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PERLPODSPEC(1)			 Perl Programmers Reference Guide		   PERLPODSPEC(1)

NAME
       perlpodspec - Plain Old Documentation: format specification and notes

DESCRIPTION
       This document is detailed notes on the Pod markup language.  Most people will only have to
       read perlpod to know how to write in Pod, but this document may answer some incidental
       questions to do with parsing and rendering Pod.

       In this document, "must" / "must not", "should" / "should not", and "may" have their con-
       ventional (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 under-
       stand it to mean any of CR (ASCII 13), LF (ASCII 10), or a CRLF (ASCII 13 followed immedi-
       ately 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 con-
       taining 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 for-
       matting code that denotes a whitespace character.)

       A Pod parser is a module meant for parsing Pod (regardless of whether this involves call-
       ing 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:

       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 appropri-
	   ately, 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 identi-
	   fier" 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 identi-
	   fier", ... "=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 paragraph is not part of Pod at all (i.e., effectively it's
	   "out-of-band"), since it's not subject to most kinds of Pod parsing; but it is speci-
	   fied 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 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.
	     sub foo { # This is the second.

	   It is an error to try to start a Pod black with a "=cut" command.  In that case, the
	   Pod processor must halt parsing of the input file, and must by default emit a warning.

       "=over"
	   This command indicates that this is the start of a list/indent region.  If there is
	   any text following the "=over", it must consist of only a nonzero positive numeral.
	   The semantics of this numeral is explained in the "About =over...=back Regions" sec-
	   tion, further below.  Formatting codes are not expanded.  Examples:

	     =over 3

	     =over 3.5

	     =over

       "=item"
	   This command indicates that an item in a list begins here.  Formatting codes are pro-
	   cessed.  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 formatname") as being
	   for some special kind of processing.  Unless "formatname" begins with a colon, the
	   contained non-command paragraphs are data paragraphs.  But if "formatname" does begin
	   with a colon, then non-command paragraphs are ordinary paragraphs or data paragraphs.
	   This is discussed in detail in the section "About Data Paragraphs and "=begin/=end"
	   Regions".

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

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

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

		=begin formatname

		text...

		=end formatname

	   That is, it creates a region consisting of a single paragraph; that paragraph is to be
	   treated as a normal paragraph if "formatname" begins with a ":"; if "formatname"
	   doesn't begin with a colon, then "text..." will constitute a data paragraph.  There is
	   no way to use "=for formatname text..." to express "text..." as a verbatim paragraph.

       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:

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

       o   A formatting code starts with a capital letter (just US-ASCII [A-Z]) followed by two
	   or more "<"'s, one or more whitespace characters, any number of characters, one or
	   more whitespace characters, and ending with the first matching sequence of two or more
	   ">"'s, where the number of ">"'s equals the number of "<"'s in the opening of this
	   formatting code.  Examples:

	       That's what I<< you >> think!

	       C<<< open(X, ">>thing.dat") || die $! >>>

	       B<< $foo->bar(); >>

	   With this syntax, the whitespace character(s) after the "C<<<" and before the ">>" (or
	   whatever letter) are not renderable -- they do not signify whitespace, are merely part
	   of the formatting codes themselves.	That is, these are all synonymous:

	       C<thing>
	       C<< thing >>
	       C<<	     thing     >>
	       C<<<   thing >>>
	       C<<<<
	       thing
			  >>>>

	   and so on.

       In parsing Pod, a notably tricky part is the correct parsing of (potentially nested!) for-
       matting 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 con-
	   tent.  Other formatters will render it with invisible codes that can be used in build-
	   ing an index of the current document.

       "Z<>" -- a null (zero-effect) formatting code
	   Discussed briefly in "Formatting Codes" in perlpod.

	   This code is unusual is that it should have no content.  That is, a processor may com-
	   plain 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 nonbreaking
	   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 nonbreaking 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<lt>bar>

       instead of as equivalent to a "C" formatting code containing only "$foo-", and then a
       "bar>" outside the "C" formatting code.	This problem has since been solved by the addi-
       tion 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 para-
       graph 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 for-
       matting 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.

       o   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 contain-
	   ing a highbit sequence that is clearly not valid as UTF-8.  A line consisting of sim-
	   ply "#", 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 pro-
	   cessors 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 expecta-
	   tion 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 not-
	   ing 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 understood (i.e., not
	   verbatim paragraphs, but including ordinary paragraphs, and command paragraphs that
	   produce renderable text, like "=head1"), literal whitespace should generally be con-
	   sidered "insignificant", in that one literal space has the same meaning as any
	   (nonzero) number of literal spaces, literal newlines, and literal tabs (as long as
	   this produces no blank lines, since those would terminate the paragraph).  Pod parsers
	   should compact literal whitespace in each processed paragraph, but may provide an
	   option for overriding this (since some processing tasks do not require it), or may
	   follow additional special rules (for example, specially treating period-space-space or
	   period-newline sequences).

       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 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.

       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 encour-
	   aged to generally translate "-" to nonbreaking 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::-" new-
	   line "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 pro-
	   cessed, 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;

		   print Foo->VERSION

	   should be unified into one paragraph ("\tuse Foo;\n\n\tprint Foo->VERSION") before
	   being passed to the formatter or other processor.  Parsers may also allow an option
	   for overriding this.

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

       o   Pod formatters, where feasible, are advised to avoid splitting short verbatim para-
	   graphs (under twelve lines, say) across pages.

       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 new-
	   lines 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 declara-
	   tions in the most recent XHTML specification at "www.W3.org".  Pod parsers must under-
	   stand at least the entities that define characters in the range 160-255 (Latin-1).
	   Pod parsers, when faced with some unknown "E<identifier>" code, shouldn't simply
	   replace it with nullstring (by default, at least), but may pass it through as a string
	   consisting of the literal characters E, less-than, identifier, greater-than.  Or Pod
	   parsers may offer the alternative option of processing such unknown "E<identifier>"
	   codes by firing an event especially for such codes, or by adding a special node-type
	   to the in-memory document tree.  Such "E<identifier>" may have special meaning to some
	   processors, or some processors may choose to add them to a special error report.

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

       o   Note that in all cases of "E<whatever>", whatever (whether an htmlname, or a number in
	   any base) must consist only of alphanumeric characters -- that is, whatever must watch
	   "m/\A\w+\z/".  So "E< 0 1 2 3 >" is invalid, because it contains spaces, which aren't
	   alphanumeric characters.  This presumably does not need special treatment by a Pod
	   processor; " 0 1 2 3 " doesn't look like a number in any base, so it would presumably
	   be looked up in the table of HTML-like names.  Since there isn't (and cannot be) an
	   HTML-like entity called " 0 1 2 3 ", this will be treated as an error.  However, Pod
	   processors may treat "E< 0 1 2 3 >" or "E<e-acute>" as syntactically invalid, poten-
	   tially 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 cur-
	   rent/native character set".	It always means only "the character represented by code-
	   point 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 code-
	   point 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 con-
	   sider 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:

	     <!ENTITY infin    "&#8734;"> <!-- infinity, U+221E ISOtech -->

	   While the mapping "infin" to the character "\x{221E}" will (hopefully) have been
	   already handled by the Pod parser, the presence of the character in this file means
	   that it's reasonably important enough to include in a formatter's table that maps from
	   notable Unicode characters to the codes necessary for rendering them.  So for a Uni-
	   code-to-*roff mapping, for example, this would merit the entry:

	     "\x{221E}" => '\(in',

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

       o   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 corresponding unac-
	   cented 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 $Cur-
	   rency 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 nonbreaking spaces as an individ-
	   ual character (which I'll call "NBSP"), and others output to formats that implement
	   nonbreaking 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 con-
	   tain "S<foo I<bar> baz>" codes, where "mere spaces" (character 32) in such codes are
	   taken to represent nonbreaking 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
	   text.  (This distinction may or may not be evident in the particular tree/event model
	   implemented by the Pod parser.)  For example, consider this unusual case:

	      S<L</Autoloaded Functions>>

	   This means that the space in the middle of the visible link text must not be broken
	   across lines.  In other words, it's the same as this:

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

	   However, a misapplied space-to-NBSP replacement could (wrongly) produce something
	   equivalent to this:

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

	   ...which is almost definitely not going to work as a hyperlink (assuming this format-
	   ter 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 for-
	   matter 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 format-
	   ters 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" com-
	   mand), 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.

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

       o   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 Func-
	       tions|perlfunc>", the link-text is "Perl Functions".  In "L<Time::HiRes>" and even
	       "L<|Time::HiRes>", there is no link text.  Note that link text may contain format-
	       ting.)

	   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 "DESCRIP-
	       TION" in Getopt::Std, "DESCRIPTION" is the section.  (Note that this is not the
	       same as a manpage section like the "5" in "man 5 crontab".  "Section Foo" in the
	       Pod sense means the part of the text that's introduced by the heading or item
	       whose text is "Foo".)

	   Pod parsers may also note additional attributes 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.

       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

	   then a link to it would look like this:

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

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

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

	     ...

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

       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 sev-
	   eral 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 eas-
	   ily 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 render-
	   ing L<...> codes.

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

	     L<B<ummE<234>stuff>|...>

	   For "L<...>" codes without a "name|" part, only "E<...>" and "Z<>" codes may occur --
	   no other formatting codes.  That is, authors should not use ""L<B<Foo::Bar>>"".

	   Note, however, that formatting codes and Z<>'s can occur in any and all parts of an
	   L<...> (i.e., in name, section, text, and url).

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

       o   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.

       o   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 pres-
	   ence 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 distinc-
	   tion is of no importance to many Pod processors, but some processors that render to
	   hypertext formats may need to distinguish them in order to know how to render a given
	   "L<foo>" code.

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

About =over...=back Regions
       "=over"..."=back" regions are used for various kinds of list-like structures.  (I use the
       term "region" here simply as a collective term for everything from the "=over" to the
       matching "=back".)

       o   The non-zero numeric indentlevel in "=over indentlevel" ...	"=back" is used for giv-
	   ing 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 docu-
	   ment's base font.  Other formatters may have to completely ignore the number.  The
	   lack of any explicit indentlevel parameter is equivalent to an indentlevel value of 4.
	   Pod processors may complain if indentlevel is present but is not a positive number
	   matching "m/\A(\d*\.)?\d+\z/".

       o   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>.

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

	   o   An "=over" ... "=back" region containing only "=item *" commands, each followed by
	       some number of ordinary/verbatim paragraphs, other nested "=over" ... "=back"
	       regions, "=for..." paragraphs, and "=begin"..."=end" regions.

	       (Pod processors must tolerate a bare "=item" as if it were "=item *".)  Whether
	       "*" is rendered as a literal asterisk, an "o", or as some kind of real bullet
	       character, is left up to the Pod formatter, and may depend on the level of nest-
	       ing.

	   o   An "=over" ... "=back" region containing only "m/\A=item\s+\d+\.?\s*\z/" para-
	       graphs, each one (or each group of them) followed by some number of ordinary/ver-
	       batim paragraphs, other nested "=over" ... "=back" regions, "=for..." paragraphs,
	       and/or "=begin"..."=end" codes.	Note that the numbers must start at 1 in each
	       section, and must proceed in order and without skipping numbers.

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

	   o   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/".

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

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

       o   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.

       o   Pod processors should tolerate "=item *" / "=item number" commands with no accompany-
	   ing 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

       o   No "=over" ... "=back" region can contain headings.	Processors may treat such a head-
	   ing as an error.

       o   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.

       o   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.

       o   Authors of Pod formatters should note that this construct:

	     =item Neque

	     =item Porro

	     =item Quisquam Est

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

	     =item Ut Enim

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

	     Neque

	     Porro

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

	     Ut Enim

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

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

	     Ut Enim

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

	     Neque

	     Porro

	     Quisquam Est

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

	     Ut Enim

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

About Data Paragraphs and ";=begin/=end" Regions
       Data paragraphs are typically used for inlining non-Pod data that is to be used (typically
       passed through) when rendering the document to a specific format:

	 =begin rtf

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

	 =end rtf

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

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

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

       Another example of a data paragraph:

	 =begin html

	 I like <em>PIE</em>!

	 <hr>Especially pecan pie!

	 =end html

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

       As a further example: At time of writing, no "biblio" identifier is supported, but suppose
       some processor were written to recognize it as a way of (say) denoting a bibliographic
       reference (necessarily containing formatting codes in ordinary paragraphs).  The fact that
       "biblio" paragraphs were meant for ordinary processing would be indicated by prefacing
       each "biblio" identifier with a colon:

	 =begin :biblio

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

	 =end :biblio

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

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

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

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

	 =begin :biblio

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

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

	 =over

	 =item

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

	 =item

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

	 =back

	 =end :biblio

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

	 =begin somedata

	 This is a data paragraph.

	 =head1 Don't do this!

	 This is a data paragraph too.

	 =end somedata

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

	 =begin somedata

	 This is a data paragraph.

	 =cut

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

	 =pod

	 This is a data paragraph too.

	 =end somedata

       And this too is valid:

	 =begin someformat

	 This is a data paragraph.

	   And this is a data paragraph.

	 =begin someotherformat

	 This is a data paragraph too.

	   And this is a data paragraph too.

	 =begin :yetanotherformat

	 =head2 This is a command paragraph!

	 This is an ordinary paragraph!

	   And this is a verbatim paragraph!

	 =end :yetanotherformat

	 =end someotherformat

	 Another data paragraph!

	 =end someformat

       The contents of the above "=begin :yetanotherformat" ...  "=end :yetanotherformat" region
       aren't data paragraphs, because the immediately containing region's identifier (":yetan-
       otherformat") begins with a colon.  In practice, most regions that contain data paragraphs
       will contain only data paragraphs; however, the above nesting is syntactically valid as
       Pod, even if it is rare.  However, the handlers for some formats, like "html", will accept
       only data paragraphs, not nested regions; and they may complain if they see (targeted for
       them) nested regions, or commands, other than "=end", "=pod", and "=cut".

       Also consider this valid structure:

	 =begin :biblio

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

	 =over

	 =item

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

	 =item

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

	 =back

	 Buy buy buy!

	 =begin html

	 <img src='wirth_spokesmodeling_book.png'>

	 <hr>

	 =end html

	 Now now now!

	 =end :biblio

       There, the "=begin html"..."=end html" region is nested inside the larger "=begin :bib-
       lio"..."=end :biblio" region.  Note that the content of the "=begin html"..."=end html"
       region is data paragraph(s), because the immediately containing region's identifier
       ("html") doesn't begin with a colon.

       Pod parsers, when processing a series of data paragraphs one after another (within a sin-
       gle region), should consider them to be one large data paragraph that happens to contain
       blank lines.  So the content of the above "=begin html"..."=end html" may be stored as two
       data paragraphs (one consisting of "<img src='wirth_spokesmodeling_book.png'>\n" and
       another consisting of "<hr>\n"), but should be stored as a single data paragraph (consist-
       ing of "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").

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

	 =for html

	 =begin html

	 =end html

	 =begin :biblio

	 =end :biblio

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

	 =begin stuff

	 =shazbot

	 =end stuff

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

	 =for stuff =shazbot

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

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

	 =begin outer

	 X

	 =begin inner

	 Y

	 =end inner

	 Z

	 =end outer

       while this is invalid:

	 =begin outer

	 X

	 =begin inner

	 Y

	 =end outer

	 Z

	 =end inner

       This latter is improper because when the "=end outer" command is seen, the currently open
       region has the formatname "inner", not "outer".	(It just happens that "outer" is the for-
       mat name of a higher-up region.)  This is an error.  Processors must by default report
       this as an error, and may halt processing the document containing that error.  A corollary
       of this is that regions cannot "overlap" -- i.e., the latter block above does not repre-
       sent 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 v5.8.0				    2003-02-18				   PERLPODSPEC(1)
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