使用gsub时如何限制替换次数？

str = \'aaaaaaaaaa\'
count = 5
p str.gsub(/a/){if count.zero? then $& else count -= 1; \'x\' end}
# => \"xxxxxaaaaa\"

str = \'aaaaaaaaaa\'
# The following is so that the variable new_string exists in this scope, 
# not just within the block
new_string = str 
5.times do 
  new_string = new_string.sub(\'a\', \'x\')
end