A "\w" matches a single alphanumeric character (an alphabetic 
character, or a decimal digit) or "_"...

##</code><code>##

s/(?:^\W+)|(?:\W+$)//g;
# versus
s/\W+$//g;
s/^\W+//g;

# my unscientific benchmark
                Rate  single_pass    two_pass
single_pass  15829/s           --        -11%
two_pass     17737/s          12%          --