What characters are valid in hash keys?

As per subject: what are the characters that can be used in hash keys or,开发者_Go百科 if it's shorter, which ones can't be used?

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

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

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.

You can use any character that's valid in a string. Length isn't an issue either. Perl will cope with just about anything :)

You can use any character in a hash key --- a hash key is just a string. But for some characters you need to quote the string. If in doubt, just put quotes round the key.

$hash{simplekey}                             # fine
$hash{/var/log/auth.log}                     # syntax error --- can't use '/' directly
$hash{"/var/log/auth.log"}                   # quoted string, so can use any character
my $key = "/var/log/auth.log";  $hash{$key}  # variable used, which can contain any character

There's no special problem with using long keys that you don't already have with long strings.