Model validation not working in rails app - ruby-on-rails

Question

I have a validation in one of my models: (The sku_code validation.)
class Vendor < ActiveRecord::Base
has_many :purchases
validates :name, format: {with: /\A[A-Za-z0-9\S\s]+\z/, message: "Vendor name is invalid."}
validates :sku_code, format: {with: /\A[A-Z]{3}\z/, message: "must follow format ABC"}
end
Controller:
class VendorsController < ApplicationController
def index
#vendors = Vendor.all
respond_to do |format|
format.js
format.html
format.json
end
end
def new
#vendor = Vendor.new
respond_to do |format|
format.js
format.html
format.json
end
end
def create
#vendor = Vendor.new(vendor_params)
respond_to do |format|
if #vendor.save
format.html { redirect_to #vendor, notice: 'Vendor successfully created.' }
format.js {}
format.json { render json: #vendor, status: :created, location: #vendor }
else
format.html { render 'new' }
format.json { render json: #vendor.errors, status: :unprocessable_entity }
end
end
end
def update
#vendor = Vendor.find(params[:id])
respond_to do |format|
if #vendor.update(vendor_params)
format.html { redirect_to #vendor, notice: 'Vendor successfully updated.' }
format.js {}
format.json { render json: #vendor, status: :updated, location: #vendor }
else
format.html { render 'edit' }
format.json { render json: #vendor.errors, status: :unprocessable_entity }
end
end
end
def edit
#vendor = Vendor.find(params[:id])
respond_to do |format|
format.js
format.html
format.json
end
end
def destroy
#vendor = Vendor.find(params[:id])
#vendor.destroy
#flash.notice="Vendor '#{#vendor.name}' was deleted."
redirect_to action: "index"
end
def show
#vendor = Vendor.find(params[:id])
respond_to do |format|
format.js
format.html
format.json
end
end
private
def vendor_params
params.require(:vendor).permit(:name, :sku_code, :contact, :phone, :email, :address_line1, :address_line2, :address_city, :address_state, :address_zip, :address_country, :lead_time)
end
end
What I expected with this regex was that it would allow for a three capital letter string to be entered into my form but prevent anything else.
Currently it is rejecting the three capital letter string.
What have I done wrong here?