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

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/