Redirect HTTP to HTTPS, for the main domain as well as all subdomains.

.htaccess

# Redirect HTTP to HTTPS, for the main domain as well as all subdomains.
#
# Example:
# http://domain.com/           => https://domain.com/
# http://subdomain.domain.com/ => https://subdomain.domain.com/
#
# Make sure you replace "replacewithyourdomain" with your actual root domain!
#
# @author: Alain Schlesser <alain.schlesser@gmail.com>

# The HTTP to HTTPS redirection code needs to go in front of the WordPress code.

# Make sure RewriteEngine is on
RewriteEngine On

# First, redirect HTTP subdomains to HTTPS subdomains
RewriteCond %{SERVER_PORT} 80
RewriteCond %{HTTP_HOST} ^(.*)\.replacewithyourdomain\.com$
RewriteRule ^(.*)$ https://%1.replacewithyourdomain.com/$1 [R=301,L]

# Now, redirect HTTP main domain to HTTPS main domain
RewriteCond %{SERVER_PORT} 80
RewriteCond %{HTTP_HOST} ^replacewithyourdomain\.com$
RewriteRule ^(.*)$ https://replacewithyourdomain.com/$1 [R=301,L]

# BEGIN WordPress

# [Here is your standard WordPress code, you should not modify this.]

# END WordPress