I need to use HTTP inspection to block certain URLs, but I need a way to informe users that they requests were blocked.
Is there anyway in the ASA to show a "page" saying "your request was blocked"? Or any way to redirect those request to a cative portal/web?
Its best if yo use a CSC module with ASA, with the CSC you have the ability to block a range of URL's and throw a notification whenever a user tried to access the website, but not with the ASA.
On the ASA we acn only blocka specific URL like facebook, using a regex but throwing a notification when a user accesses it is not possible on the ASA, for that CSC would be the best option, as it can handle URL blocking and filtering as well.
Here is a document for regex on ASA:
Let me know if you need anything else.