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.\" ========================================================================
.\"
.IX Title "re 3"
.TH re 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide"
.SH "NAME"
re \- Perl pragma to alter regular expression behaviour
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&    use re 'taint';
\&    ($x) = ($^X =~ /^(.*)$/s);     # $x is tainted here
.Ve
.PP
.Vb 3
\&    $pat = '(?{ $foo = 1 })';
\&    use re 'eval';
\&    /foo${pat}bar/;                # won't fail (when not under -T switch)
.Ve
.PP
.Vb 3
\&    {
\&        no re 'taint';             # the default
\&        ($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here
.Ve
.PP
.Vb 3
\&        no re 'eval';              # the default
\&        /foo${pat}bar/;            # disallowed (with or without -T switch)
\&    }
.Ve
.PP
.Vb 3
\&    use re 'debug';                # NOT lexically scoped (as others are)
\&    /^(.*)$/s;                     # output debugging info during
\&                                   #     compile and run time
.Ve
.PP
.Vb 2
\&    use re 'debugcolor';           # same as 'debug', but with colored output
\&    ...
.Ve
.PP
(We use $^X in these examples because it's tainted by default.)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
When \f(CW\*(C`use re 'taint'\*(C'\fR is in effect, and a tainted string is the target
of a regex, the regex memories (or values returned by the m// operator
in list context) are tainted.  This feature is useful when regex operations
on tainted data aren't meant to extract safe substrings, but to perform
other transformations.
.PP
When \f(CW\*(C`use re 'eval'\*(C'\fR is in effect, a regex is allowed to contain
\&\f(CW\*(C`(?{ ... })\*(C'\fR zero-width assertions even if regular expression contains
variable interpolation.  That is normally disallowed, since it is a
potential security risk.  Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e.  evaluation is always
disallowed with tainted regular expressions.  See \*(L"(?{ code })\*(R" in perlre.
.PP
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of \f(CW\*(C`qr//\*(C'\fR) is \fInot\fR considered variable
interpolation.  Thus:
.PP
.Vb 1
\&    /foo${pat}bar/
.Ve
.PP
\&\fIis\fR allowed if \f(CW$pat\fR is a precompiled regular expression, even
if \f(CW$pat\fR contains \f(CW\*(C`(?{ ... })\*(C'\fR assertions.
.PP
When \f(CW\*(C`use re 'debug'\*(C'\fR is in effect, perl emits debugging messages when
compiling and using regular expressions.  The output is the same as that
obtained by running a \f(CW\*(C`\-DDEBUGGING\*(C'\fR\-enabled perl interpreter with the
\&\fB\-Dr\fR switch. It may be quite voluminous depending on the complexity
of the match.  Using \f(CW\*(C`debugcolor\*(C'\fR instead of \f(CW\*(C`debug\*(C'\fR enables a
form of output that can be used to get a colorful display on terminals
that understand termcap color sequences.  Set \f(CW$ENV{PERL_RE_TC}\fR to a
comma-separated list of \f(CW\*(C`termcap\*(C'\fR properties to use for highlighting
strings on/off, pre-point part on/off.
See \*(L"Debugging regular expressions\*(R" in perldebug for additional info.
.PP
The directive \f(CW\*(C`use re 'debug'\*(C'\fR is \fInot lexically scoped\fR, as the
other directives are.  It has both compile-time and run-time effects.
.PP
See \*(L"Pragmatic Modules\*(R" in perlmodlib.