When is the leading slash (/) needed in mod_rewrite patterns?

Question

Here is an example from the Apache documentation for enforcing canonical hostnames:

RewriteCond %{HTTP_HOST} !^www\.example\.com [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/?(.*) http://www.example.com/$1 [L,R,NE]

What I would like to know is why the /? is needed. I've tried removing it and everything appears to work the same. I was under the impression that the pattern part of the rewrite rule was compared against what comes after the slash.

Answer 1

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.

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Related Reading:

• Apache Rewrite Directive

Answer 2

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/

Answer 3

All I know is that on Apache 2.4 on CentOS 7, the leading slash is required in a VHost but is forbidden in .htaccess

VHost:

<VirtualHost *:443>
  ServerName supersite-cms.example.com

  # Forward cms requests to the subsite
  # Note the RewriteRule syntax is different than in .htaccess :|
  RewriteEngine on
  RewriteRule ^/subsite/admin$ https://subsite-cms.example.com/admin [L,R=307,E]

  # ... do other stuff if not redirected
</VirtualHost>

.htaccess

RewriteRule ^subsite/admin$ https://subsite-cms.example.com/admin [L,R=307]

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Source: The last 5 hours 26 minutes of my life