# using boundaries

s//#foo foo foobar foo#/;
s/\bfoo\b//g;
print;

# using negative lookahead and lookbehind assertions

s//#foo foo foobar foo#/;
s/(?<!\w)foo(?!\w)//g;
print;

# using positive lookahead and lookbehind assertions with char classes
s//#foo foo foobar foo#/;
s/(?<=[^\w])foo(?=[^\w])//g;
print;