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

IO.foreach streams through the file, which is exactly how I want it—the bits of information I'm interested in are located in only a short section within the first 5-8% of the text file (there's some additional checking / breaking logic in the actual problem that I haven't included).

I guess compiling the regexp is my best option at this point—nice tip on the Regexp.quote, though—that would have killed me later.

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.