public function process(File $phpcsFile, $stackPtr)
    {
        $tokens  = $phpcsFile->getTokens();
        $varName = ltrim($tokens[$stackPtr]['content'], '$');

        if (!in_array($varName, $this->allowedNames)
            && preg_match('/^_/', $varName)
        ) {
            $phpcsFile->addWarning(
                'Variable names may not start with an underscore',
                $stackPtr,
                'IllegalVariableNameUnderscore'
            );
        }
    }