Apache_HTTP_server_logo_(2016) mod_rewrite is a powerful module that Apache can utilize. It is a way of rewriting URLs, modifying the request that Apache recieves. This could be a moved document, or enforcing SSL (rewriting the URL from http to https).

This is a complex subject, which cannot be covered here, but for further information, refer to the full documentation. Rewrite rules can exist in a .htaccess file, or the main configuration file, or preferably a <Directory> stanza. mod_rewrite uses Regex (compatible with Perl) for its pattern matching engine. This is nearly unlimited in searching across a range of URLs.

Enable Rewrite Engine

To enable mod_rewrite you should include the following in you Apache Documentation:

RewriteEngine on

A restart of Apache will be required to load the engine.

Declaring a Rewrite Rule

To declare a rule you will have something similar to the code below:

RewriteRule ^/old.html$ new.html [R]

This will redirect a request that Apache receives for old.html page to new.html page. The ^ indicates that it must be the initial part of the request, and the $ indicates that it is the end of the request. If /subdir/old.html is passed, it will fail this search pattern as /subdir/ is at the beginning of the pattern. This is in line with Regular Expressions pattern matching.

These rules can be embedded within a particular directory, using the <Directory stanza:

<Directory /var/www/html/subdirectory>
  RewriteEngine on
  RewriteRule "^old.html$" "new.html"
</Directory>

The above rule only applies to the directory named subddirectory.

Rewrite Flags

At the end of each RewriteRule is a set of flags that determines what should be done - these are enclosed in a set of square brackets. One of the most common is [R] which is a redirect, carried out at the browser level (issued by the webserver).

A full list of flags is documented at https://httpd.apache.org/docs/2.4/rewrite/flags.html.

Regular Expressions and mod_rewrite

Character Meaning Example
. Match any character c.t matches cat
+ Repeats the previous match one or more times a+ a, aa, aaa
* Repeats the previous match zero or more times a* matches the same as a+ but will also match an empty string
? Makes the match optional colou?r will match color and colour
\ Escape the next character . will match a . (dot) and not any single character as explained above
^ Called an anchor, matches the beginning of thes string ^a will match a string that begins with a
$ The other anchor that matches the end of a string a$ will match a string that ends with a
( ) Groups several characters into a single unit, and captures a match for use in a backreference (ab)+ matches abababab - the + applies to the group
[ ] A character class - matches one of the characters c[uoa]t matches cut, cot, cat
[^ ] Negative character class - matches any character not specified c[^/] matches cat or c=t but not c/t

mod_rewrite Log Level for < v2.4

The mod_rewrite will write to the usual apache log files.

For previous versions (pre Apache 2.4) the directive RewriteLogLevel will set the level of logging written, ranging in values from 0-9 with 0 being no logging and 0 being the most verbose. Logs will appear as pass through lines in the file.

mod_rewrite Log Level for v2.4

With the current version of Apache (2.4), mod_rewrite, the older methods of controlling logging have now been replaced by a new per-module logging method:

LogLevel alert rewrite:trace3