GNU has \_< and  \_> that matches the empty string, but only at the beginning or 
end of a symbol. 

The doc string for \_< reads: A symbol is a sequence of one or more symbol-
constituent characters.  A symbol-constituent character is a character whose 
syntax is either `w' or `_'.  `\_<' matches at the beginning of the buffer only 
if a symbol-constituent character follows.

\_> has the corresponding description for the end of a symbol.