I also prefer Oxford commas.

They're nice, unambiguous, and easy on the eyes. ;)

Awesome writeup @cmditch

I found a nicer solution that doesn't use ~

It took me a while to figure out what's happening here. In case anyone else is interested.
Basically, any Regexp without closed parentheses will throw an exception, which Insti rescues by returning false. Tricky.

Just punch this in your repl:

/asdas()/
Regexp.new("gf()((asda)asd)()sds") rescue "I won't need rescuing"
Regexp.new("gf(sds") rescue "please rescue me!!!"

The trickier part is tilde. I found the answer here - Stackoverflow & Ruby Regexp Docs very last method #~

It's matching the Regexp to the last 'gets' prompt (only if it's valid, i.e., only if it has closed or no parenthesis). The variable $_ holds the most recent and scoped value of gets, which, in the case of #valid_parentheses is nothing, nada, nil.

For illustration:

def what_does_tidle_do?(string)
  print "Type something: "
  gets.chomp
  puts "Your last gets statement was, '" + $_.chomp + "', now I'll find the first index of what you typed against your regexified method argument."
  ~ Regexp.new(string)
end

> what_does_tidle_do?("cheese")
 Type something: string cheese mmmm
 Your last gets statement was, 'string cheese mmmm', now I'll find the first index of what you typed against your regexified method argument.
=> 7