what does $1 in .htaccess file mean?

Question

I am trying to understand the meaning of this line in the .htaccess file

 RewriteRule ([a-z0-9/-]+).html $1.php [NC,L,QSA]

basically what does $1.php ? what file in the server

if we have home.html where this gonna redirect to? home.php?

Answer 1

$1 is the first captured group from your regular expression; that is, the contents between ( and ). If you had a second set of parentheses in your regex, $2 would contain the contents of those parens. Here is an example:

RewriteRule ([a-z0-9/-]+)-([a-z]+).html$ $1-$2.php [NC,L,QSA]

Say a user navigates to hello-there.html. They would be served hello-there.php. In your substitution string, $1 contains the contents of the first set of parens (hello), while $2 contains the contents of the second set (there). There will always be exactly as many "dollar" values available in your substitution string as there are sets of capturing parentheses in your regex.

If you have nested parens, say, (([a-z]+)-[a-z]+), $1 always refers to the outermost capture (in this case the whole regex), $2 is the first nested set, and so on.