re - Perl pragma to alter regular expression behaviour
- 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 $^X in these examples because it's tainted by default.)
When use re 'taint'
is in effect, and a tainted string is the target
of a regex, the regex memories (or values returned by the m// operator
in list context) are tainted. This feature is useful when regex operations
on tainted data aren't meant to extract safe substrings, but to perform
other transformations.
When use re 'eval'
is in effect, a regex is allowed to contain
(?{ ... }) zero-width assertions even if regular expression contains
variable interpolation. That is normally disallowed, since it is a
potential security risk. Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e. evaluation is always
disallowed with tainted regular expressions. See (?{ code }) in perlre.
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of qr//) is not considered variable
interpolation. Thus:
- /foo${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.