I’ve been reading Mastering Regular Expressions by Jeffrey E.F. Friedl, and since nobody in my life (aside from my wife) cares, I thought I’d share something I’m pretty proud of. My first set of regular expressions, that I wrote myself to manipulate the text I’m working with.

What’s I’m so happy about is that I wrote these expressions. I understand exactly what they do and the purpose of each character in each expression.

I’ve used regex in the past. Stuff cobbled together from stack overflow, but I never really understood how they worked or what the expressions meant, just that they did what I needed them to do at the time.

I’m only about 10% of the way through the book, but already I understand so much more than I ever did about regex (I also recognize I have a lot to learn).

I wrote the expressions to be used with egrep and sed to generate and clean up a list of filenames pulled out of tarballs. (movies I’ve ripped from my DVD collection and tarballed to archive them).

The first expression I wrote was this one used with tar and egrep to list the files in the tarball and get just the name of the video file:

tar -tzvf file.tar.gz | egrep -o '\/[^/]*\.m(kv|p4)' > movielist

Which gives me a list of movies of which this is an example:

/The.Hunger.Games.(2012).[tmdbid-70160].mp4

Then I used sed with the expression groups to remove:

  • the leading forward slash
  • Everything from .[ to the end
  • All of the periods in between words

And the last expression checks for one or more spaces and replaces them with a single space.

This is the full sed command:

sed -Eie 's/^\///; s/\.\[[a-z]+-[0-9]+\]\.m(p4|kv)//; s/[^a-zA-Z0-9\(\)&-]/ /g; s/ +/ /g' movielist

Which leaves me with a pretty list of movies that looks like this:

The Hunger Games (2012)

I’m sure this could be done more elegantly, and I’m happy for any feedback on how to do that! For now, I’m just excited that I’m beginning to understand regex and how to use it!

Edit: fixed title so it didn’t say “regex expressions”

  • bizdelnick@lemmy.ml
    1·
    11 months ago

    It is a great book, although a bit outdated. In particular, nowadays egrep is not recommended to use. grep -E is a more portable synonim.

    Some notes on you script:

    1. You don’t need to escape slashes in grep regex. In the sed s/// command better use another character like s### so you also can leave slashes unescaped.

    2. You usually don’t need to pipe grep and sed, sed -n with regex address and explicit printing command gives the same result as grep.

    3. You could omit leading slash in your egrep regex, so you won’t need to remove it later.

    So I would do the same with

    tar -tzvf file.tar.gz | sed -En '/\.(mp4|mkv)$/{s#^.*/##; s#\.\[.*##; s#[^a-zA-Z0-9()&-]# #g; s/ +/ /g; p}'
    • DefederateLemmyMl@feddit.nlEnglish
      01·
      11 months ago

      nowadays egrep is not recommended to use. grep -E is a more portable synonim

      Not directed at you personally, but this is the kind of pointless pedantry from upstream developers that grinds my gears.

      Like, I’ve used egrep for 25 years. I don’t know of a still relevant Unix variant in existence that doesn’t have the egrep command. But suddenly now, when any other Unix variant but Linux is all but extinct, and all your shell scripts are probably full of bashisms and Linuxisms anyway, now there is somehow a portability problem, and they deem it necessary to print out a warning whenever I dare to run egrep instead of grep -E? C’mon now … If anything, they have just made it less portable by spitting out spurious warnings where there weren’t any before.