Regular expression:
  - the function 'preg_match' answers:
     does a certain pattern occur INSIDE a certain string
     (not "does this string, in its entirety, match the pattern" --
      use '^' and '$' in your regexp to check if the pattern matches the entire string)

  - when writing a regexp in a language:
    How do I express the regexp                \w+\.\w+\.(edu|com)
    in Java?                      new Pattern("\\w+\\.\\w+\\.(edu|com)")
    in php?                       "/\\w+\\.\\w+\\.(edu|com)/"
                               or '/\w+\.\w+\.(edu|com)/'

  - a regexp for "0, OR 3-or-more digits":
           (\d\d\d\d*)?  


  - DTD which allows   name, followed by images and histories (in either order), 
                       followed by a reference:
    <!ELEMENT wonder (name, (image|history)*, reference) !>