re(3p) Perl Programmers Reference Guide re(3p)NAMEre - 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 $^X in these examples because it's tainted by
default.)
DESCRIPTION
When "use re 'taint'" is in effect, and a tainted string is
the target of a regex, the regex memories (or values
returned by the m// operator in list context) are tainted.
This feature is useful when regex operations on tainted data
aren't meant to extract safe substrings, but to perform
other transformations.
When "use re 'eval'" is in effect, a regex is allowed to
contain "(?{ ... })" zero-width assertions even if regular
expression contains variable interpolation. That is nor-
mally disallowed, since it is a potential security risk.
Note that this pragma is ignored when the regular expression
is obtained from tainted data, i.e. evaluation is always
disallowed with tainted regular expressions. See "(?{ code
})" in perlre.
For the purpose of this pragma, interpolation of precompiled
regular expressions (i.e., the result of "qr//") is not con-
sidered variable interpolation. Thus:
/foo${pat}bar/
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re(3p) Perl Programmers Reference Guide re(3p)
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" pro-
perties to use for highlighting strings on/off, pre-point
part on/off. See "Debugging regular expressions" in perlde-
bug 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.
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