Links to useful RegEx resources.
- Getting Started Tutorial: http://regexone.com (FCC recommended)
- RegEx 101: https://regex101.com
See also: 
Tutorials | 
Testing | 
Games | 
Blogs | 
Software
RegEx Tutorial & Guides
- RegEx Tutorial: http://www.regular-expressions.info/tutorial.html (detailed explanation)
- RegEx Cheatsheet: http://www.cheatography.com/davechild/cheat-sheets/regular-expressions
- MDN RegEx Guide: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
- StackOverflow RegEx FAQ: http://stackoverflow.com/a/22944075/1262108
- RegEx Video Tutorial: https://www.youtube.com/watch?v=EkluES9Rvak
RegEx Testing Ground
- RegEx Visualizer: https://www.debuggex.com/?flavor=javascript
- RegEx Challenges in Hackerrank: https://www.hackerrank.com/domains/algorithms/regex (practice)
- RegEx Regexplained: https://leaverou.github.io/regexplained/
RegEx Gameplay
- RegEx Golf: http://regex.alf.nu
- RegEx Crossword: http://regexcrossword.com
RegEx Blog
- Rex Egg: http://www.rexegg.com
RegEx Software Aids
- RegExBuddy: http://www.regexbuddy.com (proprietary)
- Expresso: http://www.ultrapico.com/expresso.htm (free)
- RegEx Coach: http://www.weitz.de/regex-coach/ (free)