[Httpd Wiki] Update of "RewriteRule" by RichBowen

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The following page has been changed by RichBowen:
http://wiki.apache.org/httpd/RewriteRule

New page:
'''RewriteRule''' is the main directive (heart) of mod_rewrite.

== Syntax ==
=== Overview ===
The directive consists of three arguments, separated by a space:
{{{
RewriteRule pattern substitution [flags]
}}}
The third argument (flags) is optional. Let's go now into the details for each argument.

=== Pattern ===
The pattern (rule search-pattern, rule-pattern) is a defined regular expression (RegEx) to match against the URI-path (in ["per-dir context"] local filepath) of the current request. That is, everything after ''http://hostname'', and not including any query string arguments. (For examples of how to deal with the query string, see the discussion of the ["RewriteCond"] directive.)

You must define a RegEx to say "if the URI looks like ''this'', apply the substitution".

To negate the result of the RegEx, prefix the RegEx with an exclamation mark.
{{{
RewriteRule !regex substitution [flags]
}}}

=== Substitution ===

The substitution (sometimes called ''target'') is a string to replace the current filename with it. So it's the location you want rewrite to. The syntax of the substitution depends upon in which context you're using the RewriteRule.

Once you have set the substitution value, the RewriteRule says "if the requested URI looks like ''this'' (pattern) go ''here'' (substitution) instead."

'''["per-server context"]:'''
 * /abc/def.html (rewriting to an URL-path)
 * http://example.com/abc/def.html (rewriting to an absolute URL)

'''["per-dir context"]:'''
 * /abc/def.html (rewriting to an URL-path)
 * abc/def.html (rewriting to a local filepath [note the missing leading slash]. In the end, mod_rewrite needs an URL-path to go into the [per-dir bcontext internal redirect]. So you can use a URL-path directly or mod_rewrite must built such an URL-path internally from your local filepath)
 * http://example.com/abc/def.html (rewriteing to an absolute URL)

=== Flags ===
see ["Flags"]


=== Order of processing ===
Consider the following snippet:

{{{
RewriteEngine on
RewriteCond b =b
RewriteCond c =c
RewriteRule ^/a$  /b
}}}
A wrong assumption is that the conditions are processed before the pattern of the RewriteRule. But this is wrong. The processing order is as follows:
 * The Pattern of the RewriteRule (^/a$) is checked first.
 * If the pattern results ''true'',
 * the Condition b would be checked.
 * If that Condition is true or the OR-Flag was set to Cond b,
 * the second condition would be checked.
 * If this condition is true, too,
 * the substitution from the RewriteRule is applied.


== Examples ==

{{{
# rewriting a request starting with /a to /b and last rule
RewriteRule ^/a  /b [L]

# rewriting e.g. /user.html --> /user.php
RewriteRule ^/([^./]+)\.html$ /$1.php [L]

# matching every request, do nothing (no substitution) and set ENV myenv with the value 1
RewriteRule ^ - [E=myenv:1]


}}}