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