Programming Perl, 3rd Edition

use re

This pragma controls the use of regular expressions. It has four possible invocations: "taint" and "eval", which are lexically scoped, plus "debug" and "debugcolor", which aren't.

use re 'taint';                
# Contents of $match are tainted if $dirty was also tainted.
($match) = ($dirty =~ /^(.*)$/s); 

# Allow code interpolation:
use re 'eval';
$pat = '(?{ $var = 1 })';      # embedded code execution
/alpha${pat}omega/;            # won't fail unless under -T
                               # and $pat is tainted

use re 'debug';                # like "perl -Dr"
/^(.*)$/s;                     # output debugging info during
                               #     compile time and run time

use re 'debugcolor';           # same as 'debug', 
                               #    but with colored output

When use re 'taint' is in effect and a tainted string is the target of a regex, the numbered regex variables and values returned by the m// operator in list context are all tainted. This is useful when regex operations on tainted data aren't meant to extract safe substrings, but to perform other transformations. See the discussion on tainting in Chapter 23.

When use re 'eval' is in effect, a regex is allowed to contain assertions that execute Perl code, which are of the form (?{ … }), even when the regex contains interpolated variables. Execution of code segments resulting from variable interpolation into a regex is normally disallowed for security reasons: you don't want programs that read patterns from config files, command-line arguments, or CGI form fields to suddenly start executing arbitrary code if they weren't designed to expect this possibility. This ...