Yahoo announced their support for wildcards in robots.txt. This feature allows Yahoo's robot Slurp to recognize two new symbols, * and $.

The asterisk, *, matches a sequence of characters. This means you can prevent Yahoo from indexing any URLs which contain the text you specify. So for example:

User-Agent: Yahoo! Slurp Disallow: /*?sessionid

..would prevent Yahoo from attempting to index a URL which contained the string ?sessionid. As you probably already know, you should always avoid serving session IDs to search engines. This can be one tool to help you do that.

The dollar-sign-character, $, anchors the text at the end of the URL string. That means you can prevent Yahoo from trying to index URLs that end with a certain string of text. For example:

User-Agent: Yahoo! Slurp Disallow: /*.gif$

...will keep Yahoo from indexing files which end with the word .gif.

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