Htaccess Redirect for API

I want to redirect every .htaccess call to a particular file.

I want to redirect all Get/Put/Post/Delete calls to a folder to a specific index.php file.

I have following directory structure:

\var\www\html\app

inside app i have a file index.php

Now i want any calls like

GET http(s)://myWeb.com/app 
POST http(s)://myWeb.com/app 
GET http(s)://myWeb.com/app/helloworld

to redirect to

\var\www\html\app\index.php

Here is my failed attempt:

RewriteEngine On
RewriteBase /app/
RewriteRule ^ app/index.php [QSA,L]
#RewriteRule ^ index.php [QSA,L]   //Tried this one as well

This .htaccess is inside

/var/www/html

I also placed a similar htaccess inside app folder but it doesnt works too.

Should i use 2 htaccess files ? If one what should be the format for it ?

Also, is there a tool to check htaccess regex ? something like www.regexr.com

Did you try this?

RewriteRule . app/index.php [R,L]

I think QSL is needed only if you want to pass the URL queries