> RewriteCond

The RewriteCond directive defines conditions under which the rewriting should take place.

Syntax

We should really encourage people to use the lexicographically equal operator instead of a RegEx if they want to ckeck, if test string is lexicographically equal to cond pattern.

E.g. using 

RewriteCond {HTTP_HOST} !=""
instead of 
RewriteCond {HTTP_HOST} .
# or
RewriteCond {HTTP_HOST} !^$
or using
RewriteCond {REQUEST_URI} !=/foo/bar
instead of 
RewriteCond {REQUEST_URI} !^/foo/bar$
or 
RewriteCond {SERVER_PORT} =443
instead of 
RewriteCond {SERVER_PORT} ^443$

Note: Conditions are being processed after the pattern of the RewriteRule has matched. This means that the Condition in following example would be useless (it's always true): 

RewriteCond %{REQUEST_URI} \.html$
RewriteRule \.html$ - [G]

While this one wastes performance: 

RewriteCond %{REQUEST_URI} ^/([^.]*)\.html$
RewriteRule ^/(.*) /%1.php [PT]
Every Request matches the rule-pattern and after that the condition will be checked. But you can easily check the uri value in the rule-pattern, so that there is no need for such a condition here: 
RewriteRule ^/([^.]*)\.html$ /$1.php [PT]

Examples

See ConditionalRewrites