The BrowserMatch is a special cases of the SetEnvIf directive that sets environment variables conditional on the User-Agent HTTP request header. The following two lines have the same effect:

Some additional examples:

The BrowserMatchNoCase directive is semantically identical to the BrowserMatch directive. However, it provides for case-insensitive matching. For example:

The BrowserMatch and BrowserMatchNoCase directives are special cases of the SetEnvIf and SetEnvIfNoCase directives. The following two lines have the same effect:

The SetEnvIf directive defines environment variables based on attributes of the request. The attribute specified in the first argument can be one of four things:

1. An HTTP request header field (see RFC2616 for more information about these); for example: Host, User-Agent, Referer, and Accept-Language. A regular expression may be used to specify a set of request headers.
2. One of the following aspects of the request:
   • Remote_Host - the hostname (if available) of the client making the request
   • Remote_Addr - the IP address of the client making the request
   • Server_Addr - the IP address of the server on which the request was received (only with versions later than 2.0.43)
   • Request_Method - the name of the method being used (GET, POST, et cetera)
   • Request_Protocol - the name and version of the protocol with which the request was made (e.g., "HTTP/0.9", "HTTP/1.1", etc.)
   • Request_URI - the resource requested on the HTTP request line -- generally the portion of the URL following the scheme and host portion without the query string. See the RewriteCond directive of mod_rewrite for extra information on how to match your query string.
3. The name of an environment variable in the list of those associated with the request. This allows SetEnvIf directives to test against the result of prior matches. Only those environment variables defined by earlier SetEnvIf[NoCase] directives are available for testing in this manner. 'Earlier' means that they were defined at a broader scope (such as server-wide) or previously in the current directive's scope. Environment variables will be considered only if there was no match among request characteristics and a regular expression was not used for the attribute.
4. A reference to an extension of an SSL client certificate, located by its object id oid. If a non-SSL request, or in absense of the configured oid, no variable will be set. If the oid is found multiple times, the individual strings will be concatenated, separated by a comma ','. The oid should reference a string-valued extension.

The second argument (regex) is a regular expression. If the regex matches against the attribute, then the remainder of the arguments are evaluated.

The rest of the arguments give the names of variables to set, and optionally values to which they should be set. These take the form of

1. varname, or
2. !varname, or
3. varname=value

In the first form, the value will be set to "1". The second will remove the given variable if already defined, and the third will set the variable to the literal value given by value. Since version 2.0.51 Apache will recognize occurrences of $1..$9 within value and replace them by parenthesized subexpressions of regex.

The first three will set the environment variable object_is_image if the request was for an image file, and the fourth sets intra_site_referral if the referring page was somewhere on the www.mydomain.example.com Web site.

The sixth example will set the NetscapeComment environment variable to the string found in the corresponding SSL client certificate field (if found).

The last example will set environment variable HAVE_TS if the request contains any headers that begin with "TS" whose values begins with any character in the set [a-z].

The SetEnvIfNoCase is semantically identical to the SetEnvIf directive, and differs only in that the regular expression matching is performed in a case-insensitive manner. For example:

This will cause the site environment variable to be set to "apache" if the HTTP request header field Host: was included and contained Apache.Org, apache.org, or any other combination.