(Update: whoops! jwkrahn did mention it below.)

I don't think anyone has mentioned the POSIX character class syntax: alnum. Those and more are in the perlre. They don't work in older versions of Perl, IIRC.

# match (all are alphanumeric)
/\A[[:alnum:]]+\z/

# doesn't match (any single is not alphanumeric)
/[^[:alnum:]]/
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