_array('NXP\Classes\Token\InterfaceToken', $class->getInterfaceNames())) {
-            throw new UnknownOperatorException;
+            throw new UnknownOperatorException($operatorClass);
         }
 
         $this->operators[] = $operatorClass;
@@ -69,13 +81,13 @@ class TokenFactory
     }
 
     /**
-     * Add variable
-     * @param string $name
-     * @param mixed  $value
+     * Get registered operators
+     *
+     * @return array of operator class names
      */
-    public function addVariable($name, $value)
+    public function getOperators()
     {
-
+        return $this->operators;
     }
 
     /**
@@ -89,8 +101,9 @@ class TokenFactory
         }
 
         return sprintf(
-            '/(%s)|([%s])|(%s)|(%s)|([%s%s%s])/i',
+            '/(%s)|(%s)|([%s])|(%s)|(%s)|([%s%s%s])/i',
             TokenNumber::getRegex(),
+            TokenString::getRegex(),
             $operatorsRegex,
             TokenFunction::getRegex(),
             TokenVariable::getRegex(),
@@ -119,6 +132,10 @@ class TokenFactory
             return new TokenRightBracket();
         }
 
+        if ($token[0] == '"') {
+            return new TokenString(str_replace('"', '', $token));
+        }
+
         if ($token == ',') {
             return new TokenComma();
         }
@@ -140,10 +157,10 @@ class TokenFactory
             if (isset($this->functions[$token])) {
                 return new TokenFunction($this->functions[$token]);
             } else {
-                throw new UnknownFunctionException();
+                throw new UnknownFunctionException($token);
             }
         }
 
-        throw new UnknownTokenException();
+        throw new UnknownTokenException($token);
     }
 }
diff --git a/src/NXP/Exception/IncorrectBracketsException.php b/src/NXP/Exception/IncorrectBracketsException.php
index e2aaed9..b3e8315 100644
--- a/src/NXP/Exception/IncorrectBracketsException.php
+++ b/src/NXP/Exception/IncorrectBracketsException.php
@@ -14,6 +14,6 @@ namespace NXP\Exception;
 /**
  * @author Alexander Kiryukhin <alexander@symdev.org>
  */
-class IncorrectBracketsException extends \Exception
+class IncorrectBracketsException extends MathExecutorException
 {
 }
diff --git a/src/NXP/Exception/IncorrectExpressionException.php b/src/NXP/Exception/IncorrectExpressionException.php
index ad5bc42..6f579c1 100644
--- a/src/NXP/Exception/IncorrectExpressionException.php
+++ b/src/NXP/Exception/IncorrectExpressionException.php
@@ -14,6 +14,6 @@ namespace NXP\Exception;
 /**
  * @author Vitaliy Zhuk <zhuk2205@gmail.com>
  */
-class IncorrectExpressionException extends \Exception
+class IncorrectExpressionException extends MathExecutorException
 {
 }
diff --git a/src/NXP/Exception/UnknownFunctionException.php b/src/NXP/Exception/UnknownFunctionException.php
index 5bb3658..e8ff0c0 100644
--- a/src/NXP/Exception/UnknownFunctionException.php
+++ b/src/NXP/Exception/UnknownFunctionException.php
@@ -14,6 +14,6 @@ namespace NXP\Exception;
 /**
  * @author Vitaliy Zhuk <zhuk2205@gmail.com>
  */
-class UnknownFunctionException extends \Exception
+class UnknownFunctionException extends MathExecutorException
 {
 }
diff --git a/src/NXP/Exception/UnknownOperatorException.php b/src/NXP/Exception/UnknownOperatorException.php
index b6617c3..acb4b98 100644
--- a/src/NXP/Exception/UnknownOperatorException.php
+++ b/src/NXP/Exception/UnknownOperatorException.php
@@ -14,6 +14,6 @@ namespace NXP\Exception;
 /**
  * @author Vitaliy Zhuk <zhuk2205@gmail.com>
  */
-class UnknownOperatorException extends \Exception
+class UnknownOperatorException extends MathExecutorException
 {
 }
diff --git a/src/NXP/Exception/UnknownTokenException.php b/src/NXP/Exception/UnknownTokenException.php
index b8a593f..bba28a4 100644
--- a/src/NXP/Exception/UnknownTokenException.php
+++ b/src/NXP/Exception/UnknownTokenException.php
@@ -14,6 +14,6 @@ namespace NXP\Exception;
 /**
  * @author Vitaliy Zhuk <zhuk2205@gmail.com>
  */
-class UnknownTokenException extends \Exception
+class UnknownTokenException extends MathExecutorException
 {
 }
diff --git a/src/NXP/Exception/UnknownVariableException.php b/src/NXP/Exception/UnknownVariableException.php
index 8de9493..8c215db 100644
--- a/src/NXP/Exception/UnknownVariableException.php
+++ b/src/NXP/Exception/UnknownVariableException.php
@@ -14,6 +14,6 @@ namespace NXP\Exception;
 /**
  * @author Alexander Kiryukhin <alexander@symdev.org>
  */
-class UnknownVariableException extends \Exception
+class UnknownVariableException extends MathExecutorException
 {
 }
diff --git a/src/NXP/MathExecutor.php b/src/NXP/MathExecutor.php
index 5cb4bca..d62d07a 100644
--- a/src/NXP/MathExecutor.php
+++ b/src/NXP/MathExecutor.php
@@ -15,6 +15,7 @@ use NXP\Classes\Calculator;
 use NXP\Classes\Lexer;
 use NXP\Classes\Token;
 use NXP\Classes\TokenFactory;
+use NXP\Exception\UnknownVariableException;
 
 /**
  * Class MathExecutor
@@ -27,7 +28,7 @@ class MathExecutor
      *
      * @var array
      */
-    private $variables = array();
+    private $variables = [];
 
     /**
      * @var TokenFactory
@@ -37,7 +38,7 @@ class MathExecutor
     /**
      * @var array
      */
-    private $cache = array();
+    private $cache = [];
 
     /**
      * Base math operators
@@ -75,10 +76,36 @@ class MathExecutor
         $this->tokenFactory->addFunction('max', 'max', 2);
         $this->tokenFactory->addFunction('avg', function($arg1, $arg2) { return ($arg1 + $arg2) / 2; }, 2);
 
-        $this->setVars(array(
+        $this->setVars([
             'pi' => 3.14159265359,
             'e'  => 2.71828182846
-        ));
+        ]);
+    }
+
+    /**
+     * Get all vars
+     *
+     * @return array
+     */
+    public function getVars()
+    {
+        return $this->variables;
+    }
+
+    /**
+     * Get a specific var
+     *
+     * @param  string        $variable
+     * @return integer|float
+     * @throws UnknownVariableException
+     */
+    public function getVar($variable)
+    {
+        if (! isset($this->variables[$variable])) {
+            throw new UnknownVariableException("Variable ({$variable}) not set");
+        }
+
+        return $this->variables[$variable];
     }
 
     /**
@@ -86,11 +113,14 @@ class MathExecutor
      *
      * @param  string        $variable
      * @param  integer|float $value
-     * @throws \Exception
      * @return MathExecutor
      */
     public function setVar($variable, $value)
     {
+        if (!is_numeric($value)) {
+            throw new \Exception("Variable ({$variable}) value must be a number ({$value}) type ({gettype($value)})");
+        }
+
         $this->variables[$variable] = $value;
 
         return $this;
@@ -134,7 +164,7 @@ class MathExecutor
      */
     public function removeVars()
     {
-        $this->variables = array();
+        $this->variables = [];
 
         return $this;
     }
@@ -152,6 +182,16 @@ class MathExecutor
         return $this;
     }
 
+    /**
+     * Get all registered operators to executor
+     *
+     * @return array of operator class names
+     */
+    public function getOperators()
+    {
+        return $this->tokenFactory->getOperators();
+    }
+
     /**
      * Add function to executor
      *
@@ -160,13 +200,24 @@ class MathExecutor
      * @param  int          $places   Count of arguments
      * @return MathExecutor
      */
-    public function addFunction($name, callable $function = null, $places = 1)
+    public function addFunction($name, $function = null, $places = 1)
     {
         $this->tokenFactory->addFunction($name, $function, $places);
 
         return $this;
     }
 
+    /**
+     * Get all registered functions
+     *
+     * @return array containing callback and places indexed by
+     *         function name
+     */
+    public function getFunctions()
+    {
+        return $this->tokenFactory->getFunctions();
+    }
+
     /**
      * Execute expression
      *
diff --git a/tests/MathTest.php b/tests/MathTest.php
index a83a0d4..db6e486 100644
--- a/tests/MathTest.php
+++ b/tests/MathTest.php
@@ -27,7 +27,7 @@ class MathTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($calculator->execute($expression), $phpResult);
     }
 
-    public function testZeroDevision()
+    public function testZeroDivision()
     {
         $calculator = new MathExecutor();
         $this->assertEquals($calculator->execute('1 / 0'), 0);
@@ -44,34 +44,44 @@ class MathTest extends \PHPUnit_Framework_TestCase
      */
     public function providerExpressions()
     {
-        return array(
-            array('0.1 + 0.2'),
-            array('1 + 2'),
+        return [
+            ['0.1 + 0.2'],
+            ['1 + 2'],
 
-            array('0.1 - 0.2'),
-            array('1 - 2'),
+            ['0.1 - 0.2'],
+            ['1 - 2'],
 
-            array('0.1 * 2'),
-            array('1 * 2'),
+            ['0.1 * 2'],
+            ['1 * 2'],
 
-            array('0.1 / 0.2'),
-            array('1 / 2'),
+            ['0.1 / 0.2'],
+            ['1 / 2'],
 
-            array('2 * 2 + 3 * 3'),
+            ['2 * 2 + 3 * 3'],
 
-            array('1 + 0.6 - 3 * 2 / 50'),
+            ['1 + 0.6 - 3 * 2 / 50'],
 
-            array('(5 + 3) * -1'),
+            ['(5 + 3) * -1'],
 
-            array('2+2*2'),
-            array('(2+2)*2'),
-            array('(2+2)*-2'),
-            array('(2+-2)*2'),
+            ['2+2*2'],
+            ['(2+2)*2'],
+            ['(2+2)*-2'],
+            ['(2+-2)*2'],
 
-            array('sin(10) * cos(50) / min(10, 20/2)'),
+            ['sin(10) * cos(50) / min(10, 20/2)'],
 
-            array('100500 * 3.5E5'),
-            array('100500 * 3.5E-5')
-        );
+            ['100500 * 3.5E5'],
+            ['100500 * 3.5E-5'],
+        ];
     }
-}
+
+    public function testFunction()
+    {
+        $calculator = new MathExecutor();
+
+        $calculator->addFunction('round', function ($arg) { return round($arg); }, 1);
+        /** @var float $phpResult */
+        eval('$phpResult = round(100/30);');
+        $this->assertEquals($calculator->execute('round(100/30)'), $phpResult);
+    }
+}
\ No newline at end of file