package re;
# pragma for controlling the regexp engine
use strict;
use warnings;
our $VERSION = "0.47";
our @ISA = qw(Exporter);
our @EXPORT_OK = qw{
is_regexp regexp_pattern
regname regnames regnames_count
regmust optimization
};
our %EXPORT_OK = map { $_ => 1 } @EXPORT_OK;
my %bitmask = (
taint => 0x00100000, # HINT_RE_TAINT
eval => 0x00200000, # HINT_RE_EVAL
);
my $flags_hint = 0x02000000; # HINT_RE_FLAGS
my $PMMOD_SHIFT = 0;
my %reflags = (
m => 1 << ($PMMOD_SHIFT + 0),
s => 1 << ($PMMOD_SHIFT + 1),
i => 1 << ($PMMOD_SHIFT + 2),
x => 1 << ($PMMOD_SHIFT + 3),
xx => 1 << ($PMMOD_SHIFT + 4),
n => 1 << ($PMMOD_SHIFT + 5),
p => 1 << ($PMMOD_SHIFT + 6),
strict => 1 << ($PMMOD_SHIFT + 10),
# special cases:
d => 0,
l => 1,
u => 2,
a => 3,
aa => 4,
);
sub setcolor {
eval { # Ignore errors
require Term::Cap;
my $terminal = Tgetent Term::Cap ({OSPEED => 9600}); # Avoid warning.
my $props = $ENV{PERL_RE_TC} || 'md,me,so,se,us,ue';
my @props = split /,/, $props;
my $colors = join "\t", map {$terminal->Tputs($_,1)} @props;
$colors =~ s/\0//g;
$ENV{PERL_RE_COLORS} = $colors;
};
if ($@) {
$ENV{PERL_RE_COLORS} ||= qq'\t\t> <\t> <\t\t';
}
}
my %flags = (
COMPILE => 0x0000FF,
PARSE => 0x000001,
OPTIMISE => 0x000002,
TRIEC => 0x000004,
DUMP => 0x000008,
FLAGS => 0x000010,
TEST => 0x000020,
EXECUTE => 0x00FF00,
INTUIT => 0x000100,
MATCH => 0x000200,
TRIEE => 0x000400,
EXTRA => 0x3FF0000,
TRIEM => 0x0010000,
STATE => 0x0080000,
OPTIMISEM => 0x0100000,
STACK => 0x0280000,
BUFFERS => 0x0400000,
GPOS => 0x0800000,
DUMP_PRE_OPTIMIZE => 0x1000000,
WILDCARD => 0x2000000,
);
$flags{ALL} = -1 & ~($flags{BUFFERS}
|$flags{DUMP_PRE_OPTIMIZE}
|$flags{WILDCARD}
);
$flags{All} = $flags{all} = $flags{DUMP} | $flags{EXECUTE};
$flags{Extra} = $flags{EXECUTE} | $flags{COMPILE} | $flags{GPOS};
$flags{More} = $flags{MORE} =
$flags{All} | $flags{TRIEC} | $flags{TRIEM} | $flags{STATE};
$flags{State} = $flags{DUMP} | $flags{EXECUTE} | $flags{STATE};
$flags{TRIE} = $flags{DUMP} | $flags{EXECUTE} | $flags{TRIEC};
if (defined &DynaLoader::boot_DynaLoader) {
require XSLoader;
XSLoader::load();
}
# else we're miniperl
# We need to work for miniperl, because the XS toolchain uses Text::Wrap, which
# uses re 'taint'.
sub _load_unload {
my ($on)= @_;
if ($on) {
# We call install() every time, as if we didn't, we wouldn't
# "see" any changes to the color environment var since
# the last time it was called.
# install() returns an integer, which if casted properly
# in C resolves to a structure containing the regexp
# hooks. Setting it to a random integer will guarantee
# segfaults.
$^H{regcomp} = install();
} else {
delete $^H{regcomp};
}
}
sub bits {
my $on = shift;
my $bits = 0;
my $turning_all_off = ! @_ && ! $on;
my $seen_Debug = 0;
my $seen_debug = 0;
if ($turning_all_off) {
# Pretend were called with certain parameters, which are best dealt
# with that way.
push @_, keys %bitmask; # taint and eval
push @_, 'strict';
}
# Process each subpragma parameter
ARG:
foreach my $idx (0..$#_){
my $s=$_[$idx];
if ($s eq 'Debug' or $s eq 'Debugcolor') {
if (! $seen_Debug) {
$seen_Debug = 1;
# Reset to nothing, and then add what follows. $seen_Debug
# allows, though unlikely someone would do it, more than one
# Debug and flags in the arguments
${^RE_DEBUG_FLAGS} = 0;
}
setcolor() if $s =~/color/i;
for my $idx ($idx+1..$#_) {
if ($flags{$_[$idx]}) {
if ($on) {
${^RE_DEBUG_FLAGS} |= $flags{$_[$idx]};
} else {
${^RE_DEBUG_FLAGS} &= ~ $flags{$_[$idx]};
}
} else {
require Carp;
Carp::carp("Unknown \"re\" Debug flag '$_[$idx]', possible flags: ",
join(", ",sort keys %flags ) );
}
}
_load_unload($on ? 1 : ${^RE_DEBUG_FLAGS});
last;
} elsif ($s eq 'debug' or $s eq 'debugcolor') {
# These default flags should be kept in sync with the same values
# in regcomp.h
${^RE_DEBUG_FLAGS} = $flags{'EXECUTE'} | $flags{'DUMP'};
setcolor() if $s =~/color/i;
_load_unload($on);
$seen_debug = 1;
} elsif (exists $bitmask{$s}) {
$bits |= $bitmask{$s};
} elsif ($EXPORT_OK{$s}) {
require Exporter;
re->export_to_level(2, 're', $s);
} elsif ($s eq 'strict') {
if ($on) {
$^H{reflags} |= $reflags{$s};
warnings::warnif('experimental::re_strict',
"\"use re 'strict'\" is experimental");
# Turn on warnings if not already done.
if (! warnings::enabled('regexp')) {
require warnings;
warnings->import('regexp');
$^H{re_strict} = 1;
}
}
else {
$^H{reflags} &= ~$reflags{$s} if $^H{reflags};
# Turn off warnings if we turned them on.
warnings->unimport('regexp') if $^H{re_strict};
}
if ($^H{reflags}) {
$^H |= $flags_hint;
}
else {
$^H &= ~$flags_hint;
}
} elsif ($s =~ s/^\///) {
my $reflags = $^H{reflags} || 0;
my $seen_charset;
my $x_count = 0;
while ($s =~ m/( . )/gx) {
local $_ = $1;
if (/[adul]/) {
# The 'a' may be repeated; hide this from the rest of the
# code by counting and getting rid of all of them, then
# changing to 'aa' if there is a repeat.
if ($_ eq 'a') {
my $sav_pos = pos $s;
my $a_count = $s =~ s/a//g;
pos $s = $sav_pos - 1; # -1 because got rid of the 'a'
if ($a_count > 2) {
require Carp;
Carp::carp(
qq 'The "a" flag may only appear a maximum of twice'
);
}
elsif ($a_count == 2) {
$_ = 'aa';
}
}
if ($on) {
if ($seen_charset) {
require Carp;
if ($seen_charset ne $_) {
Carp::carp(
qq 'The "$seen_charset" and "$_" flags '
.qq 'are exclusive'
);
}
else {
Carp::carp(
qq 'The "$seen_charset" flag may not appear '
.qq 'twice'
);
}
}
$^H{reflags_charset} = $reflags{$_};
$seen_charset = $_;
}
else {
delete $^H{reflags_charset}
if defined $^H{reflags_charset}
&& $^H{reflags_charset} == $reflags{$_};
}
} elsif (exists $reflags{$_}) {
if ($_ eq 'x') {
$x_count++;
if ($x_count > 2) {
require Carp;
Carp::carp(
qq 'The "x" flag may only appear a maximum of twice'
);
}
elsif ($x_count == 2) {
$_ = 'xx'; # First time through got the /x
}
}
$on
? $reflags |= $reflags{$_}
: ($reflags &= ~$reflags{$_});
} else {
require Carp;
Carp::carp(
qq'Unknown regular expression flag "$_"'
);
next ARG;
}
}
($^H{reflags} = $reflags or defined $^H{reflags_charset})
? $^H |= $flags_hint
: ($^H &= ~$flags_hint);
} else {
require Carp;
if ($seen_debug && defined $flags{$s}) {
Carp::carp("Use \"Debug\" not \"debug\", to list debug types"
. " in \"re\". \"$s\" ignored");
}
else {
Carp::carp("Unknown \"re\" subpragma '$s' (known ones are: ",
join(', ', map {qq('$_')} 'debug', 'debugcolor', sort keys %bitmask),
")");
}
}
}
if ($turning_all_off) {
_load_unload(0);
$^H{reflags} = 0;
$^H{reflags_charset} = 0;
$^H &= ~$flags_hint;
}
$bits;
}
sub import {
shift;
$^H |= bits(1, @_);
}
sub unimport {
shift;
$^H &= ~ bits(0, @_);
}
1;
__END__
=head1 NAME
re - Perl pragma to alter regular expression behaviour
=head1 SYNOPSIS
use re 'taint';
($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 '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 $^X in these examples because it's tainted by default.)
=head1 DESCRIPTION
=head2 'taint' mode
When C<use re 'taint'> is in effect, and a tainted string is the target
of a regexp, the regexp memories (or values returned by the m// operator
in list context) are tainted. This feature is useful when regexp operations
on tainted data aren't meant to extract safe substrings, but to perform
other transformations.
=head2 'eval' mode
When C<use re 'eval'> is in effect, a regexp is allowed to contain
C<(?{ ... })> zero-width assertions and C<(??{ ... })> postponed
subexpressions that are derived from variable interpolation, rather than
appearing literally within the regexp. That is normally disallowed, since
it is a
potential security risk. Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e. evaluation is always
disallowed with tainted regular expressions. See L<perlre/(?{ code })>
and L<perlre/(??{ code })>.
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of C<qr//>) is I<not> considered variable
interpolation. Thus:
/foo${pat}bar/
I<is> allowed if $pat is a precompiled regular expression, even
if $pat contains C<(?{ ... })> assertions or C<(??{ ... })> subexpressions.
=head2 'strict' mode
Note that this is an experimental feature which may be changed or removed in a
future Perl release.
When C<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 C<"regexp">
warnings category (if not already on) within its scope.
As an example of something that is caught under C<"strict'>, but not
otherwise, is the pattern
qr/\xABC/
The C<"\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 C<0xAB>, followed by the
letter C<C>. But since C<C> is a hex digit, there is a reasonable chance
that the intent was
qr/\x{ABC}/
that is the single character at C<0xABC>. Under C<'strict'> it is an error to
not follow C<\x> with exactly two hex digits. When not under C<'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 C<'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 C<experimental::re_strict> class warning,
unless that category is turned off.
Note that if a pattern compiled within C<'strict'> is recompiled, say by
interpolating into another pattern, outside of C<'strict'>, it is not checked
again for strictness. This is because if it works under strict it must work
under non-strict.
=head2 '/flags' mode
When C<use re '/I<flags>'> is specified, the given I<flags> are automatically
added to every regular expression till the end of the lexical scope.
I<flags> can be any combination of
C<'a'>,
C<'aa'>,
C<'d'>,
C<'i'>,
C<'l'>,
C<'m'>,
C<'n'>,
C<'p'>,
C<'s'>,
C<'u'>,
C<'x'>,
and/or
C<'xx'>.
C<no re '/I<flags>'> will turn off the effect of C<use re '/I<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 C</adul> flags cancel each other out. So, in this example,
use re "/u";
"ss" =~ /\xdf/;
use re "/d";
"ss" =~ /\xdf/;
the second C<use re> does an implicit C<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 C<use re> takes precedence over the
C<locale> pragma and the 'unicode_strings' C<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
Default flags are applied to wherever a pattern is compiled with the exception
of the C</x> flag, which is not applied to patterns compiled from string arguments
to C<split>. Thus `use re "/x";` does not affect the behaviour of C<split " "> but
B<does> affect the behavior of C<split / />.
=head2 'debug' mode
When C<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 C<-DDEBUGGING>-enabled perl interpreter with the
B<-Dr> switch. It may be quite voluminous depending on the complexity
of the match. Using C<debugcolor> instead of C<debug> enables a
form of output that can be used to get a colorful display on terminals
that understand termcap color sequences. Set C<$ENV{PERL_RE_TC}> to a
comma-separated list of C<termcap> properties to use for highlighting
strings on/off, pre-point part on/off.
See L<perldebug/"Debugging Regular Expressions"> for additional info.
B<NOTE> that the exact format of the C<debug> mode is B<NOT> considered
to be an officially supported API of Perl. It is intended for debugging
only and may change as the core development team deems appropriate
without notice or deprecation in any release of Perl, major or minor.
Any documentation of the output is purely advisory.
As of 5.9.5 the directive C<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 L<perlmodlib/Pragmatic Modules>.
=head2 'Debug' mode
Similarly C<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.
B<NOTE> that the options provided under the C<Debug> mode and the exact
format of the output they create is B<NOT> considered to be an
officially supported API of Perl. It is intended for debugging only and
may change as the core development team deems appropriate without notice
or deprecation in any release of Perl, major or minor. Any documentation
of the format or options available is advisory only and is subject to
change without notice.
The options are as follows:
=over 4
=item Compile related options
=over 4
=item COMPILE
Turns on all non-extra compile related debug options.
=item PARSE
Turns on debug output related to the process of parsing the pattern.
=item OPTIMISE
Enables output related to the optimisation phase of compilation.
=item TRIEC
Detailed info about trie compilation.
=item DUMP
Dump the final program out after it is compiled and optimised.
=item FLAGS
Dump the flags associated with the program
=item TEST
Print output intended for testing the internals of the compile process
=back
=item Execute related options
=over 4
=item EXECUTE
Turns on all non-extra execute related debug options.
=item MATCH
Turns on debugging of the main matching loop.
=item TRIEE
Extra debugging of how tries execute.
=item INTUIT
Enable debugging of start-point optimisations.
=back
=item Extra debugging options
=over 4
=item EXTRA
Turns on all "extra" debugging options.
=item BUFFERS
Enable debugging the capture group storage during match. Warning,
this can potentially produce extremely large output.
=item TRIEM
Enable enhanced TRIE debugging. Enhances both TRIEE
and TRIEC.
=item STATE
Enable debugging of states in the engine.
=item STACK
Enable debugging of the recursion stack in the engine. Enabling
or disabling this option automatically does the same for debugging
states as well. This output from this can be quite large.
=item GPOS
Enable debugging of the \G modifier.
=item OPTIMISEM
Enable enhanced optimisation debugging and start-point optimisations.
Probably not useful except when debugging the regexp engine itself.
=item DUMP_PRE_OPTIMIZE
Enable the dumping of the compiled pattern before the optimization phase.
=item WILDCARD
When Perl encounters a wildcard subpattern, (see L<perlunicode/Wildcards in
Property Values>), it suspends compilation of the main pattern, compiles the
subpattern, and then matches that against all legal possibilities to determine
the actual code points the subpattern matches. After that it adds these to
the main pattern, and continues its compilation.
You may very well want to see how your subpattern gets compiled, but it is
likely of less use to you to see how Perl matches that against all the legal
possibilities, as that is under control of Perl, not you. Therefore, the
debugging information of the compilation portion is as specified by the other
options, but the debugging output of the matching portion is normally
suppressed.
You can use the WILDCARD option to enable the debugging output of this
subpattern matching. Careful! This can lead to voluminous outputs, and it
may not make much sense to you what and why Perl is doing what it is.
But it may be helpful to you to see why things aren't going the way you
expect.
Note that this option alone doesn't cause any debugging information to be
output. What it does is stop the normal suppression of execution-related
debugging information during the matching portion of the compilation of
wildcards. You also have to specify which execution debugging information you
want, such as by also including the EXECUTE option.
=back
=item Other useful flags
These are useful shortcuts to save on the typing.
=over 4
=item ALL
Enable all options at once except BUFFERS, WILDCARD, and DUMP_PRE_OPTIMIZE.
(To get every single option without exception, use both ALL and EXTRA, or
starting in 5.30 on a C<-DDEBUGGING>-enabled perl interpreter, use
the B<-Drv> command-line switches.)
=item All
Enable DUMP and all non-extra execute options. Equivalent to:
use re 'debug';
=item MORE
=item More
Enable the options enabled by "All", plus STATE, TRIEC, and TRIEM.
=back
=back
As of 5.9.5 the directive C<use re 'debug'> and its equivalents are
lexically scoped, as are the other directives. However they have both
compile-time and run-time effects.
=head2 Exportable Functions
As of perl 5.9.5, the C<re> module contains a number of utility functions that
may be optionally exported into the caller's namespace. They are listed below.
=over 4
=item is_regexp($ref)
Returns true if the argument is a compiled regular expression as returned
by C<qr//>, false if it is not.
This function will not be confused by overloading or blessing. In
internals terms, this extracts the regexp pointer out of the
PERL_MAGIC_qr structure so it cannot be fooled.
=item regexp_pattern($ref)
If the argument is a compiled regular expression as returned by C<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
C<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 $ref actually is.
Like C<is_regexp> this function will not be confused by overloading
or blessing of the object.
=item regname($name,$all)
Returns the contents of a named buffer of the last successful match. If
$all is true, then returns an array ref containing one entry per buffer,
otherwise returns the first defined buffer.
=item regnames($all)
Returns a list of all of the named buffers defined in the last successful
match. If $all is true, then it returns all names defined, if not it returns
only names which were involved in the match.
=item regnames_count()
Returns the number of distinct names defined in the pattern used
for the last successful match.
B<Note:> this result is always the actual number of distinct
named buffers defined, it may not actually match that which is
returned by C<regnames()> and related routines when those routines
have not been called with the $all parameter set.
=item regmust($ref)
If the argument is a compiled regular expression as returned by C<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 I<fixed string> is defined as being a substring that must appear for the
pattern to match. An I<anchored fixed string> is a fixed string that must
appear at a particular offset from the beginning of the match. A I<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 C<here> is before the C<.*> in the pattern, its position
can be determined exactly. That's not true, however, for the C<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.
B<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 L<perlbug> utility.
=item optimization($ref)
If the argument is a compiled regular expression as returned by C<qr//>,
then this function returns a hashref of the optimization information
discovered at compile time, so we can write tests around it. If any
other argument is given, returns C<undef>.
The hash contents are expected to change from time to time as we develop
new ways to optimize - no assumption of stability should be made, not
even between minor versions of perl.
For the current version, the hash will have the following contents:
=over 4
=item minlen
An integer, the least number of characters in any string that can match.
=item minlenret
An integer, the least number of characters that can be in C<$&> after a
match. (Consider eg C< /ns(?=\d)/ >.)
=item gofs
An integer, the number of characters before C<pos()> to start match at.
=item noscan
A boolean, C<TRUE> to indicate that any anchored/floating substrings
found should not be used. (CHECKME: apparently this is set for an
anchored pattern with no floating substring, but never used.)
=item isall
A boolean, C<TRUE> to indicate that the optimizer information is all
that the regular expression contains, and thus one does not need to
enter the regexp runtime engine at all.
=item anchor SBOL
A boolean, C<TRUE> if the pattern is anchored to start of string.
=item anchor MBOL
A boolean, C<TRUE> if the pattern is anchored to any start of line
within the string.
=item anchor GPOS
A boolean, C<TRUE> if the pattern is anchored to the end of the previous
match.
=item skip
A boolean, C<TRUE> if the start class can match only the first of a run.
=item implicit
A boolean, C<TRUE> if a C</.*/> has been turned implicitly into a C</^.*/>.
=item anchored/floating
A byte string representing an anchored or floating substring respectively
that any match must contain, or undef if no such substring was found, or
if the substring would require utf8 to represent.
=item anchored utf8/floating utf8
A utf8 string representing an anchored or floating substring respectively
that any match must contain, or undef if no such substring was found, or
if the substring contains only 7-bit ASCII characters.
=item anchored min offset/floating min offset
An integer, the first offset in characters from a match location at which
we should look for the corresponding substring.
=item anchored max offset/floating max offset
An integer, the last offset in characters from a match location at which
we should look for the corresponding substring.
Ignored for anchored, so may be 0 or same as min.
=item anchored end shift/floating end shift
FIXME: not sure what this is, something to do with lookbehind. regcomp.c
says:
When the final pattern is compiled and the data is moved from the
scan_data_t structure into the regexp structure the information
about lookbehind is factored in, with the information that would
have been lost precalculated in the end_shift field for the
associated string.
=item checking
A constant string, one of "anchored", "floating" or "none" to indicate
which substring (if any) should be checked for first.
=item stclass
A string representation of a character class ("start class") that must
be the first character of any match.
TODO: explain the representations.
=back
=back
=head1 SEE ALSO
L<perlmodlib/Pragmatic Modules>.
=cut