Expressions in Apache HTTP Server

Grammar in Backus-Naur Form notation

Backus-Naur Form (BNF) is a notation technique for context-free grammars, often used to describe the syntax of languages used in computing. In most cases, expressions are used to express boolean values. For these, the starting point in the BNF is expr. However, a few directives like LogMessage accept expressions that evaluate to a string value. For those, the starting point in the BNF is string.

expr        ::= "true" | "false"
              | "!" expr
              | expr "&&" expr
              | expr "||" expr
              | "(" expr ")"
              | comp

comp        ::= stringcomp
              | integercomp
              | unaryop word
              | word binaryop word
              | word "in" "{" wordlist "}"
              | word "in" listfunction
              | word "=~" regex
              | word "!~" regex


stringcomp  ::= word "==" word
              | word "!=" word
              | word "<"  word
              | word "<=" word
              | word ">"  word
              | word ">=" word

integercomp ::= word "-eq" word | word "eq" word
              | word "-ne" word | word "ne" word
              | word "-lt" word | word "lt" word
              | word "-le" word | word "le" word
              | word "-gt" word | word "gt" word
              | word "-ge" word | word "ge" word

wordlist    ::= word
              | wordlist "," word

word        ::= word "." word
              | digit
              | "'" string "'"
              | """ string """
              | variable
              | rebackref
              | function

string      ::= stringpart
              | string stringpart

stringpart  ::= cstring
              | variable
              | rebackref

cstring     ::= ...
digit       ::= [0-9]+

variable    ::= "%{" varname "}"
              | "%{" funcname ":" funcargs "}"

rebackref   ::= "$" [0-9]

function     ::= funcname "(" word ")"

listfunction ::= listfuncname "(" word ")"

Variables

The expression parser provides a number of variables of the form %{HTTP_HOST}. Note that the value of a variable may depend on the phase of the request processing in which it is evaluated. For example, an expression used in an <If > directive is evaluated before authentication is done. Therefore, %{REMOTE_USER} will not be set in this case.

The following variables provide the values of the named HTTP request headers. The values of other headers can be obtained with the req function. Using these variables may cause the header name to be added to the Vary header of the HTTP response, except where otherwise noted for the directive accepting the expression. The req_novary function may be used to circumvent this behavior.

Name
HTTP_ACCEPT
HTTP_COOKIE
HTTP_FORWARDED
HTTP_HOST
HTTP_PROXY_CONNECTION
HTTP_REFERER
HTTP_USER_AGENT

Other request related variables

Name	Description
REQUEST_METHOD	The HTTP method of the incoming request (e.g. GET)
REQUEST_SCHEME	The scheme part of the request's URI
REQUEST_URI	The path part of the request's URI
DOCUMENT_URI	Same as REQUEST_URI
REQUEST_FILENAME	The full local filesystem path to the file or script matching the request, if this has already been determined by the server at the time REQUEST_FILENAME is referenced. Otherwise, such as when used in virtual host context, the same value as REQUEST_URI
SCRIPT_FILENAME	Same as REQUEST_FILENAME
LAST_MODIFIED	The date and time of last modification of the file in the format 20101231235959, if this has already been determined by the server at the time LAST_MODIFIED is referenced.
SCRIPT_USER	The user name of the owner of the script.
SCRIPT_GROUP	The group name of the group of the script.
PATH_INFO	The trailing path name information, see AcceptPathInfo
QUERY_STRING	The query string of the current request
IS_SUBREQ	"true" if the current request is a subrequest, "false" otherwise
THE_REQUEST	The complete request line (e.g., "GET /index.html HTTP/1.1")
REMOTE_ADDR	The IP address of the remote host
REMOTE_PORT	The port of the remote host (2.4.26 and later)
REMOTE_HOST	The host name of the remote host
REMOTE_USER	The name of the authenticated user, if any (not available during <If>)
REMOTE_IDENT	The user name set by mod_ident
SERVER_NAME	The ServerName of the current vhost
SERVER_PORT	The server port of the current vhost, see ServerName
SERVER_ADMIN	The ServerAdmin of the current vhost
SERVER_PROTOCOL	The protocol used by the request
DOCUMENT_ROOT	The DocumentRoot of the current vhost
AUTH_TYPE	The configured AuthType (e.g. "basic")
CONTENT_TYPE	The content type of the response (not available during <If>)
HANDLER	The name of the handler creating the response
HTTP2	"on" if the request uses http/2, "off" otherwise
HTTPS	"on" if the request uses https, "off" otherwise
IPV6	"on" if the connection uses IPv6, "off" otherwise
REQUEST_STATUS	The HTTP error status of the request (not available during <If>)
REQUEST_LOG_ID	The error log id of the request (see ErrorLogFormat)
CONN_LOG_ID	The error log id of the connection (see ErrorLogFormat)
CONN_REMOTE_ADDR	The peer IP address of the connection (see the mod_remoteip module)
CONTEXT_PREFIX
CONTEXT_DOCUMENT_ROOT

Misc variables

Name	Description
TIME_YEAR	The current year (e.g. 2010)
TIME_MON	The current month (01, ..., 12)
TIME_DAY	The current day of the month (01, ...)
TIME_HOUR	The hour part of the current time (00, ..., 23)
TIME_MIN	The minute part of the current time
TIME_SEC	The second part of the current time
TIME_WDAY	The day of the week (starting with 0 for Sunday)
TIME	The date and time in the format 20101231235959
SERVER_SOFTWARE	The server version string
API_VERSION	The date of the API version (module magic number)

Some modules register additional variables, see e.g.