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/; # wont 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; # output debugging info during
/^(.*)$/s; # compile and run time
use re debugcolor; # same as debug, but with colored output
...
use re qw(Debug All); # Finer tuned debugging options.
use re qw(Debug More);
no re qw(Debug ALL); # Turn of all re debugging in this scope
use re qw(is_regexp regexp_pattern); # import utility functions
my ($pat,$mods)=regexp_pattern(qr/foo/i);
if (is_regexp($obj)) {
print "Got regexp: ",
scalar regexp_pattern($obj); # just as perl would stringify it
} # but no hassle with blessed res.
(We use $^X in these examples because its tainted by default.)
DESCRIPTION
'taint' mode
When
use re taintis in effect, and a tainted string is the target of a regex, the regex memories (or values returned by the m// operator in list context) are tainted. This feature is useful when regex operations on tainted data arent meant to extract safe substrings, but to perform other transformations.
'eval' mode
When
use re evalis in effect, a regex is allowed to contain
(?{ ... })zero-width assertions even if regular expression contains variable interpolation. That is normally disallowed, since it is a potential security risk. Note that this pragma is ignored when the regular expression is obtained from tainted data, i.e. evaluation is always disallowed with tainted regular expressions. See (?{ code }) in perlre.
For the purpose of this pragma, interpolation of precompiled regular expressions (i.e., the result of
qr//) is not considered variable interpolation. Thus:
/foo${pat}bar/
is allowed if
$patis a precompiled regular expression, even if
$patcontains
(?{ ... })assertions.
'debug' mode
When
use re debugis 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
debugcolorinstead of
debugenables a form of output that can be used to get a colorful display on terminals that understand termcap color sequences. Set
$ENV{PERL_RE_TC}to a comma-separated list of
termcapproperties to use for highlighting strings on/off, pre-point part on/off. See Debugging regular expressions in perldebug for additional info.
As of 5.9.5 the directive
use re debugand its equivalents are lexically scoped, as the other directives are. However they have both compile-time and run-time effects.
See Pragmatic Modules in perlmodlib.
'Debug' mode
Similarly
use re Debugproduces debugging output, the difference being that it allows the fine tuning of what debugging output will be emitted. Options are divided into three groups, those related to compilation, those related to execution and those related to special purposes. The options are as follows:
Compile related options |
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Execute related options |
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Extra debugging options |
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Other useful flags |
These are useful shortcuts to save on the typing.
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use re debugand its equivalents are lexically scoped, as the other directives are. However they have both compile-time and run-time effects.
Exportable Functions
As of perl 5.9.5 re debug contains a number of utility functions that may be optionally exported into the callers namespace. They are listed below.
is_regexp($ref) |
Returns true if the argument is a compiled regular expression as returned
by qr//, false if it is not. This function will not be confused by overloading or blessing. In internals terms, this extracts the regexp pointer out of the PERL_MAGIC_qr structure so it it cannot be fooled. |
regexp_pattern($ref) |
If the argument is a compiled regular expression as returned by qr//, then this function returns the pattern. In list context it returns a two element list, the first element containing the pattern and the second containing the modifiers used when the pattern was compiled.
my ($pat, $mods) = regexp_pattern($ref);
In scalar context it returns the same as perl would when strigifying a raw qr//with the same pattern inside. If the argument is not a compiled reference then this routine returns false but defined in scalar context, and the empty list in list context. Thus the following
if (regexp_pattern($ref) eq (?i-xsm:foo))
will be warning free regardless of what $refactually is. Like is_regexpthis function will not be confused by overloading or blessing of the object. |
regmust($ref) |
If the argument is a compiled regular expression as returned by qr//, then this function returns what the optimiser consiers to be the longest anchored fixed string and longest floating fixed string in the pattern. A fixed string is defined as being a substring that must appear for the pattern to match. An anchored fixed string is a fixed string that must appear at a particular offset from the beginning of the match. A floating fixed string is defined as a fixed string that can appear at any point in a range of positions relative to the start of the match. For example,
my $qr = qr/here .* there/x;
my ($anchored, $floating) = regmust($qr);
print "anchored:$anchored\nfloating:$floating\n";
results in
anchored:here
floating:there
Because the hereis before the .*in the pattern, its position can be determined exactly. Thats not true, however, for the there; it could appear at any point after where the anchored string appeared. Perl uses both for its optimisations, prefering the longer, or, if they are equal, the floating. NOTE: This may not necessarily be the definitive longest anchored and floating string. This will be what the optimiser of the Perl that you are using thinks is the longest. If you believe that the result is wrong please report it via the perlbug utility. |
regname($name,$all) |
Returns the contents of a named buffer of the last successful match. If
$allis true, then returns an array ref containing one entry per buffer, otherwise returns the first defined buffer. |
regnames($all) |
Returns a list of all of the named buffers defined in the last successful
match. If $allis true, then it returns all names defined, if not it returns only names which were involved in the match. |
regnames_count() |
Returns the number of distinct names defined in the pattern used
for the last successful match.
Note: this result is always the actual number of distinct named buffers defined, it may not actually match that which is returned by regnames()and related routines when those routines have not been called with the $allparameter set. |
SEE ALSO
Pragmatic Modules in perlmodlib.