You can also do it using an eval which is perhaps more obvious than no_slogan's method but not as elegant. 

$_ = 'foo(';

$regex = 'FOO(';
$modifiers = 'i';

# a bit of error checking never goes astray
die "Illegal modifier!" unless $modifiers =~ m/^[ismxo]\z/;

# escape everything except the essential regex char and \w \s
# this is a security measure as discussed below
$regex =~ s/([^\w\s\\|()[\]{}^\$*+?.])/\\$1/g;

# generate the regex with error checking
$regex = eval "qr/$regex/$modifiers";
die "Eval failed $@\n" if $@;

# test it
print "match" if /$regex/;

[download]

There is a significant security issue eval-ing user supplied data, or in fact using raw user data in many places in a script. The problem is scary. If $regex contains a string like this '/; `some bad command`; $hacked =1' then when this interpolates we will eval 

$regex = eval "qr//; `some bad command`; $hacked =1/$modifiers"

This may execute some bad command on your system with the same permissions as the script. Both the solutions presented suffer from this problem. To solve it (at least partially) you need to escape all the non regex metachars. 

$regex =~ s/([^\w\s\\|()[\]{}^\$*+?.])/\\$1/g;

[download]

This will leave \w \s and the regex chars unescaped but throw in escapes for everything else including semicolons and backtics. It can probably still be hacked - did someone say merlyn - but the escapes should throw a spanner in the works for the casual hacker.

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

In reply to Re: Store regex modifiers in variables? by tachyon
in thread Store regex modifiers in variables? by derek3000

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.