Underscores to hyphens dynamic url - php

Question

I have the following existing .htaccess in place
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.example\.com
RewriteRule (.*) http://www.example.com/$1 [R=301,L]
RewriteRule home index.php [L]
RewriteRule ^([^/]*)/$ /index.php?TopicID=$1 [L]
RewriteRule ^([^/]*)/([^/]*)/$ /index.php?RollID=$1&FrameID=$2 [L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule (.*) $1.php [L]
RedirectMatch 301 /cache(.*) //$1
ErrorDocument 404 http://www.example.com/error.php
I have many dynamically generated URLs which are being rewritten to this
www.example.com/abc_def_ghi/ OR
www.example.com/abc_def_ghi/abc_def_ghi/
and i'd like to end up with this
www.example.com/abc-def-ghi/ OR
www.example.com/abc-def-ghi/abc-def-ghi/
I've tried adding this to the end of my .htaccess
Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !.(html|php)($|?) [NC]
RewriteRule ^([^_]*)_+(.*)$ $1-$2 [E=underscores:Yes,N]
RewriteCond %{ENV:underscores} ^Yes$
RewriteRule (.*) http://%{HTTP_HOST}/$1 [R=301,L]
but i'm getting a 500 error.
Can anyone help?

Answer 1

Rearrange your rules like this:
ErrorDocument 404 http://www.example.com/error.php
RewriteEngine on
RewriteRule ^cache(.*) /$1 [L,R=301]
RewriteCond %{HTTP_HOST} !^www\.example\.com
RewriteRule (.*) http://www.example.com/$1 [R=301,L]
# if there is only one underscore then repalce it by - and redirect
RewriteRule ^([^_]*)_([^_]*)$ /$1-$2 [L,R=302]
# if there are more than one underscores then "repeatedly" repalce it by - and set env var
RewriteRule ^([^_]*)_([^_]*)_(.*) $1-$2-$3 [E=USCORE:1]
# if USCORE env var is set and there is no underscore left then redirect
RewriteCond %{ENV:REDIRECT_USCORE} =1
RewriteRule ^([^_]+)$ /$1 [L,R=302]
RewriteRule home index.php [L]
RewriteCond %{REQUEST_FILENAME} -d
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
RewriteRule ^([^/]+)/?$ /index.php?TopicID=$1 [L,QSA]
RewriteRule ^([^/]+)/([^/]+)/?$ /index.php?RollID=$1&FrameID=$2 [L,QSA]
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule (.*) $1.php [L]