The worst thing about regular expressions is that most people only need them occasionally. You invest a few hours becoming an expert and then forget a lot of it by the next time you need them.
For this reason I really appreciate tools that explain expressions in plain english. For example check out how RegEx buddy parses this expression: http://www.regexbuddy.com/screens/analyze.png
Another great site I use for highlighting regular expressions is regex101.com
It additionally supports PCRE and Python syntax. And has also a unit testing feature.
http://jex.im/regulex/
has beautiful railroad diagrams, but limited functionality otherwise.
https://www.debuggex.com/
also has railroad diagrams, and a ton of other functionality