feat: Add the most important regex playgrounds (#6227)

* feat(playground): Add regex section

* feat(playgrounds): Add regex101.com by Firas Dib

* feat(playgrounds): Add regexr.com by GSkinner

* feat(playgrounds): Add regextester.com @ Dan Tools

* feat(playgrounds); Add iHateRegex.io by GeonGeorge

more/free-programming-playgrounds.md

@@ -30,6 +30,7 @@
 * [R](#r)
 * [React](#react)
 * [Redis](#redis)
+* [Regular Expressions](#regular-expressions)
 * [Ruby](#ruby)
 * [Rust](#rust)
 * [RxJS](#rxjs)
@@ -230,6 +231,14 @@
 * [Try Redis](https://try.redis.io)
 
 
+### Regular Expressions
+
+* [iHateRegex: regex for playground](https://ihateregex.io/playground) - Geon George
+* [Regex Tester and Debugger Online - Javascript, PCRE, PHP](https://www.regextester.com) - Dan's Tools
+* [Regex101: build, test, and debug regex](https://regex101.com) - Firas Dib (regex101.com)
+* [RegExr: Learn, Build, \& Test RegEx](https://regexr.com) - GSkinner Inc.
+
+
 ### Ruby
 
 * [Codepad](http://codepad.org)