See `Regexps' in the online manual.
WARNING: The "or" operator is `\|', not `|', and the grouping operators are `\(' and `\)'. Also, the string syntax for a backslash is "\\". Thus, the string syntax for a regular expression like xxx\(foo\|bar\) is "xxx\\(foo\\|bar\\)". Notice the duplicated 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 (eg. `\sw') are not meaningful inside character set regexps (eg. `[aeiou]'). (This is actually typical for regexp syntax.)