Updating a field/subfield value with ruby-marc

update_sfield.rb

require 'marc'

TAG = '856'
SUB = 'y'
STR = 'Img100'
REP = 'Img200'

count = 0
w = MARC::Writer.new('dla_fixed.mrc')
MARC::ForgivingReader.new('records/dla.mrc').each do |r|
	if f = r.find { |f| f.tag == TAG and f[SUB] =~ /#{STR}/ }
		f[SUB].gsub!(/#{STR}/, REP)
		w.write r
		count += 1
	end
end

w.close
puts count