(I asked [this question on stackoverflow](https://stackoverflow.com/questions/2338812/how-can-i-view-log-files-in-linux-and-apply-custom-filers-while-viewing) but it might be better off here...)

I need to read through some gigantic log files on a Linux system. There's a lot of clutter in the logs. At the moment I'm doing something like this:

```
cat logfile.txt | grep -v "IgnoreThis\|IgnoreThat" | less

```

But it's cumbersome -- every time I want to add another filter, I need to quit `less` and edit the command line. Some of the filters are relatively complicated and may be multi-line.

I'd like some way to apply filters as I am reading through the log, and a way to save these filters somewhere.

Is there a tool that can do this for me? I can't install new software so hopefully it's something that would already be installed -- e.g., less, vi, something in a Python or Perl lib, etc.

Changing the code that generates the log to generate less is not an option.