MirOS Manual: perlpodspec(1)


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    "&#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 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 "&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
         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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