Match unless the line is made up entirely of exceptions and non-matches:

my $exceptions = qr/_ALPHA|#ALPHA|XL5 ALPHA/;
my $keepers = qr/ALPHA|BETA/;
while (my $srch = <DATA>) {
  print "Matched ($srch)!\n" unless $srch=~/^(?:$exceptions|(?!$keeper
+s).)*$/;
}
__DATA__
one with #ALPHA and good ALPHA
one with just XL5 ALPHA
one with just BETA
one with _ALPHA and BETA
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