Question

I have migrated a WordPress Mu blog under Linux to WP 3 under IIS 7 so I need to redirect http://mydomain.com/blog/blahblah/ to http://mydomain.com/blahblah/

I added to wordpress web.config

        <rule name="rewrite /blog/">
            <match url="^/blog/([_0-9a-z-]+)/" />
            <conditions>                
            </conditions>
            <action type="Rewrite" url="/{R:1}/" />
        </rule>

so that web.config now contains:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <rewrite>
      <rules>
                <rule name="wordpress" patternSyntax="Wildcard">
                    <match url="*" />
                        <conditions>
                            <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
                        </conditions>
                    <action type="Rewrite" url="index.php" />
                </rule>
        <rule name="rewrite /blog/">
            <match url="^/blog/([_0-9a-z-]+)/" />
            <conditions>                
            </conditions>
            <action type="Rewrite" url="/{R:1}/" />
        </rule>
      </rules>
    </rewrite>
  </system.webServer>
</configuration>

WordPress continues to work but /blog/ is not redirected. Why ?

Answer 1

1) Better move this rule above Wildcard rule

2) You had leading slash ^/blog/... — you do not need it: ^blog/...

3) Because no conditions were used I have also removed <conditions></conditions>

<rule name="rewrite /blog/">
    <match url="^blog/([_0-9a-zA-Z\-]+)/$"/>
    <action type="Rewrite" url="/{R:1}/"/>
</rule>

The above is Rewrite rule — you see the same url in browser but behind it executes differently.

If you want Redirect then use the below:

<rule name="rewrite /blog/">
    <match url="^blog/([_0-9a-zA-Z\-]+)/$"/>
    <action type="Redirect" url="/{R:1}/" redirectType="Permanent" />
</rule>