How to fix error Call to undefined function link_to_route() laravel 5?

First I am sorry my endlish so bad ,Hi all I am a new of laravel I just studying laravel. and I have problame error with link_to_route() function I try to fix and search google but I still can not.and I got error:

Whoops, looks like something went wrong.
1/1 FatalErrorException in cb4e66236ac7f3bb7bc9c7abce945dc30f4b8bf1.php line 33: Call to undefined function link_to_route() 

and here is my route.php

Route::group(['prefix'=>'auth'], function(){
    Route::get('register',[
        'as' => 'get_register',
        'uses' => 'Auth\AuthController@getRegister'
        ]);
    Route::post('register',[
        'as' => 'post_register',
        'uses' => 'Auth\AuthController@postRegister'
        ]);
});

And here is my code nav.blade.php

 <li><?php echo link_to_route('get_register', 'Register'); ?></li>

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