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PERLPODSPEC(1)	       Perl Programmers	Reference Guide		PERLPODSPEC(1)

NAME
       perlpodspec - Plain Old Documentation: format specification and notes

DESCRIPTION
       This document is	detailed notes on the Pod markup language.  Most
       people will only	have to	read perlpod to	know how to write in Pod, but
       this document may answer	some incidental	questions to do	with parsing
       and rendering Pod.

       In this document, "must"	/ "must	not", "should" / "should not", and
       "may" have their	conventional (cf. RFC 2119) meanings: "X must do Y"
       means that if X doesn't do Y, it's against this specification, and
       should really be	fixed.	"X should do Y"	means that it's	recommended,
       but X may fail to do Y, if there's a good reason.  "X may do Y" is
       merely a	note that X can	do Y at	will (although it is up	to the reader
       to detect any connotation of "and I think it would be nice if X did Y"
       versus "it wouldn't really bother me if X did Y").

       Notably,	when I say "the	parser should do Y", the parser	may fail to do
       Y, if the calling application explicitly	requests that the parser not
       do Y.  I	often phrase this as "the parser should, by default, do	Y."
       This doesn't require the	parser to provide an option for	turning	off
       whatever	feature	Y is (like expanding tabs in verbatim paragraphs),
       although	it implicates that such	an option may be provided.

Pod Definitions
       Pod is embedded in files, typically Perl	source files, although you can
       write a file that's nothing but Pod.

       A line in a file	consists of zero or more non-newline characters,
       terminated by either a newline or the end of the	file.

       A newline sequence is usually a platform-dependent concept, but Pod
       parsers should understand it to mean any	of CR (ASCII 13), LF (ASCII
       10), or a CRLF (ASCII 13	followed immediately by	ASCII 10), in addition
       to any other system-specific meaning.  The first	CR/CRLF/LF sequence in
       the file	may be used as the basis for identifying the newline sequence
       for parsing the rest of the file.

       A blank line is a line consisting entirely of zero or more spaces
       (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-
       file.  A	non-blank line is a line containing one	or more	characters
       other than space	or tab (and terminated by a newline or end-of-file).

       (Note: Many older Pod parsers did not accept a line consisting of
       spaces/tabs and then a newline as a blank line. The only	lines they
       considered blank	were lines consisting of no characters at all,
       terminated by a newline.)

       Whitespace is used in this document as a	blanket	term for spaces, tabs,
       and newline sequences.  (By itself, this	term usually refers to literal
       whitespace.  That is, sequences of whitespace characters	in Pod source,
       as opposed to "E<32>", which is a formatting code that denotes a
       whitespace character.)

       A Pod parser is a module	meant for parsing Pod (regardless of whether
       this involves calling callbacks or building a parse tree	or directly
       formatting it).	A Pod formatter	(or Pod	translator) is a module	or
       program that converts Pod to some other format (HTML, plaintext,	TeX,
       PostScript, RTF).  A Pod	processor might	be a formatter or translator,
       or might	be a program that does something else with the Pod (like
       counting	words, scanning	for index points, etc.).

       Pod content is contained	in Pod blocks.	A Pod block starts with	a line
       that matches "m/\A=[a-zA-Z]/", and continues up to the next line	that
       matches "m/\A=cut/" or up to the	end of the file	if there is no
       "m/\A=cut/" line.

       Note that a parser is not expected to distinguish between something
       that looks like pod, but	is in a	quoted string, such as a here
       document.

       Within a	Pod block, there are Pod paragraphs.  A	Pod paragraph consists
       of non-blank lines of text, separated by	one or more blank lines.

       For purposes of Pod processing, there are four types of paragraphs in a
       Pod block:

       o   A command paragraph (also called a "directive").  The first line of
	   this	paragraph must match "m/\A=[a-zA-Z]/".	Command	paragraphs are
	   typically one line, as in:

	     =head1 NOTES

	     =item *

	   But they may	span several (non-blank) lines:

	     =for comment
	     Hm, I wonder what it would	look like if
	     you tried to write	a BNF for Pod from this.

	     =head3 Dr.	Strangelove, or: How I Learned to
	     Stop Worrying and Love the	Bomb

	   Some	command	paragraphs allow formatting codes in their content
	   (i.e., after	the part that matches "m/\A=[a-zA-Z]\S*\s*/"), as in:

	     =head1 Did	You Remember to	C<use strict;>?

	   In other words, the Pod processing handler for "head1" will apply
	   the same processing to "Did You Remember to C<use strict;>?"	that
	   it would to an ordinary paragraph (i.e., formatting codes like
	   "C<...>") are parsed	and presumably formatted appropriately,	and
	   whitespace in the form of literal spaces and/or tabs	is not
	   significant.

       o   A verbatim paragraph.  The first line of this paragraph must	be a
	   literal space or tab, and this paragraph must not be	inside a
	   "=begin identifier",	... "=end identifier" sequence unless
	   "identifier"	begins with a colon (":").  That is, if	a paragraph
	   starts with a literal space or tab, but is inside a "=begin
	   identifier",	... "=end identifier" region, then it's	a data
	   paragraph, unless "identifier" begins with a	colon.

	   Whitespace is significant in	verbatim paragraphs (although, in
	   processing, tabs are	probably expanded).

       o   An ordinary paragraph.  A paragraph is an ordinary paragraph	if its
	   first line matches neither "m/\A=[a-zA-Z]/" nor "m/\A[ \t]/", and
	   if it's not inside a	"=begin	identifier", ... "=end identifier"
	   sequence unless "identifier"	begins with a colon (":").

       o   A data paragraph.  This is a	paragraph that is inside a "=begin
	   identifier" ... "=end identifier" sequence where "identifier" does
	   not begin with a literal colon (":").  In some sense, a data
	   paragraph is	not part of Pod	at all (i.e., effectively it's "out-
	   of-band"), since it's not subject to	most kinds of Pod parsing; but
	   it is specified here, since Pod parsers need	to be able to call an
	   event for it, or store it in	some form in a parse tree, or at least
	   just	parse around it.

       For example: consider the following paragraphs:

	 # <- that's the 0th column

	 =head1	Foo

	 Stuff

	   $foo->bar

	 =cut

       Here, "=head1 Foo" and "=cut" are command paragraphs because the	first
       line of each matches "m/\A=[a-zA-Z]/".  "[space][space]$foo->bar" is a
       verbatim	paragraph, because its first line starts with a	literal
       whitespace character (and there's no "=begin"..."=end" region around).

       The "=begin identifier" ... "=end identifier" commands stop paragraphs
       that they surround from being parsed as ordinary	or verbatim
       paragraphs, if identifier doesn't begin with a colon.  This is
       discussed in detail in the section "About Data Paragraphs and
       "=begin/=end" Regions".

Pod Commands
       This section is intended	to supplement and clarify the discussion in
       "Command	Paragraph" in perlpod.	These are the currently	recognized Pod
       commands:

       "=head1", "=head2", "=head3", "=head4", "=head5", "=head6"
	   This	command	indicates that the text	in the remainder of the
	   paragraph is	a heading.  That text may contain formatting codes.
	   Examples:

	     =head1 Object Attributes

	     =head3 What B<Not>	to Do!

	   Both	"=head5" and "=head6" were added in 2020 and might not be
	   supported on	all Pod	parsers. Pod::Simple 3.41 was released on
	   October 2020	and supports both of these providing support for all
	   Pod::Simple-based Pod parsers.

       "=pod"
	   This	command	indicates that this paragraph begins a Pod block.  (If
	   we are already in the middle	of a Pod block,	this command has no
	   effect at all.)  If there is	any text in this command paragraph
	   after "=pod", it must be ignored.  Examples:

	     =pod

	     This is a plain Pod paragraph.

	     =pod This text is ignored.

       "=cut"
	   This	command	indicates that this line is the	end of this previously
	   started Pod block.  If there	is any text after "=cut" on the	line,
	   it must be ignored.	Examples:

	     =cut

	     =cut The documentation ends here.

	     =cut
	     # This is the first line of program text.
	     sub foo { # This is the second.

	   It is an error to try to start a Pod	block with a "=cut" command.
	   In that case, the Pod processor must	halt parsing of	the input
	   file, and must by default emit a warning.

       "=over"
	   This	command	indicates that this is the start of a list/indent
	   region.  If there is	any text following the "=over",	it must
	   consist of only a nonzero positive numeral.	The semantics of this
	   numeral is explained	in the "About =over...=back Regions" section,
	   further below.  Formatting codes are	not expanded.  Examples:

	     =over 3

	     =over 3.5

	     =over

       "=item"
	   This	command	indicates that an item in a list begins	here.
	   Formatting codes are	processed.  The	semantics of the (optional)
	   text	in the remainder of this paragraph are explained in the	"About
	   =over...=back Regions" section, further below.  Examples:

	     =item

	     =item *

	     =item	*

	     =item 14

	     =item   3.

	     =item C<< $thing->stuff(I<dodad>) >>

	     =item For transporting us beyond seas to be tried for pretended
	     offenses

	     =item He is at this time transporting large armies	of foreign
	     mercenaries to complete the works of death, desolation and
	     tyranny, already begun with circumstances of cruelty and perfidy
	     scarcely paralleled in the	most barbarous ages, and totally
	     unworthy the head of a civilized nation.

       "=back"
	   This	command	indicates that this is the end of the region begun by
	   the most recent "=over" command.  It	permits	no text	after the
	   "=back" command.

       "=begin formatname"
       "=begin formatname parameter"
	   This	marks the following paragraphs (until the matching "=end
	   formatname")	as being for some special kind of processing.  Unless
	   "formatname"	begins with a colon, the contained non-command
	   paragraphs are data paragraphs.  But	if "formatname"	does begin
	   with	a colon, then non-command paragraphs are ordinary paragraphs
	   or data paragraphs.	This is	discussed in detail in the section
	   "About Data Paragraphs and "=begin/=end" Regions".

	   It is advised that formatnames match	the regexp
	   "m/\A:?[-a-zA-Z0-9_]+\z/".  Everything following whitespace after
	   the formatname is a parameter that may be used by the formatter
	   when	dealing	with this region.  This	parameter must not be repeated
	   in the "=end" paragraph.  Implementors should anticipate future
	   expansion in	the semantics and syntax of the	first parameter	to
	   "=begin"/"=end"/"=for".

       "=end formatname"
	   This	marks the end of the region opened by the matching "=begin
	   formatname" region.	If "formatname"	is not the formatname of the
	   most	recent open "=begin formatname"	region,	then this is an	error,
	   and must generate an	error message.	This is	discussed in detail in
	   the section "About Data Paragraphs and "=begin/=end"	Regions".

       "=for formatname	text..."
	   This	is synonymous with:

		=begin formatname

		text...

		=end formatname

	   That	is, it creates a region	consisting of a	single paragraph; that
	   paragraph is	to be treated as a normal paragraph if "formatname"
	   begins with a ":"; if "formatname" doesn't begin with a colon, then
	   "text..." will constitute a data paragraph.	There is no way	to use
	   "=for formatname text..." to	express	"text..." as a verbatim
	   paragraph.

       "=encoding encodingname"
	   This	command, which should occur early in the document (at least
	   before any non-US-ASCII data!), declares that this document is
	   encoded in the encoding encodingname, which must be an encoding
	   name	that Encode recognizes.	 (Encode's list	of supported
	   encodings, in Encode::Supported, is useful here.)  If the Pod
	   parser cannot decode	the declared encoding, it should emit a
	   warning and may abort parsing the document altogether.

	   A document having more than one "=encoding" line should be
	   considered an error.	 Pod processors	may silently tolerate this if
	   the not-first "=encoding" lines are just duplicates of the first
	   one (e.g., if there's a "=encoding utf8" line, and later on another
	   "=encoding utf8" line).  But	Pod processors should complain if
	   there are contradictory "=encoding" lines in	the same document
	   (e.g., if there is a	"=encoding utf8" early in the document and
	   "=encoding big5" later).  Pod processors that recognize BOMs	may
	   also	complain if they see an	"=encoding" line that contradicts the
	   BOM (e.g., if a document with a UTF-16LE BOM	has an "=encoding
	   shiftjis" line).

       If a Pod	processor sees any command other than the ones listed above
       (like "=head", or "=haed1", or "=stuff",	or "=cuttlefish", or "=w123"),
       that processor must by default treat this as an error.  It must not
       process the paragraph beginning with that command, must by default warn
       of this as an error, and	may abort the parse.  A	Pod parser may allow a
       way for particular applications to add to the above list	of known
       commands, and to	stipulate, for each additional command,	whether
       formatting codes	should be processed.

       Future versions of this specification may add additional	commands.

Pod Formatting Codes
       (Note that in previous drafts of	this document and of perlpod,
       formatting codes	were referred to as "interior sequences", and this
       term may	still be found in the documentation for	Pod parsers, and in
       error messages from Pod processors.)

       There are two syntaxes for formatting codes:

       o   A formatting	code starts with a capital letter (just	US-ASCII
	   [A-Z]) followed by a	"<", any number	of characters, and ending with
	   the first matching ">".  Examples:

	       That's what I<you> think!

	       What's C<CORE::dump()> for?

	       X<C<chmod> and C<unlink()> Under	Different Operating Systems>

       o   A formatting	code starts with a capital letter (just	US-ASCII
	   [A-Z]) followed by two or more "<"'s, one or	more whitespace
	   characters, any number of characters, one or	more whitespace
	   characters, and ending with the first matching sequence of two or
	   more	">"'s, where the number	of ">"'s equals	the number of "<"'s in
	   the opening of this formatting code.	 Examples:

	       That's what I<< you >> think!

	       C<<< open(X, ">>thing.dat") || die $! >>>

	       B<< $foo->bar();	>>

	   With	this syntax, the whitespace character(s) after the "C<<<" and
	   before the ">>>" (or	whatever letter) are not renderable. They do
	   not signify whitespace, are merely part of the formatting codes
	   themselves.	That is, these are all synonymous:

	       C<thing>
	       C<< thing >>
	       C<<	     thing     >>
	       C<<<   thing >>>
	       C<<<<
	       thing
			  >>>>

	   and so on.

	   Finally, the	multiple-angle-bracket form does not alter the
	   interpretation of nested formatting codes, meaning that the
	   following four example lines	are identical in meaning:

	     B<example:	C<$a E<lt>=E<gt> $b>>

	     B<example:	C<< $a <=> $b >>>

	     B<example:	C<< $a E<lt>=E<gt> $b >>>

	     B<<< example: C<< $a E<lt>=E<gt> $b >> >>>

       In parsing Pod, a notably tricky	part is	the correct parsing of
       (potentially nested!) formatting	codes.	Implementors should consult
       the code	in the "parse_text" routine in Pod::Parser as an example of a
       correct implementation.

       "I<text>" -- italic text
	   See the brief discussion in "Formatting Codes" in perlpod.

       "B<text>" -- bold text
	   See the brief discussion in "Formatting Codes" in perlpod.

       "C<code>" -- code text
	   See the brief discussion in "Formatting Codes" in perlpod.

       "F<filename>" --	style for filenames
	   See the brief discussion in "Formatting Codes" in perlpod.

       "X<topic	name>" -- an index entry
	   See the brief discussion in "Formatting Codes" in perlpod.

	   This	code is	unusual	in that	most formatters	completely discard
	   this	code and its content.  Other formatters	will render it with
	   invisible codes that	can be used in building	an index of the
	   current document.

       "Z<>" --	a null (zero-effect) formatting	code
	   Discussed briefly in	"Formatting Codes" in perlpod.

	   This	code is	unusual	in that	it should have no content.  That is, a
	   processor may complain if it	sees "Z<potatoes>".  Whether or	not it
	   complains, the potatoes text	should ignored.

       "L<name>" -- a hyperlink
	   The complicated syntaxes of this code are discussed at length in
	   "Formatting Codes" in perlpod, and implementation details are
	   discussed below, in "About L<...> Codes".  Parsing the contents of
	   L<content> is tricky.  Notably, the content has to be checked for
	   whether it looks like a URL,	or whether it has to be	split on
	   literal "|" and/or "/" (in the right	order!), and so	on, before
	   E<...> codes	are resolved.

       "E<escape>" -- a	character escape
	   See "Formatting Codes" in perlpod, and several points in "Notes on
	   Implementing	Pod Processors".

       "S<text>" -- text contains non-breaking spaces
	   This	formatting code	is syntactically simple, but semantically
	   complex.  What it means is that each	space in the printable content
	   of this code	signifies a non-breaking space.

	   Consider:

	       C<$x ? $y    :  $z>

	       S<C<$x ?	$y     :  $z>>

	   Both	signify	the monospace (c[ode] style) text consisting of	"$x",
	   one space, "?", one space, ":", one space, "$z".  The difference is
	   that	in the latter, with the	S code,	those spaces are not "normal"
	   spaces, but instead are non-breaking	spaces.

       If a Pod	processor sees any formatting code other than the ones listed
       above (as in "N<...>", or "Q<...>", etc.), that processor must by
       default treat this as an	error.	A Pod parser may allow a way for
       particular applications to add to the above list	of known formatting
       codes; a	Pod parser might even allow a way to stipulate,	for each
       additional command, whether it requires some form of special
       processing, as L<...> does.

       Future versions of this specification may add additional	formatting
       codes.

       Historical note:	 A few older Pod processors would not see a ">"	as
       closing a "C<" code, if the ">" was immediately preceded	by a "-".
       This was	so that	this:

	   C<$foo->bar>

       would parse as equivalent to this:

	   C<$foo-E<gt>bar>

       instead of as equivalent	to a "C" formatting code containing only
       "$foo-",	and then a "bar>" outside the "C" formatting code.  This
       problem has since been solved by	the addition of	syntaxes like this:

	   C<< $foo->bar >>

       Compliant parsers must not treat	"->" as	special.

       Formatting codes	absolutely cannot span paragraphs.  If a code is
       opened in one paragraph,	and no closing code is found by	the end	of
       that paragraph, the Pod parser must close that formatting code, and
       should complain (as in "Unterminated I code in the paragraph starting
       at line 123: 'Time objects are not...'").  So these two paragraphs:

	 I<I told you not to do	this!

	 Don't make me say it again!>

       ...must not be parsed as	two paragraphs in italics (with	the I code
       starting	in one paragraph and starting in another.)  Instead, the first
       paragraph should	generate a warning, but	that aside, the	above code
       must parse as if	it were:

	 I<I told you not to do	this!>

	 Don't make me say it again!E<gt>

       (In SGMLish jargon, all Pod commands are	like block-level elements,
       whereas all Pod formatting codes	are like inline-level elements.)

Notes on Implementing Pod Processors
       The following is	a long section of miscellaneous	requirements and
       suggestions to do with Pod processing.

       o   Pod formatters should tolerate lines	in verbatim blocks that	are of
	   any length, even if that means having to break them (possibly
	   several times, for very long	lines) to avoid	text running off the
	   side	of the page.  Pod formatters may warn of such line-breaking.
	   Such	warnings are particularly appropriate for lines	are over 100
	   characters long, which are usually not intentional.

       o   Pod parsers must recognize all of the three well-known newline
	   formats: CR,	LF, and	CRLF.  See perlport.

       o   Pod parsers should accept input lines that are of any length.

       o   Since Perl recognizes a Unicode Byte	Order Mark at the start	of
	   files as signaling that the file is Unicode encoded as in UTF-16
	   (whether big-endian or little-endian) or UTF-8, Pod parsers should
	   do the same.	 Otherwise, the	character encoding should be
	   understood as being UTF-8 if	the first highbit byte sequence	in the
	   file	seems valid as a UTF-8 sequence, or otherwise as CP-1252
	   (earlier versions of	this specification used	Latin-1	instead	of
	   CP-1252).

	   Future versions of this specification may specify how Pod can
	   accept other	encodings.  Presumably treatment of other encodings in
	   Pod parsing would be	as in XML parsing: whatever the	encoding
	   declared by a particular Pod	file, content is to be stored in
	   memory as Unicode characters.

       o   The well known Unicode Byte Order Marks are as follows:  if the
	   file	begins with the	two literal byte values	0xFE 0xFF, this	is the
	   BOM for big-endian UTF-16.  If the file begins with the two literal
	   byte	value 0xFF 0xFE, this is the BOM for little-endian UTF-16.  On
	   an ASCII platform, if the file begins with the three	literal	byte
	   values 0xEF 0xBB 0xBF, this is the BOM for UTF-8.  A	mechanism
	   portable to EBCDIC platforms	is to:

	     my	$utf8_bom = "\x{FEFF}";
	     utf8::encode($utf8_bom);

       o   A naive, but	often sufficient heuristic on ASCII platforms, for
	   testing the first highbit byte-sequence in a	BOM-less file (whether
	   in code or in Pod!),	to see whether that sequence is	valid as UTF-8
	   (RFC	2279) is to check whether that the first byte in the sequence
	   is in the range 0xC2	- 0xFD and whether the next byte is in the
	   range 0x80 -	0xBF.  If so, the parser may conclude that this	file
	   is in UTF-8,	and all	highbit	sequences in the file should be
	   assumed to be UTF-8.	 Otherwise the parser should treat the file as
	   being in CP-1252.  (A better	check, and which works on EBCDIC
	   platforms as	well, is to pass a copy	of the sequence	to
	   utf8::decode() which	performs a full	validity check on the sequence
	   and returns TRUE if it is valid UTF-8, FALSE	otherwise.  This
	   function is always pre-loaded, is fast because it is	written	in C,
	   and will only get called at most once, so you don't need to avoid
	   it out of performance concerns.)  In	the unlikely circumstance that
	   the first highbit sequence in a truly non-UTF-8 file	happens	to
	   appear to be	UTF-8, one can cater to	our heuristic (as well as any
	   more	intelligent heuristic) by prefacing that line with a comment
	   line	containing a highbit sequence that is clearly not valid	as
	   UTF-8.  A line consisting of	simply "#", an e-acute,	and any	non-
	   highbit byte, is sufficient to establish this file's	encoding.

       o   Pod processors must treat a "=for [label] [content...]" paragraph
	   as meaning the same thing as	a "=begin [label]" paragraph, content,
	   and an "=end	[label]" paragraph.  (The parser may conflate these
	   two constructs, or may leave	them distinct, in the expectation that
	   the formatter will nevertheless treat them the same.)

       o   When	rendering Pod to a format that allows comments (i.e., to
	   nearly any format other than	plaintext), a Pod formatter must
	   insert comment text identifying its name and	version	number,	and
	   the name and	version	numbers	of any modules it might	be using to
	   process the Pod.  Minimal examples:

	    %% POD::Pod2PS v3.14159, using POD::Parser v1.92

	    <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->

	    {\doccomm generated	by Pod::Tree::RTF 3.14159 using	Pod::Tree 1.08}

	    .\"	Pod::Man version 3.14159, using	POD::Parser version 1.92

	   Formatters may also insert additional comments, including: the
	   release date	of the Pod formatter program, the contact address for
	   the author(s) of the	formatter, the current time, the name of input
	   file, the formatting	options	in effect, version of Perl used, etc.

	   Formatters may also choose to note errors/warnings as comments,
	   besides or instead of emitting them otherwise (as in	messages to
	   STDERR, or "die"ing).

       o   Pod parsers may emit	warnings or error messages ("Unknown E code
	   E<zslig>!") to STDERR (whether through printing to STDERR, or
	   "warn"ing/"carp"ing,	or "die"ing/"croak"ing), but must allow
	   suppressing all such	STDERR output, and instead allow an option for
	   reporting errors/warnings in	some other way,	whether	by triggering
	   a callback, or noting errors	in some	attribute of the document
	   object, or some similarly unobtrusive mechanism -- or even by
	   appending a "Pod Errors" section to the end of the parsed form of
	   the document.

       o   In cases of exceptionally aberrant documents, Pod parsers may abort
	   the parse.  Even then, using	"die"ing/"croak"ing is to be avoided;
	   where possible, the parser library may simply close the input file
	   and add text	like "*** Formatting Aborted ***" to the end of	the
	   (partial) in-memory document.

       o   In paragraphs where formatting codes	(like E<...>, B<...>) are
	   understood (i.e., not verbatim paragraphs, but including ordinary
	   paragraphs, and command paragraphs that produce renderable text,
	   like	"=head1"), literal whitespace should generally be considered
	   "insignificant", in that one	literal	space has the same meaning as
	   any (nonzero) number	of literal spaces, literal newlines, and
	   literal tabs	(as long as this produces no blank lines, since	those
	   would terminate the paragraph).  Pod	parsers	should compact literal
	   whitespace in each processed	paragraph, but may provide an option
	   for overriding this (since some processing tasks do not require
	   it),	or may follow additional special rules (for example, specially
	   treating period-space-space or period-newline sequences).

       o   Pod parsers should not, by default, try to coerce apostrophe	(')
	   and quote (") into smart quotes (little 9's,	66's, 99's, etc), nor
	   try to turn backtick	(`) into anything else but a single backtick
	   character (distinct from an open quote character!), nor "--"	into
	   anything but	two minus signs.  They must never do any of those
	   things to text in C<...> formatting codes, and never	ever to	text
	   in verbatim paragraphs.

       o   When	rendering Pod to a format that has two kinds of	hyphens	(-),
	   one that's a	non-breaking hyphen, and another that's	a breakable
	   hyphen (as in "object-oriented", which can be split across lines as
	   "object-", newline, "oriented"), formatters are encouraged to
	   generally translate "-" to non-breaking hyphen, but may apply
	   heuristics to convert some of these to breaking hyphens.

       o   Pod formatters should make reasonable efforts to keep words of Perl
	   code	from being broken across lines.	 For example, "Foo::Bar" in
	   some	formatting systems is seen as eligible for being broken	across
	   lines as "Foo::" newline "Bar" or even "Foo::-" newline "Bar".
	   This	should be avoided where	possible, either by disabling all
	   line-breaking in mid-word, or by wrapping particular	words with
	   internal punctuation	in "don't break	this across lines" codes
	   (which in some formats may not be a single code, but	might be a
	   matter of inserting non-breaking zero-width spaces between every
	   pair	of characters in a word.)

       o   Pod parsers should, by default, expand tabs in verbatim paragraphs
	   as they are processed, before passing them to the formatter or
	   other processor.  Parsers may also allow an option for overriding
	   this.

       o   Pod parsers should, by default, remove newlines from	the end	of
	   ordinary and	verbatim paragraphs before passing them	to the
	   formatter.  For example, while the paragraph	you're reading now
	   could be considered,	in Pod source, to end with (and	contain) the
	   newline(s) that end it, it should be	processed as ending with (and
	   containing) the period character that ends this sentence.

       o   Pod parsers,	when reporting errors, should make some	effort to
	   report an approximate line number ("Nested E<>'s in Paragraph #52,
	   near	line 633 of Thing/Foo.pm!"), instead of	merely noting the
	   paragraph number ("Nested E<>'s in Paragraph	#52 of
	   Thing/Foo.pm!").  Where this	is problematic,	the paragraph number
	   should at least be accompanied by an	excerpt	from the paragraph
	   ("Nested E<>'s in Paragraph #52 of Thing/Foo.pm, which begins
	   'Read/write accessor	for the	C<interest rate> attribute...'").

       o   Pod parsers,	when processing	a series of verbatim paragraphs	one
	   after another, should consider them to be one large verbatim
	   paragraph that happens to contain blank lines.  I.e., these two
	   lines, which	have a blank line between them:

		   use Foo;

		   print Foo->VERSION

	   should be unified into one paragraph	("\tuse	Foo;\n\n\tprint
	   Foo->VERSION") before being passed to the formatter or other
	   processor.  Parsers may also	allow an option	for overriding this.

	   While this might be too cumbersome to implement in event-based Pod
	   parsers, it is straightforward for parsers that return parse	trees.

       o   Pod formatters, where feasible, are advised to avoid	splitting
	   short verbatim paragraphs (under twelve lines, say) across pages.

       o   Pod parsers must treat a line with only spaces and/or tabs on it as
	   a "blank line" such as separates paragraphs.	 (Some older parsers
	   recognized only two adjacent	newlines as a "blank line" but would
	   not recognize a newline, a space, and a newline, as a blank line.
	   This	is noncompliant	behavior.)

       o   Authors of Pod formatters/processors	should make every effort to
	   avoid writing their own Pod parser.	There are already several in
	   CPAN, with a	wide range of interface	styles -- and one of them,
	   Pod::Simple,	comes with modern versions of Perl.

       o   Characters in Pod documents may be conveyed either as literals, or
	   by number in	E<n> codes, or by an equivalent	mnemonic, as in
	   E<eacute> which is exactly equivalent to E<233>.  The numbers are
	   the Latin1/Unicode values, even on EBCDIC platforms.

	   When	referring to characters	by using a E<n>	numeric	code, numbers
	   in the range	32-126 refer to	those well known US-ASCII characters
	   (also defined there by Unicode, with	the same meaning), which all
	   Pod formatters must render faithfully.  Characters whose E<>
	   numbers are in the ranges 0-31 and 127-159 should not be used
	   (neither as literals, nor as	E<number> codes), except for the
	   literal byte-sequences for newline (ASCII 13, ASCII 13 10, or ASCII
	   10),	and tab	(ASCII 9).

	   Numbers in the range	160-255	refer to Latin-1 characters (also
	   defined there by Unicode, with the same meaning).  Numbers above
	   255 should be understood to refer to	Unicode	characters.

       o   Be warned that some formatters cannot reliably render characters
	   outside 32-126; and many are	able to	handle 32-126 and 160-255, but
	   nothing above 255.

       o   Besides the well-known "E<lt>" and "E<gt>" codes for	less-than and
	   greater-than, Pod parsers must understand "E<sol>" for "/"
	   (solidus, slash), and "E<verbar>" for "|" (vertical bar, pipe).
	   Pod parsers should also understand "E<lchevron>" and	"E<rchevron>"
	   as legacy codes for characters 171 and 187, i.e., "left-pointing
	   double angle	quotation mark"	= "left	pointing guillemet" and
	   "right-pointing double angle	quotation mark"	= "right pointing
	   guillemet".	(These look like little	"<<" and ">>", and they	are
	   now preferably expressed with the HTML/XHTML	codes "E<laquo>" and
	   "E<raquo>".)

       o   Pod parsers should understand all "E<html>" codes as	defined	in the
	   entity declarations in the most recent XHTML	specification at
	   "www.W3.org".  Pod parsers must understand at least the entities
	   that	define characters in the range 160-255 (Latin-1).  Pod
	   parsers, when faced with some unknown "E<identifier>" code,
	   shouldn't simply replace it with nullstring (by default, at least),
	   but may pass	it through as a	string consisting of the literal
	   characters E, less-than, identifier,	greater-than.  Or Pod parsers
	   may offer the alternative option of processing such unknown
	   "E<identifier>" codes by firing an event especially for such	codes,
	   or by adding	a special node-type to the in-memory document tree.
	   Such	"E<identifier>"	may have special meaning to some processors,
	   or some processors may choose to add	them to	a special error
	   report.

       o   Pod parsers must also support the XHTML codes "E<quot>" for
	   character 34	(doublequote, "), "E<amp>" for character 38
	   (ampersand, &), and "E<apos>" for character 39 (apostrophe, ').

       o   Note	that in	all cases of "E<whatever>", whatever (whether an
	   htmlname, or	a number in any	base) must consist only	of
	   alphanumeric	characters -- that is, whatever	must match
	   "m/\A\w+\z/".  So "E< 0 1 2 3 >" is invalid,	because	it contains
	   spaces, which aren't	alphanumeric characters.  This presumably does
	   not need special treatment by a Pod processor; " 0 1	2 3 " doesn't
	   look	like a number in any base, so it would presumably be looked up
	   in the table	of HTML-like names.  Since there isn't (and cannot be)
	   an HTML-like	entity called "	0 1 2 3	", this	will be	treated	as an
	   error.  However, Pod	processors may treat "E< 0 1 2 3 >" or
	   "E<e-acute>"	as syntactically invalid, potentially earning a
	   different error message than	the error message (or warning, or
	   event) generated by a merely	unknown	(but theoretically valid)
	   htmlname, as	in "E<qacute>" [sic].  However,	Pod parsers are	not
	   required to make this distinction.

       o   Note	that E<number> must not	be interpreted as simply "codepoint
	   number in the current/native	character set".	 It always means only
	   "the	character represented by codepoint number in Unicode."	(This
	   is identical	to the semantics of &#number; in XML.)

	   This	will likely require many formatters to have tables mapping
	   from	treatable Unicode codepoints (such as the "\xE9" for the
	   e-acute character) to the escape sequences or codes necessary for
	   conveying such sequences in the target output format.  A converter
	   to *roff would, for example know that "\xE9"	(whether conveyed
	   literally, or via a E<...> sequence)	is to be conveyed as "e\\*'".
	   Similarly, a	program	rendering Pod in a Mac OS application window,
	   would presumably need to know that "\xE9" maps to codepoint 142 in
	   MacRoman encoding that (at time of writing) is native for Mac OS.
	   Such	Unicode2whatever mappings are presumably already widely
	   available for common	output formats.	 (Such mappings	may be
	   incomplete!	Implementers are not expected to bend over backwards
	   in an attempt to render Cherokee syllabics, Etruscan	runes,
	   Byzantine musical symbols, or any of	the other weird	things that
	   Unicode can encode.)	 And if	a Pod document uses a character	not
	   found in such a mapping, the	formatter should consider it an
	   unrenderable	character.

       o   If, surprisingly, the implementor of	a Pod formatter	can't find a
	   satisfactory	pre-existing table mapping from	Unicode	characters to
	   escapes in the target format	(e.g., a decent	table of Unicode
	   characters to *roff escapes), it will be necessary to build such a
	   table.  If you are in this circumstance, you	should begin with the
	   characters in the range 0x00A0 - 0x00FF, which is mostly the
	   heavily used	accented characters.  Then proceed (as patience
	   permits and fastidiousness compels) through the characters that the
	   (X)HTML standards groups judged important enough to merit mnemonics
	   for.	 These are declared in the (X)HTML specifications at the
	   www.W3.org site.  At	time of	writing	(September 2001), the most
	   recent entity declaration files are:

	     http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
	     http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
	     http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent

	   Then	you can	progress through any remaining notable Unicode
	   characters in the range 0x2000-0x204D (consult the character	tables
	   at www.unicode.org),	and whatever else strikes your fancy.  For
	   example, in xhtml-symbol.ent, there is the entry:

	     <!ENTITY infin    "&#8734;"> <!-- infinity, U+221E	ISOtech	-->

	   While the mapping "infin" to	the character "\x{221E}" will
	   (hopefully) have been already handled by the	Pod parser, the
	   presence of the character in	this file means	that it's reasonably
	   important enough to include in a formatter's	table that maps	from
	   notable Unicode characters to the codes necessary for rendering
	   them.  So for a Unicode-to-*roff mapping, for example, this would
	   merit the entry:

	     "\x{221E}"	=> '\(in',

	   It is eagerly hoped that in the future, increasing numbers of
	   formats (and	formatters) will support Unicode characters directly
	   (as (X)HTML does with "&infin;", "&#8734;", or "&#x221E;"),
	   reducing the	need for idiosyncratic mappings	of
	   Unicode-to-my_escapes.

       o   It is up to individual Pod formatter	to display good	judgement when
	   confronted with an unrenderable character (which is distinct	from
	   an unknown E<thing> sequence	that the parser	couldn't resolve to
	   anything, renderable	or not).  It is	good practice to map Latin
	   letters with	diacritics (like "E<eacute>"/"E<233>") to the
	   corresponding unaccented US-ASCII letters (like a simple character
	   101,	"e"), but clearly this is often	not feasible, and an
	   unrenderable	character may be represented as	"?", or	the like.  In
	   attempting a	sane fallback (as from E<233> to "e"), Pod formatters
	   may use the %Latin1Code_to_fallback table in	Pod::Escapes, or
	   Text::Unidecode, if available.

	   For example,	this Pod text:

	     magic is enabled if you set C<$Currency> to 'E<euro>'.

	   may be rendered as: "magic is enabled if you	set $Currency to '?'"
	   or as "magic	is enabled if you set $Currency	to '[euro]'", or as
	   "magic is enabled if	you set	$Currency to '[x20AC]',	etc.

	   A Pod formatter may also note, in a comment or warning, a list of
	   what	unrenderable characters	were encountered.

       o   E<...> may freely appear in any formatting code (other than in
	   another E<...> or in	an Z<>).  That is, "X<The E<euro>1,000,000
	   Solution>" is valid,	as is "L<The E<euro>1,000,000
	   Solution|Million::Euros>".

       o   Some	Pod formatters output to formats that implement	non-breaking
	   spaces as an	individual character (which I'll call "NBSP"), and
	   others output to formats that implement non-breaking	spaces just as
	   spaces wrapped in a "don't break this across	lines" code.  Note
	   that	at the level of	Pod, both sorts	of codes can occur: Pod	can
	   contain a NBSP character (whether as	a literal, or as a "E<160>" or
	   "E<nbsp>" code); and	Pod can	contain	"S<foo I<bar> baz>" codes,
	   where "mere spaces" (character 32) in such codes are	taken to
	   represent non-breaking spaces.  Pod parsers should consider
	   supporting the optional parsing of "S<foo I<bar> baz>" as if	it
	   were	"fooNBSPI<bar>NBSPbaz",	and, going the other way, the optional
	   parsing of groups of	words joined by	NBSP's as if each group	were
	   in a	S<...> code, so	that formatters	may use	the representation
	   that	maps best to what the output format demands.

       o   Some	processors may find that the "S<...>" code is easiest to
	   implement by	replacing each space in	the parse tree under the
	   content of the S, with an NBSP.  But	note: the replacement should
	   apply not to	spaces in all text, but	only to	spaces in printable
	   text.  (This	distinction may	or may not be evident in the
	   particular tree/event model implemented by the Pod parser.)	For
	   example, consider this unusual case:

	      S<L</Autoloaded Functions>>

	   This	means that the space in	the middle of the visible link text
	   must	not be broken across lines.  In	other words, it's the same as
	   this:

	      L<"AutoloadedE<160>Functions"/Autoloaded Functions>

	   However, a misapplied space-to-NBSP replacement could (wrongly)
	   produce something equivalent	to this:

	      L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>

	   ...which is almost definitely not going to work as a	hyperlink
	   (assuming this formatter outputs a format supporting	hypertext).

	   Formatters may choose to just not support the S format code,
	   especially in cases where the output	format simply has no NBSP
	   character/code and no code for "don't break this stuff across
	   lines".

       o   Besides the NBSP character discussed	above, implementors are
	   reminded of the existence of	the other "special" character in
	   Latin-1, the	"soft hyphen" character, also known as "discretionary
	   hyphen", i.e. "E<173>" = "E<0xAD>" =	"E<shy>").  This character
	   expresses an	optional hyphenation point.  That is, it normally
	   renders as nothing, but may render as a "-" if a formatter breaks
	   the word at that point.  Pod	formatters should, as appropriate, do
	   one of the following:  1) render this with a	code with the same
	   meaning (e.g., "\-" in RTF),	2) pass	it through in the expectation
	   that	the formatter understands this character as such, or 3)	delete
	   it.

	   For example:

	     sigE<shy>action
	     manuE<shy>script
	     JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi

	   These signal	to a formatter that if it is to	hyphenate "sigaction"
	   or "manuscript", then it should be done as "sig-[linebreak]action"
	   or "manu-[linebreak]script" (and if it doesn't hyphenate it,	then
	   the "E<shy>"	doesn't	show up	at all).  And if it is to hyphenate
	   "Jarkko" and/or "Hietaniemi", it can	do so only at the points where
	   there is a "E<shy>" code.

	   In practice,	it is anticipated that this character will not be used
	   often, but formatters should	either support it, or delete it.

       o   If you think	that you want to add a new command to Pod (like, say,
	   a "=biblio" command), consider whether you could get	the same
	   effect with a for or	begin/end sequence: "=for biblio ..." or
	   "=begin biblio" ... "=end biblio".  Pod processors that don't
	   understand "=for biblio", etc, will simply ignore it, whereas they
	   may complain	loudly if they see "=biblio".

       o   Throughout this document, "Pod" has been the	preferred spelling for
	   the name of the documentation format.  One may also use "POD" or
	   "pod".  For the documentation that is (typically) in	the Pod
	   format, you may use "pod", or "Pod",	or "POD".  Understanding these
	   distinctions	is useful; but obsessing over how to spell them,
	   usually is not.

About L<...> Codes
       As you can tell from a glance at	perlpod, the L<...> code is the	most
       complex of the Pod formatting codes.  The points	below will hopefully
       clarify what it means and how processors	should deal with it.

       o   In parsing an L<...>	code, Pod parsers must distinguish at least
	   four	attributes:

	   First:
	       The link-text.  If there	is none, this must be "undef".	(E.g.,
	       in "L<Perl Functions|perlfunc>",	the link-text is "Perl
	       Functions".  In "L<Time::HiRes>"	and even "L<|Time::HiRes>",
	       there is	no link	text.  Note that link text may contain
	       formatting.)

	   Second:
	       The possibly inferred link-text;	i.e., if there was no real
	       link text, then this is the text	that we'll infer in its	place.
	       (E.g., for "L<Getopt::Std>", the	inferred link text is
	       "Getopt::Std".)

	   Third:
	       The name	or URL,	or "undef" if none.  (E.g., in "L<Perl
	       Functions|perlfunc>", the name (also sometimes called the page)
	       is "perlfunc".  In "L</CAVEATS>", the name is "undef".)

	   Fourth:
	       The section (AKA	"item" in older	perlpods), or "undef" if none.
	       E.g., in	"L<Getopt::Std/DESCRIPTION>", "DESCRIPTION" is the
	       section.	 (Note that this is not	the same as a manpage section
	       like the	"5" in "man 5 crontab".	 "Section Foo" in the Pod
	       sense means the part of the text	that's introduced by the
	       heading or item whose text is "Foo".)

	   Pod parsers may also	note additional	attributes including:

	   Fifth:
	       A flag for whether item 3 (if present) is a URL (like
	       "http://lists.perl.org" is), in which case there	should be no
	       section attribute; a Pod	name (like "perldoc" and "Getopt::Std"
	       are); or	possibly a man page name (like "crontab(5)" is).

	   Sixth:
	       The raw original	L<...> content,	before text is split on	"|",
	       "/", etc, and before E<...> codes are expanded.

	   (The	above were numbered only for concise reference below.  It is
	   not a requirement that these	be passed as an	actual list or array.)

	   For example:

	     L<Foo::Bar>
	       =>  undef,			  # link text
		   "Foo::Bar",			  # possibly inferred link text
		   "Foo::Bar",			  # name
		   undef,			  # section
		   'pod',			  # what sort of link
		   "Foo::Bar"			  # original content

	     L<Perlport's section on NL's|perlport/Newlines>
	       =>  "Perlport's section on NL's",  # link text
		   "Perlport's section on NL's",  # possibly inferred link text
		   "perlport",			  # name
		   "Newlines",			  # section
		   'pod',			  # what sort of link
		   "Perlport's section on NL's|perlport/Newlines"
						  # original content

	     L<perlport/Newlines>
	       =>  undef,			  # link text
		   '"Newlines" in perlport',	  # possibly inferred link text
		   "perlport",			  # name
		   "Newlines",			  # section
		   'pod',			  # what sort of link
		   "perlport/Newlines"		  # original content

	     L<crontab(5)/"DESCRIPTION">
	       =>  undef,			  # link text
		   '"DESCRIPTION" in crontab(5)', # possibly inferred link text
		   "crontab(5)",		  # name
		   "DESCRIPTION",		  # section
		   'man',			  # what sort of link
		   'crontab(5)/"DESCRIPTION"'	  # original content

	     L</Object Attributes>
	       =>  undef,			  # link text
		   '"Object Attributes"',	  # possibly inferred link text
		   undef,			  # name
		   "Object Attributes",		  # section
		   'pod',			  # what sort of link
		   "/Object Attributes"		  # original content

	     L<https://www.perl.org/>
	       =>  undef,			  # link text
		   "https://www.perl.org/",	  # possibly inferred link text
		   "https://www.perl.org/",	  # name
		   undef,			  # section
		   'url',			  # what sort of link
		   "https://www.perl.org/"	   # original content

	     L<Perl.org|https://www.perl.org/>
	       =>  "Perl.org",			  # link text
		   "https://www.perl.org/",	  # possibly inferred link text
		   "https://www.perl.org/",	  # name
		   undef,			  # section
		   'url',			  # what sort of link
		   "Perl.org|https://www.perl.org/" # original content

	   Note	that you can distinguish URL-links from	anything else by the
	   fact	that they match	"m/\A\w+:[^:\s]\S*\z/".	 So
	   "L<http://www.perl.com>" is a URL, but "L<HTTP::Response>" isn't.

       o   In case of L<...> codes with	no "text|" part	in them, older
	   formatters have exhibited great variation in	actually displaying
	   the link or cross reference.	 For example, L<crontab(5)> would
	   render as "the crontab(5) manpage", or "in the crontab(5) manpage"
	   or just "crontab(5)".

	   Pod processors must now treat "text|"-less links as follows:

	     L<name>	     =>	 L<name|name>
	     L</section>     =>	 L<"section"|/section>
	     L<name/section> =>	 L<"section" in	name|name/section>

       o   Note	that section names might contain markup.  I.e.,	if a section
	   starts with:

	     =head2 About the C<-M> Operator

	   or with:

	     =item About the C<-M> Operator

	   then	a link to it would look	like this:

	     L<somedoc/About the C<-M> Operator>

	   Formatters may choose to ignore the markup for purposes of
	   resolving the link and use only the renderable characters in	the
	   section name, as in:

	     <h1><a name="About_the_-M_Operator">About the <code>-M</code>
	     Operator</h1>

	     ...

	     <a	href="somedoc#About_the_-M_Operator">About the <code>-M</code>
	     Operator" in somedoc</a>

       o   Previous versions of	perlpod	distinguished "L<name/"section">"
	   links from "L<name/item>" links (and	their targets).	 These have
	   been	merged syntactically and semantically in the current
	   specification, and section can refer	either to a "=headn Heading
	   Content" command or to a "=item Item	Content" command.  This
	   specification does not specify what behavior	should be in the case
	   of a	given document having several things all seeming to produce
	   the same section identifier (e.g., in HTML, several things all
	   producing the same anchorname in <a name="anchorname">...</a>
	   elements).  Where Pod processors can	control	this behavior, they
	   should use the first	such anchor.  That is, "L<Foo/Bar>" refers to
	   the first "Bar" section in Foo.

	   But for some	processors/formats this	cannot be easily controlled;
	   as with the HTML example, the behavior of multiple ambiguous	<a
	   name="anchorname">...</a> is	most easily just left up to browsers
	   to decide.

       o   In a	"L<text|...>" code, text may contain formatting	codes for
	   formatting or for E<...> escapes, as	in:

	     L<B<ummE<234>stuff>|...>

	   For "L<...>"	codes without a	"name|"	part, only "E<...>" and	"Z<>"
	   codes may occur.  That is, authors should not use
	   ""L<B<Foo::Bar>>"".

	   Note, however, that formatting codes	and Z<>'s can occur in any and
	   all parts of	an L<...> (i.e., in name, section, text, and url).

	   Authors must	not nest L<...>	codes.	For example, "L<The
	   L<Foo::Bar> man page>" should be treated as an error.

       o   Note	that Pod authors may use formatting codes inside the "text"
	   part	of "L<text|name>" (and so on for L<text|/"sec">).

	   In other words, this	is valid:

	     Go	read L<the docs	on C<$.>|perlvar/"$.">

	   Some	output formats that do allow rendering "L<...>"	codes as
	   hypertext, might not	allow the link-text to be formatted; in	that
	   case, formatters will have to just ignore that formatting.

       o   At time of writing, "L<name>" values	are of two types: either the
	   name	of a Pod page like "L<Foo::Bar>" (which	might be a real	Perl
	   module or program in	an @INC	/ PATH directory, or a .pod file in
	   those places); or the name of a Unix	man page, like
	   "L<crontab(5)>".  In	theory,	"L<chmod>" is ambiguous	between	a Pod
	   page	called "chmod",	or the Unix man	page "chmod" (in whatever man-
	   section).  However, the presence of a string	in parens, as in
	   "crontab(5)", is sufficient to signal that what is being discussed
	   is not a Pod	page, and so is	presumably a Unix man page.  The
	   distinction is of no	importance to many Pod processors, but some
	   processors that render to hypertext formats may need	to distinguish
	   them	in order to know how to	render a given "L<foo>"	code.

       o   Previous versions of	perlpod	allowed	for a "L<section>" syntax (as
	   in "L<Object	Attributes>"), which was not easily distinguishable
	   from	"L<name>" syntax and for "L<"section">"	which was only
	   slightly less ambiguous.  This syntax is no longer in the
	   specification, and has been replaced	by the "L</section>" syntax
	   (where the slash was	formerly optional).  Pod parsers should
	   tolerate the	"L<"section">" syntax, for a while at least.  The
	   suggested heuristic for distinguishing "L<section>" from "L<name>"
	   is that if it contains any whitespace, it's a section.  Pod
	   processors should warn about	this being deprecated syntax.

About =over...=back Regions
       "=over"..."=back" regions are used for various kinds of list-like
       structures.  (I use the term "region" here simply as a collective term
       for everything from the "=over" to the matching "=back".)

       o   The non-zero	numeric	indentlevel in "=over indentlevel" ...
	   "=back" is used for giving the formatter a clue as to how many
	   "spaces" (ems, or roughly equivalent	units) it should tab over,
	   although many formatters will have to convert this to an absolute
	   measurement that may	not exactly match with the size	of spaces (or
	   M's)	in the document's base font.  Other formatters may have	to
	   completely ignore the number.  The lack of any explicit indentlevel
	   parameter is	equivalent to an indentlevel value of 4.  Pod
	   processors may complain if indentlevel is present but is not	a
	   positive number matching "m/\A(\d*\.)?\d+\z/".

       o   Authors of Pod formatters are reminded that "=over" ... "=back" may
	   map to several different constructs in your output format.  For
	   example, in converting Pod to (X)HTML, it can map to	any of
	   <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
	   <blockquote>...</blockquote>.  Similarly, "=item" can map to	<li>
	   or <dt>.

       o   Each	"=over"	... "=back" region should be one of the	following:

	   o   An "=over" ... "=back" region containing	only "=item *"
	       commands, each followed by some number of ordinary/verbatim
	       paragraphs, other nested	"=over"	... "=back" regions, "=for..."
	       paragraphs, and "=begin"..."=end" regions.

	       (Pod processors must tolerate a bare "=item" as if it were
	       "=item *".)  Whether "*"	is rendered as a literal asterisk, an
	       "o", or as some kind of real bullet character, is left up to
	       the Pod formatter, and may depend on the	level of nesting.

	   o   An "=over" ... "=back" region containing	only
	       "m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each	group
	       of them)	followed by some number	of ordinary/verbatim
	       paragraphs, other nested	"=over"	... "=back" regions, "=for..."
	       paragraphs, and/or "=begin"..."=end" codes.  Note that the
	       numbers must start at 1 in each section,	and must proceed in
	       order and without skipping numbers.

	       (Pod processors must tolerate lines like	"=item 1" as if	they
	       were "=item 1.",	with the period.)

	   o   An "=over" ... "=back" region containing	only "=item [text]"
	       commands, each one (or each group of them) followed by some
	       number of ordinary/verbatim paragraphs, other nested "=over"
	       ... "=back" regions, or "=for..." paragraphs, and
	       "=begin"..."=end" regions.

	       The "=item [text]" paragraph should not match
	       "m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor
	       should it match just "m/\A=item\s*\z/".

	   o   An "=over" ... "=back" region containing	no "=item" paragraphs
	       at all, and containing only some	number of ordinary/verbatim
	       paragraphs, and possibly	also some nested "=over" ... "=back"
	       regions,	"=for..." paragraphs, and "=begin"..."=end" regions.
	       Such an itemless	"=over"	... "=back" region in Pod is
	       equivalent in meaning to	a "<blockquote>...</blockquote>"
	       element in HTML.

	   Note	that with all the above	cases, you can determine which type of
	   "=over" ... "=back" you have, by examining the first	(non-"=cut",
	   non-"=pod") Pod paragraph after the "=over" command.

       o   Pod formatters must tolerate	arbitrarily large amounts of text in
	   the "=item text..." paragraph.  In practice,	most such paragraphs
	   are short, as in:

	     =item For cutting off our trade with all parts of the world

	   But they may	be arbitrarily long:

	     =item For transporting us beyond seas to be tried for pretended
	     offenses

	     =item He is at this time transporting large armies	of foreign
	     mercenaries to complete the works of death, desolation and
	     tyranny, already begun with circumstances of cruelty and perfidy
	     scarcely paralleled in the	most barbarous ages, and totally
	     unworthy the head of a civilized nation.

       o   Pod processors should tolerate "=item *" / "=item number" commands
	   with	no accompanying	paragraph.  The	middle item is an example:

	     =over

	     =item 1

	     Pick up dry cleaning.

	     =item 2

	     =item 3

	     Stop by the store.	 Get Abba Zabas, Stoli,	and cheap lawn chairs.

	     =back

       o   No "=over" ... "=back" region can contain headings.	Processors may
	   treat such a	heading	as an error.

       o   Note	that an	"=over"	... "=back" region should have some content.
	   That	is, authors should not have an empty region like this:

	     =over

	     =back

	   Pod processors seeing such a	contentless "=over" ...	"=back"
	   region, may ignore it, or may report	it as an error.

       o   Processors must tolerate an "=over" list that goes off the end of
	   the document	(i.e., which has no matching "=back"), but they	may
	   warn	about such a list.

       o   Authors of Pod formatters should note that this construct:

	     =item Neque

	     =item Porro

	     =item Quisquam Est

	     Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
	     velit, sed	quia non numquam eius modi tempora incidunt ut
	     labore et dolore magnam aliquam quaerat voluptatem.

	     =item Ut Enim

	   is semantically ambiguous, in a way that makes formatting decisions
	   a bit difficult.  On	the one	hand, it could be mention of an	item
	   "Neque", mention of another item "Porro", and mention of another
	   item	"Quisquam Est",	with just the last one requiring the
	   explanatory paragraph "Qui dolorem ipsum quia dolor..."; and	then
	   an item "Ut Enim".  In that case, you'd want	to format it like so:

	     Neque

	     Porro

	     Quisquam Est
	       Qui dolorem ipsum quia dolor sit	amet, consectetur, adipisci
	       velit, sed quia non numquam eius	modi tempora incidunt ut
	       labore et dolore	magnam aliquam quaerat voluptatem.

	     Ut	Enim

	   But it could	equally	well be	a discussion of	three (related or
	   equivalent) items, "Neque", "Porro",	and "Quisquam Est", followed
	   by a	paragraph explaining them all, and then	a new item "Ut Enim".
	   In that case, you'd probably	want to	format it like so:

	     Neque
	     Porro
	     Quisquam Est
	       Qui dolorem ipsum quia dolor sit	amet, consectetur, adipisci
	       velit, sed quia non numquam eius	modi tempora incidunt ut
	       labore et dolore	magnam aliquam quaerat voluptatem.

	     Ut	Enim

	   But (for the	foreseeable future), Pod does not provide any way for
	   Pod authors to distinguish which grouping is	meant by the above
	   "=item"-cluster structure.  So formatters should format it like so:

	     Neque

	     Porro

	     Quisquam Est

	       Qui dolorem ipsum quia dolor sit	amet, consectetur, adipisci
	       velit, sed quia non numquam eius	modi tempora incidunt ut
	       labore et dolore	magnam aliquam quaerat voluptatem.

	     Ut	Enim

	   That	is, there should be (at	least roughly) equal spacing between
	   items as between paragraphs (although that spacing may well be less
	   than	the full height	of a line of text).  This leaves it to the
	   reader to use (con)textual cues to figure out whether the "Qui
	   dolorem ipsum..." paragraph applies to the "Quisquam	Est" item or
	   to all three	items "Neque", "Porro",	and "Quisquam Est".  While not
	   an ideal situation, this is preferable to providing formatting cues
	   that	may be actually	contrary to the	author's intent.

About Data Paragraphs and "=begin/=end"	Regions
       Data paragraphs are typically used for inlining non-Pod data that is to
       be used (typically passed through) when rendering the document to a
       specific	format:

	 =begin	rtf

	 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}

	 =end rtf

       The exact same effect could, incidentally, be achieved with a single
       "=for" paragraph:

	 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}

       (Although that is not formally a	data paragraph,	it has the same
       meaning as one, and Pod parsers may parse it as one.)

       Another example of a data paragraph:

	 =begin	html

	 I like	<em>PIE</em>!

	 <hr>Especially	pecan pie!

	 =end html

       If these	were ordinary paragraphs, the Pod parser would try to expand
       the "E</em>" (in	the first paragraph) as	a formatting code, just	like
       "E<lt>" or "E<eacute>".	But since this is in a "=begin
       identifier"..."=end identifier" region and the identifier "html"
       doesn't begin have a ":"	prefix,	the contents of	this region are	stored
       as data paragraphs, instead of being processed as ordinary paragraphs
       (or if they began with a	spaces and/or tabs, as verbatim	paragraphs).

       As a further example: At	time of	writing, no "biblio" identifier	is
       supported, but suppose some processor were written to recognize it as a
       way of (say) denoting a bibliographic reference (necessarily containing
       formatting codes	in ordinary paragraphs).  The fact that	"biblio"
       paragraphs were meant for ordinary processing would be indicated	by
       prefacing each "biblio" identifier with a colon:

	 =begin	:biblio

	 Wirth,	Niklaus.  1976.	 I<Algorithms +	Data Structures	=
	 Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

	 =end :biblio

       This would signal to the	parser that paragraphs in this begin...end
       region are subject to normal handling as	ordinary/verbatim paragraphs
       (while still tagged as meant only for processors	that understand	the
       "biblio"	identifier).  The same effect could be had with:

	 =for :biblio
	 Wirth,	Niklaus.  1976.	 I<Algorithms +	Data Structures	=
	 Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

       The ":" on these	identifiers means simply "process this stuff normally,
       even though the result will be for some special target".	 I suggest
       that parser APIs	report "biblio"	as the target identifier, but also
       report that it had a ":"	prefix.	 (And similarly, with the above
       "html", report "html" as	the target identifier, and note	the lack of a
       ":" prefix.)

       Note that a "=begin identifier"..."=end identifier" region where
       identifier begins with a	colon, can contain commands.  For example:

	 =begin	:biblio

	 Wirth's classic is available in several editions, including:

	 =for comment
	  hm, check abebooks.com for how much used copies cost.

	 =over

	 =item

	 Wirth,	Niklaus.  1975.	 I<Algorithmen und Datenstrukturen.>
	 Teubner, Stuttgart.  [Yes, it's in German.]

	 =item

	 Wirth,	Niklaus.  1976.	 I<Algorithms +	Data Structures	=
	 Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

	 =back

	 =end :biblio

       Note, however, a	"=begin	identifier"..."=end identifier"	region where
       identifier does not begin with a	colon, should not directly contain
       "=head1"	... "=head4" commands, nor "=over", nor	"=back", nor "=item".
       For example, this may be	considered invalid:

	 =begin	somedata

	 This is a data	paragraph.

	 =head1	Don't do this!

	 This is a data	paragraph too.

	 =end somedata

       A Pod processor may signal that the above (specifically the "=head1"
       paragraph) is an	error.	Note, however, that the	following should not
       be treated as an	error:

	 =begin	somedata

	 This is a data	paragraph.

	 =cut

	 # Yup,	this isn't Pod anymore.
	 sub excl { (rand() > .5) ? "hoo!" : "hah!" }

	 =pod

	 This is a data	paragraph too.

	 =end somedata

       And this	too is valid:

	 =begin	someformat

	 This is a data	paragraph.

	   And this is a data paragraph.

	 =begin	someotherformat

	 This is a data	paragraph too.

	   And this is a data paragraph	too.

	 =begin	:yetanotherformat

	 =head2	This is	a command paragraph!

	 This is an ordinary paragraph!

	   And this is a verbatim paragraph!

	 =end :yetanotherformat

	 =end someotherformat

	 Another data paragraph!

	 =end someformat

       The contents of the above "=begin :yetanotherformat" ...	 "=end
       :yetanotherformat" region aren't	data paragraphs, because the
       immediately containing region's identifier (":yetanotherformat")	begins
       with a colon.  In practice, most	regions	that contain data paragraphs
       will contain only data paragraphs; however, the above nesting is
       syntactically valid as Pod, even	if it is rare.	However, the handlers
       for some	formats, like "html", will accept only data paragraphs,	not
       nested regions; and they	may complain if	they see (targeted for them)
       nested regions, or commands, other than "=end", "=pod", and "=cut".

       Also consider this valid	structure:

	 =begin	:biblio

	 Wirth's classic is available in several editions, including:

	 =over

	 =item

	 Wirth,	Niklaus.  1975.	 I<Algorithmen und Datenstrukturen.>
	 Teubner, Stuttgart.  [Yes, it's in German.]

	 =item

	 Wirth,	Niklaus.  1976.	 I<Algorithms +	Data Structures	=
	 Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

	 =back

	 Buy buy buy!

	 =begin	html

	 <img src='wirth_spokesmodeling_book.png'>

	 <hr>

	 =end html

	 Now now now!

	 =end :biblio

       There, the "=begin html"..."=end	html" region is	nested inside the
       larger "=begin :biblio"..."=end :biblio"	region.	 Note that the content
       of the "=begin html"..."=end html" region is data paragraph(s), because
       the immediately containing region's identifier ("html") doesn't begin
       with a colon.

       Pod parsers, when processing a series of	data paragraphs	one after
       another (within a single	region), should	consider them to be one	large
       data paragraph that happens to contain blank lines.  So the content of
       the above "=begin html"..."=end html" may be stored as two data
       paragraphs (one consisting of "<img
       src='wirth_spokesmodeling_book.png'>\n" and another consisting of
       "<hr>\n"), but should be	stored as a single data	paragraph (consisting
       of "<img	src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").

       Pod processors should tolerate empty "=begin something"..."=end
       something" regions, empty "=begin :something"..."=end :something"
       regions,	and contentless	"=for something" and "=for :something"
       paragraphs.  I.e., these	should be tolerated:

	 =for html

	 =begin	html

	 =end html

	 =begin	:biblio

	 =end :biblio

       Incidentally, note that there's no easy way to express a	data paragraph
       starting	with something that looks like a command.  Consider:

	 =begin	stuff

	 =shazbot

	 =end stuff

       There, "=shazbot" will be parsed	as a Pod command "shazbot", not	as a
       data paragraph "=shazbot\n".  However, you can express a	data paragraph
       consisting of "=shazbot\n" using	this code:

	 =for stuff =shazbot

       The situation where this	is necessary, is presumably quite rare.

       Note that =end commands must match the currently	open =begin command.
       That is,	they must properly nest.  For example, this is valid:

	 =begin	outer

	 X

	 =begin	inner

	 Y

	 =end inner

	 Z

	 =end outer

       while this is invalid:

	 =begin	outer

	 X

	 =begin	inner

	 Y

	 =end outer

	 Z

	 =end inner

       This latter is improper because when the	"=end outer" command is	seen,
       the currently open region has the formatname "inner", not "outer".  (It
       just happens that "outer" is the	format name of a higher-up region.)
       This is an error.  Processors must by default report this as an error,
       and may halt processing the document containing that error.  A
       corollary of this is that regions cannot	"overlap". That	is, the	latter
       block above does	not represent a	region called "outer" which contains X
       and Y, overlapping a region called "inner" which	contains Y and Z.  But
       because it is invalid (as all apparently	overlapping regions would be),
       it doesn't represent that, or anything at all.

       Similarly, this is invalid:

	 =begin	thing

	 =end hting

       This is an error	because	the region is opened by	"thing", and the
       "=end" tries to close "hting" [sic].

       This is also invalid:

	 =begin	thing

	 =end

       This is invalid because every "=end" command must have a	formatname
       parameter.

SEE ALSO
       perlpod,	"PODs: Embedded	Documentation" in perlsyn, podchecker

AUTHOR
       Sean M. Burke

perl v5.35.5			  2021-09-26			PERLPODSPEC(1)

NAME | DESCRIPTION | Pod Definitions | Pod Commands | Pod Formatting Codes | Notes on Implementing Pod Processors | About L<...> Codes | About =over...=back Regions | About Data Paragraphs and "=begin/=end" Regions | SEE ALSO | AUTHOR

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