The Core

Notepad ++ as mentioned already

There's also grepWin, dnGRep, winGrep, Astrogrep

But if you are going to be lots of this and you want complicated processing, multi-step and all sorts of stuff like that, there is NOTHING to match PowerGREP

Yeah, it's $159 and the ones mentioned so far are all free or close to it. But for 10 (?) years, it has been my ultimate text processing toolkit. I've had processes that involved a dozen complicated and sequential regex transformations and you just load them all into PowerGREP, save the definition file and for the rest of your life you can load it and run those transformations on a text file.

I just used it recently to take a logfile with 2.4 million entries from a dozen domains and pull out the 404s for a given domain

https://www.powergrep.com/index.html

I will see your Powergrep and raise you Funduc.

I call Funduc and raise you history, unlimited undo, "collect data", filter file list (i.e. files that are searched) with regular expressions

I haven't tried Funduc, but I've tried a lot of the competitors when I've had to work on another machine without PowerGREP, but none of them are so fine.

I can knock up HTML parsers pretty quick if you can't find something that does the job you want.