Well, both  foo and (?-xism:$STUFF) are regular expressions and valid strings. A better way to distinguish the two is to look at the reference: 
$reg = qr'foo';
$str = 'foo';
$str2 = '(?-xism:foo)';
$reg_ref = \$reg;
$str_ref = \$str;
$str2_ref = \$str2;
print "$reg -> $reg_ref\n";
print "$str -> $str_ref\n";
print "$str2 -> $str2_ref\n";

[download]
yields 
(?-xism:foo) -> REF(0x80635ac)
foo -> SCALAR(0x80635dc)
(?-xism:foo) -> SCALAR(0x8063600)

[download]

-Mark

In reply to Re: Walks like a regex, quacks like a regex... by kvale
in thread Walks like a regex, quacks like a regex... by legLess

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