This won't work if the perl code on the back end works like this: 
my $string = "a_sample_string";
my $re = q[(?{ tr|_| | })(.*)]; 
# or my $re = some CGI parameter extraction
$string =~ m/$re/;
print $1;

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Which is, you must admit much more likely than forming a string and passing it to eval (but some people do some amazingly unsecure things). Of course, if they pass it through the qr operator, we're back in business. (I assume they aren't using the obviously insecure use re 'eval';)

The lesson here for anyone writing CGI scripts that do regexp manipulation is this:
Using qr allows regexps to execute arbitrary code. DO NOT take arbitrary user input and feed it to qr unless you mean to allow arbitrary code execution. 

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/

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In reply to Re^4: a small regexp question by fizbin
in thread a small regexp question by newroz

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