Пункт 21. Expression Parsing in Apache
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
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.
HTTP_ACCEPT |
HTTP_COOKIE |
HTTP_FORWARDED |
HTTP_HOST |
HTTP_PROXY_CONNECTION |
HTTP_REFERER |
HTTP_USER_AGENT |
Other request related variables
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
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
== |
= |
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
-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.
-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.
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 | |
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 |
filemod |
Return last modification time of a file (or 0 if file does not exist
or is not regular file) | restricted |
filesize |
Return size of a file (or 0 if file does not exist or is not
regular file) | restricted |
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$#"
Other
-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.
