Question

I’m trying to understand mod_rewrite better and have one particular problem I think I need to get my head round first.

I am rewriting http://www.somesite.tld/a/b/c to index.php?path=a/b/c using the following

RewriteRule ^(?!index.php)(.*)$ index.php?path=$1 [NC,L]

An equivalent rewrite would, in this case, be

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ index.php?path=$1 [NC,L]

• This does not work without the RewriteCond — path=index.php would be the result without specifically ignoring files or saying ‘not index.php’. Why is this?
• Also, what is the ?! and ?: syntax that I sometimes see used? I do not understand the use of the ? when it is not prefixed by anything.
• And why, in the first RewriteRule above, do the second pair of brackets return a match for $1?

Cheers

Answer 1

(?= ...) and (?! ...) is special syntax in Perl regular expressions and in PCRE, which is the regex library that Apache uses. They are, respectively, positive and negative lookahead assertions: they match an empty string if the text after it matches or does not match the content in the brackets.

They are non-capturing, so they don’t define any $n (it would be pointless, since they match an empty string). (?: ...) is also non-capturing, it is used to group subexpressions.

Your first rule should work in .htaccess (but not in a virtual host configuration file), though it would be more correct to write it as

RewriteRule ^(?!index\.php$)(.*)$ index.php?path=$1 [L]

Perhaps another rule is interacting with it. You can check what exactly is being matched and rewritten with RewriteLog and RewriteLogLevel.