perlpodspec


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

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

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

             =head1 NOTES

             =item *

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

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

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

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

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

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

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

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

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

       o   A data paragraph.  This is a paragraph that is inside a "=begin
           identifier" ... "=end identifier" sequence where "identifier" does
           not begin with a literal colon (":").  In some sense, a data

         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"
           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!

       "=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.

             =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:?[XaXzAXZ0X9_]+\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

                =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:

           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.
           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 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>

       at line 123: 'Time objects are not...'").  So these two paragraphs:

         I<I told you not to do this!

         Don't make me say it again!>

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

         I<I told you not to do this!>

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

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

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

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

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

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

       o   Since Perl recognizes a Unicode Byte Order Mark at the start of
           files as signaling that the file is Unicode encoded as in UTF-16
           (whether big-endian or little-endian) or UTF-8, Pod parsers should
           do the same.  Otherwise, the character encoding should be
           understood as being UTF-8 if the first highbit byte sequence in the
           file seems valid as a UTF-8 sequence, or otherwise as 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 parsing: whatever the encoding
           declared by a particular Pod file, content is to be stored in
           memory as Unicode characters.

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

           not valid as UTF-8.  A line consisting of simply "#", an e-acute,
           and any non-highbit byte, is sufficient to establish this file's
           encoding.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                   use Foo;

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

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

       o   Characters in Pod documents may be conveyed either as literals, or
           by number in E<n> codes, or by an equivalent mnemonic, as in
           E<eacute> which is exactly equivalent to E<233>.

           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 characters (also
           defined there by Unicode, with the same meaning).  Characters above
           255 should be understood to refer to Unicode characters.

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

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

       o   Pod parsers should understand all "E<html>" codes as defined in the
           entity declarations in the most recent XHTML specification at
           "www.W3.org".  Pod parsers must understand at least the entities
           that define characters in the range 160-255 (Latin-1).  Pod
           parsers, when faced with some unknown "E<identifier>" code,
           shouldn't simply replace it with nullstring (by default, at least),
           but may pass it through as a string consisting of the literal
           characters E, less-than, identifier, greater-than.  Or Pod parsers
           may offer the alternative option of processing such unknown
           "E<identifier>" codes by firing an event especially for such codes,
           or by adding a special node-type to the in-memory document tree.
           Such "E<identifier>" may have special meaning to some processors,
           or some processors may choose to add them to a special error
           look like a number in any base, so it would presumably be looked up
           in the table of HTML-like names.  Since there isn't (and cannot be)
           an HTML-like entity called " 0 1 2 3 ", this will be treated as an
           error.  However, Pod processors may treat "E< 0 1 2 3 >" or
           "E<e-acute>" as syntactically invalid, potentially earning a
           different error message than the error message (or warning, or
           event) generated by a merely unknown (but theoretically valid)
           htmlname, as in "E<qacute>" [sic].  However, Pod parsers are not
           required to make this distinction.

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

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

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

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

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

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

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

           For example, this Pod text:

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

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

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

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

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

       o   Some processors may find that the "S<...>" code is easiest to
           implement by replacing each space in the parse tree under the
           content of the S, with an NBSP.  But note: the replacement should
           apply not to spaces in all text, but only to spaces in printable
           However, a misapplied space-to-NBSP replacement could (wrongly)
           produce something equivalent to this:

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

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

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

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

           For example:

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

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

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

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

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

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

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

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

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

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

             =head2 About the C<-M> Operator

           or with:

             =item About the C<-M> Operator
             ...

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

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

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

       o   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.

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

             L<B
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