Ruby code - 7 lines

Link:

Ruby, pasted on Mar 12:

class String
  def to_case_insensitive_regex
    /#{gsub(/./) {|c| "[#{c.downcase}#{c.upcase}]"}}/
  end
end

puts "teStEr".to_case_insensitive_regex

Output:

1 2	(?-mix:[tT][eE][sS][tT][eE][rR])