Question

How to remove ? sign from URL with query string

Answer 1

RewriteEngine On

# This is to physically change what's in the browser's address bar using a client redirect
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /\?([^\ ]+)
RewriteRule ^$ /%1? [R=301,L]

# This is to internally rewrite on the server side
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^/?(.+)$ /?$1 [L]

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Also please could you explain me what does mean [A-Z]{3,9}\ and ([^\ ]+) on its own? Where I can read about building this type of structures?

That regular expression is used to match the HTTP Request, the first line is going to look something like this:

GET /path/to/file HTTP/1.1

The first word is the METHOD, which can be anywhere between 3 to 9 characters long (thus the {3,9}, the second word is the unaltered URI of the request. This is what goes through the rewrite engine, but before any path processing is done on it. The last bit is the protocol version and is somewhat optional, and determines whether there will be headers afterwards.