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.
A line in a file consists of zero or more non-newline characters, terminated by either a newline or the end of the file.
A newline sequence is usually a platform-dependent concept, but Pod parsers should understand it to mean any of CR (ASCII 13), LF (ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in addition to any other system-specific meaning. The first CR/CRLF/LF sequence in the file may be used as the basis for identifying the newline sequence for parsing the rest of the file.
A blank line is a line consisting entirely of zero or more spaces (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file. A non-blank line is a line containing one or more characters other than space or tab (and terminated by a newline or end-of-file).
(Note: Many older Pod parsers did not accept a line consisting of spaces/tabs and then a newline as a blank line. The only lines they considered blank were lines consisting of no characters at all, terminated by a newline.)
Whitespace is used in this document as a blanket term for spaces, tabs, and newline sequences. (By itself, this term usually refers to literal whitespace. That is, sequences of whitespace characters in Pod source, as opposed to ``E<32>'', which is a formatting code that denotes a whitespace character.)
A Pod parser is a module meant for parsing Pod (regardless of whether this involves calling callbacks or building a parse tree or directly formatting it). A Pod formatter (or Pod translator) is a module or program that converts Pod to some other format (HTML, plaintext, TeX, PostScript, RTF). A Pod processor might be a formatter or translator, or might be a program that does something else with the Pod (like counting words, scanning for index points, etc.).
Pod content is contained in Pod blocks. A Pod block starts with a line that matches "m/\A=[a-zA-Z]/", and continues up to the next line that matches "m/\A=cut/" or up to the end of the file if there is no "m/\A=cut/" line.
Note that a parser is not expected to distinguish between something that looks like pod, but is in a quoted string, such as a here document.
Within a Pod block, there are Pod paragraphs. A Pod paragraph consists of non-blank lines of text, separated by one or more blank lines.
For purposes of Pod processing, there are four types of paragraphs in a Pod block:
=head1 NOTES =item *
But they may span several (non-blank) lines:
=for comment Hm, I wonder what it would look like if you tried to write a BNF for Pod from this. =head3 Dr. Strangelove, or: How I Learned to Stop Worrying and Love the Bomb
Some command paragraphs allow formatting codes in their content (i.e., after the part that matches "m/\A=[a-zA-Z]\S*\s*/"), as in:
=head1 Did You Remember to C<use strict;>?
In other words, the Pod processing handler for ``head1'' will apply the same processing to ``Did You Remember to C<use strict;>?'' that it would to an ordinary paragraph (i.e., formatting codes like ``C<...>'') are parsed and presumably formatted appropriately, and whitespace in the form of literal spaces and/or tabs is not significant.
Whitespace is significant in verbatim paragraphs (although, in processing, tabs are probably expanded).
For example: consider the following paragraphs:
# <- that's the 0th column
=head1 Foo
Stuff
$foo->bar
=cut
Here, ``=head1 Foo'' and ``=cut'' are command paragraphs because the first line of each matches "m/\A=[a-zA-Z]/". "[space][space]$foo->bar`` is a verbatim paragraph, because its first line starts with a literal whitespace character (and there's no ''=begin``...''=end" region around).
The "=begin identifier`` ... ''=end identifier" commands stop paragraphs that they surround from being parsed as ordinary or verbatim paragraphs, if identifier doesn't begin with a colon. This is discussed in detail in the section ``About Data Paragraphs and ''=begin/=end`` Regions''.
=head1 Object Attributes =head3 What B<Not> to Do!
=pod This is a plain Pod paragraph. =pod This text is ignored.
=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 3 =over 3.5 =over
=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.
It is advised that formatnames match the regexp "m/\A:?[-a-zA-Z0-9_]+\z/". Everything following whitespace after the formatname is a parameter that may be used by the formatter when dealing with this region. This parameter must not be repeated in the ``=end'' paragraph. Implementors should anticipate future expansion in the semantics and syntax of the first parameter to ``=begin''/``=end''/``=for''.
=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.
A document having more than one ``=encoding'' line should be considered an error. Pod processors may silently tolerate this if the not-first ``=encoding'' lines are just duplicates of the first one (e.g., if there's a ``=encoding utf8'' line, and later on another ``=encoding utf8'' line). But Pod processors should complain if there are contradictory ``=encoding'' lines in the same document (e.g., if there is a ``=encoding utf8'' early in the document and ``=encoding big5'' later). Pod processors that recognize BOMs may also complain if they see an ``=encoding'' line that contradicts the BOM (e.g., if a document with a UTF-16LE BOM has an ``=encoding shiftjis'' line).
If a Pod processor sees any command other than the ones listed above (like ``=head'', or ``=haed1'', or ``=stuff'', or ``=cuttlefish'', or ``=w123''), that processor must by default treat this as an error. It must not process the paragraph beginning with that command, must by default warn of this as an error, and may abort the parse. A Pod parser may allow a way for particular applications to add to the above list of known commands, and to stipulate, for each additional command, whether formatting codes should be processed.
Future versions of this specification may add additional commands.
There are two syntaxes for formatting codes:
That's what I<you> think!
What's C<CORE::dump()> for?
X<C<chmod> and C<unlink()> Under Different Operating Systems>
That's what I<< you >> think!
C<<< open(X, ">>thing.dat") || die $! >>>
B<< $foo->bar(); >>
With this syntax, the whitespace character(s) after the ``C<<<'' and before the ``>>>'' (or whatever letter) are not renderable. They do not signify whitespace, are merely part of the formatting codes themselves. That is, these are all synonymous:
C<thing>
C<< thing >>
C<< thing >>
C<<< thing >>>
C<<<<
thing
>>>>
and so on.
Finally, the multiple-angle-bracket form does not alter the interpretation of nested formatting codes, meaning that the following four example lines are identical in meaning:
B<example: C<$a E<lt>=E<gt> $b>> B<example: C<< $a <=> $b >>> B<example: C<< $a E<lt>=E<gt> $b >>> B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
In parsing Pod, a notably tricky part is the correct parsing of (potentially nested!) formatting codes. Implementors should consult the code in the "parse_text" routine in Pod::Parser as an example of a correct implementation.
This code is unusual in that most formatters completely discard this code and its content. Other formatters will render it with invisible codes that can be used in building an index of the current document.
This code is unusual in that it should have no content. That is, a processor may complain if it sees "Z<potatoes>". Whether or not it complains, the potatoes text should ignored.
Consider:
C<$x ? $y : $z>
S<C<$x ? $y : $z>>
Both signify the monospace (c[ode] style) text consisting of ``$x'', one space, ``?'', one space, ``:'', one space, ``$z''. The difference is that in the latter, with the S code, those spaces are not ``normal'' spaces, but instead are non-breaking spaces.
If a Pod processor sees any formatting code other than the ones listed above (as in ``N<...>'', or ``Q<...>'', etc.), that processor must by default treat this as an error. A Pod parser may allow a way for particular applications to add to the above list of known formatting codes; a Pod parser might even allow a way to stipulate, for each additional command, whether it requires some form of special processing, as L<...> does.
Future versions of this specification may add additional formatting codes.
Historical note: A few older Pod processors would not see a ``>'' as closing a ``C<'' code, if the ``>'' was immediately preceded by a ``-''. This was so that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-E<gt>bar>
instead of as equivalent to a ``C'' formatting code containing only ``$foo-'', and then a ``bar>'' outside the ``C'' formatting code. This problem has since been solved by the addition of syntaxes like this:
C<< $foo->bar >>
Compliant parsers must not treat ``->'' as special.
Formatting codes absolutely cannot span paragraphs. If a code is opened in one paragraph, and no closing code is found by the end of that paragraph, the Pod parser must close that formatting code, and should complain (as in ``Unterminated I code in the paragraph starting at line 123: 'Time objects are not...'''). So these two paragraphs:
I<I told you not to do this! Don't make me say it again!>
...must not be parsed as two paragraphs in italics (with the I code starting in one paragraph and starting in another.) Instead, the first paragraph should generate a warning, but that aside, the above code must parse as if it were:
I<I told you not to do this!> Don't make me say it again!E<gt>
(In SGMLish jargon, all Pod commands are like block-level elements, whereas all Pod formatting codes are like inline-level elements.)
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.
my $utf8_bom = "\x{FEFF}";
utf8::encode($utf8_bom);
%% 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).
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.
When referring to characters by using a E<n> numeric code, numbers in the range 32-126 refer to those well known US-ASCII characters (also defined there by Unicode, with the same meaning), which all Pod formatters must render faithfully. Characters whose E<> numbers are in the ranges 0-31 and 127-159 should not be used (neither as literals, nor as E<number> codes), except for the literal byte-sequences for newline (ASCII 13, ASCII 13 10, or ASCII 10), and tab (ASCII 9).
Numbers in the range 160-255 refer to Latin-1 characters (also defined there by Unicode, with the same meaning). Numbers above 255 should be understood to refer to Unicode characters.
This will likely require many formatters to have tables mapping from treatable Unicode codepoints (such as the ``\xE9'' for the e-acute character) to the escape sequences or codes necessary for conveying such sequences in the target output format. A converter to *roff would, for example know that ``\xE9'' (whether conveyed literally, or via a E<...> sequence) is to be conveyed as ``e\\*'''. Similarly, a program rendering Pod in a Mac OS application window, would presumably need to know that ``\xE9'' maps to codepoint 142 in MacRoman encoding that (at time of writing) is native for Mac OS. Such Unicode2whatever mappings are presumably already widely available for common output formats. (Such mappings may be incomplete! Implementers are not expected to bend over backwards in an attempt to render Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any of the other weird things that Unicode can encode.) And if a Pod document uses a character not found in such a mapping, the formatter should consider it an unrenderable character.
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 "∞"> <!-- infinity, U+221E ISOtech -->
While the mapping ``infin'' to the character ``\x{221E}'' will (hopefully) have been already handled by the Pod parser, the presence of the character in this file means that it's reasonably important enough to include in a formatter's table that maps from notable Unicode characters to the codes necessary for rendering them. So for a Unicode-to-*roff mapping, for example, this would merit the entry:
"\x{221E}" => '\(in',
It is eagerly hoped that in the future, increasing numbers of formats (and formatters) will support Unicode characters directly (as (X)HTML does with "∞", "∞", or "∞"), reducing the need for idiosyncratic mappings of Unicode-to-my_escapes.
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.
S<L</Autoloaded Functions>>
This means that the space in the middle of the visible link text must not be broken across lines. In other words, it's the same as this:
L<"AutoloadedE<160>Functions"/Autoloaded Functions>
However, a misapplied space-to-NBSP replacement could (wrongly) produce something equivalent to this:
L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
...which is almost definitely not going to work as a hyperlink (assuming this formatter outputs a format supporting hypertext).
Formatters may choose to just not support the S format code, especially in cases where the output format simply has no NBSP character/code and no code for ``don't break this stuff across lines''.
For example:
sigE<shy>action manuE<shy>script JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
These signal to a formatter that if it is to hyphenate ``sigaction'' or ``manuscript'', then it should be done as "sig-[linebreak]action`` or ''manu-[linebreak]script" (and if it doesn't hyphenate it, then the "E<shy>" doesn't show up at all). And if it is to hyphenate ``Jarkko'' and/or ``Hietaniemi'', it can do so only at the points where there is a "E<shy>" code.
In practice, it is anticipated that this character will not be used often, but formatters should either support it, or delete it.
Pod parsers may also note additional attributes including:
(The above were numbered only for concise reference below. It is not a requirement that these be passed as an actual list or array.)
For example:
L<Foo::Bar>
=> undef, # link text
"Foo::Bar", # possibly inferred link text
"Foo::Bar", # name
undef, # section
'pod', # what sort of link
"Foo::Bar" # original content
L<Perlport's section on NL's|perlport/Newlines>
=> "Perlport's section on NL's", # link text
"Perlport's section on NL's", # possibly inferred link text
"perlport", # name
"Newlines", # section
'pod', # what sort of link
"Perlport's section on NL's|perlport/Newlines"
# original content
L<perlport/Newlines>
=> undef, # link text
'"Newlines" in perlport', # possibly inferred link text
"perlport", # name
"Newlines", # section
'pod', # what sort of link
"perlport/Newlines" # original content
L<crontab(5)/"DESCRIPTION">
=> undef, # link text
'"DESCRIPTION" in crontab(5)', # possibly inferred link text
"crontab(5)", # name
"DESCRIPTION", # section
'man', # what sort of link
'crontab(5)/"DESCRIPTION"' # original content
L</Object Attributes>
=> undef, # link text
'"Object Attributes"', # possibly inferred link text
undef, # name
"Object Attributes", # section
'pod', # what sort of link
"/Object Attributes" # original content
L<https://www.perl.org/>
=> undef, # link text
"https://www.perl.org/", # possibly inferred link text
"https://www.perl.org/", # name
undef, # section
'url', # what sort of link
"https://www.perl.org/" # original content
L<Perl.org|https://www.perl.org/>
=> "Perl.org", # link text
"https://www.perl.org/", # possibly inferred link text
"https://www.perl.org/", # name
undef, # section
'url', # what sort of link
"Perl.org|https://www.perl.org/" # original content
Note that you can distinguish URL-links from anything else by the fact that they match "m/\A\w+:[^:\s]\S*\z/". So "L<http://www.perl.com>" is a URL, but "L<HTTP::Response>" isn't.
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>
=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>
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.
L<B<ummE<234>stuff>|...>
For "L<...>" codes without a ``name|'' part, only "E<...>" and "Z<>" codes may occur. That is, authors should not use ""L<B<Foo::Bar>>"".
Note, however, that formatting codes and Z<>'s can occur in any and all parts of an L<...> (i.e., in name, section, text, and url).
Authors must not nest L<...> codes. For example, ``L<The L<Foo::Bar> man page>'' should be treated as an error.
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.
(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.
(Pod processors must tolerate lines like ``=item 1'' as if they were ``=item 1.'', with the period.)
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/".
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.
=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.
=over =item 1 Pick up dry cleaning. =item 2 =item 3 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs. =back
=over =back
Pod processors seeing such a contentless ``=over'' ... ``=back'' region, may ignore it, or may report it as an error.
=item Neque =item Porro =item Quisquam Est Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. =item Ut Enim
is semantically ambiguous, in a way that makes formatting decisions a bit difficult. On the one hand, it could be mention of an item ``Neque'', mention of another item ``Porro'', and mention of another item ``Quisquam Est'', with just the last one requiring the explanatory paragraph ``Qui dolorem ipsum quia dolor...''; and then an item ``Ut Enim''. In that case, you'd want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But it could equally well be a discussion of three (related or equivalent) items, ``Neque'', ``Porro'', and ``Quisquam Est'', followed by a paragraph explaining them all, and then a new item ``Ut Enim''. In that case, you'd probably want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But (for the foreseeable future), Pod does not provide any way for Pod authors to distinguish which grouping is meant by the above ``=item''-cluster structure. So formatters should format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
That is, there should be (at least roughly) equal spacing between items as between paragraphs (although that spacing may well be less than the full height of a line of text). This leaves it to the reader to use (con)textual cues to figure out whether the ``Qui dolorem ipsum...'' paragraph applies to the ``Quisquam Est'' item or to all three items ``Neque'', ``Porro'', and ``Quisquam Est''. While not an ideal situation, this is preferable to providing formatting cues that may be actually contrary to the author's intent.
=begin rtf
\par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
=end rtf
The exact same effect could, incidentally, be achieved with a single ``=for'' paragraph:
=for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
(Although that is not formally a data paragraph, it has the same meaning as one, and Pod parsers may parse it as one.)
Another example of a data paragraph:
=begin html I like <em>PIE</em>! <hr>Especially pecan pie! =end html
If these were ordinary paragraphs, the Pod parser would try to expand the ``E</em>'' (in the first paragraph) as a formatting code, just like ``E<lt>'' or ``E<eacute>''. But since this is in a "=begin identifier``...''=end identifier" region and the identifier ``html'' doesn't begin have a ``:'' prefix, the contents of this region are stored as data paragraphs, instead of being processed as ordinary paragraphs (or if they began with a spaces and/or tabs, as verbatim paragraphs).
As a further example: At time of writing, no ``biblio'' identifier is supported, but suppose some processor were written to recognize it as a way of (say) denoting a bibliographic reference (necessarily containing formatting codes in ordinary paragraphs). The fact that ``biblio'' paragraphs were meant for ordinary processing would be indicated by prefacing each ``biblio'' identifier with a colon:
=begin :biblio Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ. =end :biblio
This would signal to the parser that paragraphs in this begin...end region are subject to normal handling as ordinary/verbatim paragraphs (while still tagged as meant only for processors that understand the ``biblio'' identifier). The same effect could be had with:
=for :biblio Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ.
The ``:'' on these identifiers means simply ``process this stuff normally, even though the result will be for some special target''. I suggest that parser APIs report ``biblio'' as the target identifier, but also report that it had a ``:'' prefix. (And similarly, with the above ``html'', report ``html'' as the target identifier, and note the lack of a ``:'' prefix.)
Note that a "=begin identifier``...''=end identifier" region where identifier begins with a colon, can contain commands. For example:
=begin :biblio Wirth's classic is available in several editions, including: =for comment hm, check abebooks.com for how much used copies cost. =over =item Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.> Teubner, Stuttgart. [Yes, it's in German.] =item Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ. =back =end :biblio
Note, however, a "=begin identifier``...''=end identifier" region where identifier does not begin with a colon, should not directly contain ``=head1'' ... ``=head4'' commands, nor ``=over'', nor ``=back'', nor ``=item''. For example, this may be considered invalid:
=begin somedata This is a data paragraph. =head1 Don't do this! This is a data paragraph too. =end somedata
A Pod processor may signal that the above (specifically the ``=head1'' paragraph) is an error. Note, however, that the following should not be treated as an error:
=begin somedata
This is a data paragraph.
=cut
# Yup, this isn't Pod anymore.
sub excl { (rand() > .5) ? "hoo!" : "hah!" }
=pod
This is a data paragraph too.
=end somedata
And this too is valid:
=begin someformat
This is a data paragraph.
And this is a data paragraph.
=begin someotherformat
This is a data paragraph too.
And this is a data paragraph too.
=begin :yetanotherformat
=head2 This is a command paragraph!
This is an ordinary paragraph!
And this is a verbatim paragraph!
=end :yetanotherformat
=end someotherformat
Another data paragraph!
=end someformat
The contents of the above ``=begin :yetanotherformat'' ... ``=end :yetanotherformat'' region aren't data paragraphs, because the immediately containing region's identifier (``:yetanotherformat'') begins with a colon. In practice, most regions that contain data paragraphs will contain only data paragraphs; however, the above nesting is syntactically valid as Pod, even if it is rare. However, the handlers for some formats, like ``html'', will accept only data paragraphs, not nested regions; and they may complain if they see (targeted for them) nested regions, or commands, other than ``=end'', ``=pod'', and ``=cut''.
Also consider this valid structure:
=begin :biblio Wirth's classic is available in several editions, including: =over =item Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.> Teubner, Stuttgart. [Yes, it's in German.] =item Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ. =back Buy buy buy! =begin html <img src='wirth_spokesmodeling_book.png'> <hr> =end html Now now now! =end :biblio
There, the ``=begin html''...``=end html'' region is nested inside the larger ``=begin :biblio''...``=end :biblio'' region. Note that the content of the ``=begin html''...``=end html'' region is data paragraph(s), because the immediately containing region's identifier (``html'') doesn't begin with a colon.
Pod parsers, when processing a series of data paragraphs one after another (within a single region), should consider them to be one large data paragraph that happens to contain blank lines. So the content of the above ``=begin html''...``=end html'' may be stored as two data paragraphs (one consisting of ``<img src='wirth_spokesmodeling_book.png'>\n'' and another consisting of ``<hr>\n''), but should be stored as a single data paragraph (consisting of ``<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n'').
Pod processors should tolerate empty "=begin something``...''=end something`` regions, empty ''=begin :something``...''=end :something`` regions, and contentless ''=for something`` and ''=for :something" paragraphs. I.e., these should be tolerated:
=for html =begin html =end html =begin :biblio =end :biblio
Incidentally, note that there's no easy way to express a data paragraph starting with something that looks like a command. Consider:
=begin stuff =shazbot =end stuff
There, ``=shazbot'' will be parsed as a Pod command ``shazbot'', not as a data paragraph ``=shazbot\n''. However, you can express a data paragraph consisting of ``=shazbot\n'' using this code:
=for stuff =shazbot
The situation where this is necessary, is presumably quite rare.
Note that =end commands must match the currently open =begin command. That is, they must properly nest. For example, this is valid:
=begin outer X =begin inner Y =end inner Z =end outer
while this is invalid:
=begin outer X =begin inner Y =end outer Z =end inner
This latter is improper because when the ``=end outer'' command is seen, the currently open region has the formatname ``inner'', not ``outer''. (It just happens that ``outer'' is the format name of a higher-up region.) This is an error. Processors must by default report this as an error, and may halt processing the document containing that error. A corollary of this is that regions cannot ``overlap''. That is, the latter block above does not represent a region called ``outer'' which contains X and Y, overlapping a region called ``inner'' which contains Y and Z. But because it is invalid (as all apparently overlapping regions would be), it doesn't represent that, or anything at all.
Similarly, this is invalid:
=begin thing =end hting
This is an error because the region is opened by ``thing'', and the ``=end'' tries to close ``hting'' [sic].
This is also invalid:
=begin thing =end
This is invalid because every ``=end'' command must have a formatname parameter.