Hash Key vs. Quantifier Ambiguity in Regex Context

QM has asked for the wisdom of the Perl Monks concerning the following question:

While building up regex strings, I came across this conundrum. For example,

my $x = "(quantifier)";
my %x = ( "1" => "(hash_value)" );
my $z = 'quantifier hash_value';

print "_$1_\n" if $z =~ /$x{1}/;         # {1} is quantifier
print "_$1_\n" if $z =~ /@{[$x{1}]}[0]/; # {1} is hash key 

__END__

_quantifier_
_hash_value_
[download]

(1) How does Perl disambiguate (a) exactly 1 occurence of $x and (b) the hash value $x{1}?

(2) You can see that Perl chooses (a) for /$x{1}/. If I want (b), is there a better way than @{[$x{1}]}[0]?

(3) If we had it to do over, is there a better way for Perl to resolve this?

-QM
--
Quantum Mechanics: The dreams stuff is made of

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Re: Hash Key vs. Quantifier Ambiguity in Regex Context by jarich (Curate) on Apr 01, 2004 at 00:08 UTC
(2) You can see that Perl chooses (a) for /$x{1}/. If I want (b), is there a better way than `@{[$x{1}]}[0]`? Absolutely. Quote your hash key. `my $x = "(quantifier)"; my %x = ( "1" => "(hash_value)" ); my $z = 'quantifier hash_value'; print "_$1_\n" if $z =~ /$x{'1'}/; # $x{'1'} is hash key __END__ _hash_value_` [download] Hope this helps. jarich	[reply] [d/l] [select]
Re: Re: Hash Key vs. Quantifier Ambiguity in Regex Context by QM (Parson) on Apr 01, 2004 at 00:24 UTC
Thanks! [Seems I've left my Obvious Hat at home today :( ] -QM -- Quantum Mechanics: The dreams stuff is made of	[reply]
Re: Hash Key vs. Quantifier Ambiguity in Regex Context by kvale (Monsignor) on Apr 01, 2004 at 00:20 UTC
(1) How regex strings are interpolated is a messy business. Usually it DWIM, but you hit a case where it doesn't. See Gory-details-of-parsing-quoted-constructs for more details. (2), I'd rename one of the x variables or do the interpolation my self: `my $x = "(quantifier)"; my %x = ( "1" => "(hash_value)" ); my $z = 'quantifier hash_value'; my $ele = $x{1}; print "_$1_\n" if $z =~ /$ele/;` [download] For the third question, I'd probably play at perturbing perl's probabilistic parser, and not get anywhere :) -Mark	[reply] [d/l]
Re: Hash Key vs. Quantifier Ambiguity in Regex Context by borisz (Canon) on Apr 01, 2004 at 00:13 UTC
Use `/$x{'1'}/` for the regex. Perl treat `/$x{1}/` as match $x exactly 1 time {1}. Boris	[reply] [d/l] [select]