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

Have you looked at sed yet?

Not helpful at all for learning, but I've found RegExper[1] is fantastic for visualizing existing expressions, particularly ones you haven't written but need to understand or edit.

[1] http://www.regexper.com/

for JavaScript devs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guid...

also http://overapi.com/regex

This is also a great tool to play around with / construct regexes: http://regex101.com/