$ perl -ple ' $_ = eval "sub { /$_/ }" ? "OK\n" : "$@" ' a OK a( Unmatched ( in regex; marked by <-- HERE in m/a( <-- HERE / at (eval 2) line 1, <> line 2. a[ Unmatched [ in regex; marked by <-- HERE in m/a[ <-- HERE / at (eval 3) line 1, <> line 3. a() OK