Reading regex from file in a filter

I created a log stash filter plugin which runs the regex match against a field, and if it matches, create a new field 'flag' and sets the value as 'Y'
Currently I am using the regex inside the ruby file of my filter but ideally I have to read the regex from some file , as this regex is gonna be a huge string.

This is how my filter code looks like now
def filter(event)

    `if @message`
      `products = event.get("products")`
	  `if ( products =~ /apple(?:ipad|laptop|watch)?|nike(?:jersey|shoes)?/ )`
		`event.set("flag", 'Y')`
	 `  end `
   ` end`

   ` filter_matched(event)`
`  end `

end

Kindly guide me on how to proceed on this further.