Add "Convert PascalCase string into snake_case" kata

Author: juan-antonio-ledesma

5-kyu/convert-pascal-case-string-into-snake-case.md

@@ -0,0 +1,31 @@
+<h1>Convert PascalCase string into snake_case <sup><sup>5 Kyu</sup></sup></h1>
+
+<sup>
+  <a href="https://www.codewars.com/kata/529b418d533b76924600085d">
+    <strong>LINK TO THE KATA</strong>
+  </a> - <code>STRINGS</code> <code>ALGORITHMS</code>
+</sup>
+
+## Description
+
+Complete the function/method so that it takes a `PascalCase` string and returns the string in `snake_case` notation. Lowercase characters can be numbers. If the method gets a number as input, it should return a string.
+
+**Examples**
+
+```
+"TestController"  -->  "test_controller"
+"MoviesAndBooks"  -->  "movies_and_books"
+"App7Test"        -->  "app7_test"
+1                 -->  "1"
+```
+
+## Solution
+
+```javascript
+const toUnderscore = string => {
+  return string
+    .toString()
+    .replace(/(.)([A-Z])/g, '$1_$2')
+    .toLowerCase()
+}
+```

README.md

@@ -34,6 +34,7 @@ JavaScript katas on Codewars are programming challenges that help you improve yo
 
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 - **[Convert A Hex String To RGB](./5-kyu/convert-a-hex-string-to-rgb.md)**<br><sup>`PARSING` `STRINGS` `ALGORITHMS`</sup>
+- **[Convert PascalCase string into snake_case](./5-kyu/convert-pascal-case-string-into-snake-case.md)**<br><sup>`STRINGS` `ALGORITHMS`</sup>
 - **[Don't Drink the Water](./5-kyu/dont-drink-the-water.md)**<br><sup>`ALGORITHMS` `ARRAYS` `SORTING` `LISTS`</sup>
 - **[Extract the domain name from a URL](./5-kyu/extract-the-domain-name-from-a-url.md)**<br><sup>`PARSING` `REGULAR EXPRESSIONS`</sup>
 - **[First non-repeating character](./5-kyu/first-non-repeating-character.md)**<br><sup>`STRINGS` `ALGORITHMS`</sup>