When is the leading slash (/) needed in mod_rewrite patterns?
I recently happened across this issue myself. There is a difference between using RewriteRule in the VirtualHost and as part of an .htaccess file. The following rule will match (with a URL like this: example.com/fruit/apple) if it appears in a .htaccess file:
RewriteRule ^(fruit|fruits)/apple http://newfruitwebsite.com$1 [R=301,L]
but will not match in the VirtualHost context. If we check out the apache docs we can see that:
When using the rewrite engine in .htaccess files the per-directory prefix is automatically removed for the RewriteRule pattern matching and automatically added after any relative (not starting with a slash or protocol name) substitution encounters the end of a rule set.
...but what does this mean? The above rule would not match the same URL if it was placed into the VirtualHost because it doesn't allow a leading slash (/) to be at the start of the string. The apache docs go on to say:
The removed prefix always ends with a slash, meaning the matching occurs against a string which never has a leading slash. Therefore, a Pattern with ^/ never matches in per-directory context.
...so if we need a slash for .htaccess and no slash for VirtualHost, how do we modify the pattern? You guessed it. We need to add /? to add the following condition in our regex (where our question mark (?) is the quantifier):
- Has between 0 and 1
/at the beginning of the string.
Now our URL (example.com/fruit/apple) string will match because we are handling the fact that if the rule is in the .htaccess file it will have 0 slashes and if it comes from the virtualHost it will have 1 slashes.
Related Reading:
- Apache Rewrite Directive
The {REQUEST_URI} string starts with a / character. Apache changed regex engines when it changed versions, so Apache version 1 requires the leading slash while Apache 2 forbids it! We can satisfy both versions by making the leading slash optional with the expression ^/? (? is the metacharacter for zero or one of the preceding character).
Source: http://www.sitepoint.com/apache-mod_rewrite-examples/
A few more resources which talk about the leading slash
http://forum.modrewrite.com/viewtopic.php?f=4&t=4627
and http://net.tutsplus.com/tutorials/other/a-deeper-look-at-mod_rewrite-for-apache/