Apache HTTP Server Version 2.4

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# Expressions in Apache HTTP Server

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>`
- `ErrorDocument`
- `Alias`
- `ScriptAlias`
- `Redirect`
- `AuthBasicFake`
- `AuthFormLoginRequiredLocation`
- `AuthFormLoginSuccessLocation`
- `AuthFormLogoutLocation`
- `AuthName`
- `AuthType`
- `RewriteCond`
- `SetEnvIfExpr`
- `Header`
- `RequestHeader`
- `FilterProvider`
- Require expr
- Require ldap-user
- Require ldap-group
- Require ldap-dn
- Require ldap-attribute
- Require ldap-filter
- Require ldap-search
- Require dbd-group
- Require dbm-group
- Require group
- Require host
- `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. 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. `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
`=~` |  | String matches the regular expression
`!~` |  | String does not match the regular expression
`-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 | Special notes
--- | --- | ---
`req`, `http` | Get HTTP request header; header names may be added to the Vary header, see below |
`req_novary` | Same as `req`, but header names will not be added to the Vary header |
`resp` | Get HTTP response header (most response headers will not yet be set during `<If>`) |
`reqenv` | Lookup request environment variable (as a shortcut, `v` can also be used to access variables). | ordering
`osenv` | Lookup operating system environment variable |
`note` | Lookup request note | ordering
`env` | Return first match of `note`, `reqenv`, `osenv` | ordering
`tolower` | Convert string to lower case |
`toupper` | Convert string to upper case |
`escape` | Escape special characters in %hex encoding |
`unescape` | Unescape %hex encoded string, leaving encoded slashes alone; return empty string if %00 is found |
`base64` | Encode the string using base64 encoding |
`unbase64` | Decode base64 encoded string, return truncated string if 0x00 is found |
`md5` | Hash the string using MD5, then encode the hash with hexadecimal encoding |
`sha1` | Hash the string using SHA1, then encode the hash with hexadecimal encoding |
`file` | Read contents from a file (including line endings, when present) | restricted
`filesize` | Return size of a file (or 0 if file does not exist or is not regular file) | restricted
`ldap` | Escape characters as required by LDAP distinguished name escaping (RFC4514) and LDAP filter escaping (RFC4515). (Available in httpd 2.4.53 and later) |

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.

### Environment variable ordering

When environment variables are looked up within an <`If`> 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. `SetEnvIf` 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`).

## 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} -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'}"

## Other

Name | Alternative | Description
--- | --- | ---
`-in` | `in` | string contained in wordlist
`/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.

## Version History

The `req_novary` function is available for versions 2.4.4 and later.