Emacs regular expressions

Emacs regular expressions have some unusual features.

\B matches the empty string everywhere except at the beginning and end of a word.
\_< matches the empty string, but only at the beginning of a “symbol”; i.e., a sequence of one or more characters with syntax class w or _.
\_> matches the empty string, but only at the end of a “symbol”; i.e., a sequence of one or more characters with syntax class w or _.
\< matches the empty string, but only at the beginning of a word.
\> matches the empty string, but only at the end of a word.
\s<c>, where <c> is any character, matches any character with syntax class c.