mod_rewrite: encode only certain matches in URL - apache

Question

I am trying to rewrite a URL using mod_rewrite and encode a substring in my URL which is between brackets. My URL:
http://localhost/something?var_a=A&var_b=(B&2/3&)&var_c=C
and my .htaccess:
RewriteEngine On
RewriteCond %{QUERY_STRING} (.+)\((.*?)\)(.+)
RewriteRule ^.*$ somedir/%1%2%3? [R,B]
so I capture three strings, anything before the brackets, anything within, and anything after.
Result is:
somedir/var_a%3dA%26var_b%3dB%262%2f3%26%26var_c%3dC
but I would like to encode only the text which was within the brackets of my initial URL, such that
somedir/var_a=A&var_b=dB%262%2f3%26&var_c=C
The problem seems to be that the [B] option decodes the whole string. Is there a way to do this selectively? Also, my solution could only capture an occurrence of brackets once, it would be nice to have this more generic; could someone give me a hint?
Note that this question is related to my previous one, where I was trying to capture text between brackets.

Answer 1

This is extremely tricky for mod_rewrite but I took a shot at it. Solution is not pretty as it involves 2 redirects and use of cookies, but it works.
RewriteEngine On
# store non bracket query string in cookie while redirecting value in brackets using B flag
RewriteCond %{QUERY_STRING} ^(.+?&)?(var_b=)\(([^)]*)\)(.*)$ [NC]
RewriteRule !^somedir/ /somedir/%3? [L,CO=QS:%1-%2-%4:%{HTTP_HOST},B,R]
# retrieve value from cookie and use it to construct full URL
RewriteCond %{QUERY_STRING} ^$
RewriteCond %{HTTP_COOKIE} QS=([^-]*)-([^-]+)-([^;]+) [NC]
RewriteRule ^(somedir)/(.*)$ /$1/%1%2$2%3? [L,R,NE]
Using these rules when I visit this URL: http://localhost/something?var_a=A&var_b=(B&2/3&)&var_c=C
it gets redirected to: http://localhost/somedir/var_a=A&var_b=B%262%2f3%26&var_c=C