What about the ^ $in the example of Apache rewrite non-www to www official documents?

there is a paragraph

in the example of the conversion of URLs from non-www to www in the official Apache HTTPD documentation.

RewriteCond %{HTTP_HOST} !^www\.example\.com [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/?(.*) http://www.example.com/$1 [L,R,NE]

this rewrite rule will be something like http://example.com/index.php...://www.example.com/index.php.

The! (exclamation mark) in the first line of

indicates no, the NC indicates case insensitivity, and the backslash indicates the middle of the domain name. Symbols. The ^ in the third line of
may start with / (slash), followed by any string that ends. This pairs the index.php above, rewrites the original address to http://www.example.com/index.php, and temporarily turns to the end of the sentence without converting special characters.

but what does the ! ^ $ in the second line mean? Literally, it seems to be the end of the beginning. If there is no second line of code, I think this non-www to www domain name redirection can also be successful, right?