re(3) Perl Programmers Reference Guide re(3)NAME
re - Perl pragma to alter regular expression behaviour
SYNOPSIS
use re 'taint';
($x) = ($^X =~ /^(.*)$/s); # $x is tainted here
$pat = '(?{ $foo = 1 })';
use re 'eval';
/foo${pat}bar/; # won't fail (when not under -T switch)
{
no re 'taint'; # the default
($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here
no re 'eval'; # the default
/foo${pat}bar/; # disallowed (with or without -T switch)
}
use re 'debug'; # NOT lexically scoped (as others are)
/^(.*)$/s; # output debugging info during
# compile and run time
use re 'debugcolor'; # same as 'debug', but with colored output
...
(We use $^X in these examples because it's tainted by default.)
DESCRIPTION
When "use re 'taint'" is in effect, and a tainted string is the target
of a regex, the regex memories (or values returned by the m// operator
in list context) are tainted. This feature is useful when regex opera‐
tions on tainted data aren't meant to extract safe substrings, but to
perform other transformations.
When "use re 'eval'" is in effect, a regex is allowed to contain "(?{
... })" zero-width assertions even if regular expression contains vari‐
able interpolation. That is normally disallowed, since it is a poten‐
tial security risk. Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e. evaluation is always
disallowed with tainted regular expressions. See "(?{ code })" in
perlre.
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of "qr//") is not considered variable
interpolation. Thus:
/foo${pat}bar/
is allowed if $pat is a precompiled regular expression, even if $pat
contains "(?{ ... })" assertions.
When "use re 'debug'" is in effect, perl emits debugging messages when
compiling and using regular expressions. The output is the same as
that obtained by running a "-DDEBUGGING"-enabled perl interpreter with
the -Dr switch. It may be quite voluminous depending on the complexity
of the match. Using "debugcolor" instead of "debug" enables a form of
output that can be used to get a colorful display on terminals that
understand termcap color sequences. Set $ENV{PERL_RE_TC} to a comma-
separated list of "termcap" properties to use for highlighting strings
on/off, pre-point part on/off. See "Debugging regular expressions" in
perldebug for additional info.
The directive "use re 'debug'" is not lexically scoped, as the other
directives are. It has both compile-time and run-time effects.
See "Pragmatic Modules" in perlmodlib.
perl v5.8.8 2008-09-19 re(3)
