| re - Perl pragma to alter regular expression behaviour |
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 'strict'; # Raise warnings for more conditions
use re '/ix';
"FOO" =~ / foo /; # /ix implied
no re '/x';
"FOO" =~ /foo/; # just /i implied
use re 'debug'; # output debugging info during
/^(.*)$/s; # compile and run time
use re 'debugcolor'; # same as 'debug', but with colored
# output
...
use re qw(Debug All); # Same as "use re 'debug'", but you
# can use "Debug" with things other
# than 'All'
use re qw(Debug More); # 'All' plus output more details
no re qw(Debug ALL); # Turn on (almost) all re debugging
# in this scope
use re qw(is_regexp regexp_pattern); # import utility functions
my ($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
# re's.
(We use $^X in these examples because it's tainted by default.)
When use re 'taint' is in effect, and a tainted string is the target
of a regexp, the regexp memories (or values returned by the m// operator
in list context) are tainted. This feature is useful when regexp operations
on tainted data aren't meant to extract safe substrings, but to perform
other transformations.
When use re 'eval' is in effect, a regexp is allowed to contain
(?{ ... }) zero-width assertions and (??{ ... }) postponed
subexpressions that are derived from variable interpolation, rather than
appearing literally within the regexp. That is normally disallowed, since
it is a
potential security risk. Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e. evaluation is always
disallowed with tainted regular expressions. See (?{ code }) in the perlre manpage
and (??{ code }) in the perlre manpage.
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of qr//) is not considered variable
interpolation. Thus:
/foo${pat}bar/
is allowed if $pat is a precompiled regular expression, even
if $pat contains (?{ ... }) assertions or (??{ ... }) subexpressions.
Note that this is an experimental feature which may be changed or removed in a future Perl release.
When use re 'strict' is in effect, stricter checks are applied than
otherwise when compiling regular expressions patterns. These may cause more
warnings to be raised than otherwise, and more things to be fatal instead of
just warnings. The purpose of this is to find and report at compile time some
things, which may be legal, but have a reasonable possibility of not being the
programmer's actual intent. This automatically turns on the "regexp"
warnings category (if not already on) within its scope.
As an example of something that is caught under "strict', but not
otherwise, is the pattern
qr/\xABC/
The "\x" construct without curly braces should be followed by exactly two
hex digits; this one is followed by three. This currently evaluates as
equivalent to
qr/\x{AB}C/
that is, the character whose code point value is 0xAB, followed by the
letter C. But since C is a a hex digit, there is a reasonable chance
that the intent was
qr/\x{ABC}/
that is the single character at 0xABC. Under 'strict' it is an error to
not follow \x with exactly two hex digits. When not under 'strict' a
warning is generated if there is only one hex digit, and no warning is raised
if there are more than two.
It is expected that what exactly 'strict' does will evolve over time as we
gain experience with it. This means that programs that compile under it in
today's Perl may not compile, or may have more or fewer warnings, in future
Perls. There is no backwards compatibility promises with regards to it. Also
there are already proposals for an alternate syntax for enabling it. For
these reasons, using it will raise a experimental::re_strict class warning,
unless that category is turned off.
Note that if a pattern compiled within 'strict' is recompiled, say by
interpolating into another pattern, outside of 'strict', it is not checked
again for strictness. This is because if it works under strict it must work
under non-strict.
When use re '/flags' is specified, the given flags are automatically
added to every regular expression till the end of the lexical scope.
flags can be any combination of
'a',
'aa',
'd',
'i',
'l',
'm',
'n',
'p',
's',
'u',
'x',
and/or
'xx'.
no re '/flags' will turn off the effect of use re '/flags' for the
given flags.
For example, if you want all your regular expressions to have /msxx on by default, simply put
use re '/msxx';
at the top of your code.
The character set /adul flags cancel each other out. So, in this example,
use re "/u";
"ss" =~ /\xdf/;
use re "/d";
"ss" =~ /\xdf/;
the second use re does an implicit no re '/u'.
Similarly,
use re "/xx"; # Doubled-x
...
use re "/x"; # Single x from here on
...
Turning on one of the character set flags with use re takes precedence over the
locale pragma and the 'unicode_strings' feature, for regular
expressions. Turning off one of these flags when it is active reverts to
the behaviour specified by whatever other pragmata are in scope. For
example:
use feature "unicode_strings";
no re "/u"; # does nothing
use re "/l";
no re "/l"; # reverts to unicode_strings behaviour
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 $ENV{PERL_RE_TC} to a
comma-separated list of termcap properties to use for highlighting
strings on/off, pre-point part on/off.
See Debugging Regular Expressions in the perldebug manpage for additional info.
As of 5.9.5 the directive use re 'debug' and its equivalents are
lexically scoped, as the other directives are. However they have both
compile-time and run-time effects.
See perlmodlib/Pragmatic Modules.
Similarly use re 'Debug' produces debugging output, the difference
being that it allows the fine tuning of what debugging output will be
emitted. Options are divided into three groups, those related to
compilation, those related to execution and those related to special
purposes. The options are as follows:
NODENUM:POSITION[LENGTH]
Where 1 is the position of the first char in the string. Note that position can be 0, or larger than the actual length of the pattern, likewise length can be zero.
Almost definitely only useful to people hacking on the offsets part of the debug engine.
-DDEBUGGING-enabled perl interpreter, use
the -Drv command-line switches.)
use re 'debug';
As of 5.9.5 the directive use re 'debug' and its equivalents are
lexically scoped, as are the other directives. However they have both
compile-time and run-time effects.
As of perl 5.9.5 're' debug contains a number of utility functions that may be optionally exported into the caller's namespace. They are listed below.
is_regexp($ref)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 cannot be fooled.
regexp_pattern($ref)qr//,
then this function returns the pattern.
In list context it returns a two element list, the first element containing the pattern and the second containing the modifiers used when the pattern was compiled.
my ($pat, $mods) = regexp_pattern($ref);
In scalar context it returns the same as perl would when stringifying 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:foo)')
will be warning free regardless of what $ref actually is.
Like is_regexp this function will not be confused by overloading
or blessing of the object.
regmust($ref)qr//,
then this function returns what the optimiser considers 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 here is before the .* in the pattern, its position
can be determined exactly. That's not true, however, for the there;
it could appear at any point after where the anchored string appeared.
Perl uses both for its optimisations, preferring 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)regnames($all)regnames_count()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 $all parameter set.
perlmodlib/Pragmatic Modules.
| re - Perl pragma to alter regular expression behaviour |