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<manualpage metafile="expr.xml.meta">

  <title>Expressions in Apache HTTP Server</title>

  <summary>
    <p>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 <em>ap_expr</em>, for all configuration directives.
    This document describes the <em>ap_expr</em> expression parser.
    </p>
    <p>The <em>ap_expr</em> expression is intended to replace most other
    expression variants in HTTPD. For example, the deprecated <directive
    module="mod_ssl">SSLRequire</directive> expressions can be replaced
    by <a href="mod/mod_authz_core.html#reqexpr">Require expr</a>.  </p>
  </summary>

<seealso><directive module="core" type="section">If</directive></seealso>
<seealso><directive module="core" type="section">ElseIf</directive></seealso>
<seealso><directive module="core" type="section">Else</directive></seealso>
<seealso><directive module="core">ErrorDocument</directive></seealso>
<seealso><directive module="mod_alias">Alias</directive></seealso>
<seealso><directive module="mod_alias">ScriptAlias</directive></seealso>
<seealso><directive module="mod_alias">Redirect</directive></seealso>
<seealso><directive module="mod_auth_basic">AuthBasicFake</directive></seealso>
<seealso><directive module="mod_auth_form">AuthFormLoginRequiredLocation</directive></seealso>
<seealso><directive module="mod_auth_form">AuthFormLoginSuccessLocation</directive></seealso>
<seealso><directive module="mod_auth_form">AuthFormLogoutLocation</directive></seealso>
<seealso><directive module="mod_authn_core">AuthName</directive></seealso>
<seealso><directive module="mod_authn_core">AuthType</directive></seealso>
<seealso><directive module="mod_rewrite">RewriteCond</directive></seealso>
<seealso><directive module="mod_setenvif">SetEnvIfExpr</directive></seealso>
<seealso><directive module="mod_headers">Header</directive></seealso>
<seealso><directive module="mod_headers">RequestHeader</directive></seealso>
<seealso><directive module="mod_filter">FilterProvider</directive></seealso>
<seealso><directive module="mod_crypto">CryptoKey</directive></seealso>
<seealso><directive module="mod_crypto">CryptoIV</directive></seealso>
<seealso><a href="mod/mod_authz_core.html#reqexpr">Require expr</a></seealso>
<seealso><a href="mod/mod_authnz_ldap.html#requser">Require ldap-user</a></seealso>
<seealso><a href="mod/mod_authnz_ldap.html#reqgroup">Require ldap-group</a></seealso>
<seealso><a href="mod/mod_authnz_ldap.html#reqdn">Require ldap-dn</a></seealso>
<seealso><a href="mod/mod_authnz_ldap.html#reqattribute">Require ldap-attribute</a></seealso>
<seealso><a href="mod/mod_authnz_ldap.html#reqfilter">Require ldap-filter</a></seealso>
<seealso><a href="mod/mod_authnz_ldap.html#reqsearch">Require ldap-search</a></seealso>
<seealso><a href="mod/mod_authz_dbd.html#reqgroup">Require dbd-group</a></seealso>
<seealso><a href="mod/mod_authz_dbm.html#reqgroup">Require dbm-group</a></seealso>
<seealso><a href="mod/mod_authz_groupfile.html#reqgroup">Require group</a></seealso>
<seealso><a href="mod/mod_authz_host.html#reqhost">Require host</a></seealso>
<seealso><directive module="mod_ssl">SSLRequire</directive></seealso>
<seealso><directive module="mod_log_debug">LogMessage</directive></seealso>
<seealso><module>mod_include</module></seealso>

  <section id="grammar">
    <title>Grammar in Backus-Naur Form notation</title>
      <p><a
      href="http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form">Backus-Naur
      Form</a> (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 <code>expr</code>.
      However, a few directives like <directive
      module="mod_log_debug">LogMessage</directive> accept expressions
      that evaluate to a string value. For those, the starting point in
      the BNF is <code>string</code>.
      </p>
<blockquote>
<pre>
expr        ::= "<strong>true</strong>" | "<strong>false</strong>"
              | "<strong>!</strong>" expr
              | expr "<strong>&amp;&amp;</strong>" expr
              | expr "<strong>||</strong>" expr
              | "<strong>(</strong>" expr "<strong>)</strong>"
              | comp

comp        ::= stringcomp
              | integercomp
              | unaryop word
              | word binaryop word
              | word "<strong>in</strong>" "<strong>{</strong>" wordlist "<strong>}</strong>"
              | word "<strong>in</strong>" listfunction
              | word "<strong>=~</strong>" regex
              | word "<strong>!~</strong>" regex


stringcomp  ::= word "<strong>==</strong>" word
              | word "<strong>!=</strong>" word
              | word "<strong>&lt;</strong>"  word
              | word "<strong>&lt;=</strong>" word
              | word "<strong>&gt;</strong>"  word
              | word "<strong>&gt;=</strong>" word

integercomp ::= word "<strong>-eq</strong>" word | word "<strong>eq</strong>" word
              | word "<strong>-ne</strong>" word | word "<strong>ne</strong>" word
              | word "<strong>-lt</strong>" word | word "<strong>lt</strong>" word
              | word "<strong>-le</strong>" word | word "<strong>le</strong>" word
              | word "<strong>-gt</strong>" word | word "<strong>gt</strong>" word
              | word "<strong>-ge</strong>" word | word "<strong>ge</strong>" word

wordlist    ::= word
              | wordlist "<strong>,</strong>" word

word        ::= word "<strong>.</strong>" word
              | digit
              | "<strong>'</strong>" string "<strong>'</strong>"
              | "<strong>"</strong>" string "<strong>"</strong>"
              | variable
              | rebackref
              | function

string      ::= stringpart
              | string stringpart

stringpart  ::= cstring
              | variable
              | rebackref

cstring     ::= ...
digit       ::= [0-9]+

variable    ::= "<strong>%{</strong>" varname "<strong>}</strong>"
              | "<strong>%{</strong>" funcname "<strong>:</strong>" funcargs "<strong>}</strong>"

rebackref   ::= "<strong>$</strong>" [0-9]

function     ::= funcname "<strong>(</strong>" wordlist "<strong>)</strong>"

listfunction ::= listfuncname "<strong>(</strong>" word "<strong>)</strong>"
</pre>
</blockquote>

</section>

<section id="vars">
    <title>Variables</title>

    <p>The expression parser provides a number of variables of the form
    <code>%{HTTP_HOST}</code>. 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 <directive>&lt;If &gt;</directive>
    directive is evaluated before authentication is done. Therefore,
    <code>%{REMOTE_USER}</code> will not be set in this case.</p>

    <p>The following variables provide the values of the named HTTP request
    headers. The values of other headers can be obtained with the
    <code>req</code> <a href="#functions">function</a>. 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 <code>req_novary</code>
    <a href="#functions">function</a> may be used to circumvent this
    behavior.</p>

    <table border="1" style="zebra">
    <columnspec><column width="1"/></columnspec>

    <tr><th>Name</th></tr>
    <tr><td><code>HTTP_ACCEPT</code></td></tr>
    <tr><td><code>HTTP_COOKIE</code></td></tr>
    <tr><td><code>HTTP_FORWARDED</code></td></tr>
    <tr><td><code>HTTP_HOST</code></td></tr>
    <tr><td><code>HTTP_PROXY_CONNECTION</code></td></tr>
    <tr><td><code>HTTP_REFERER</code></td></tr>
    <tr><td><code>HTTP_USER_AGENT</code></td></tr>

    </table>

    <p>Other request related variables</p>

    <table border="1" style="zebra">
    <columnspec><column width=".4"/><column width=".6"/></columnspec>

    <tr><th>Name</th><th>Description</th></tr>
    <tr><td><code>REQUEST_METHOD</code></td>
        <td>The HTTP method of the incoming request (e.g.
            <code>GET</code>)</td></tr>
    <tr><td><code>REQUEST_SCHEME</code></td>
        <td>The scheme part of the request's URI</td></tr>
    <tr><td><code>REQUEST_URI</code></td>
        <td>The path part of the request's URI</td></tr>
    <tr><td><code>DOCUMENT_URI</code></td>
        <td>Same as <code>REQUEST_URI</code></td></tr>
    <tr><td><code>REQUEST_FILENAME</code></td>
        <td>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 <code>REQUEST_FILENAME</code> is referenced. Otherwise, such
            as when used in virtual host context, the same value as
            <code>REQUEST_URI</code> </td></tr>
    <tr><td><code>SCRIPT_FILENAME</code></td>
        <td>Same as <code>REQUEST_FILENAME</code></td></tr>
    <tr><td><code>LAST_MODIFIED</code></td>
        <td>The date and time of last modification of the file in the format
            <code>20101231235959</code>, if this has already been determined by
            the server at the time <code>LAST_MODIFIED</code> is referenced.
            </td></tr>
    <tr><td><code>SCRIPT_USER</code></td>
        <td>The user name of the owner of the script.</td></tr>
    <tr><td><code>SCRIPT_GROUP</code></td>
        <td>The group name of the group of the script.</td></tr>
    <tr><td><code>PATH_INFO</code></td>
        <td>The trailing path name information, see
            <directive module="core">AcceptPathInfo</directive></td></tr>
    <tr><td><code>QUERY_STRING</code></td>
        <td>The query string of the current request</td></tr>
    <tr><td><code>IS_SUBREQ</code></td>
        <td>"<code>true</code>" if the current request is a subrequest,
            "<code>false</code>" otherwise</td></tr>
    <tr><td><code>THE_REQUEST</code></td>
        <td>The complete request line (e.g.,
            "<code>GET /index.html HTTP/1.1</code>")</td></tr>
    <tr><td><code>REMOTE_ADDR</code></td>
        <td>The IP address of the remote host</td></tr>
    <tr><td><code>REMOTE_PORT</code></td>
        <td>The port of the remote host (2.4.26 and later)</td></tr>
    <tr><td><code>REMOTE_HOST</code></td>
        <td>The host name of the remote host</td></tr>
    <tr><td><code>REMOTE_USER</code></td>
        <td>The name of the authenticated user, if any (not available during <directive>&lt;If&gt;</directive>)</td></tr>
    <tr><td><code>REMOTE_IDENT</code></td>
        <td>The user name set by <module>mod_ident</module></td></tr>
    <tr><td><code>SERVER_NAME</code></td>
        <td>The <directive module="core">ServerName</directive> of
            the current vhost</td></tr>
    <tr><td><code>SERVER_PORT</code></td>
        <td>The server port of the current vhost, see
            <directive module="core">ServerName</directive></td></tr>
    <tr><td><code>SERVER_ADMIN</code></td>
        <td>The <directive module="core">ServerAdmin</directive> of
            the current vhost</td></tr>
    <tr><td><code>SERVER_PROTOCOL</code></td>
        <td>The protocol used by the request (e.g. HTTP/1.1). In some types of
            internal subrequests, this variable has the value
            <code>INCLUDED</code>.</td></tr>
    <tr><td><code>SERVER_PROTOCOL_VERSION</code></td>
        <td>A number that encodes the HTTP version of the request:
            <code>1000 * major + minor</code>. For example, <code>1001</code>
            corresponds to HTTP/1.1 and <code>9</code> corresponds
            to HTTP/0.9</td></tr>
    <tr><td><code>SERVER_PROTOCOL_VERSION_MAJOR</code></td>
        <td>The major version part of the HTTP version of the request,
            e.g. <code>1</code> for HTTP/1.0</td></tr>
    <tr><td><code>SERVER_PROTOCOL_VERSION_MINOR</code></td>
        <td>The minor version part of the HTTP version of the request,
            e.g. <code>0</code> for HTTP/1.0</td></tr>
    <tr><td><code>DOCUMENT_ROOT</code></td>
        <td>The <directive module="core">DocumentRoot</directive> of
            the current vhost</td></tr>
    <tr><td><code>AUTH_TYPE</code></td>
        <td>The configured <directive
        module="mod_authn_core">AuthType</directive> (e.g.
        "<code>basic</code>")</td></tr>
    <tr><td><code>CONTENT_TYPE</code></td>
        <td>The content type of the response (not available during <directive>&lt;If&gt;</directive>)</td></tr>
    <tr><td><code>HANDLER</code></td>
        <td>The name of the <a href="handler.html">handler</a> creating
            the response</td></tr>
    <tr><td><code>HTTP2</code></td>
        <td>"<code>on</code>" if the request uses http/2,
            "<code>off</code>" otherwise</td></tr>
    <tr><td><code>HTTPS</code></td>
        <td>"<code>on</code>" if the request uses https,
            "<code>off</code>" otherwise</td></tr>
    <tr><td><code>IPV6</code></td>
        <td>"<code>on</code>" if the connection uses IPv6,
            "<code>off</code>" otherwise</td></tr>
    <tr><td><code>REQUEST_STATUS</code></td>
        <td>The HTTP error status of the request (not available during <directive>&lt;If&gt;</directive>)</td></tr>
    <tr><td><code>REQUEST_LOG_ID</code></td>
        <td>The error log id of the request (see
            <directive module="core">ErrorLogFormat</directive>)</td></tr>
    <tr><td><code>CONN_LOG_ID</code></td>
        <td>The error log id of the connection (see
            <directive module="core">ErrorLogFormat</directive>)</td></tr>
    <tr><td><code>CONN_REMOTE_ADDR</code></td>
        <td>The peer IP address of the connection (see the
            <module>mod_remoteip</module> module)</td></tr>
    <tr><td><code>CONTEXT_PREFIX</code></td>
        <td></td></tr>
    <tr><td><code>CONTEXT_DOCUMENT_ROOT</code></td>
        <td></td></tr>

    </table>

    <p>Misc variables</p>

    <table border="1" style="zebra">
    <columnspec><column width=".4"/><column width=".6"/></columnspec>

    <tr><th>Name</th><th>Description</th></tr>
    <tr><td><code>TIME_YEAR</code></td>
        <td>The current year (e.g. <code>2010</code>)</td></tr>
    <tr><td><code>TIME_MON</code></td>
        <td>The current month (<code>01</code>, ..., <code>12</code>)</td></tr>
    <tr><td><code>TIME_DAY</code></td>
        <td>The current day of the month (<code>01</code>, ...)</td></tr>
    <tr><td><code>TIME_HOUR</code></td>
        <td>The hour part of the current time
            (<code>00</code>, ..., <code>23</code>)</td></tr>
    <tr><td><code>TIME_MIN</code></td>
        <td>The minute part of the current time </td></tr>
    <tr><td><code>TIME_SEC</code></td>
        <td>The second part of the current time </td></tr>
    <tr><td><code>TIME_WDAY</code></td>
        <td>The day of the week (starting with <code>0</code>
            for Sunday)</td></tr>
    <tr><td><code>TIME</code></td>
        <td>The date and time in the format
        <code>20101231235959</code></td></tr>
    <tr><td><code>SERVER_SOFTWARE</code></td>
        <td>The server version string</td></tr>
    <tr><td><code>API_VERSION</code></td>
        <td>The date of the API version (module magic number)</td></tr>
    </table>

    <p>Some modules register additional variables, see e.g.
    <module>mod_ssl</module>.</p>

</section>

<section id="binop">
    <title>Binary operators</title>

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

    <section id="comp">
    <title>Comparison operators</title>

    <table border="1" style="zebra">
    <columnspec><column width=".2"/><column width=".2"/><column width=".6"/></columnspec>

    <tr><th>Name</th><th>Alternative</th> <th>Description</th></tr>
    <tr><td><code>==</code></td>
        <td><code>=</code></td>
        <td>String equality</td></tr>
    <tr><td><code>!=</code></td>
        <td></td>
        <td>String inequality</td></tr>
    <tr><td><code>&lt;</code></td>
        <td></td>
        <td>String less than</td></tr>
    <tr><td><code>&lt;=</code></td>
        <td></td>
        <td>String less than or equal</td></tr>
    <tr><td><code>&gt;</code></td>
        <td></td>
        <td>String greater than</td></tr>
    <tr><td><code>&gt;=</code></td>
        <td></td>
        <td>String greater than or equal</td></tr>
    <tr><td><code>=~</code></td>
        <td></td>
        <td>String matches the regular expression</td></tr>
    <tr><td><code>!~</code></td>
        <td></td>
        <td>String does not match the regular expression</td></tr>
    <tr><td><code>-eq</code></td>
        <td><code>eq</code></td>
        <td>Integer equality</td></tr>
    <tr><td><code>-ne</code></td>
        <td><code>ne</code></td>
        <td>Integer inequality</td></tr>
    <tr><td><code>-lt</code></td>
        <td><code>lt</code></td>
        <td>Integer less than</td></tr>
    <tr><td><code>-le</code></td>
        <td><code>le</code></td>
        <td>Integer less than or equal</td></tr>
    <tr><td><code>-gt</code></td>
        <td><code>gt</code></td>
        <td>Integer greater than</td></tr>
    <tr><td><code>-ge</code></td>
        <td><code>ge</code></td>
        <td>Integer greater than or equal</td></tr>
    </table>
    </section>

    <section id="binaryother">
    <title>Other binary operators</title>

    <table border="1" style="zebra">
    <columnspec><column width=".2"/><column width=".8"/></columnspec>

    <tr><th>Name</th><th>Description</th></tr>
    <tr><td><code>-ipmatch</code></td>
        <td>IP address matches address/netmask</td></tr>
    <tr><td><code>-strmatch</code></td>
        <td>left string matches pattern given by right string (containing
            wildcards *, ?, [])</td></tr>
    <tr><td><code>-strcmatch</code></td>
        <td>same as <code>-strmatch</code>, but case insensitive</td></tr>
    <tr><td><code>-fnmatch</code></td>
        <td>same as <code>-strmatch</code>, but slashes are not matched by
            wildcards</td></tr>
    </table>
    </section>

</section>

<section id="unnop">
    <title>Unary operators</title>

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

    <table border="1" style="zebra">
    <columnspec><column width=".2"/><column width=".2"/><column width=".6"/></columnspec>

    <tr><th>Name</th><th>Description</th><th>Restricted</th></tr>
    <tr><td><code>-d</code></td>
        <td>The argument is treated as a filename.
            True if the file exists and is a directory</td><td>yes</td></tr>
    <tr><td><code>-e</code></td>
        <td>The argument is treated as a filename.
            True if the file (or dir or special) exists</td><td>yes</td></tr>
    <tr><td><code>-f</code></td>
        <td>The argument is treated as a filename.
            True if the file exists and is regular file</td><td>yes</td></tr>
    <tr><td><code>-s</code></td>
        <td>The argument is treated as a filename.
            True if the file exists and is not empty</td><td>yes</td></tr>
    <tr><td><code>-L</code></td>
        <td>The argument is treated as a filename.
            True if the file exists and is symlink</td><td>yes</td></tr>
    <tr><td><code>-h</code></td>
        <td>The argument is treated as a filename.
            True if the file exists and is symlink
            (same as <code>-L</code>)</td><td>yes</td></tr>
    <tr><td><code>-F</code></td>
        <td>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!</td><td></td></tr>
    <tr><td><code>-U</code></td>
        <td>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!</td><td></td></tr>
    <tr><td><code>-A</code></td>
        <td>Alias for <code>-U</code></td><td></td></tr>
    <tr><td><code>-n</code></td>
        <td>True if string is not empty</td><td></td></tr>
    <tr><td><code>-z</code></td>
        <td>True if string is empty</td><td></td></tr>
    <tr><td><code>-T</code></td>
        <td>False if string is empty, "<code>0</code>", "<code>off</code>",
            "<code>false</code>", or "<code>no</code>" (case insensitive).
            True otherwise.</td><td></td></tr>
    <tr><td><code>-R</code></td>
        <td>Same as "<code>%{REMOTE_ADDR} -ipmatch ...</code>", but more
        efficient
        </td><td></td></tr>
    </table>

    <p>The operators marked as "restricted" are not available in some modules
    like <module>mod_include</module>.</p>
</section>

<section id="functions">
    <title>Functions</title>

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

    <table border="1" style="zebra">
    <columnspec><column width=".2"/><column width=".4"/><column width=".4"/></columnspec>

    <tr><th>Name</th><th>Description</th><th>Special notes</th></tr>
    <tr><td><code>req</code>, <code>http</code></td>
        <td>Get HTTP request header; header names may be added to the Vary
            header, see below</td><td></td></tr>
    <tr><td><code>req_novary</code></td>
        <td>Same as <code>req</code>, but header names will not be added to the
            Vary header</td><td></td></tr>
    <tr><td><code>resp</code></td>
        <td>Get HTTP response header (most response headers will not yet be set
            during <directive>&lt;If&gt;</directive>)</td><td></td></tr>
    <tr><td><code>reqenv</code></td>
        <td>Lookup request environment variable (as a shortcut,
        <code>v</code> can also be used to access variables). 
        </td>
        <td>ordering</td></tr>
    <tr><td><code>osenv</code></td>
        <td>Lookup operating system environment variable</td><td></td></tr>
    <tr><td><code>note</code></td>
        <td>Lookup request note</td><td>ordering</td></tr>
    <tr><td><code>env</code></td>
        <td>Return first match of <code>note</code>, <code>reqenv</code>,
            <code>osenv</code></td><td>ordering</td></tr>
    <tr><td><code>tolower</code></td>
        <td>Convert string to lower case</td><td></td></tr>
    <tr><td><code>toupper</code></td>
        <td>Convert string to upper case</td><td></td></tr>
    <tr><td><code>escape</code></td>
        <td>Escape special characters in %hex encoding</td><td></td></tr>
    <tr><td><code>unescape</code></td>
        <td>Unescape %hex encoded string, leaving encoded slashes alone;
            return empty string if %00 is found</td><td></td></tr>
    <tr><td><code>base64</code></td>
        <td>Encode the string using base64 encoding</td><td></td></tr>
    <tr><td><code>unbase64</code></td>
        <td>Decode base64 encoded string, return truncated string if 0x00 is
            found</td><td></td></tr>
    <tr><td><code>md5</code></td>
        <td>Hash the string using MD5, then encode the hash with hexadecimal
            encoding</td><td></td></tr>
    <tr><td><code>sha1</code></td>
        <td>Hash the string using SHA1, then encode the hash with hexadecimal
            encoding</td><td></td></tr>
    <tr><td><code>file</code></td>
        <td>Read contents from a file (including line endings, when present)
        </td><td>restricted</td></tr>
    <tr><td><code>filemod</code></td>
        <td>Return last modification time of a file (or 0 if file does not exist
            or is not regular file)</td><td>restricted</td></tr>
    <tr><td><code>filesize</code></td>
        <td>Return size of a file (or 0 if file does not exist or is not
            regular file)</td><td>restricted</td></tr>
    <tr><td><code>ldap</code></td>
        <td>Escape characters as required by LDAP distinguished name escaping
            (RFC4514) and LDAP filter escaping (RFC4515).</td><td></td></tr>
    <tr><td><code>replace</code></td>
        <td>replace(string, "from", "to") replaces all occurrences of "from"
            in the string with "to".</td><td></td></tr>

    </table>

    <p>The functions marked as "restricted" in the final column are not 
    available in some modules like <module>mod_include</module>.</p>

    <p>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 
    &lt;<directive module="core">If</directive>&gt; directive which is
    evaluated relatively early.</p>
    <note>
    <title>Environment variable ordering</title>
    When environment variables are looked up within an 
    &lt;<directive module="core">If</directive>&gt; 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. <directive module="mod_setenvif">SetEnvIf</directive>
    in virtual host scope is one directive that runs prior to this resolution
    <br/>
    <br/>
    When <code>reqenv</code> is used outside of &lt;<directive module="core"
    >If</directive>&gt;, the resolution will generally occur later, but the 
    exact timing depends on the directive the expression has been used within.
    </note>

    <p>When the functions <code>req</code> or <code>http</code> 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 <code>req_novary</code> function can be used to
    prevent names from being added to the Vary header.</p>

    <p>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 <code>-in</code> operator.  Functions names are not case
    sensitive.  Modules may register additional functions.</p>

    <p>There are no built-in list-valued functions. <module>mod_ssl</module>
    provides <code>PeerExtList</code>.  See the description of
    <directive module="mod_ssl">SSLRequire</directive> for details
    (but <code>PeerExtList</code> is also usable outside
    of <directive module="mod_ssl">SSLRequire</directive>).</p>

</section>

<section id="examples">

    <title>Example expressions</title>
    <p>The following examples show how expressions might be used to
    evaluate requests:</p>

    <!-- This section should probably be extended with more, useful examples -->
    <highlight language="config">
# Compare the host name to example.com and redirect to www.example.com if it matches
&lt;If "%{HTTP_HOST} == 'example.com'"&gt;
    Redirect permanent "/" "http://www.example.com/"
&lt;/If&gt;

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

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

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

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

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

# Check against the client IP
&lt;If "-R '192.168.1.0/24'"&gt;
    Header set matched true
&lt;/If&gt;

# Function examples in boolean context
&lt;If "md5('foo') == 'acbd18db4cc2f85cedef654fccc4a4d8'"&gt;
  Header set checksum-matched true
&lt;/If&gt;
&lt;If "md5('foo') == replace('md5:XXXd18db4cc2f85cedef654fccc4a4d8', 'md5:XXX', 'acb')"&gt;
  Header set checksum-matched-2 true
&lt;/If&gt;

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

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

    </highlight>
</section>

<section id="other">
    <title>Other</title>

    <table border="1" style="zebra">
    <columnspec><column width=".2"/><column width=".2"/><column width=".6"/></columnspec>

    <tr><th>Name</th><th>Alternative</th> <th>Description</th></tr>
    <tr><td><code>-in</code></td>
        <td><code>in</code></td>
        <td>string contained in wordlist</td></tr>
    <tr><td><code>/regexp/</code></td>
        <td><code>m#regexp#</code></td>
        <td>Regular expression (the second form allows different
        delimiters than /)</td></tr>
    <tr><td><code>/regexp/i</code></td>
        <td><code>m#regexp#i</code></td>
        <td>Case insensitive regular expression</td></tr>
    <tr><td><code>$0 ... $9</code></td>
        <td></td>
        <td>Regular expression backreferences</td></tr>
    </table>

    <section id="rebackref">
        <title>Regular expression backreferences</title>
        <p>The strings <code>$0</code> ... <code>$9</code> 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.</p>
    </section>

</section>

<section id="sslrequire">
    <title>Comparison with SSLRequire</title>
    <p>The <em>ap_expr</em> syntax is mostly a superset of the syntax of the
    deprecated <directive module="mod_ssl">SSLRequire</directive> directive.
    The differences are described in <directive
    module="mod_ssl">SSLRequire</directive>'s documentation.</p>
</section>

<section id="compatibility">
    <title>Version History</title>
    <p>The <code>req_novary</code> <a href="#functions">function</a>
    is available for versions 2.4.4 and later.</p>
    <p>The <code>SERVER_PROTOCOL_VERSION</code>,
    <code>SERVER_PROTOCOL_VERSION_MAJOR</code> and
    <code>SERVER_PROTOCOL_VERSION_MINOR</code>
    <a href="#vars">variables</a>
    are available for versions 2.5.0 and later.</p>
</section>

</manualpage>