.htaccess code showing 500 internal error

Hi there I'm trying to make a blog demo and I'm having some pretty URL codes already.

i am having a url www.xyz.com

and a search url www.xyz.com/search/this+is+a+search+text

in search url the parameter search is a page name and this+is+a+search+text is a parameter that i'll be parsing

I'm having a .htaccess code below already

# code to make pretty URLS | we're using this code to achieve /category/slug
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.+)/([\w-]+)/([\d]+)$ app/post.php?&category=$2&page=$3 [L,QSA]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.+)/([\w-]+)/([\w-]+)$ app/post.php?&category=$2&slug=$3 [L,QSA]

# code to make pretty URLS for search page
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.+)/([\w-]+)/([\d]+)$ app/search.php?&searchstring=$2&page=$3 [L,QSA]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.+)/([\d]+)$ app/index.php?page=$2 [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.+)/([\w-]+)$ app/post.php?category=$2 [L]

I'm using the below code for serach page

# code to make pretty URLS for search page
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?(.+)/([\w-]+)/([\d]+)$ app/search.php&searchstring=$2&page=$3 [L,QSA]

But while using the code I get a 500 internal error message, I'm not able to figure out what's the error!

I would really appreciate if anyone could help me out with this logic.

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