SassScriptOperation::isa PHP Method

    public static function isa($subject)
    {
        # begins with a "/x", almost always a path without quotes.
        if (preg_match('/^\\/[^0-9\\.\\-\\s]+/', $subject)) {
            return FALSE;
        }
        return preg_match(self::MATCH, $subject, $matches) ? trim($matches[1]) : false;
    }

 /**
  * Lex an expression into SassScript tokens.
  * @param string expression to lex
  * @param SassContext the context in which the expression is lexed
  * @return array tokens
  */
 public function lex($string, $context)
 {
     $tokens = array();
     while ($string !== false) {
         if (($match = $this->isWhitespace($string)) !== false) {
             $tokens[] = null;
         } elseif (($match = SassScriptFunction::isa($string)) !== false) {
             preg_match(SassScriptFunction::MATCH_FUNC, $match, $matches);
             $args = array();
             foreach (SassScriptFunction::extractArgs($matches[SassScriptFunction::ARGS]) as $expression) {
                 $args[] = $this->parser->evaluate($expression, $context);
             }
             $tokens[] = new SassScriptFunction($matches[SassScriptFunction::NAME], $args);
         } elseif (($match = SassString::isa($string)) !== false) {
             $tokens[] = new SassString($match);
         } elseif (($match = SassBoolean::isa($string)) !== false) {
             $tokens[] = new SassBoolean($match);
         } elseif (($match = SassColour::isa($string)) !== false) {
             $tokens[] = new SassColour($match);
         } elseif (($match = SassNumber::isa($string)) !== false) {
             $tokens[] = new SassNumber($match);
         } elseif (($match = SassScriptOperation::isa($string)) !== false) {
             $tokens[] = new SassScriptOperation($match);
         } elseif (($match = SassScriptVariable::isa($string)) !== false) {
             $tokens[] = new SassScriptVariable($match);
         } else {
             $_string = $string;
             $match = '';
             while (strlen($_string) && !$this->isWhitespace($_string)) {
                 foreach (SassScriptOperation::$inStrOperators as $operator) {
                     if (substr($_string, 0, strlen($operator)) == $operator) {
                         break 2;
                     }
                 }
                 $match .= $_string[0];
                 $_string = substr($_string, 1);
             }
             $tokens[] = new SassString($match);
         }
         $string = substr($string, strlen($match));
     }
     return $tokens;
 }