//I want to search for this code inside "file.php"...
<?php foreach comments as comment { ?>

<h1><?php the_comment_author() ?> says</h1>
}

//And let's suppose I want to replace it for...

<?php foreach comments as comment { ?>

<h3>Let's hear what <?php the_comment_author() ?> has to say, while <?php the_loggedin_user() ?> hears him talk</h3>
}