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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) mean-
     ings: "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 char-
     acters, 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 contain-
     ing one or more characters other than space or tab (and ter-
     minated by a newline or end-of-file).

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     (Note: Many older Pod parsers did not accept a line consist-
     ing 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 charac-
     ter.)

     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
     wordcounting it, 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.

     Within a Pod block, there are Pod paragraphs.  A Pod para-
     graph 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:

     +	 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

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	 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 signifi-
	 cant.

     +	 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 identif-
	 ier" 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).

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

     +	 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

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       =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 data 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 dis-
     cussion in "Command Paragraph" in perlpod.	 These are the
     currently recognized Pod commands:

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

	   =head1 Object Attributes

	   =head3 What B<Not> to Do!

     "=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.

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	   =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" sec-
	 tion, 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"

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	 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"
	 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 para-
	 graphs.  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/".  Implementors should antici-
	 pate 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 sec-
	 tion "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 "format-
	 name" 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 ver-
	 batim 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 Encoding recognizes.
	 (Encoding's list of supported encodings, in
	 Encoding::Supported, is useful here.) If the Pod parser

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	 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
	 "=use utf8" line, and later on another "=use 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 docu-
	 ment and "=encoding big5" later).  Pod processors that
	 recognize BOMs may also complain if they see an "=encod-
	 ing" 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 com-
     mand, 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 perl-
     pod, formatting codes were referred to as "interior
     sequences", and this term may still be found in the documen-
     tation for Pod parsers, and in error messages from Pod pro-
     cessors.)

     There are two syntaxes for formatting codes:

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

	     That's what I<you> think!

	     What's C<dump()> for?

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

     +	 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

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	 first matching sequence of two or more ">"'s, where the
	 number of ">"'s equals the number of "<"'s in the open-
	 ing 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.

     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 perl-
	 pod.

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

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

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

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

	 This code is unusual in that most formatters completely

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

     "L<name>" -- a hyperlink
	 The complicated syntaxes of this code are discussed at
	 length in "Formatting Codes" in perlpod, and implementa-
	 tion 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 consist-
	 ing 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.

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     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 pre-
     ceded 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 require-
     ments and suggestions to do with Pod processing.

     +	 Pod formatters should tolerate lines in verbatim blocks
	 that are of any length, even if that means having to

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	 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 warn-
	 ings are particularly appropriate for lines are over 100
	 characters long, which are usually not intentional.

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

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

     +	 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
	 Latin-1.

	 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 pars-
	 ing: whatever the encoding declared by a particular Pod
	 file, content is to be stored in memory as Unicode char-
	 acters.

     +	 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.  If the file
	 begins with the three literal byte values 0xEF 0xBB
	 0xBF, this is the BOM for UTF-8.

     +	 A naive but sufficient heuristic 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 0xC0 - 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 Latin-1.  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-

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	 highbit byte, is sufficient to establish this file's
	 encoding.

     +	 This document's requirements and suggestions about
	 encodings do not apply to Pod processors running on
	 non-ASCII platforms, notably EBCDIC platforms.

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

     +	 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, includ-
	 ing: 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).

     +	 Pod parsers may emit warnings or error messages ("Unk-
	 nown 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 attri-
	 bute of the document object, or some similarly unob-
	 trusive mechanism -- or even by appending a "Pod Errors"
	 section to the end of the parsed form of the document.

     +	 In cases of exceptionally aberrant documents, Pod
	 parsers may abort the parse.  Even then, using

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	 "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.

     +	 In paragraphs where formatting codes (like E<...>,
	 B<...>) are understood (i.e., not verbatim paragraphs,
	 but including ordinary paragraphs, and command para-
	 graphs 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 pro-
	 duces no blank lines, since those would terminate the
	 paragraph).  Pod parsers should compact literal whi-
	 tespace 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).

     +	 Pod parsers should not, by default, try to coerce apos-
	 trophe (') and quote (") into smart quotes (little 9's,
	 66's, 99's, etc), nor try to turn backtick (`) into any-
	 thing else but a single backtick character (distinct
	 from an openquote 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.

     +	 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.

     +	 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 charac-
	 ters in a word.)

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     +	 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.

     +	 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.

     +	 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 para-
	 graph 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...'").

     +	 Pod parsers, when processing a series of verbatim para-
	 graphs 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.

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

     +	 Pod parsers must treat a line with only spaces and/or
	 tabs on it as a "blank line" such as separates para-
	 graphs.  (Some older parsers recognized only two adja-
	 cent newlines as a "blank line" but would not recognize
	 a newline, a space, and a newline, as a blank line.

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	 This is noncompliant behavior.)

     +	 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::Parser, comes with
	 modern versions of Perl.

     +	 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>.

	 Characters 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 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 (13, 13 10, or 10), and tab (9).

	 Characters in the range 160-255 refer to Latin-1 charac-
	 ters (also defined there by Unicode, with the same mean-
	 ing).	Characters above 255 should be understood to
	 refer to Unicode characters.

     +	 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.

     +	 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>".)

     +	 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

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	 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.

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

     +	 Note that in all cases of "E<whatever>", whatever
	 (whether an htmlname, or a number in any base) must con-
	 sist only of alphanumeric characters -- that is, what-
	 ever must watch "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 earn-
	 ing 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 dis-
	 tinction.

     +	 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 con-
	 veyed literally, or via a E<...> sequence) is to be con-
	 veyed 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 map-
	 pings may be incomplete!  Implementers are not expected
	 to bend over backwards in an attempt to render Cherokee

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	 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 unrender-
	 able character.

     +	 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    "∞"> <!-- 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 charac-
	 ters 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 "∞",
	 "∞", or "∞"), reducing the need for
	 idiosyncratic mappings of Unicode-to-my_escapes.

     +	 It is up to individual Pod formatter to display good
	 judgment when confronted with an unrenderable character

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	 (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.

     +	 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>".

     +	 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.

     +	 Some processors may find that the "S<...>" code is easi-
	 est 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

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	 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 sup-
	 porting 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".

     +	 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 for-
	 matter 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

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	 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.

     +	 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".

     +	 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 proces-
     sors should deal with it.

     +	 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 some-
	     times called the page -- is "perlfunc".  In
	     "L</CAVEATS>", the name is undef.)

	 Fourth:

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	     The section (AKA "item" in older perlpods), or undef
	     if none.  E.g., in "DESCRIPTION" in Getopt::Std,
	     "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 includ-
	 ing:

	 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" # orig. 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

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

     +	 In case of L<...> codes with no "text|" part in them,
	 older formatters have exhibited great variation in actu-
	 ally displaying the link or cross reference.  For exam-
	 ple, L<crontab(5)> would render as "the crontab(5) man-
	 page", or "in the crontab(5) manpage" or just "cron-
	 tab(5)".

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

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

     +	 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

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	 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 char-
	 acters 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>

     +	 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 sec-
	 tion can refer either to a "=headn Heading Content" com-
	 mand or to a "=item Item Content" command.  This specif-
	 ication does not specify what behavior should be in the
	 case of a given document having several things all seem-
	 ing 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.

     +	 Authors wanting to link to a particular (absolute) URL,
	 must do so only with "L<scheme:...>" codes (like
	 L<http://www.perl.org>), and must not attempt "L<Some
	 Site Name|scheme:...>" codes.	This restriction avoids
	 many problems in parsing and rendering L<...> codes.

     +	 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 -- no other formatting codes.
	 That is, authors should not use ""L<B<Foo::Bar>>"".

	 Note, however, that formatting codes and Z<>'s can occur

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	 in any and all parts of an L<...> (i.e., in name, sec-
	 tion, text, and url).

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

     +	 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.

     +	 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>" in 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.

     +	 Previous versions of perlpod allowed for a "L<section>"
	 syntax (as in ""L<Object Attributes>""), which was not
	 easily distinguishable from "L<name>" syntax.	This syn-
	 tax is no longer in the specification, and has been
	 replaced by the "L<"section">" syntax (where the quotes
	 were formerly optional).  Pod parsers should tolerate
	 the "L<section>" syntax, for a while at least.	 The sug-
	 gested heuristic for distinguishing "L<section>" from
	 "L<name>" is that if it contains any whitespace, it's a
	 section.  Pod processors may 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".)

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     +	 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 com-
	 pletely 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/".

     +	 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>.

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

	 +   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.

	 +   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.)

	 +   An "=over" ... "=back" region containing only "=item
	     [text]" commands, each one (or each group of them)
	     followed by some number of ordinary/verbatim para-
	     graphs, other nested "=over" ... "=back" regions, or

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	     "=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/".

	 +   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.

     +	 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.

     +	 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

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

	   =back

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

     +	 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.

     +	 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.

     +	 Authors of Pod formatters should note that this con-
	 struct:

	   =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 format-
	 ting 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

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	   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 forseeable future), Pod does not provide
	 any way for Pod authors to distinguish which grouping is
	 meant by the above "=item"-cluster structure.	So for-
	 matters 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 spac-
	 ing 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 con-
	 trary to the author's intent.

About Data Paragraphs and "=begin/=end" Regions
     Data paragraphs are typically used for inlining non-Pod data

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     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 format-
     ting 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 para-
     graphs).

     As a further example: At time of writing, no "biblio" iden-
     tifier 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 ordi-
     nary paragraphs).	The fact that "biblio" paragraphs were
     meant for ordinary processing would be indicated by prefac-
     ing each "biblio" identifier with a colon:

       =begin :biblio

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

       =end :biblio

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     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" identif-
     ier).  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 spe-
     cial 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:

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       =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!

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       =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 (tar-
     geted 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!

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       =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 con-
     taining 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

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     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" com-
     mand 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.  Pro-
     cessors 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" -- i.e.,
     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:

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       =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, pod-
     checker

AUTHOR
     Sean M. Burke

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