Nginx 支持在if 语句中使用正则. 例如:

if ( condition ){
do_something
}
if ( $http_user_agent = "wget" ){
do_something
}
if ( $http_user_agent ~ MSIE ){
return 403;
}

一些常用规则:

1) The comparison of variable with the line with using the = and != operators;	
2) Pattern matching with regular expressions using the symbols ~* and ~:~ 
   is case-sensitive match
3) ~* specifies a case-insensitive match (firefox matches FireFox)
4) ~ and !~* mean the opposite, “doesn’t match”
5) checking for the existence of a file using the -f and !-f operators;
6) checking existence of a directory using the -d and !-d operators;
7) checking existence of a file, directory or symbolic link using 
   the -e and !-e operators;
8) checking whether a file is executable using the -x and !-x operators.
9) Parts of the regular expressions can be in parentheses, whose value can 
   then later be accessed in the $1 to $9 variables

一般的使用建议是多用map，少用if

if ($http_user_agent ~ (iPhone|Android) ) {
rewrite ^(.*) https://m.domain1.com$1 permanent;
}
if ($http_user_agent ~ (MSIE|Mozilla) ) {
rewrite ^(.*) https://domain2.com$1 permanent;
}

在nginx 0.9.6以后，支持使用map功能，上面的例子是配合map，可以改成

http {
map $http_user_agent $ua_redirect {
default '';
~(iPhone|Android) m.domain1.com;
~(MSIE|Mozilla) domain2.com;
}

server {
if ($ua_redirect != '') {
rewrite ^ https://$ua_redirect$request_uri? permanent;
}
}
}

 

另外一个例子:

### Testing if the client is a mobile or a desktop.
### The selection is based on the usual UA strings for desktop browsers.

## Testing a user agent using a method that reverts the logic of the
## UA detection. Inspired by notnotmobile.appspot.com.
map $http_user_agent $is_desktop {
    default 0;
    ~*linux.*android|windows\s+(?:ce|phone) 0; # exceptions to the rule
    ~*spider|crawl|slurp|bot 1; # bots
    ~*windows|linux|os\s+x\s*[\d\._]+|solaris|bsd 1; # OSes
}

## Revert the logic.
map $is_desktop $is_mobile {
    1 0;
    0 1;
}