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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:

       •   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.

       •   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).

       •   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
           (":").

       •   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:

       •   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>

       •   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.

       •   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.

       •   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 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.

       •   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);

       •   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.

       •   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.)

       •   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).

       •   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.

       •   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.

       •   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).

       •   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.

       •   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 characters in a word.)

       •   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 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...'").

       •   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.

       •   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 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.)

       •   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.

       •   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.

       •   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 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 character 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 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.

       •   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.

       •   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.

       •   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.

       •   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 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".

       •   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.

       •   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
       processors 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 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.

       •   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>

       •   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>

       •   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.

       •   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.

       •   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>" 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.

       •   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".)

       •   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/".

       •   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 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/".

           •   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

             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 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