my $dle = sprintf "\\%03o", DLE;
my $regex = "\\A[$dle][^$dle]";
$regex = qr/$regex/;
if ($string =~ $regex) { ... }
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$msg =~ /(\A$dle)(?<!\1)/;
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That seems to generate a "variable length lookbehind not implemented" error :( Also, I don't understand why we are looking *behind*, don't we want to test the second string character with look *ahead* ? If so, how do we confine the lookahead test to only the *next* character ? Doesn't it look all the way to the end of the string ? Thanks, Scott.

That'll teach me to blab about features I don't know without testing them first. Yes, positive lookahead would definitely be possible. You'd have to flip your test, though.

$x = 'aba';
 
print $x =~ /(.)(?=\1)/
        ? "double!\n"
        : "not double!\n";
 
$x = 'abba';
print $x =~ /(.)(?=\1)/
        ? "double!\n"
        : "not double!\n";
----
not double!
double!
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------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

use constant DLE => chr(0x10);

my $rx = do { local $_ = quotemeta DLE; qr/ \A $_ [^$_] /x };

$msg =~ $rx;
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So the answer to your question is that your construction works because you would expect a double-quoted string to work that way. You could also use the octal excape syntax: /\A\020[^\020]/.

The reason \x{$a} doesn't work is the same reason that it doesn't work in a double-quoted string - probably that the double-quoted string interpolation mechanism is single-pass and not recursive. (This is only a guess.)