here a way you could go, to counter the problems listed and explained here

  1. code injection by string interpolation, like /@{[ do_evil() ]}/
  2. code injection by regex, like /(?{ do_evil() })/
  3. exponential time regexes with excessive backtracking, something like /((x*)*)*/ IIRC

This will compile a regex into an anonymous sub without executing it 

use re qw(debug);
my $sub = eval "sub { m/$evil_re/ }";

[download]

the re debug will emit regex-opcodes for the regexes involved to STDERR 

Final program:
   1: EVAL (4)
   4: EXACT <\n> (6)
   6: END (0)

[download]

the 1: EVAL here tells you that an EVAL was involved which you need to reject, you don't want embedded Perl code

$evil_re = "(?{ BEGIN { do_evil() } })";

with Keyword::Simple disabling BEGIN,END,... etc you won't risk that the compilation of the sub inside the eval will run any code (see here)

with Safe you'll be able to additionally disable a bunch of external commands. (see here)

For this to work you need to spawn an external command for each regex and capture STDERR, you can use this to also limit the maximal runtime.

Since your code looks a lot like a test suite, you might wanna use the TAP protocol anyway.

NB: No guaranties whatsoever!

HTH! :)

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re: Is it safe to use external strings for regexes? by LanX
in thread Is it safe to use external strings for regexes? by stevieb

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.