Rewrite URL term(s) separated by equal sign to query string(s) - regex

Question

Using mod_rewrite, what RewriteRule(s) would make the following two examples function properly?
Example #1 (one term):
http://example.net/dir/thanks
-to-
http://example.net/dir/index.php?a=thanks
Example #2 (two terms):
http://example.net/dir/thanks=stackoverflow
-to-
http://example.net/dir/index.php?a=thanks&b=stackoverflow
Note: The .htaccess file is located in /dir/ outside of domain root.
I've gotten close with this:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]*)=([^/]*)/?$ index.php?a=$1&b=$2 [L]
However, to work with example #1 a trailing "=" is required (which I want to avoid). I've tried changing "=" to "=?" in the regex to make it optional but while it then works for example #1 it fails for example #2.
Thank you kindly for any consideration to my question. I'm new to mod_rewrite and regex and am truly stumped.

Answer 1

I see two main approaches:
1. More safer/easier to understand and extend if necessary. Make two rules: 1st will catch thanks=stackoverflow while 2nd will work with thanks only:
RewriteEngine On
# do not do anything for already existing files
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule .+ - [L]
# will work with /thanks=stackoverflow
RewriteRule ^([^/=]+)=([^/=]+)/?$ index.php?a=$1&b=$2 [L]
# will work with /thanks
RewriteRule ^([^/=]+)/?$ index.php?a=$1 [L]
2. Combine those two rules into a single rule. In this case parameter b= will always be present, but will be empty for thanks scenario:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/=]+)(=([^/=]+))?/?$ index.php?a=$1&b=$3 [L]
Tested both -- working fine on my Apache box.

Answer 2

Why not create two seperate rewrite rules:
i) One that takes care of example 1 - with the added check that it does not contain an equal sign [so that you don't match example 2 with rewrite rule 1]
ii) Another one that takes care of example 2