Redirect subdomain to root domain

Redirecting pages of a specific subdomain to root domain pages by means of .htaccess, when subdomain pages may be both with or without www:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(?:www\.)?subdomain\.(domain\.tld)$ [NC]
RewriteRule ^(.*)$ https://www.domain.tld/$1 [R=301,NC,L]

Commentary

Enable runtime rewriting engine:

RewriteEngine On

If there is a subdomain with or without www:

RewriteCond %{HTTP_HOST} ^(?:www\.)?subdomain\.(domain\.tld)$ [NC]

Then redirect to root domain page:

RewriteRule ^(.*)$ https://www.domain.tld/$1 [R=301,NC,L]

Links

1. Making www optional in htaccess rule: stackoverflow.com/questions/35124505/making-www-optional-in-htaccess-rule
2. htaccess redirect page from subdomain to main domain: stackoverflow.com/questions/31477917/htaccess-redirect-page-from-subdomain-to-main-domain
3. Redirect any sub domain to a page on main domain: stackoverflow.com/questions/8730298/redirect-any-sub-domain-to-a-page-on-main-domain

Web servers

• Apache

More

• Change default index page
• Deny access to a specific page by IP
• Deny access to a specific page for an IP address range
• HTTP to HTTPS redirect in .htaccess
• HTTPS to HTTP redirect for a single page
• Redirect index.php to root in .htaccess