58:  How do I use regexps (regular expressions) in Emacs?

  See "Regexps" in the on-line manual.

  WARNING: The "or" operator is `\|', not `|', and the grouping operators
  are `\(' and `\)'.  Also, the string syntax for a backslash is `\\'.  To
  specify a regular expression like xxx\(foo\|bar\) in a Lisp string, use
   
     "xxx\\(foo\\|bar\\)"

  Notice the doubled backslashes!

  WARNING: Unlike in Unix grep, sed, etc., a complement character set
  ([^...])  can match a newline character (LFD aka C-j aka \n), unless
  newline is mentioned as one of the characters not to match.

  WARNING: The character syntax regexps (e.g., "\sw") are not meaningful
  inside character set regexps (e.g., "[aeiou]").  (This is actually
  typical for regexp syntax.)