There is no escaping character in an URL Rewrite expression, but here is how you include curly bracket within an URL Rewrite expression. For opening { curly bracket you will always require to use the UrlDecode function to generate the { character from
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Read the complete post at http://blogs.iis.net/danielvl/archive/2011/07/29/how-to-escape-curly-brackets-in-url-rewrite-rule.aspx