perlpodspec



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 peo-
       ple 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, ter-
       minated 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 charac-
       ters 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, termi-
       nated by a newline.)

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

       A Pod parser is a module meant for parsing Pod (regardless of whether
       this involves calling callbacks or building a parse tree or directly
       formatting it).  A Pod formatter (or Pod translator) is a module or
       program that converts Pod to some other format (HTML, plaintext, TeX,
       PostScript, RTF).  A Pod processor might be a formatter or translator,
       or might be a program that does something else with the Pod (like
       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 paragraph consists
       of non-blank lines of text, separated by one or more blank lines.

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

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

             =head1 NOTES

             =item *

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

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

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

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

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

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

       ·   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 "identi-
           fier" 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 pro-
           cessing, tabs are probably expanded).

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

       ·   A data paragraph.  This is a paragraph that is inside a "=begin
           identifier" ... "=end identifier" sequence where "identifier" does
           not begin with a literal colon (":").  In some sense, a data para-
           graph 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 pars-
           ing; but it is specified here, since Pod parsers need to be able to
           call an event for it, or store it in some form in a parse tree, or
           at least just parse around it.

       For example: consider the following paragraphs:

         # <- that’s the 0th column

         =head1 Foo

         Stuff

           $foo->bar

         =cut

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

       The "=begin identifier" ... "=end identifier" commands stop paragraphs
       that they surround from being parsed as 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 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 para-
           graph is a heading.  That text may contain formatting codes.  Exam-
           ples:

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

           It is advised that formatnames match the regexp
           "m/\A:?[-a-zA-Z0-9_]+\z/".  Implementors should anticipate future
           expansion in the semantics and syntax of the first parameter to
           "=begin"/"=end"/"=for".

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

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

                =begin formatname

                text...

                =end formatname

           That is, it creates a region consisting of a single paragraph; that
           paragraph is to be treated as a normal paragraph if "formatname"
           begins with a ":"; if "formatname" doesnt 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 para-
           graph.

       "=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 cannot decode the declared encoding, it should emit a warn-
           ing and may abort parsing the document altogether.

           A document having more than one "=encoding" line should be consid-
           ered 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 con-
           tradictory "=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 com-
       mands, and to stipulate, for each additional command, whether format-
       ting 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, format-
       ting codes were referred to as "interior sequences", and this term may
       still be found in the documentation for Pod parsers, and in error mes-
       sages from Pod processors.)

       There are two syntaxes for formatting codes:

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

               That’s what I<you> think!

               What’s C<dump()> for?

               X<C

               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 (poten-
       tially nested!) formatting codes.  Implementors should consult the code
       in the "parse_text" routine in Pod::Parser as an example of a correct
       implementation.

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

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

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

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

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

           This code is unusual in that most formatters completely discard
           this code and its content.  Other formatters will render it with
           invisible codes that can be used in building an index of the cur-
           rent 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 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 dis-
           cussed 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 lit-
           eral "│" 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 com-
           plex.  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 par-
       ticular applications to add to the above list of known formatting
       codes; a Pod parser might even allow a way to stipulate, for each addi-
       tional 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<lt>bar>

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

           C<< $foo->bar >>

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

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

         I<I told you not to do this!

         Don’t make me say it again!>

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

         I<I told you not to do this!>

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

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


Notes on Implementing Pod Processors

       The following is a long section of miscellaneous requirements and sug-
       gestions to do with Pod processing.

       ·   Pod formatters should tolerate lines in verbatim blocks that are of
           any length, even if that means having to break them (possibly sev-
           eral 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 char-
           acters long, which are usually not intentional.

       ·   Pod parsers must recognize all of the three well-known newline for-
           mats: 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 under-
           stood 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 mem-
           ory as Unicode characters.

       ·   The well known Unicode Byte Order Marks are as follows:  if the
           file begins with the two literal byte values 0xFE 0xFF, this is the
           BOM for big-endian UTF-16.  If the file begins with the two literal
           byte value 0xFF 0xFE, this is the BOM for little-endian UTF-16.  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-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]" paragraph.  (The parser may conflate these
           two constructs, or may leave them distinct, in the expectation that
           the formatter will nevertheless treat them the same.)

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

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

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

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

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

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

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

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

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

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

       ·   Pod parsers should not, by default, try to coerce apostrophe (’)
           and quote (") into smart quotes (little 9’s, 66’s, 99’s, etc), nor
           try to turn backtick (‘) into anything else but a single backtick
           character (distinct from an 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 gen-
           erally translate "-" to non-breaking hyphen, but may apply heuris-
           tics to convert some of these to breaking hyphens.

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

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

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

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

       ·   Pod parsers, when processing a series of verbatim paragraphs one
           after another, should consider them to be one large verbatim para-
           graph 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 pro-
           cessor.  Parsers may also allow an option for overriding this.

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

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

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

       ·   Authors of Pod formatters/processors should make every effort to
           avoid writing their own Pod parser.  There are already several in
           CPAN, with a wide range of interface styles -- and one of them,
           Pod::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 characters (also
           defined there by Unicode, with the same meaning).  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 char-
           acters E, less-than, identifier, greater-than.  Or Pod parsers may
           offer the alternative option of processing such unknown "E<identi-
           fier>" 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 charac-
           ter 34 (doublequote, "), "E<amp>" for character 38 (ampersand, &),
           and "E<apos>" for character 39 (apostrophe, ’).

       ·   Note that in all cases of "E<whatever>", whatever (whether an html-
           name, or a number in any base) must consist only of alphanumeric
           characters -- that is, whatever 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 num-
           ber 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.  How-
           ever, Pod processors may treat "E< 0 1 2 3 >" or "E<e-acute>" as
           syntactically invalid, potentially earning a different error mes-
           sage than the error message (or warning, or event) generated by a
           merely unknown (but theoretically valid) htmlname, as in
           "E<qacute>" [sic].  However, Pod parsers are not required to make
           this distinction.

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

           This will likely require many formatters to have tables mapping
           from treatable Unicode codepoints (such as the "\xE9" for the
           e-acute character) to the escape sequences or codes necessary for
           conveying such sequences in the target output format.  A converter
           to *roff would, for example know that "\xE9" (whether conveyed lit-
           erally, 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 avail-
           able for common output formats.  (Such mappings may be incomplete!
           Implementers are not expected to bend over backwards in an attempt
           to render Cherokee syllabics, Etruscan runes, Byzantine musical
           symbols, or any of the other weird things that Unicode can encode.)
           And if a Pod document uses a character not found in such a mapping,
           the formatter should consider it an unrenderable character.

       ·   If, surprisingly, the implementor of a Pod formatter can’t find a
           satisfactory pre-existing table mapping from Unicode characters to
           escapes in the target format (e.g., a decent table of Unicode char-
           acters to *roff escapes), it will be necessary to build such a ta-
           ble.  If you are in this circumstance, you should begin with the
           characters in the range 0x00A0 - 0x00FF, which is mostly the heav-
           ily 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 charac-
           ters in the range 0x2000-0x204D (consult the character tables at
           www.unicode.org), and whatever else strikes your fancy.  For exam-
           ple, 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 (hope-
           fully) have been already handled by the Pod parser, the presence of
           the character in this file means that it’s reasonably important
           enough to include in a formatter’s table that maps from notable
           Unicode characters to the codes necessary for rendering them.  So
           for a Unicode-to-*roff mapping, for example, this would merit the
           entry:

             "\x{221E}" => ’\(in’,

           It is eagerly hoped that in the future, increasing numbers of for-
           mats (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 (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 corre-
           sponding 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│Mil-
           lion::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 rep-
           resent non-breaking spaces.  Pod parsers should consider supporting
           the optional parsing of "S<foo I<bar> baz>" as if it were "fooNB-
           SPI<bar>NBSPbaz", and, going the other way, the optional parsing of
           groups of words joined by NBSP’s as if each group were in a S<...>
           code, so that formatters may use the representation that maps best
           to what the output format demands.

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

              S<L[linebreak]action"
           or "manu-[linebreak]script" (and if it doesn’t hyphenate it, then
           the "E<shy>" doesn’t show up at all).  And if it is to hyphenate
           "Jarkko" and/or "Hietaniemi", it can do so only at the points where
           there is a "E<shy>" code.

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

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

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


About L<...> Codes

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

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

           First:
               The link-text.  If there is none, this must be undef.  (E.g.,
               in "L<Perl Functions│perlfunc>", the link-text is "Perl Func-
               tions".  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 Func-
               tions│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 "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 head-
               ing or item whose text is "Foo".)

           Pod parsers may also note additional attributes including:

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

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

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

           For example:

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

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

             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 format-
           ters have exhibited great variation in actually displaying the link
           or cross reference.  For example, L<crontab(5)> would render as
           "the crontab(5) manpage", or "in the crontab(5) manpage" or just
           "crontab(5)".

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

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

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

             =head2 About the C<-M> Operator

           or with:

             =item About the C<-M> Operator

           then a link to it would look like this:

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

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

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

             ...

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

       ·   Previous versions of perlpod distinguished "L<name/"section">"
           links from "L<name/item>" links (and their targets).  These have
           been merged syntactically and semantically in the current specifi-
           cation, 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 sec-
           tion 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" sec-
           tion 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 for-
           matting or for E<...> escapes, as in:

             L<Bname, section, text, and url).

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

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

           In other words, this is valid:

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

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

       ·   At time of writing, "L<name>" values are of two types: either the
           name of a Pod page like "L<Foo::Bar>" (which might be a real Perl
           module or program in an @INC / PATH directory, or a .pod file in
           those places); or the name of a UNIX man page, like
           "L<crontab(5)>".  In theory, "L<chmod>" 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 dis-
           tinction is of no importance to many Pod processors, but some pro-
           cessors 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 syntax is no longer in the specifica-
           tion, 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 suggested heuristic
           for distinguishing "L<section>" from "L<name>" is that if it con-
           tains 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".)

       ·   The non-zero numeric indentlevel in "=over indentlevel" ...
           "=back" is used for giving the formatter a clue as to how many
           "spaces" (ems, or roughly equivalent units) it should tab over,
           although many formatters will have to convert this to an absolute
           measurement that may not exactly match with the size of spaces (or
           M’s) in the document’s base font.  Other formatters may have to
           completely ignore the number.  The lack of any explicit indentlevel
           parameter is equivalent to an indentlevel value of 4.  Pod proces-
           sors 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 <block-
           quote>...</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 *" com-
               mands, each followed by some number of ordinary/verbatim para-
               graphs, 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 para-
               graphs, other nested "=over" ... "=back" regions, "=for..."
               paragraphs, and/or "=begin"..."=end" codes.  Note that the num-
               bers must start at 1 in each section, and must proceed in order
               and without skipping numbers.

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

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

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

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

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

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

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

           But they may be arbitrarily long:

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

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

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

             =over

             =item 1

             Pick up dry cleaning.

             =item 2

             =item 3

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

             =back

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

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

             =over

             =back

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

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

       ·   Authors of Pod formatters should note that this construct:

             =item Neque

             =item Porro

             =item Quisquam Est

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

             =item Ut Enim

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

             Neque

             Porro

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

             Ut Enim

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

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

             Ut Enim

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

             Neque

             Porro

             Quisquam Est

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

             Ut Enim

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


About Data Paragraphs and "=begin/=end" Regions

       Data paragraphs are typically used for inlining non-Pod data that is to
       be used (typically passed through) when rendering the document to a
       specific format:

         =begin rtf

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

         =end rtf

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

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

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

       Another example of a data paragraph:

         =begin html

         I like <em>PIE</em>!

         <hr>Especially pecan pie!

         =end html

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

       As a further example: At time of writing, no "biblio" identifier is
       supported, but suppose some processor were written to recognize it as a
       way of (say) denoting a bibliographic reference (necessarily containing
       formatting codes in ordinary paragraphs).  The fact that "biblio" para-
       graphs 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

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

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

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

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

         =begin :biblio

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

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

         =over

         =item

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

         =item

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

         =back

         =end :biblio

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

         =begin somedata

         This is a data paragraph.

         =head1 Don’t do this!

         This is a data paragraph too.

         =end somedata

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

         =begin somedata

         This is a data paragraph.

         =cut

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

         =pod

         This is a data paragraph too.

         =end somedata

       And this too is valid:

         =begin someformat

         This is a data paragraph.

           And this is a data paragraph.

         =begin someotherformat

         This is a data paragraph too.

           And this is a data paragraph too.

         =begin :yetanotherformat

         =head2 This is a command paragraph!

         This is an ordinary paragraph!

           And this is a verbatim paragraph!

         =end :yetanotherformat

         =end someotherformat

         Another data paragraph!

         =end someformat

       The contents of the above "=begin :yetanotherformat" ...  "=end :yetan-
       otherformat" region arent 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 syntacti-
       cally valid as Pod, even if it is rare.  However, the handlers for some
       formats, like "html", will accept only data paragraphs, not nested
       regions; and they may complain if they see (targeted for them) nested
       regions, or commands, other than "=end", "=pod", and "=cut".

       Also consider this valid structure:

         =begin :biblio

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

         =over

         =item

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

         =item

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

         =back

         Buy buy buy!

         =begin html

         <img src=’wirth_spokesmodeling_book.png’>

         <hr>

         =end html

         Now now now!

         =end :biblio

       There, the "=begin html"..."=end html" region is nested inside the
       larger "=begin :biblio"..."=end :biblio" region.  Note that the content
       of the "=begin html"..."=end html" region is data paragraph(s), because
       the immediately containing region’s identifier ("html") doesnt 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 para-
       graphs (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_spokesmodel-
       ing_book.png’>\n\n<hr>\n").

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

         =for html

         =begin html

         =end html

         =begin :biblio

         =end :biblio

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

         =begin stuff

         =shazbot

         =end stuff

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

         =for stuff =shazbot

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

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

         =begin outer

         X

         =begin inner

         Y

         =end inner

         Z

         =end outer

       while this is invalid:

         =begin outer

         X

         =begin inner

         Y

         =end outer

         Z

         =end inner

       This latter is improper because when the "=end outer" command is seen,
       the currently open region has the formatname "inner", not "outer".  (It
       just happens that "outer" is the format name of a higher-up region.)
       This is an error.  Processors must by default report this as an error,
       and may halt processing the document containing that error.  A corol-
       lary 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:

         =begin thing

         =end hting

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

       This is also invalid:

         =begin thing

         =end

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


SEE ALSO

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


AUTHOR

       Sean M. Burke



perl v5.8.6                       2004-11-05                    PERLPODSPEC(1)

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