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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.

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 ")"

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.

Other request related variables

Misc variables

Some modules register additional variables, see e.g. mod_ssl.

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.

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.

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

Normal string-valued functions take one string as argument and return a string. Functions names are not case sensitive. Modules may register additional functions.

The functions marked as "restricted" in the final column are not available in some modules like mod_include.

The functions marked as "ordering" in the final column require some consideration for the ordering of different components of the server, especially when the function is used within the <If> directive which is evaluated relatively early.

When environment variables are looked up within an <

> condition, it's important to consider how extremely early in request processing that this resolution occurs. As a guideline, any directive defined outside of virtual host context (directory, location, htaccess) is not likely to have yet had a chance to execute.

in virtual host scope is one directive that runs prior to this resolution

When reqenv is used outside of <If>, the resolution will generally occur later, but the exact timing depends on the directive the expression has been used within.

When the functions req or http are used, the header name will automatically be added to the Vary header of the HTTP response, except where otherwise noted for the directive accepting the expression. The req_novary function can be used to prevent names from being added to the Vary header.

In addition to string-valued functions, there are also list-valued functions which take one string as argument and return a wordlist, i.e. a list of strings. The wordlist can be used with the special -in operator. Functions names are not case sensitive. Modules may register additional functions.

There are no built-in list-valued functions. mod_ssl provides PeerExtList. See the description of SSLRequire for details (but PeerExtList is also usable outside of SSLRequire).

The following examples show how expressions might be used to evaluate requests:

# Compare the host name to example.com and redirect to www.example.com if it matches
<If "%{HTTP_HOST} == 'example.com'">
    Redirect permanent "/" "http://www.example.com/"
</If>

# Force text/plain if requesting a file with the query string contains 'forcetext'
<If "%{QUERY_STRING} =~ /forcetext/">
    ForceType text/plain
</If>

# Only allow access to this content during business hours
<Directory "/foo/bar/business">
    Require expr %{TIME_HOUR} -gt 9 && %{TIME_HOUR} -lt 17
</Directory>

# Check a HTTP header for a list of values
<If "%{HTTP:X-example-header} in { 'foo', 'bar', 'baz' }">
    Header set matched true
</If>

# Check an environment variable for a regular expression, negated.
<If "! reqenv('REDIRECT_FOO') =~ /bar/">
    Header set matched true
</If>

# Check result of URI mapping by running in Directory context with -f
<Directory "/var/www">
    AddEncoding x-gzip gz
<If "-f '%{REQUEST_FILENAME}.unzipme' && ! %{HTTP:Accept-Encoding} =~ /gzip/">
      SetOutputFilter INFLATE
</If>
</Directory>

# Check against the client IP
<If "-R '192.168.1.0/24'">
    Header set matched true
</If>

# Function example in boolean context
<If "md5('foo') == 'acbd18db4cc2f85cedef654fccc4a4d8'">
  Header set checksum-matched true
</If>

# Function example in string context
Header set foo-checksum "expr=%{md5:foo}"

# This delays the evaluation of the condition clause compared to <If>
Header always set CustomHeader my-value "expr=%{REQUEST_URI} =~ m#^/special_path\.php$#"

# Conditional logging
CustomLog logs/access-errors.log common "expr=%{REQUEST_STATUS} >= 400"
CustomLog logs/access-errors-specific.log common "expr=%{REQUEST_STATUS} -in {'405','410'}"

The strings $0 ... $9 allow to reference the capture groups from a previously executed, successfully matching regular expressions. They can normally only be used in the same expression as the matching regex, but some modules allow special uses.

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