Perl defines the following zero-width assertions:    
\b	Match a word boundary   
\B	Match except at a word boundary   
\A	Match only at beginning of string      
\Z	Match only at end of string, or before newline at   
 the end    
\z	Match only at end of string       
\G	Match only at pos() (e.g. at the end-of-match position        of prior m//g)   
  
A word boundary (\b ) is a spot between two characters that has a \w on one side of it and a \W on the other side of it (in either order), counting the imaginary characters    
off the beginning and end of the string as matching a \W . (Within character classes \b represents backspace rather than a word boundary, just as it normally does in any double-quoted string.)