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Apache > HTTP Server > Documentation > Version 2.4

Expressions in Apache HTTP Server

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Historically, there are several syntax variants for expressions used to express a condition in the different modules of the Apache HTTP Server. There is some ongoing effort to only use a single variant, called ap_expr, for all configuration directives. This document describes the ap_expr expression parser.

The ap_expr expression is intended to replace most other expression variants in HTTPD. For example, the deprecated SSLRequire expressions can be replaced by Require expr.

See also

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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 ")"
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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

NameDescription
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_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

NameDescription
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. mod_ssl.

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Binary operators

With the exception of some built-in comparison operators, binary operators have the form "-[a-zA-Z][a-zA-Z0-9_]+", i.e. a minus and at least two characters. The name is not case sensitive. Modules may register additional binary operators.

Comparison operators

NameAlternative Description
== = String equality
!= String inequality
< String less than
<= String less than or equal
> String greater than
>= String greater than or equal
-eq eq Integer equality
-ne ne Integer inequality
-lt lt Integer less than
-le le Integer less than or equal
-gt gt Integer greater than
-ge ge Integer greater than or equal

Other binary operators

NameDescription
-ipmatch IP address matches address/netmask
-strmatch left string matches pattern given by right string (containing wildcards *, ?, [])
-strcmatch same as -strmatch, but case insensitive
-fnmatch same as -strmatch, but slashes are not matched by wildcards
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Unary operators

Unary operators take one argument and have the form "-[a-zA-Z]", i.e. a minus and one character. The name is case sensitive. Modules may register additional unary operators.

NameDescriptionRestricted
-d The argument is treated as a filename. True if the file exists and is a directoryyes
-e The argument is treated as a filename. True if the file (or dir or special) existsyes
-f The argument is treated as a filename. True if the file exists and is regular fileyes
-s The argument is treated as a filename. True if the file exists and is not emptyyes
-L The argument is treated as a filename. True if the file exists and is symlinkyes
-h The argument is treated as a filename. True if the file exists and is symlink (same as -L)yes
-F True if string is a valid file, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact your server's performance!
-U True if string is a valid URL, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact your server's performance!
-A Alias for -U
-n True if string is not empty
-z True if string is empty
-T False if string is empty, "0", "off", "false", or "no" (case insensitive). True otherwise.
-R Same as "%{REMOTE_ADDR} -ipmatch ...", but more efficient

The operators marked as "restricted" are not available in some modules like mod_include.

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Functions

Normal string-valued