Remove rel="nofollow" attribute from internal links into comment text

remove_nofollow_attribute_from_internal_links_comment.php

<?php 
/**
 * Filter
 * Remove rel="nofollow" attribute from internal links into comment text
 */
function vd_comment_text( $comment_text, $comment, $args ) {
    // find all <a> tags
    $regex = '#<\s*?a\b[^>]*>.*?</a\b[^>]*>#s';
    preg_match_all( $regex, $comment_text, $matches );

    foreach ( $matches as $atag ) {

        // find rel="nofollow"
        $nof_regex = '/rel\=[\"\']nofollow.*[\"\']\S*?/i';
        preg_match( $nof_regex, $comment_text, $nof_matches );

        if ( empty( $nof_matches[ 0 ] ) )
            continue;

        // find link domain
        $href_regex = '/[https]+\:\/\/([a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,3})\/\S*?/i';
        preg_match( $href_regex, $comment_text, $href_matches );

        if ( empty( $href_matches[ 1 ] ) )
            continue;

        // check if the domain is th current domain
        if( strpos( site_url(), $href_matches[ 1 ] ) !== false ){
            $comment_text = str_replace( $nof_matches[0], '', $comment_text );
        }

    }

    return $comment_text;
}
add_filter( 'comment_text', 'vd_comment_text', 10, 3 );