var myString:String = "/";
trace(myString);
var urlEncodeForwardSlashedRegExp:RegExp = new RegExp("/", "gi");
myString = myString.replace(urlEncodeForwardSlashedRegExp, "%2F");
trace(myString);

// OUTPUT
// /
// %2F

In AS2 forward slashes got url-encoded to %2F when you escaped them. But in AS3 they get ignored and left as they are. In some situations I have found that this can cause problems. So you can do it manually using this regular expression.

AS3 Convert a Forward Slash to URLEncoded Character %2F

regex, replace, regexp

ActionScript 3