What characters are valid in hash keys?

Question

As per subject: what are the characters that can be used in hash keys or, if it's shorter, which ones can't be used?

Also, are there any problems in using long hash keys (like full path names)?

Answer 1

See How Hashes Really Work for a discussion on this topic. In short, as long as you quote the key (non-interpolating q{}), you can use whatever characters you want.

Regarding Dana's answer, no, it won't take longer for longer keys to get matched: it will take infinitesimally longer to hash the key, but that's all.

For reference, this is the hashing function in Perl 5.10.0:

#define PERL_HASH(hash,str,len)
 STMT_START {
    register const char * const s_PeRlHaSh_tmp = str;
    register const unsigned char *s_PeRlHaSh = (const unsigned char *)s_PeRlHaSh_tmp;
    register I32 i_PeRlHaSh = len;
    register U32 hash_PeRlHaSh = PERL_HASH_SEED;
    while (i_PeRlHaSh--) {
        hash_PeRlHaSh += *s_PeRlHaSh++;
        hash_PeRlHaSh += (hash_PeRlHaSh << 10);
        hash_PeRlHaSh ^= (hash_PeRlHaSh >> 6);
    }
    hash_PeRlHaSh += (hash_PeRlHaSh << 3);
    hash_PeRlHaSh ^= (hash_PeRlHaSh >> 11);
    (hash) = (hash_PeRlHaSh + (hash_PeRlHaSh << 15));
} STMT_END

Answer 2

One point not brought up yet is that you can use any valid string as a hash key. If you try to use something other than a string, it will be automatically stringified, which means that, e.g.,

my $ref = [];
$hash{$ref} = 'foo';

will use the string "ARRAY(0xdeadbeef)" (or whatever address) as the hash key, not the actual array reference.