Then it won't work, but the instructions say: Create a five_num method that takes an array of integers as an argument
so checking for invalid input is not a requirement.

Thanks for the tip. I think the Kata description has changed since I wrote my solution.

The de-facto community style guide for Ruby suggests using parens if the method accepts paramters and leaving off the parens if the method takes no parameters.

Assignment in Ruby (and in most other programming languages) works right to left, so the result of the expression on the right of the = is stored in the variable or variables on the left.

https://www.oreilly.com/library/view/the-ruby-programming/9780596516178/ch04s05.html
https://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Operators#Assignment

Huh, I never noticed that when writing the solution!
Looks like the Kata needs more tests.

grep returns an Array of every element that matches the pattern.

matches = arr.grep(/.*foo.*/) is the equivalent of matches = arr.select { |e| /.*foo.*/ === e }

The pattern in my solution is looking for any word in the dictionary that starts with the input string, ignoring case.

lazy returns a lazy enumerator, which when used with methods like grep, map, etc. will only enumerate values as they are needed.

When first(5) is called it forces the lazy enumeration to be evaluated and will only search through the dictionary until it finds 5 matching items.

Without the lazy, grep would search through the entire dictionary for matches which would be wasteful when only the first 5 are required.