str = "[ASDF ASDF]"
from, to = [/\[([A-Z][ A-Z_0-9]*)\]/, proc{|s| "\"#{$1.downcase}\""}]
def doit(str, from, to)
  str.gsub(from, &to)
end
doit(str, from, to)

def doit2(str, from, to)
  to = proc{|s| "\"#{$1.downcase}\""}
  str.gsub(from, &to)
end
doit2(str, from, to)

str.gsub(from, &to)