Learning regular expressions
 

I have discovered Regular Expressions in LibreOffice (Search and Replace) and I have found it very useful. I have a few questions on RegEx. What is the best way to learn it? Is regex for libreoffice the same as in the grep command? Is regex in the grep-command the same as in the sed-command?

As you probably can guess from my questions, I don't have much experience with this, so be lenient please.

Those to gave me a good learning experience.

This ones a primar and very good to get ones feet wet. You will also find some more complex stuff.
http://regexone.com/

Every little thing one needs to know about regex. Also I sometimes found it hard to follow along. Kinda bloated on some topics.
http://www.regular-expressions.info/tutorial.html

I used to use http://www.regexr.com/ for building / testing with bulks of text.

There will be slight variations, but in general regex follows between languages and applications