apache mod_rewrite

The caret, ^, signifies the start of an URL, under the current directory. This directory is whatever directory the .htaccess file is in. You’ll start almost all matches with a caret.

The dollar sign, $, signifies the end of the string to be matched. You should add this in to stop your rules matching the first part of longer URLs.

Query string is read until "?"

example

RewriteCond %{QUERY_STRING} ^resid=ID

RewriteRule ^/eid?  http://store.datascrip.com/? [R=301,L]

Force SSL
RewriteEngine On

RewriteCond %{HTTPS} off

RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI}

Redirrect everything to new URL
RewriteRule /.* http://www.new-domain.com/ [R]

redirrect certain query string match + sub path to new URL
RewriteCond %{QUERY_STRING} ^resid=ID$
RewriteRule ^/eid? http://new.com/ [R,L]

this will redirrect testing.com/eid?resid=ID to http://new.com
the symbol $ is to indicate the string end

 RewriteCond %{HTTP_HOST} ^gab.com

 RewriteRule "^(.*)/p/7074689$" "https://gab.com/p/7075394" [R=301,L]

this will rewrite request go to gab.com/* and end with that redirrect to different place

Block all access to /test/* except /test/gab/*

RewriteCond %{HTTP_HOST} ^abc.com
RewriteCond %{REQUEST_URI} ^/test
RewriteRule  "!^/test/gab(.*)" "-" [F]

# rewrite rule base on akamai country code for maintenance pages
 RewriteCond %{HTTP:X-Akamai-Edgescape} code=MY
 RewriteCond %{REQUEST_URI} !^/maintenance
 RewriteRule ^(.*)$ /maintenance/ES/maintenance.html [R=301,L]

https://www.cheatography.com/davechild/cheat-sheets/mod-rewrite/