I am moving a custom coded blog to Wordpress, and also have to setup a redirect which will handle all the blogs.

I have to redirect out of this:

/oldblogdirectory/Old_Blog_Posts_Looked_Like_This.htm

to:

/newblogdirectory/new-blog-posts-look-like-this/

Any applying for grants the regex for any redirect such as this?

Do this mod_rewrite rules:

RewriteEngine on
RewriteRule ^(oldblogdirectory/[^_]*)_([^_]*)_(.*) /$1-$2-$3 [N]
RewriteRule ^(oldblogdirectory/[^_]*)_(.*) /$1-$2
RewriteRule ^oldblogdirectory/(.+)\.htm$ /newdirectory/$1/ [L,R=301]

As well as the uppercase to lowercase conversion you’ll either require a mapping such as the internal [cde] function or else you use PHP for.

Gumbo's approach is unquestionably the best way to get it done. I made two test sites:

tolower

And also the following RewriteRules redirect effectively. They're only slightly transformed to Gumbo's proposal:

oldblogdir/archives/blog_posts_look_like_this.htm
newblogdir/archives/blog-posts-look-like-this

Observe that the RewriteEngine on RewriteBase / RewriteRule ^(oldblogdir/archives/[^_]*)_(.*) $1-$2 [N] RewriteRule ^oldblogdir/archives/(.*?)\.htm$ newblogdir/archives/$1 [R,NC,L] causes the .htaccess file to become re-examined before the RegEx no more matches. So that you should place it towards the top from the file.