I hope this isn't flippant, but have you read the Perl regex man pages [0]? "Perl compatible" regexes are very common, and are explained _really_ well. Similarly, the Perl Cookbook (from O'Reilly) has a whole chapter on regular expressions -- even crazy ones like detecting US phone numbers (hard!) -- with explanations. That chapter is QUITE good, and I highly recommend it even you don't write Perl.

For your specific problem at hand, I assume you want to replace spaces with a tab character.

In Perl, you could do:

  # replace four spaces with a tab, globally
  $line =~ s/    /\t/g;

You can do this directly on the command line as well:

  perl -e 'while (<>) { $line = $_; $line =~ s/    /\t/g; print $line; }' < foo_spaces.py > foo_tabs.py

I have a gist [1] which has very slightly more comments, if you prefer. To paraphrase XKCD, there are now N+1 regex explanations, and I apologize that mine is comparatively worse than all of them. :)

0: http://perldoc.perl.org/perlre.html 1: https://gist.github.com/gknoy/94c382299c4543f2e863

Thanks for sharing the man page link. I am reading it now! :)

Have you looked at sed yet?

Sed was the problem. On Mac the syntax is slightly different and it doesn't take \t as tab. Those two curve balls took some extra research.

I didn't actually take the entire day. :)