Apache HTTP Server Version 2.4

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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.

### See also

- `<If>`
- `<ElseIf>`
- `<Else>`
- `RewriteCond`
- `SetEnvIfExpr`
- `Header`
- `RequestHeader`
- `FilterProvider`
- Require expr
- `SSLRequire`
- `LogMessage`
- `mod_include`

## 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.

>     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.

Name
`HTTP_ACCEPT`
`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_HOST` | The host name of the remote host
`REMOTE_USER` | The name of the authenticated user (if any)
`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
`HANDLER` | The name of the handler creating the response
`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
`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)

Misc variables

Name | Description
--- | ---
`TIME_YEAR` | The current year (e.g. `2010`)
`TIME_MON` | The current month (`1`, ..., `12`)
`TIME_DAY` | The current day of the month
`TIME_HOUR` | The hour part of the current time (`0`, ..., `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`.

## 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

Name | Alternative | 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

Name | Description
--- | ---
`-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

## 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.

Name | Description | Restricted
--- | --- | ---
`-d` | The argument is treated as a filename. True if the file exists and is a directory | yes
`-e` | The argument is treated as a filename. True if the file (or dir or special) exists | yes
`-f` | The argument is treated as a filename. True if the file exists and is regular file | yes
`-s` | The argument is treated as a filename. True if the file exists and is not empty | yes
`-L` | The argument is treated as a filename. True if the file exists and is symlink | yes
`-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`.

## Functions

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

Name | Description | Restricted
--- | --- | ---
`req`, `http` | Get HTTP request header |
`resp` | Get HTTP response header |
`reqenv` | Lookup request environment variable |
`osenv` | Lookup operating system environment variable |
`note` | Lookup request note |
`env` | Return first match of `note`, `reqenv`, `osenv` |
`tolower` | Convert string to lower case |
`toupper` | Convert string to uppser case |
`escape` | Escape special characters in %hex encoding |
`unescape` | Unescape %hex encoded string, leaving encoded slashes alone; return empty string if %00 is found |
`file` | Read contents from a file | yes
`filesize` | Return size of a file (or 0 if file does not exist or is not regular file) | yes

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

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`).

## Example expressions

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} >= 9 && %{TIME_HOUR} <= 17
</Directory>`

## Other

Name | Alternative | Description
--- | --- | ---
`-in` | `in` | string contained in string list
`/regexp/` | `m#regexp#` | Regular expression (the second form allows different delimiters than /)
`/regexp/i` | `m#regexp#i` | Case insensitive regular expression
`$0 ... $9` |  | Regular expression backreferences

### Regular expression backreferences

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.

## Comparison with SSLRequire

The ap_expr syntax is mostly a superset of the syntax of the deprecated `SSLRequire` directive. The differences are described in `SSLRequire`'s documentation.

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