As you note it does rather depend on what the problem is. Depending on the situation and the complexity of the language a pre-generated hash lookup table will be potentially much faster than a regex solution.

Consider a simple alphabet that may only contain words in the form: aa ab ac ad .... az. Including the overhead of generating the hash lookup table the hash method is much faster than a comparable regex method as well as being far more flexible. 

use Benchmark;

$string = 'aa ab ac ad ae af ' x 10000 . ' ff';
@string = split /\s/, $string;

$regex = <<'CODE';
&regex;
sub regex {
    do{return 0 unless /^a[a-z]$/} for @string;
    return 1;
}
CODE

$hash = <<'CODE';
$hash{$_}++ for aa..az;
&hash;
sub hash {
    do{return 0 unless defined $hash{$_}} for @string;
    return 1;
}
CODE

timethese ( 100, { 'regex' => $regex, 'hash' => $hash } );

__END__

Benchmark: timing 100 iterations of hash, regex...
      hash: 21 wallclock secs (20.37 usr +  0.00 sys = 20.37 CPU) @  4
+.91/s (n=100)
     regex: 45 wallclock secs (45.10 usr +  0.00 sys = 45.10 CPU) @  2
+.22/s (n=100)

[download]

doc

print(s??cod??scalar reverse :p)

In reply to Re: Re: Re: finite automata by doc
in thread finite automata by Anonymous Monk

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