Version 4.1 - 4.6 Content filtering and packet inspection
Since this is under the 4.x topics, I'm going to expect that the blueprint is referring to content filtering / packet inspection in general and not Cisco IOS Content Filtering - which is EOS/EOL.
This is going to be covered more in depth in multiple sections of 5.x and 6.x. So let's talk about it in general. In order to filter something, you have to first detect it. Where you detect it is mainly the difference between content filtering and packet inspection. Think of the WSA (or any proxy). They filter out content based on URLs. If a specific word is found in the URL, the site is blocked. Or if the site is listed in a category of sites (gambling, games, etc), it can also be blocked. For packet inspection, you are likely looking at traffic being tunneled in another protocol - or checking for protocol non-compliance. That's it in a nutshell. So what else could fall under this?
Regex falls under this category. For all the filtering and inspection questions, it will probably be something that is indirectly incorporated into the questions. Remember - ACLs are going to match on IP addresses and ports. Content filtering and packet inspection is going to match on strings - and therefore you are going to need to know Regex.
If you're not really good with Regex, or don't have a lot of experience with it, don't just read the Regex site. You will never remember it that way. You really have to use it a lot to get familiar with it and be able to remember the little ins and outs of this. Fortunately there are a ton of sites on the Internet to learn regex. It's worth taking the time to learn because not only will it be on the exam, but you'll be able to use it in a million ways.
Again, I'm not going to milk this section. 4.x is only 10% of the exam and I would only expect maybe one direct question at most. However, let me reinforce the Regex suggestion above - since this could be indirectly on many other questions.