why does my ReWrite rule not rewrite? - .htaccess

Question

I want to rewrite the urls like
site.com/eng?sentence=a-sentence
to be like:
site.com/eng/a-sentence
I only want the text shown in the brower url to change, I dont want a redirect.
RewriteCond %{THE_REQUEST} ^GET\s/+eng?sentence=([^/?&\s]+) [NC]
RewriteRule ^ /eng/%1 [NC,L,NE]
I have fiddled with it, comparing other rewrite rules i found for a long time, but just cant get it to work, And i really can't figure out why. I tried removing all other rewrite from my htaccess everytime i tested.
here is my htaccess file:
# Use PHP5.3 Single php.ini as default
AddHandler application/x-httpd-php54s .php
AddType application/x-httpd-php .htm .html
Options -Indexes -MultiViews
ErrorDocument 404 /404.php
ErrorDocument 500 /500.php
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*)$ $1.php [L]
RewriteCond %{THE_REQUEST} ^GET\s/+eng?sentence=([^/?&\s]+) [NC]
RewriteRule ^ /eng/%1 [NC,L,NE]
<FilesMatch "\.(jpg|jpeg|png|gif|swf)$">
Header set Cache-Control "max-age=604800, public"
</FilesMatch>
preferably, if you could fix my rule rather than write your own one that I won't likely understand, that would be great.

Answer 1

I only want the text shown in the browser url to change, I dont want a
redirect.
To change URL in browser you need external redirect using R flag.
Also ? is special regex symbol and to match a literal ? you need to escape it like \?. Also you would need a ? in the target URI's end to strip off query string.
Your complete .htaccess:
RewriteEngine On
# actual oneto pretty URL - external redirect
RewriteCond %{THE_REQUEST} \s/+eng\.php\?sentence=([^&\s]+) [NC]
RewriteRule ^ /eng/%1? [L,NE,R=302]
# pretty URL to actual one - internal rewrite
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^eng/([^/]+)/?$ /eng.php?sentence=$1 [L,QSA,NC]
# To internally forward /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{DOCUMENT_ROOT}/$1.php -f
RewriteRule ^(.+?)/?$ /$1.php [L]
I highly recommend you to read: Apache mod_rewrite Introduction