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(efaq)Using regular expressions

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How do I use regexps (regular expressions) in Emacs?
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   Note: Regexps.

   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\\)'.

   Note the doubled backslashes!

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

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