path: root/upstream/debian-unstable/man3/re.3perl

.\" -*- mode: troff; coding: utf-8 -*-
.\" Automatically generated by Pod::Man 5.01 (Pod::Simple 3.43)
.\"
.\" Standard preamble:
.\" ========================================================================
.de Sp \" Vertical space (when we can't use .PP)
.if t .sp .5v
.if n .sp
..
.de Vb \" Begin verbatim text
.ft CW
.nf
.ne \\$1
..
.de Ve \" End verbatim text
.ft R
.fi
..
.\" \*(C` and \*(C' are quotes in nroff, nothing in troff, for use with C<>.
.ie n \{\
.    ds C` ""
.    ds C' ""
'br\}
.el\{\
.    ds C`
.    ds C'
'br\}
.\"
.\" Escape single quotes in literal strings from groff's Unicode transform.
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\"
.\" If the F register is >0, we'll generate index entries on stderr for
.\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
.\" entries marked with X<> in POD.  Of course, you'll have to process the
.\" output yourself in some meaningful fashion.
.\"
.\" Avoid warning from groff about undefined register 'F'.
.de IX
..
.nr rF 0
.if \n(.g .if rF .nr rF 1
.if (\n(rF:(\n(.g==0)) \{\
.    if \nF \{\
.        de IX
.        tm Index:\\$1\t\\n%\t"\\$2"
..
.        if !\nF==2 \{\
.            nr % 0
.            nr F 2
.        \}
.    \}
.\}
.rr rF
.\" ========================================================================
.\"
.IX Title "re 3perl"
.TH re 3perl 2024-01-12 "perl v5.38.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
re \- Perl pragma to alter regular expression behaviour
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 2
\&    use re \*(Aqtaint\*(Aq;
\&    ($x) = ($^X =~ /^(.*)$/s);     # $x is tainted here
\&
\&    $pat = \*(Aq(?{ $foo = 1 })\*(Aq;
\&    use re \*(Aqeval\*(Aq;
\&    /foo${pat}bar/;                # won\*(Aqt fail (when not under \-T
\&                                   # switch)
\&
\&    {
\&        no re \*(Aqtaint\*(Aq;             # the default
\&        ($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here
\&
\&        no re \*(Aqeval\*(Aq;              # the default
\&        /foo${pat}bar/;            # disallowed (with or without \-T
\&                                   # switch)
\&    }
\&
\&    use re \*(Aqstrict\*(Aq;               # Raise warnings for more conditions
\&
\&    use re \*(Aq/ix\*(Aq;
\&    "FOO" =~ / foo /; # /ix implied
\&    no re \*(Aq/x\*(Aq;
\&    "FOO" =~ /foo/; # just /i implied
\&
\&    use re \*(Aqdebug\*(Aq;                # output debugging info during
\&    /^(.*)$/s;                     # compile and run time
\&
\&
\&    use re \*(Aqdebugcolor\*(Aq;           # same as \*(Aqdebug\*(Aq, but with colored
\&                                   # output
\&    ...
\&
\&    use re qw(Debug All);          # Same as "use re \*(Aqdebug\*(Aq", but you
\&                                   # can use "Debug" with things other
\&                                   # than \*(AqAll\*(Aq
\&    use re qw(Debug More);         # \*(AqAll\*(Aq plus output more details
\&    no re qw(Debug ALL);           # Turn on (almost) all re debugging
\&                                   # in this scope
\&
\&    use re qw(is_regexp regexp_pattern); # import utility functions
\&    my ($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\*(Aqs.
.Ve
.PP
(We use $^X in these examples because it's tainted by default.)
.SH DESCRIPTION
.IX Header "DESCRIPTION"
.SS "'taint' mode"
.IX Subsection "'taint' mode"
When \f(CW\*(C`use re \*(Aqtaint\*(Aq\*(C'\fR is in effect, and a tainted string is the target
of a regexp, the regexp memories (or values returned by the m// operator
in list context) are tainted.  This feature is useful when regexp operations
on tainted data aren't meant to extract safe substrings, but to perform
other transformations.
.SS "'eval' mode"
.IX Subsection "'eval' mode"
When \f(CW\*(C`use re \*(Aqeval\*(Aq\*(C'\fR is in effect, a regexp is allowed to contain
\&\f(CW\*(C`(?{ ... })\*(C'\fR zero-width assertions and \f(CW\*(C`(??{ ... })\*(C'\fR postponed
subexpressions that are derived from variable interpolation, rather than
appearing literally within the regexp.  That is normally disallowed, since
it is a
potential security risk.  Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e.  evaluation is always
disallowed with tainted regular expressions.  See "(?{ code })" in perlre 
and "(??{ code })" in perlre.
.PP
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of \f(CW\*(C`qr//\*(C'\fR) is \fInot\fR considered variable
interpolation.  Thus:
.PP
.Vb 1
\&    /foo${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 or \f(CW\*(C`(??{ ... })\*(C'\fR subexpressions.
.SS "'strict' mode"
.IX Subsection "'strict' mode"
Note that this is an experimental feature which may be changed or removed in a
future Perl release.
.PP
When \f(CW\*(C`use re \*(Aqstrict\*(Aq\*(C'\fR 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 \f(CW"regexp"\fR
warnings category (if not already on) within its scope.
.PP
As an example of something that is caught under \f(CW\*(C`"strict\*(Aq\*(C'\fR, but not
otherwise, is the pattern
.PP
.Vb 1
\& qr/\exABC/
.Ve
.PP
The \f(CW"\ex"\fR construct without curly braces should be followed by exactly two
hex digits; this one is followed by three.  This currently evaluates as
equivalent to
.PP
.Vb 1
\& qr/\ex{AB}C/
.Ve
.PP
that is, the character whose code point value is \f(CW0xAB\fR, followed by the
letter \f(CW\*(C`C\*(C'\fR.  But since \f(CW\*(C`C\*(C'\fR is a hex digit, there is a reasonable chance
that the intent was
.PP
.Vb 1
\& qr/\ex{ABC}/
.Ve
.PP
that is the single character at \f(CW0xABC\fR.  Under \f(CW\*(Aqstrict\*(Aq\fR it is an error to
not follow \f(CW\*(C`\ex\*(C'\fR with exactly two hex digits.  When not under \f(CW\*(Aqstrict\*(Aq\fR a
warning is generated if there is only one hex digit, and no warning is raised
if there are more than two.
.PP
It is expected that what exactly \f(CW\*(Aqstrict\*(Aq\fR 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 \f(CW\*(C`experimental::re_strict\*(C'\fR class warning,
unless that category is turned off.
.PP
Note that if a pattern compiled within \f(CW\*(Aqstrict\*(Aq\fR is recompiled, say by
interpolating into another pattern, outside of \f(CW\*(Aqstrict\*(Aq\fR, it is not checked
again for strictness.  This is because if it works under strict it must work
under non-strict.
.SS "'/flags' mode"
.IX Subsection "'/flags' mode"
When \f(CW\*(C`use re \*(Aq/\fR\f(CIflags\fR\f(CW\*(Aq\*(C'\fR is specified, the given \fIflags\fR are automatically
added to every regular expression till the end of the lexical scope.
\&\fIflags\fR can be any combination of
\&\f(CW\*(Aqa\*(Aq\fR,
\&\f(CW\*(Aqaa\*(Aq\fR,
\&\f(CW\*(Aqd\*(Aq\fR,
\&\f(CW\*(Aqi\*(Aq\fR,
\&\f(CW\*(Aql\*(Aq\fR,
\&\f(CW\*(Aqm\*(Aq\fR,
\&\f(CW\*(Aqn\*(Aq\fR,
\&\f(CW\*(Aqp\*(Aq\fR,
\&\f(CW\*(Aqs\*(Aq\fR,
\&\f(CW\*(Aqu\*(Aq\fR,
\&\f(CW\*(Aqx\*(Aq\fR,
and/or
\&\f(CW\*(Aqxx\*(Aq\fR.
.PP
\&\f(CW\*(C`no re \*(Aq/\fR\f(CIflags\fR\f(CW\*(Aq\*(C'\fR will turn off the effect of \f(CW\*(C`use re \*(Aq/\fR\f(CIflags\fR\f(CW\*(Aq\*(C'\fR for the
given flags.
.PP
For example, if you want all your regular expressions to have /msxx on by
default, simply put
.PP
.Vb 1
\&    use re \*(Aq/msxx\*(Aq;
.Ve
.PP
at the top of your code.
.PP
The character set \f(CW\*(C`/adul\*(C'\fR flags cancel each other out. So, in this example,
.PP
.Vb 4
\&    use re "/u";
\&    "ss" =~ /\exdf/;
\&    use re "/d";
\&    "ss" =~ /\exdf/;
.Ve
.PP
the second \f(CW\*(C`use re\*(C'\fR does an implicit \f(CW\*(C`no re \*(Aq/u\*(Aq\*(C'\fR.
.PP
Similarly,
.PP
.Vb 4
\&    use re "/xx";   # Doubled\-x
\&    ...
\&    use re "/x";    # Single x from here on
\&    ...
.Ve
.PP
Turning on one of the character set flags with \f(CW\*(C`use re\*(C'\fR takes precedence over the
\&\f(CW\*(C`locale\*(C'\fR pragma and the 'unicode_strings' \f(CW\*(C`feature\*(C'\fR, 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:
.PP
.Vb 4
\&    use feature "unicode_strings";
\&    no re "/u"; # does nothing
\&    use re "/l";
\&    no re "/l"; # reverts to unicode_strings behaviour
.Ve
.SS "'debug' mode"
.IX Subsection "'debug' mode"
When \f(CW\*(C`use re \*(Aqdebug\*(Aq\*(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 "Debugging Regular Expressions" in perldebug for additional info.
.PP
\&\fBNOTE\fR that the exact format of the \f(CW\*(C`debug\*(C'\fR mode is \fBNOT\fR 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.
.PP
As of 5.9.5 the directive \f(CW\*(C`use re \*(Aqdebug\*(Aq\*(C'\fR and its equivalents are
lexically scoped, as the other directives are.  However they have both
compile-time and run-time effects.
.PP
See "Pragmatic Modules" in perlmodlib.
.SS "'Debug' mode"
.IX Subsection "'Debug' mode"
Similarly \f(CW\*(C`use re \*(AqDebug\*(Aq\*(C'\fR 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.
.PP
\&\fBNOTE\fR that the options provided under the \f(CW\*(C`Debug\*(C'\fR mode and the exact
format of the output they create is \fBNOT\fR 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.
.PP
The options are as follows:
.IP "Compile related options" 4
.IX Item "Compile related options"
.RS 4
.PD 0
.IP COMPILE 4
.IX Item "COMPILE"
.PD
Turns on all non-extra compile related debug options.
.IP PARSE 4
.IX Item "PARSE"
Turns on debug output related to the process of parsing the pattern.
.IP OPTIMISE 4
.IX Item "OPTIMISE"
Enables output related to the optimisation phase of compilation.
.IP TRIEC 4
.IX Item "TRIEC"
Detailed info about trie compilation.
.IP DUMP 4
.IX Item "DUMP"
Dump the final program out after it is compiled and optimised.
.IP FLAGS 4
.IX Item "FLAGS"
Dump the flags associated with the program
.IP TEST 4
.IX Item "TEST"
Print output intended for testing the internals of the compile process
.RE
.RS 4
.RE
.IP "Execute related options" 4
.IX Item "Execute related options"
.RS 4
.PD 0
.IP EXECUTE 4
.IX Item "EXECUTE"
.PD
Turns on all non-extra execute related debug options.
.IP MATCH 4
.IX Item "MATCH"
Turns on debugging of the main matching loop.
.IP TRIEE 4
.IX Item "TRIEE"
Extra debugging of how tries execute.
.IP INTUIT 4
.IX Item "INTUIT"
Enable debugging of start-point optimisations.
.RE
.RS 4
.RE
.IP "Extra debugging options" 4
.IX Item "Extra debugging options"
.RS 4
.PD 0
.IP EXTRA 4
.IX Item "EXTRA"
.PD
Turns on all "extra" debugging options.
.IP BUFFERS 4
.IX Item "BUFFERS"
Enable debugging the capture group storage during match. Warning,
this can potentially produce extremely large output.
.IP TRIEM 4
.IX Item "TRIEM"
Enable enhanced TRIE debugging. Enhances both TRIEE
and TRIEC.
.IP STATE 4
.IX Item "STATE"
Enable debugging of states in the engine.
.IP STACK 4
.IX 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.
.IP GPOS 4
.IX Item "GPOS"
Enable debugging of the \eG modifier.
.IP OPTIMISEM 4
.IX Item "OPTIMISEM"
Enable enhanced optimisation debugging and start-point optimisations.
Probably not useful except when debugging the regexp engine itself.
.IP DUMP_PRE_OPTIMIZE 4
.IX Item "DUMP_PRE_OPTIMIZE"
Enable the dumping of the compiled pattern before the optimization phase.
.IP WILDCARD 4
.IX Item "WILDCARD"
When Perl encounters a wildcard subpattern, (see "Wildcards in
Property Values" in perlunicode), 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.
.Sp
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.
.Sp
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.
.Sp
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.
.RE
.RS 4
.RE
.IP "Other useful flags" 4
.IX Item "Other useful flags"
These are useful shortcuts to save on the typing.
.RS 4
.IP ALL 4
.IX 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 \f(CW\*(C`\-DDEBUGGING\*(C'\fR\-enabled perl interpreter, use
the \fB\-Drv\fR command-line switches.)
.IP All 4
.IX Item "All"
Enable DUMP and all non-extra execute options. Equivalent to:
.Sp
.Vb 1
\&  use re \*(Aqdebug\*(Aq;
.Ve
.IP MORE 4
.IX Item "MORE"
.PD 0
.IP More 4
.IX Item "More"
.PD
Enable the options enabled by "All", plus STATE, TRIEC, and TRIEM.
.RE
.RS 4
.RE
.PP
As of 5.9.5 the directive \f(CW\*(C`use re \*(Aqdebug\*(Aq\*(C'\fR and its equivalents are
lexically scoped, as are the other directives.  However they have both
compile-time and run-time effects.
.SS "Exportable Functions"
.IX Subsection "Exportable Functions"
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.
.IP is_regexp($ref) 4
.IX Item "is_regexp($ref)"
Returns true if the argument is a compiled regular expression as returned
by \f(CW\*(C`qr//\*(C'\fR, false if it is not.
.Sp
This function will not be confused by overloading or blessing. In
internals terms, this extracts the regexp pointer out of the
PERL_MAGIC_qr structure so it cannot be fooled.
.IP regexp_pattern($ref) 4
.IX Item "regexp_pattern($ref)"
If the argument is a compiled regular expression as returned by \f(CW\*(C`qr//\*(C'\fR,
then this function returns the pattern.
.Sp
In list context it returns a two element list, the first element
containing the pattern and the second containing the modifiers used when
the pattern was compiled.
.Sp
.Vb 1
\&  my ($pat, $mods) = regexp_pattern($ref);
.Ve
.Sp
In scalar context it returns the same as perl would when stringifying a raw
\&\f(CW\*(C`qr//\*(C'\fR 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
.Sp
.Vb 1
\&    if (regexp_pattern($ref) eq \*(Aq(?^i:foo)\*(Aq)
.Ve
.Sp
will be warning free regardless of what \f(CW$ref\fR actually is.
.Sp
Like \f(CW\*(C`is_regexp\*(C'\fR this function will not be confused by overloading
or blessing of the object.
.IP regname($name,$all) 4
.IX Item "regname($name,$all)"
Returns the contents of a named buffer of the last successful match. If
\&\f(CW$all\fR is true, then returns an array ref containing one entry per buffer,
otherwise returns the first defined buffer.
.IP regnames($all) 4
.IX Item "regnames($all)"
Returns a list of all of the named buffers defined in the last successful
match. If \f(CW$all\fR is true, then it returns all names defined, if not it returns
only names which were involved in the match.
.IP \fBregnames_count()\fR 4
.IX Item "regnames_count()"
Returns the number of distinct names defined in the pattern used
for the last successful match.
.Sp
\&\fBNote:\fR this result is always the actual number of distinct
named buffers defined, it may not actually match that which is
returned by \f(CWregnames()\fR and related routines when those routines
have not been called with the \f(CW$all\fR parameter set.
.IP regmust($ref) 4
.IX Item "regmust($ref)"
If the argument is a compiled regular expression as returned by \f(CW\*(C`qr//\*(C'\fR,
then this function returns what the optimiser considers to be the longest
anchored fixed string and longest floating fixed string in the pattern.
.Sp
A \fIfixed string\fR is defined as being a substring that must appear for the
pattern to match. An \fIanchored fixed string\fR is a fixed string that must
appear at a particular offset from the beginning of the match. A \fIfloating
fixed string\fR 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,
.Sp
.Vb 3
\&    my $qr = qr/here .* there/x;
\&    my ($anchored, $floating) = regmust($qr);
\&    print "anchored:\*(Aq$anchored\*(Aq\enfloating:\*(Aq$floating\*(Aq\en";
.Ve
.Sp
results in
.Sp
.Vb 2
\&    anchored:\*(Aqhere\*(Aq
\&    floating:\*(Aqthere\*(Aq
.Ve
.Sp
Because the \f(CW\*(C`here\*(C'\fR is before the \f(CW\*(C`.*\*(C'\fR in the pattern, its position
can be determined exactly. That's not true, however, for the \f(CW\*(C`there\*(C'\fR;
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.
.Sp
\&\fBNOTE:\fR 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.
.IP optimization($ref) 4
.IX Item "optimization($ref)"
If the argument is a compiled regular expression as returned by \f(CW\*(C`qr//\*(C'\fR,
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 \f(CW\*(C`undef\*(C'\fR.
.Sp
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.
.Sp
For the current version, the hash will have the following contents:
.RS 4
.IP minlen 4
.IX Item "minlen"
An integer, the least number of characters in any string that can match.
.IP minlenret 4
.IX Item "minlenret"
An integer, the least number of characters that can be in \f(CW$&\fR after a
match. (Consider eg \f(CW\*(C` /ns(?=\ed)/ \*(C'\fR.)
.IP gofs 4
.IX Item "gofs"
An integer, the number of characters before \f(CWpos()\fR to start match at.
.IP noscan 4
.IX Item "noscan"
A boolean, \f(CW\*(C`TRUE\*(C'\fR 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.)
.IP isall 4
.IX Item "isall"
A boolean, \f(CW\*(C`TRUE\*(C'\fR 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.
.IP "anchor SBOL" 4
.IX Item "anchor SBOL"
A boolean, \f(CW\*(C`TRUE\*(C'\fR if the pattern is anchored to start of string.
.IP "anchor MBOL" 4
.IX Item "anchor MBOL"
A boolean, \f(CW\*(C`TRUE\*(C'\fR if the pattern is anchored to any start of line
within the string.
.IP "anchor GPOS" 4
.IX Item "anchor GPOS"
A boolean, \f(CW\*(C`TRUE\*(C'\fR if the pattern is anchored to the end of the previous
match.
.IP skip 4
.IX Item "skip"
A boolean, \f(CW\*(C`TRUE\*(C'\fR if the start class can match only the first of a run.
.IP implicit 4
.IX Item "implicit"
A boolean, \f(CW\*(C`TRUE\*(C'\fR if a \f(CW\*(C`/.*/\*(C'\fR has been turned implicitly into a \f(CW\*(C`/^.*/\*(C'\fR.
.IP anchored/floating 4
.IX 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.
.IP "anchored utf8/floating utf8" 4
.IX 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.
.IP "anchored min offset/floating min offset" 4
.IX 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.
.IP "anchored max offset/floating max offset" 4
.IX 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.
.Sp
Ignored for anchored, so may be 0 or same as min.
.IP "anchored end shift/floating end shift" 4
.IX 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.
.IP checking 4
.IX Item "checking"
A constant string, one of "anchored", "floating" or "none" to indicate
which substring (if any) should be checked for first.
.IP stclass 4
.IX Item "stclass"
A string representation of a character class ("start class") that must
be the first character of any match.
.Sp
TODO: explain the representations.
.RE
.RS 4
.RE
.SH "SEE ALSO"
.IX Header "SEE ALSO"
"Pragmatic Modules" in perlmodlib.