Add RegExp Playground (#5885)

* Update free-programming-interactive-tutorials-en.md

* Update free-programming-interactive-tutorials-en.md

* Fix linting errors

* Fix linting errors

* Update more/free-programming-interactive-tutorials-en.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update free-programming-interactive-tutorials-en.md

Add another resource and rename the section

* Update free-programming-interactive-tutorials-en.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

more/free-programming-interactive-tutorials-en.md

@@ -37,6 +37,7 @@
 * [PostgreSQL](#postgresql)
 * [Python](#python)
 * [R](#r)
+* [Regular Expressions](#regular-expressions)
 * [Ruby](#ruby)
 * [Rust](#rust)
 * [Scala](#scala)
@@ -315,6 +316,12 @@
 * [R Tutorial](https://www.w3schools.com/r) - W3Schools
 
 
+### Regular Expressions
+
+* [Regex Crossword](https://regexcrossword.com) - Ole Michelsen, Maria Hagsten Michelsen
+* [RegExp Playground](https://projects.verou.me/regexplained) - Lea Verou
+
+
 ### Ruby
 
 * [CodeCademy Ruby](https://www.codecademy.com/learn/ruby)