gistfile1.rb

# Given a big (~25MB) text file, I want to extract
# and process lines that are prefixed with a certain
# string. In the actual case, known_prefixes is 
# approximately 50 elements long with potential for growth

known_prefixes = %w(aa ab ax fy fx)
regex = "^#{known_prefixes.join("|")} "

IO.foreach(big_ass_text_file).each |row|
  if row =~ regex
    process
  end
end

I guess the point I'm trying to illustrate is that passing a block into String#scan might be faster than if =~ - it's worth testing.