Expressions in Apache HTTP Server - Apache HTTP Server Version 2.4








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Apache HTTP Server Version 2.4



Apache > HTTP Server > Documentation > Version 2.4Expressions 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.  
  
 Grammar in Backus-Naur Form notation
 Variables
 Binary operators
 Unary operators
 Functions
 Example expressions
 Other
 Comparison with SSLRequire
 Version History
See also<If><ElseIf><Else>ErrorDocumentAliasScriptAliasRedirectAuthBasicFakeAuthFormLoginRequiredLocationAuthFormLoginSuccessLocationAuthFormLogoutLocationAuthNameAuthTypeRewriteCondSetEnvIfExprHeaderRequestHeaderFilterProviderRequire exprRequire ldap-userRequire ldap-groupRequire ldap-dnRequire ldap-attributeRequire ldap-filterRequire ldap-searchRequire dbd-groupRequire dbm-groupRequire groupRequire hostSSLRequireLogMessagemod_includeComments


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

    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_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

    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.



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
=~
        
        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
    

    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

    



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.


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.

    NameDescriptionSpecial 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 noteordering
env
        Return first match of note, reqenv,
            osenvordering
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
    

    NameAlternative 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.


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