generic solution for Rewriteurl for all URL's [duplicate] - .htaccess

Question

My website runs a script called -> WSS wallpaper script
My Problem -> I have been trying to force remove or add trailing slash to the end of my URL to prevent duplicated content and also to clean up my URLs.
I have tried all sorts and tried everything I could think of and loads from the interwebs but no such luck yet! It might be a quick fix but I have looked at it so much I am probably blind to something dead obvious.
So I present you with all my .htaccess code:
DirectoryIndex index.php
RewriteEngine on
RewriteRule ^download/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+) image.php?id=$1&width=$2&height=$3&cropratio=$4&download=1 [L]
RewriteRule ^file/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+) image.php?id=$1&width=$2&height=$3&cropratio=$4 [L]
RewriteRule ^preview/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+) wallpaper_preview.php?id=$1&width=$2&height=$3&name=$4 [L]
RewriteRule ^thumbnail/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+)/([^/\.]+)/([^/\.]+)/([^/]+) image.php?wallpaper_id=$1&width=$2&height=$3&cropratio=$4&align=$5&valign=$6&file=$7 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+)/p([0-9]+) index.php?task=category&id=$1&name=$2&page=$3 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+)/([0-9a-zA-Z?-]+)/p([0-9]+) index.php?task=category&id=$1&name=$2&sortby=$3&page=$4 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+)/([0-9a-zA-Z?-]+)-([0-9]+) index.php?task=category&id=$1&sortby=$3&page=$4 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+) index.php?task=category&id=$1&name=$2 [L]
RewriteRule ^tag/([^/\.]+)/([0-9a-zA-Z?-]+)/([0-9]+) index.php?task=tag&t=$1&sortby=$2&page=$3 [L]
RewriteRule ^tag/([^/\.]+) index.php?task=tag&t=$1 [L]
RewriteRule ^profile/([0-9]+)?/([^/\.]+) index.php?task=profile&id=$1&name=$2 [L]
RewriteRule ^profile/comments/([0-9]+)?/([^/\.]+) index.php?task=users_comments&id=$1&name=$2 [L]
RewriteRule ^page/([0-9]+) index.php?task=view_page&id=$1 [L]
RewriteRule ^register index.php?task=register [L]
RewriteRule ^lost-password index.php?task=lost_pass [L]
RewriteRule ^links index.php?task=links [L]
RewriteRule ^news/item/([0-9]+)/([^/\.]+) index.php?task=news&id=$1 [L]
RewriteRule ^news/page([0-9]+) index.php?task=news&page=$1 [L]
RewriteRule ^members/([^/\.]+)-([^/\.]+)/page([0-9]+)? index.php?task=member_list&sort=$1&order=$2&page=$3 [L]
RewriteRule ^members index.php?task=member_list [L]
RewriteRule ^messages index.php?task=messages [L]
RewriteRule ^submit index.php?task=submit [L]
RewriteRule ^search/([^/\.]+) index.php?task=search&q=$1 [L]
RewriteRule ^search index.php?task=search [L]
RewriteRule ^submit index.php?task=submit [L]
RewriteRule ^r-([0-9]+)?-([0-9]+)? go.php?id=$1&ref=$2 [L]
RewriteRule ^r-([0-9]+)? go.php?id=$1 [L]
RewriteRule ^([^/\.]+)/([0-9]+)/([^/\.]+) index.php?task=view&id=$2&name=$3 [L]
RewriteRule ^news/([^/\.]+) index.php?task=news&name=$1 [L]
RewriteRule ^profile/([^/\.]+) index.php?task=profile&name=$1 [L]
RewriteRule ^news index.php?task=news [L]
RewriteRule ^page/([^/\.]+) index.php?task=view_page&name=$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+)/([0-9a-zA-Z'?-]+)/([0-9]+) index.php?task=category&name=$1&sortby=$2&page=$3 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+)/([^/\.]+) index.php?task=view&name=$2 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+) index.php?task=category&name=$1 [L]
## www reslove ##
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
## www reslove ##
## index reslove ##
Options +FollowSymLinks
RewriteCond %{THE_REQUEST} ^.*/index\.php
RewriteRule ^(.*)index.php$ http://www.epicwallpaper.net/$1 [R=301,L]
## index reslove ##

Answer 1

Right below the RewriteEngine On line, add:
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)/$ /$1 [L,R] # <- for test, for prod use [L,R=301]
to enforce a no-trailing-slash policy.
To enforce a trailing-slash policy:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*[^/])$ /$1/ [L,R] # <- for test, for prod use [L,R=301]
EDIT: commented the R=301 parts because, as explained in a comment:
Be careful with that R=301! Having it there makes many browsers cache the .htaccess-file indefinitely: It somehow becomes irreversible if you can't clear the browser-cache on all machines that opened it. When testing, better go with simple R or R=302
After you've completed your tests, you can use R=301.

Answer 2

To complement Jon Lin's answer, here is a no-trailing-slash technique that also works if the website is located in a directory (like example.org/blog/):
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} (.+)/$
RewriteRule ^ %1 [R=301,L]
For the sake of completeness, here is an alternative emphasizing that REQUEST_URI starts with a slash (at least in .htaccess files):
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} /(.*)/$
RewriteRule ^ /%1 [R=301,L] <-- added slash here too, don't forget it
Just don't use %{REQUEST_URI} (.*)/$. Because in the root directory REQUEST_URI equals /, the leading slash, and it would be misinterpreted as a trailing slash.
If you are interested in more reading:
PR 3145 for Laravel
A discussion on commit 343c31e
(update: this technique is now implemented in Laravel 5.5)

Answer 3

This is what I've used for my latest app.
# redirect the main page to landing
##RedirectMatch 302 ^/$ /landing
# remove php ext from url
# https://stackoverflow.com/questions/4026021/remove-php-extension-with-htaccess
RewriteEngine on
# File exists but has a trailing slash
# https://stackoverflow.com/questions/21417263/htaccess-add-remove-trailing-slash-from-url
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.*)/+$ /$1 [R=302,L,QSA]
# ok. It will still find the file but relative assets won't load
# e.g. page: /landing/ -> assets/js/main.js/main
# that's we have the rules above.
RewriteCond %{REQUEST_FILENAME} !\.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^/?(.*?)/?$ $1.php

Answer 4

Options +FollowSymLinks
RewriteEngine On
RewriteBase /
## hide .html extension
# To externally redirect /dir/foo.html to /dir/foo
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+).html
RewriteRule ^ %1 [R=301,L]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)/\s
RewriteRule ^ %1 [R=301,L]
## To internally redirect /dir/foo to /dir/foo.html
RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^([^\.]+)$ $1.html [L]
<Files ~"^.*\.([Hh][Tt][Aa])">
order allow,deny
deny from all
satisfy all
</Files>
This removes html code or php if you supplement it. Allows you to add trailing slash and it come up as well as the url without the trailing slash all bypassing the 404 code. Plus a little added security.