A simple way is to invert the logic: reject the string if there's anything found that doesn't match with what you like. So you invert your character class, and you negate your test. 
if(/[^xyz]/) {
    # It matches, though it shouldn't
    print "No good.";
    return 0;
} else {
    print "Acceptable";
    return 1;
}

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In reply to Re: matching all characters within a string? by bart
in thread matching all characters within a string? by Anonymous Monk

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