But you can't be sure that $x starts and ends with an alphanumeric character, in which case \b$x\b wouldn't match when $x is surrounded by spaces.
Jeff japhy Pinyan,
P.L., P.M., P.O.D, X.S.:
Perl,
regex,
and perlhacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart