5 perlpodspec - Plain Old Documentation: format specification and notes
9 This document is detailed notes on the Pod markup language. Most
10 people will only have to read L<perlpod|perlpod> to know how to write
11 in Pod, but this document may answer some incidental questions to do
12 with parsing and rendering Pod.
14 In this document, "must" / "must not", "should" /
15 "should not", and "may" have their conventional (cf. RFC 2119)
16 meanings: "X must do Y" means that if X doesn't do Y, it's against
17 this specification, and should really be fixed. "X should do Y"
18 means that it's recommended, but X may fail to do Y, if there's a
19 good reason. "X may do Y" is merely a note that X can do Y at
20 will (although it is up to the reader to detect any connotation of
21 "and I think it would be I<nice> if X did Y" versus "it wouldn't
22 really I<bother> me if X did Y").
24 Notably, when I say "the parser should do Y", the
25 parser may fail to do Y, if the calling application explicitly
26 requests that the parser I<not> do Y. I often phrase this as
27 "the parser should, by default, do Y." This doesn't I<require>
28 the parser to provide an option for turning off whatever
29 feature Y is (like expanding tabs in verbatim paragraphs), although
30 it implicates that such an option I<may> be provided.
32 =head1 Pod Definitions
34 Pod is embedded in files, typically Perl source files, although you
35 can write a file that's nothing but Pod.
37 A B<line> in a file consists of zero or more non-newline characters,
38 terminated by either a newline or the end of the file.
40 A B<newline sequence> is usually a platform-dependent concept, but
41 Pod parsers should understand it to mean any of CR (ASCII 13), LF
42 (ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in
43 addition to any other system-specific meaning. The first CR/CRLF/LF
44 sequence in the file may be used as the basis for identifying the
45 newline sequence for parsing the rest of the file.
47 A B<blank line> is a line consisting entirely of zero or more spaces
48 (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file.
49 A B<non-blank line> is a line containing one or more characters other
50 than space or tab (and terminated by a newline or end-of-file).
52 (I<Note:> Many older Pod parsers did not accept a line consisting of
53 spaces/tabs and then a newline as a blank line. The only lines they
54 considered blank were lines consisting of I<no characters at all>,
55 terminated by a newline.)
57 B<Whitespace> is used in this document as a blanket term for spaces,
58 tabs, and newline sequences. (By itself, this term usually refers
59 to literal whitespace. That is, sequences of whitespace characters
60 in Pod source, as opposed to "EE<lt>32>", which is a formatting
61 code that I<denotes> a whitespace character.)
63 A B<Pod parser> is a module meant for parsing Pod (regardless of
64 whether this involves calling callbacks or building a parse tree or
65 directly formatting it). A B<Pod formatter> (or B<Pod translator>)
66 is a module or program that converts Pod to some other format (HTML,
67 plaintext, TeX, PostScript, RTF). A B<Pod processor> might be a
68 formatter or translator, or might be a program that does something
69 else with the Pod (like counting words, scanning for index points,
72 Pod content is contained in B<Pod blocks>. A Pod block starts with a
73 line that matches C<m/\A=[a-zA-Z]/>, and continues up to the next line
74 that matches C<m/\A=cut/> or up to the end of the file if there is
78 The current perlsyn says:
80 Note that pod translators should look at only paragraphs beginning
81 with a pod directive (it makes parsing easier), whereas the compiler
82 actually knows to look for pod escapes even in the middle of a
83 paragraph. This means that the following secret stuff will be ignored
84 by both the compiler and the translators.
87 warn "Neither POD nor CODE!?"
90 You probably shouldn't rely upon the warn() being podded out forever.
91 Not all pod translators are well-behaved in this regard, and perhaps
92 the compiler will become pickier.
94 I think that those paragraphs should just be removed; paragraph-based
95 parsing seems to have been largely abandoned, because of the hassle
96 with non-empty blank lines messing up what people meant by "paragraph".
97 Even if the "it makes parsing easier" bit were especially true,
98 it wouldn't be worth the confusion of having perl and pod2whatever
99 actually disagree on what can constitute a Pod block.
101 Note that a parser is not expected to distinguish between something that
102 looks like pod, but is in a quoted string, such as a here document.
104 Within a Pod block, there are B<Pod paragraphs>. A Pod paragraph
105 consists of non-blank lines of text, separated by one or more blank
108 For purposes of Pod processing, there are four types of paragraphs in
115 A command paragraph (also called a "directive"). The first line of
116 this paragraph must match C<m/\A=[a-zA-Z]/>. Command paragraphs are
117 typically one line, as in:
123 But they may span several (non-blank) lines:
126 Hm, I wonder what it would look like if
127 you tried to write a BNF for Pod from this.
129 =head3 Dr. Strangelove, or: How I Learned to
130 Stop Worrying and Love the Bomb
132 I<Some> command paragraphs allow formatting codes in their content
133 (i.e., after the part that matches C<m/\A=[a-zA-Z]\S*\s*/>), as in:
135 =head1 Did You Remember to C<use strict;>?
137 In other words, the Pod processing handler for "head1" will apply the
138 same processing to "Did You Remember to CE<lt>use strict;>?" that it
139 would to an ordinary paragraph (i.e., formatting codes like
140 "CE<lt>...>") are parsed and presumably formatted appropriately, and
141 whitespace in the form of literal spaces and/or tabs is not
146 A B<verbatim paragraph>. The first line of this paragraph must be a
147 literal space or tab, and this paragraph must not be inside a "=begin
148 I<identifier>", ... "=end I<identifier>" sequence unless
149 "I<identifier>" begins with a colon (":"). That is, if a paragraph
150 starts with a literal space or tab, but I<is> inside a
151 "=begin I<identifier>", ... "=end I<identifier>" region, then it's
152 a data paragraph, unless "I<identifier>" begins with a colon.
154 Whitespace I<is> significant in verbatim paragraphs (although, in
155 processing, tabs are probably expanded).
159 An B<ordinary paragraph>. A paragraph is an ordinary paragraph
160 if its first line matches neither C<m/\A=[a-zA-Z]/> nor
161 C<m/\A[ \t]/>, I<and> if it's not inside a "=begin I<identifier>",
162 ... "=end I<identifier>" sequence unless "I<identifier>" begins with
167 A B<data paragraph>. This is a paragraph that I<is> inside a "=begin
168 I<identifier>" ... "=end I<identifier>" sequence where
169 "I<identifier>" does I<not> begin with a literal colon (":"). In
170 some sense, a data paragraph is not part of Pod at all (i.e.,
171 effectively it's "out-of-band"), since it's not subject to most kinds
172 of Pod parsing; but it is specified here, since Pod
173 parsers need to be able to call an event for it, or store it in some
174 form in a parse tree, or at least just parse I<around> it.
178 For example: consider the following paragraphs:
180 # <- that's the 0th column
190 Here, "=head1 Foo" and "=cut" are command paragraphs because the first
191 line of each matches C<m/\A=[a-zA-Z]/>. "I<[space][space]>$foo->bar"
192 is a verbatim paragraph, because its first line starts with a literal
193 whitespace character (and there's no "=begin"..."=end" region around).
195 The "=begin I<identifier>" ... "=end I<identifier>" commands stop
196 paragraphs that they surround from being parsed as ordinary or verbatim
197 paragraphs, if I<identifier> doesn't begin with a colon. This
198 is discussed in detail in the section
199 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
203 This section is intended to supplement and clarify the discussion in
204 L<perlpod/"Command Paragraph">. These are the currently recognized
209 =item "=head1", "=head2", "=head3", "=head4"
211 This command indicates that the text in the remainder of the paragraph
212 is a heading. That text may contain formatting codes. Examples:
214 =head1 Object Attributes
216 =head3 What B<Not> to Do!
220 This command indicates that this paragraph begins a Pod block. (If we
221 are already in the middle of a Pod block, this command has no effect at
222 all.) If there is any text in this command paragraph after "=pod",
223 it must be ignored. Examples:
227 This is a plain Pod paragraph.
229 =pod This text is ignored.
233 This command indicates that this line is the end of this previously
234 started Pod block. If there is any text after "=cut" on the line, it must be
239 =cut The documentation ends here.
242 # This is the first line of program text.
243 sub foo { # This is the second.
245 It is an error to try to I<start> a Pod block with a "=cut" command. In
246 that case, the Pod processor must halt parsing of the input file, and
247 must by default emit a warning.
251 This command indicates that this is the start of a list/indent
252 region. If there is any text following the "=over", it must consist
253 of only a nonzero positive numeral. The semantics of this numeral is
254 explained in the L</"About =over...=back Regions"> section, further
255 below. Formatting codes are not expanded. Examples:
265 This command indicates that an item in a list begins here. Formatting
266 codes are processed. The semantics of the (optional) text in the
267 remainder of this paragraph are
268 explained in the L</"About =over...=back Regions"> section, further
281 =item C<< $thing->stuff(I<dodad>) >>
283 =item For transporting us beyond seas to be tried for pretended
286 =item He is at this time transporting large armies of foreign
287 mercenaries to complete the works of death, desolation and
288 tyranny, already begun with circumstances of cruelty and perfidy
289 scarcely paralleled in the most barbarous ages, and totally
290 unworthy the head of a civilized nation.
294 This command indicates that this is the end of the region begun
295 by the most recent "=over" command. It permits no text after the
298 =item "=begin formatname"
300 =item "=begin formatname parameter"
302 This marks the following paragraphs (until the matching "=end
303 formatname") as being for some special kind of processing. Unless
304 "formatname" begins with a colon, the contained non-command
305 paragraphs are data paragraphs. But if "formatname" I<does> begin
306 with a colon, then non-command paragraphs are ordinary paragraphs
307 or data paragraphs. This is discussed in detail in the section
308 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
310 It is advised that formatnames match the regexp
311 C<m/\A:?[-a-zA-Z0-9_]+\z/>. Everything following whitespace after the
312 formatname is a parameter that may be used by the formatter when dealing
313 with this region. This parameter must not be repeated in the "=end"
314 paragraph. Implementors should anticipate future expansion in the
315 semantics and syntax of the first parameter to "=begin"/"=end"/"=for".
317 =item "=end formatname"
319 This marks the end of the region opened by the matching
320 "=begin formatname" region. If "formatname" is not the formatname
321 of the most recent open "=begin formatname" region, then this
322 is an error, and must generate an error message. This
323 is discussed in detail in the section
324 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
326 =item "=for formatname text..."
328 This is synonymous with:
336 That is, it creates a region consisting of a single paragraph; that
337 paragraph is to be treated as a normal paragraph if "formatname"
338 begins with a ":"; if "formatname" I<doesn't> begin with a colon,
339 then "text..." will constitute a data paragraph. There is no way
340 to use "=for formatname text..." to express "text..." as a verbatim
343 =item "=encoding encodingname"
345 This command, which should occur early in the document (at least
346 before any non-US-ASCII data!), declares that this document is
347 encoded in the encoding I<encodingname>, which must be
348 an encoding name that L<Encode> recognizes. (Encode's list
349 of supported encodings, in L<Encode::Supported>, is useful here.)
350 If the Pod parser cannot decode the declared encoding, it
351 should emit a warning and may abort parsing the document
354 A document having more than one "=encoding" line should be
355 considered an error. Pod processors may silently tolerate this if
356 the not-first "=encoding" lines are just duplicates of the
357 first one (e.g., if there's a "=encoding utf8" line, and later on
358 another "=encoding utf8" line). But Pod processors should complain if
359 there are contradictory "=encoding" lines in the same document
360 (e.g., if there is a "=encoding utf8" early in the document and
361 "=encoding big5" later). Pod processors that recognize BOMs
362 may also complain if they see an "=encoding" line
363 that contradicts the BOM (e.g., if a document with a UTF-16LE
364 BOM has an "=encoding shiftjis" line).
368 If a Pod processor sees any command other than the ones listed
369 above (like "=head", or "=haed1", or "=stuff", or "=cuttlefish",
370 or "=w123"), that processor must by default treat this as an
371 error. It must not process the paragraph beginning with that
372 command, must by default warn of this as an error, and may
373 abort the parse. A Pod parser may allow a way for particular
374 applications to add to the above list of known commands, and to
375 stipulate, for each additional command, whether formatting
376 codes should be processed.
378 Future versions of this specification may add additional
383 =head1 Pod Formatting Codes
385 (Note that in previous drafts of this document and of perlpod,
386 formatting codes were referred to as "interior sequences", and
387 this term may still be found in the documentation for Pod parsers,
388 and in error messages from Pod processors.)
390 There are two syntaxes for formatting codes:
396 A formatting code starts with a capital letter (just US-ASCII [A-Z])
397 followed by a "<", any number of characters, and ending with the first
398 matching ">". Examples:
400 That's what I<you> think!
402 What's C<CORE::dump()> for?
404 X<C<chmod> and C<unlink()> Under Different Operating Systems>
408 A formatting code starts with a capital letter (just US-ASCII [A-Z])
409 followed by two or more "<"'s, one or more whitespace characters,
410 any number of characters, one or more whitespace characters,
411 and ending with the first matching sequence of two or more ">"'s, where
412 the number of ">"'s equals the number of "<"'s in the opening of this
413 formatting code. Examples:
415 That's what I<< you >> think!
417 C<<< open(X, ">>thing.dat") || die $! >>>
421 With this syntax, the whitespace character(s) after the "CE<lt><<"
422 and before the ">>>" (or whatever letter) are I<not> renderable. They
423 do not signify whitespace, are merely part of the formatting codes
424 themselves. That is, these are all synonymous:
436 Finally, the multiple-angle-bracket form does I<not> alter the interpretation
437 of nested formatting codes, meaning that the following four example lines are
438 identical in meaning:
440 B<example: C<$a E<lt>=E<gt> $b>>
442 B<example: C<< $a <=> $b >>>
444 B<example: C<< $a E<lt>=E<gt> $b >>>
446 B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
450 In parsing Pod, a notably tricky part is the correct parsing of
451 (potentially nested!) formatting codes. Implementors should
452 consult the code in the C<parse_text> routine in Pod::Parser as an
453 example of a correct implementation.
457 =item C<IE<lt>textE<gt>> -- italic text
459 See the brief discussion in L<perlpod/"Formatting Codes">.
461 =item C<BE<lt>textE<gt>> -- bold text
463 See the brief discussion in L<perlpod/"Formatting Codes">.
465 =item C<CE<lt>codeE<gt>> -- code text
467 See the brief discussion in L<perlpod/"Formatting Codes">.
469 =item C<FE<lt>filenameE<gt>> -- style for filenames
471 See the brief discussion in L<perlpod/"Formatting Codes">.
473 =item C<XE<lt>topic nameE<gt>> -- an index entry
475 See the brief discussion in L<perlpod/"Formatting Codes">.
477 This code is unusual in that most formatters completely discard
478 this code and its content. Other formatters will render it with
479 invisible codes that can be used in building an index of
480 the current document.
482 =item C<ZE<lt>E<gt>> -- a null (zero-effect) formatting code
484 Discussed briefly in L<perlpod/"Formatting Codes">.
486 This code is unusual in that it should have no content. That is,
487 a processor may complain if it sees C<ZE<lt>potatoesE<gt>>. Whether
488 or not it complains, the I<potatoes> text should ignored.
490 =item C<LE<lt>nameE<gt>> -- a hyperlink
492 The complicated syntaxes of this code are discussed at length in
493 L<perlpod/"Formatting Codes">, and implementation details are
494 discussed below, in L</"About LE<lt>...E<gt> Codes">. Parsing the
495 contents of LE<lt>content> is tricky. Notably, the content has to be
496 checked for whether it looks like a URL, or whether it has to be split
497 on literal "|" and/or "/" (in the right order!), and so on,
498 I<before> EE<lt>...> codes are resolved.
500 =item C<EE<lt>escapeE<gt>> -- a character escape
502 See L<perlpod/"Formatting Codes">, and several points in
503 L</Notes on Implementing Pod Processors>.
505 =item C<SE<lt>textE<gt>> -- text contains non-breaking spaces
507 This formatting code is syntactically simple, but semantically
508 complex. What it means is that each space in the printable
509 content of this code signifies a non-breaking space.
517 Both signify the monospace (c[ode] style) text consisting of
518 "$x", one space, "?", one space, ":", one space, "$z". The
519 difference is that in the latter, with the S code, those spaces
520 are not "normal" spaces, but instead are non-breaking spaces.
525 If a Pod processor sees any formatting code other than the ones
526 listed above (as in "NE<lt>...>", or "QE<lt>...>", etc.), that
527 processor must by default treat this as an error.
528 A Pod parser may allow a way for particular
529 applications to add to the above list of known formatting codes;
530 a Pod parser might even allow a way to stipulate, for each additional
531 command, whether it requires some form of special processing, as
534 Future versions of this specification may add additional
537 Historical note: A few older Pod processors would not see a ">" as
538 closing a "CE<lt>" code, if the ">" was immediately preceded by
539 a "-". This was so that this:
543 would parse as equivalent to this:
547 instead of as equivalent to a "C" formatting code containing
548 only "$foo-", and then a "bar>" outside the "C" formatting code. This
549 problem has since been solved by the addition of syntaxes like this:
553 Compliant parsers must not treat "->" as special.
555 Formatting codes absolutely cannot span paragraphs. If a code is
556 opened in one paragraph, and no closing code is found by the end of
557 that paragraph, the Pod parser must close that formatting code,
558 and should complain (as in "Unterminated I code in the paragraph
559 starting at line 123: 'Time objects are not...'"). So these
562 I<I told you not to do this!
564 Don't make me say it again!>
566 ...must I<not> be parsed as two paragraphs in italics (with the I
567 code starting in one paragraph and starting in another.) Instead,
568 the first paragraph should generate a warning, but that aside, the
569 above code must parse as if it were:
571 I<I told you not to do this!>
573 Don't make me say it again!E<gt>
575 (In SGMLish jargon, all Pod commands are like block-level
576 elements, whereas all Pod formatting codes are like inline-level
581 =head1 Notes on Implementing Pod Processors
583 The following is a long section of miscellaneous requirements
584 and suggestions to do with Pod processing.
590 Pod formatters should tolerate lines in verbatim blocks that are of
591 any length, even if that means having to break them (possibly several
592 times, for very long lines) to avoid text running off the side of the
593 page. Pod formatters may warn of such line-breaking. Such warnings
594 are particularly appropriate for lines are over 100 characters long, which
595 are usually not intentional.
599 Pod parsers must recognize I<all> of the three well-known newline
600 formats: CR, LF, and CRLF. See L<perlport|perlport>.
604 Pod parsers should accept input lines that are of any length.
608 Since Perl recognizes a Unicode Byte Order Mark at the start of files
609 as signaling that the file is Unicode encoded as in UTF-16 (whether
610 big-endian or little-endian) or UTF-8, Pod parsers should do the
611 same. Otherwise, the character encoding should be understood as
612 being UTF-8 if the first highbit byte sequence in the file seems
613 valid as a UTF-8 sequence, or otherwise as CP-1252 (earlier versions of
614 this specification used Latin-1 instead of CP-1252).
616 Future versions of this specification may specify
617 how Pod can accept other encodings. Presumably treatment of other
618 encodings in Pod parsing would be as in XML parsing: whatever the
619 encoding declared by a particular Pod file, content is to be
620 stored in memory as Unicode characters.
624 The well known Unicode Byte Order Marks are as follows: if the
625 file begins with the two literal byte values 0xFE 0xFF, this is
626 the BOM for big-endian UTF-16. If the file begins with the two
627 literal byte value 0xFF 0xFE, this is the BOM for little-endian
628 UTF-16. On an ASCII platform, if the file begins with the three literal
630 0xEF 0xBB 0xBF, this is the BOM for UTF-8.
631 A mechanism portable to EBCDIC platforms is to:
633 my $utf8_bom = "\x{FEFF}";
634 utf8::encode($utf8_bom);
637 use bytes; print map sprintf(" 0x%02X", ord $_), split '', "\x{feff}";
641 If toke.c is modified to support UTF-32, add mention of those here.
645 A naive, but often sufficient heuristic on ASCII platforms, for testing
647 byte-sequence in a BOM-less file (whether in code or in Pod!), to see
648 whether that sequence is valid as UTF-8 (RFC 2279) is to check whether
649 that the first byte in the sequence is in the range 0xC2 - 0xFD
650 I<and> whether the next byte is in the range
651 0x80 - 0xBF. If so, the parser may conclude that this file is in
652 UTF-8, and all highbit sequences in the file should be assumed to
653 be UTF-8. Otherwise the parser should treat the file as being
654 in CP-1252. (A better check, and which works on EBCDIC platforms as
655 well, is to pass a copy of the sequence to
656 L<utf8::decode()|utf8> which performs a full validity check on the
657 sequence and returns TRUE if it is valid UTF-8, FALSE otherwise. This
658 function is always pre-loaded, is fast because it is written in C, and
659 will only get called at most once, so you don't need to avoid it out of
660 performance concerns.)
661 In the unlikely circumstance that the first highbit
662 sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one
663 can cater to our heuristic (as well as any more intelligent heuristic)
664 by prefacing that line with a comment line containing a highbit
665 sequence that is clearly I<not> valid as UTF-8. A line consisting
666 of simply "#", an e-acute, and any non-highbit byte,
667 is sufficient to establish this file's encoding.
670 If/WHEN some brave soul makes these heuristics into a generic
671 text-file class (or PerlIO layer?), we can presumably delete
672 mention of these icky details from this file, and can instead
673 tell people to just use appropriate class/layer.
674 Auto-recognition of newline sequences would be another desirable
675 feature of such a class/layer.
679 "The probability that a string of characters
680 in any other encoding appears as valid UTF-8 is low" - RFC2279
684 Pod processors must treat a "=for [label] [content...]" paragraph as
685 meaning the same thing as a "=begin [label]" paragraph, content, and
686 an "=end [label]" paragraph. (The parser may conflate these two
687 constructs, or may leave them distinct, in the expectation that the
688 formatter will nevertheless treat them the same.)
692 When rendering Pod to a format that allows comments (i.e., to nearly
693 any format other than plaintext), a Pod formatter must insert comment
694 text identifying its name and version number, and the name and
695 version numbers of any modules it might be using to process the Pod.
698 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
700 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
702 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
704 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
706 Formatters may also insert additional comments, including: the
707 release date of the Pod formatter program, the contact address for
708 the author(s) of the formatter, the current time, the name of input
709 file, the formatting options in effect, version of Perl used, etc.
711 Formatters may also choose to note errors/warnings as comments,
712 besides or instead of emitting them otherwise (as in messages to
713 STDERR, or C<die>ing).
717 Pod parsers I<may> emit warnings or error messages ("Unknown E code
718 EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or
719 C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> allow
720 suppressing all such STDERR output, and instead allow an option for
721 reporting errors/warnings
722 in some other way, whether by triggering a callback, or noting errors
723 in some attribute of the document object, or some similarly unobtrusive
724 mechanism -- or even by appending a "Pod Errors" section to the end of
725 the parsed form of the document.
729 In cases of exceptionally aberrant documents, Pod parsers may abort the
730 parse. Even then, using C<die>ing/C<croak>ing is to be avoided; where
731 possible, the parser library may simply close the input file
732 and add text like "*** Formatting Aborted ***" to the end of the
733 (partial) in-memory document.
737 In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>)
738 are understood (i.e., I<not> verbatim paragraphs, but I<including>
739 ordinary paragraphs, and command paragraphs that produce renderable
740 text, like "=head1"), literal whitespace should generally be considered
741 "insignificant", in that one literal space has the same meaning as any
742 (nonzero) number of literal spaces, literal newlines, and literal tabs
743 (as long as this produces no blank lines, since those would terminate
744 the paragraph). Pod parsers should compact literal whitespace in each
745 processed paragraph, but may provide an option for overriding this
746 (since some processing tasks do not require it), or may follow
747 additional special rules (for example, specially treating
748 period-space-space or period-newline sequences).
752 Pod parsers should not, by default, try to coerce apostrophe (') and
753 quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to
754 turn backtick (`) into anything else but a single backtick character
755 (distinct from an open quote character!), nor "--" into anything but
756 two minus signs. They I<must never> do any of those things to text
757 in CE<lt>...> formatting codes, and never I<ever> to text in verbatim
762 When rendering Pod to a format that has two kinds of hyphens (-), one
763 that's a non-breaking hyphen, and another that's a breakable hyphen
764 (as in "object-oriented", which can be split across lines as
765 "object-", newline, "oriented"), formatters are encouraged to
766 generally translate "-" to non-breaking hyphen, but may apply
767 heuristics to convert some of these to breaking hyphens.
771 Pod formatters should make reasonable efforts to keep words of Perl
772 code from being broken across lines. For example, "Foo::Bar" in some
773 formatting systems is seen as eligible for being broken across lines
774 as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should
775 be avoided where possible, either by disabling all line-breaking in
776 mid-word, or by wrapping particular words with internal punctuation
777 in "don't break this across lines" codes (which in some formats may
778 not be a single code, but might be a matter of inserting non-breaking
779 zero-width spaces between every pair of characters in a word.)
783 Pod parsers should, by default, expand tabs in verbatim paragraphs as
784 they are processed, before passing them to the formatter or other
785 processor. Parsers may also allow an option for overriding this.
789 Pod parsers should, by default, remove newlines from the end of
790 ordinary and verbatim paragraphs before passing them to the
791 formatter. For example, while the paragraph you're reading now
792 could be considered, in Pod source, to end with (and contain)
793 the newline(s) that end it, it should be processed as ending with
794 (and containing) the period character that ends this sentence.
798 Pod parsers, when reporting errors, should make some effort to report
799 an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near
800 line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph
801 number ("Nested EE<lt>>'s in Paragraph #52 of Thing/Foo.pm!"). Where
802 this is problematic, the paragraph number should at least be
803 accompanied by an excerpt from the paragraph ("Nested EE<lt>>'s in
804 Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for
805 the CE<lt>interest rate> attribute...'").
809 Pod parsers, when processing a series of verbatim paragraphs one
810 after another, should consider them to be one large verbatim
811 paragraph that happens to contain blank lines. I.e., these two
812 lines, which have a blank line between them:
818 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
819 Foo->VERSION") before being passed to the formatter or other
820 processor. Parsers may also allow an option for overriding this.
822 While this might be too cumbersome to implement in event-based Pod
823 parsers, it is straightforward for parsers that return parse trees.
827 Pod formatters, where feasible, are advised to avoid splitting short
828 verbatim paragraphs (under twelve lines, say) across pages.
832 Pod parsers must treat a line with only spaces and/or tabs on it as a
833 "blank line" such as separates paragraphs. (Some older parsers
834 recognized only two adjacent newlines as a "blank line" but would not
835 recognize a newline, a space, and a newline, as a blank line. This
836 is noncompliant behavior.)
840 Authors of Pod formatters/processors should make every effort to
841 avoid writing their own Pod parser. There are already several in
842 CPAN, with a wide range of interface styles -- and one of them,
843 Pod::Simple, comes with modern versions of Perl.
847 Characters in Pod documents may be conveyed either as literals, or by
848 number in EE<lt>n> codes, or by an equivalent mnemonic, as in
849 EE<lt>eacute> which is exactly equivalent to EE<lt>233>. The numbers
850 are the Latin1/Unicode values, even on EBCDIC platforms.
852 When referring to characters by using a EE<lt>n> numeric code, numbers
853 in the range 32-126 refer to those well known US-ASCII characters (also
854 defined there by Unicode, with the same meaning), which all Pod
855 formatters must render faithfully. Characters whose EE<lt>E<gt> numbers
856 are in the ranges 0-31 and 127-159 should not be used (neither as
858 nor as EE<lt>number> codes), except for the literal byte-sequences for
859 newline (ASCII 13, ASCII 13 10, or ASCII 10), and tab (ASCII 9).
861 Numbers in the range 160-255 refer to Latin-1 characters (also
862 defined there by Unicode, with the same meaning). Numbers above
863 255 should be understood to refer to Unicode characters.
868 that some formatters cannot reliably render characters outside 32-126;
869 and many are able to handle 32-126 and 160-255, but nothing above
874 Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for
875 less-than and greater-than, Pod parsers must understand "EE<lt>sol>"
876 for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar,
877 pipe). Pod parsers should also understand "EE<lt>lchevron>" and
878 "EE<lt>rchevron>" as legacy codes for characters 171 and 187, i.e.,
879 "left-pointing double angle quotation mark" = "left pointing
880 guillemet" and "right-pointing double angle quotation mark" = "right
881 pointing guillemet". (These look like little "<<" and ">>", and they
882 are now preferably expressed with the HTML/XHTML codes "EE<lt>laquo>"
887 Pod parsers should understand all "EE<lt>html>" codes as defined
888 in the entity declarations in the most recent XHTML specification at
889 C<www.W3.org>. Pod parsers must understand at least the entities
890 that define characters in the range 160-255 (Latin-1). Pod parsers,
891 when faced with some unknown "EE<lt>I<identifier>>" code,
892 shouldn't simply replace it with nullstring (by default, at least),
893 but may pass it through as a string consisting of the literal characters
894 E, less-than, I<identifier>, greater-than. Or Pod parsers may offer the
895 alternative option of processing such unknown
896 "EE<lt>I<identifier>>" codes by firing an event especially
897 for such codes, or by adding a special node-type to the in-memory
898 document tree. Such "EE<lt>I<identifier>>" may have special meaning
899 to some processors, or some processors may choose to add them to
900 a special error report.
904 Pod parsers must also support the XHTML codes "EE<lt>quot>" for
905 character 34 (doublequote, "), "EE<lt>amp>" for character 38
906 (ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, ').
910 Note that in all cases of "EE<lt>whateverE<gt>", I<whatever> (whether
911 an htmlname, or a number in any base) must consist only of
912 alphanumeric characters -- that is, I<whatever> must match
913 C<m/\A\w+\z/>. So S<"EE<lt> 0 1 2 3 E<gt>"> is invalid, because
914 it contains spaces, which aren't alphanumeric characters. This
915 presumably does not I<need> special treatment by a Pod processor;
916 S<" 0 1 2 3 "> doesn't look like a number in any base, so it would
917 presumably be looked up in the table of HTML-like names. Since
918 there isn't (and cannot be) an HTML-like entity called S<" 0 1 2 3 ">,
919 this will be treated as an error. However, Pod processors may
920 treat S<"EE<lt> 0 1 2 3 E<gt>"> or "EE<lt>e-acute>" as I<syntactically>
921 invalid, potentially earning a different error message than the
922 error message (or warning, or event) generated by a merely unknown
923 (but theoretically valid) htmlname, as in "EE<lt>qacute>"
924 [sic]. However, Pod parsers are not required to make this
929 Note that EE<lt>number> I<must not> be interpreted as simply
930 "codepoint I<number> in the current/native character set". It always
931 means only "the character represented by codepoint I<number> in
932 Unicode." (This is identical to the semantics of &#I<number>; in XML.)
934 This will likely require many formatters to have tables mapping from
935 treatable Unicode codepoints (such as the "\xE9" for the e-acute
936 character) to the escape sequences or codes necessary for conveying
937 such sequences in the target output format. A converter to *roff
938 would, for example know that "\xE9" (whether conveyed literally, or via
939 a EE<lt>...> sequence) is to be conveyed as "e\\*'".
940 Similarly, a program rendering Pod in a Mac OS application window, would
941 presumably need to know that "\xE9" maps to codepoint 142 in MacRoman
942 encoding that (at time of writing) is native for Mac OS. Such
943 Unicode2whatever mappings are presumably already widely available for
944 common output formats. (Such mappings may be incomplete! Implementers
945 are not expected to bend over backwards in an attempt to render
946 Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any
947 of the other weird things that Unicode can encode.) And
948 if a Pod document uses a character not found in such a mapping, the
949 formatter should consider it an unrenderable character.
953 If, surprisingly, the implementor of a Pod formatter can't find a
954 satisfactory pre-existing table mapping from Unicode characters to
955 escapes in the target format (e.g., a decent table of Unicode
956 characters to *roff escapes), it will be necessary to build such a
957 table. If you are in this circumstance, you should begin with the
958 characters in the range 0x00A0 - 0x00FF, which is mostly the heavily
959 used accented characters. Then proceed (as patience permits and
960 fastidiousness compels) through the characters that the (X)HTML
961 standards groups judged important enough to merit mnemonics
962 for. These are declared in the (X)HTML specifications at the
963 www.W3.org site. At time of writing (September 2001), the most recent
964 entity declaration files are:
966 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
967 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
968 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
970 Then you can progress through any remaining notable Unicode characters
971 in the range 0x2000-0x204D (consult the character tables at
972 www.unicode.org), and whatever else strikes your fancy. For example,
973 in F<xhtml-symbol.ent>, there is the entry:
975 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
977 While the mapping "infin" to the character "\x{221E}" will (hopefully)
978 have been already handled by the Pod parser, the presence of the
979 character in this file means that it's reasonably important enough to
980 include in a formatter's table that maps from notable Unicode characters
981 to the codes necessary for rendering them. So for a Unicode-to-*roff
982 mapping, for example, this would merit the entry:
984 "\x{221E}" => '\(in',
986 It is eagerly hoped that in the future, increasing numbers of formats
987 (and formatters) will support Unicode characters directly (as (X)HTML
988 does with C<∞>, C<∞>, or C<∞>), reducing the need
989 for idiosyncratic mappings of Unicode-to-I<my_escapes>.
993 It is up to individual Pod formatter to display good judgement when
994 confronted with an unrenderable character (which is distinct from an
995 unknown EE<lt>thing> sequence that the parser couldn't resolve to
996 anything, renderable or not). It is good practice to map Latin letters
997 with diacritics (like "EE<lt>eacute>"/"EE<lt>233>") to the corresponding
998 unaccented US-ASCII letters (like a simple character 101, "e"), but
999 clearly this is often not feasible, and an unrenderable character may
1000 be represented as "?", or the like. In attempting a sane fallback
1001 (as from EE<lt>233> to "e"), Pod formatters may use the
1002 %Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or
1003 L<Text::Unidecode|Text::Unidecode>, if available.
1005 For example, this Pod text:
1007 magic is enabled if you set C<$Currency> to 'E<euro>'.
1010 "magic is enabled if you set C<$Currency> to 'I<?>'" or as
1011 "magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as
1012 "magic is enabled if you set C<$Currency> to '[x20AC]', etc.
1014 A Pod formatter may also note, in a comment or warning, a list of what
1015 unrenderable characters were encountered.
1019 EE<lt>...> may freely appear in any formatting code (other than
1020 in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The
1021 EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The
1022 EE<lt>euro>1,000,000 Solution|Million::Euros>".
1026 Some Pod formatters output to formats that implement non-breaking
1027 spaces as an individual character (which I'll call "NBSP"), and
1028 others output to formats that implement non-breaking spaces just as
1029 spaces wrapped in a "don't break this across lines" code. Note that
1030 at the level of Pod, both sorts of codes can occur: Pod can contain a
1031 NBSP character (whether as a literal, or as a "EE<lt>160>" or
1032 "EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo
1033 IE<lt>barE<gt> baz>" codes, where "mere spaces" (character 32) in
1034 such codes are taken to represent non-breaking spaces. Pod
1035 parsers should consider supporting the optional parsing of "SE<lt>foo
1036 IE<lt>barE<gt> baz>" as if it were
1037 "fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the
1038 optional parsing of groups of words joined by NBSP's as if each group
1039 were in a SE<lt>...> code, so that formatters may use the
1040 representation that maps best to what the output format demands.
1044 Some processors may find that the C<SE<lt>...E<gt>> code is easiest to
1045 implement by replacing each space in the parse tree under the content
1046 of the S, with an NBSP. But note: the replacement should apply I<not> to
1047 spaces in I<all> text, but I<only> to spaces in I<printable> text. (This
1048 distinction may or may not be evident in the particular tree/event
1049 model implemented by the Pod parser.) For example, consider this
1052 S<L</Autoloaded Functions>>
1054 This means that the space in the middle of the visible link text must
1055 not be broken across lines. In other words, it's the same as this:
1057 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
1059 However, a misapplied space-to-NBSP replacement could (wrongly)
1060 produce something equivalent to this:
1062 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
1064 ...which is almost definitely not going to work as a hyperlink (assuming
1065 this formatter outputs a format supporting hypertext).
1067 Formatters may choose to just not support the S format code,
1068 especially in cases where the output format simply has no NBSP
1069 character/code and no code for "don't break this stuff across lines".
1073 Besides the NBSP character discussed above, implementors are reminded
1074 of the existence of the other "special" character in Latin-1, the
1075 "soft hyphen" character, also known as "discretionary hyphen",
1076 i.e. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> =
1077 C<EE<lt>shyE<gt>>). This character expresses an optional hyphenation
1078 point. That is, it normally renders as nothing, but may render as a
1079 "-" if a formatter breaks the word at that point. Pod formatters
1080 should, as appropriate, do one of the following: 1) render this with
1081 a code with the same meaning (e.g., "\-" in RTF), 2) pass it through
1082 in the expectation that the formatter understands this character as
1083 such, or 3) delete it.
1089 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
1091 These signal to a formatter that if it is to hyphenate "sigaction"
1092 or "manuscript", then it should be done as
1093 "sig-I<[linebreak]>action" or "manu-I<[linebreak]>script"
1094 (and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> doesn't
1095 show up at all). And if it is
1096 to hyphenate "Jarkko" and/or "Hietaniemi", it can do
1097 so only at the points where there is a C<EE<lt>shyE<gt>> code.
1099 In practice, it is anticipated that this character will not be used
1100 often, but formatters should either support it, or delete it.
1104 If you think that you want to add a new command to Pod (like, say, a
1105 "=biblio" command), consider whether you could get the same
1106 effect with a for or begin/end sequence: "=for biblio ..." or "=begin
1107 biblio" ... "=end biblio". Pod processors that don't understand
1108 "=for biblio", etc, will simply ignore it, whereas they may complain
1109 loudly if they see "=biblio".
1113 Throughout this document, "Pod" has been the preferred spelling for
1114 the name of the documentation format. One may also use "POD" or
1115 "pod". For the documentation that is (typically) in the Pod
1116 format, you may use "pod", or "Pod", or "POD". Understanding these
1117 distinctions is useful; but obsessing over how to spell them, usually
1126 =head1 About LE<lt>...E<gt> Codes
1128 As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...>
1129 code is the most complex of the Pod formatting codes. The points below
1130 will hopefully clarify what it means and how processors should deal
1137 In parsing an LE<lt>...> code, Pod parsers must distinguish at least
1144 The link-text. If there is none, this must be C<undef>. (E.g., in
1145 "LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions".
1146 In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no
1147 link text. Note that link text may contain formatting.)
1151 The possibly inferred link-text; i.e., if there was no real link
1152 text, then this is the text that we'll infer in its place. (E.g., for
1153 "LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".)
1157 The name or URL, or C<undef> if none. (E.g., in "LE<lt>Perl
1158 Functions|perlfunc>", the name (also sometimes called the page)
1159 is "perlfunc". In "LE<lt>/CAVEATS>", the name is C<undef>.)
1163 The section (AKA "item" in older perlpods), or C<undef> if none. E.g.,
1164 in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "DESCRIPTION" is the section. (Note
1165 that this is not the same as a manpage section like the "5" in "man 5
1166 crontab". "Section Foo" in the Pod sense means the part of the text
1167 that's introduced by the heading or item whose text is "Foo".)
1171 Pod parsers may also note additional attributes including:
1177 A flag for whether item 3 (if present) is a URL (like
1178 "http://lists.perl.org" is), in which case there should be no section
1179 attribute; a Pod name (like "perldoc" and "Getopt::Std" are); or
1180 possibly a man page name (like "crontab(5)" is).
1184 The raw original LE<lt>...> content, before text is split on
1185 "|", "/", etc, and before EE<lt>...> codes are expanded.
1189 (The above were numbered only for concise reference below. It is not
1190 a requirement that these be passed as an actual list or array.)
1195 => undef, # link text
1196 "Foo::Bar", # possibly inferred link text
1199 'pod', # what sort of link
1200 "Foo::Bar" # original content
1202 L<Perlport's section on NL's|perlport/Newlines>
1203 => "Perlport's section on NL's", # link text
1204 "Perlport's section on NL's", # possibly inferred link text
1206 "Newlines", # section
1207 'pod', # what sort of link
1208 "Perlport's section on NL's|perlport/Newlines"
1211 L<perlport/Newlines>
1212 => undef, # link text
1213 '"Newlines" in perlport', # possibly inferred link text
1215 "Newlines", # section
1216 'pod', # what sort of link
1217 "perlport/Newlines" # original content
1219 L<crontab(5)/"DESCRIPTION">
1220 => undef, # link text
1221 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
1222 "crontab(5)", # name
1223 "DESCRIPTION", # section
1224 'man', # what sort of link
1225 'crontab(5)/"DESCRIPTION"' # original content
1227 L</Object Attributes>
1228 => undef, # link text
1229 '"Object Attributes"', # possibly inferred link text
1231 "Object Attributes", # section
1232 'pod', # what sort of link
1233 "/Object Attributes" # original content
1235 L<https://www.perl.org/>
1236 => undef, # link text
1237 "https://www.perl.org/", # possibly inferred link text
1238 "https://www.perl.org/", # name
1240 'url', # what sort of link
1241 "https://www.perl.org/" # original content
1243 L<Perl.org|https://www.perl.org/>
1244 => "Perl.org", # link text
1245 "https://www.perl.org/", # possibly inferred link text
1246 "https://www.perl.org/", # name
1248 'url', # what sort of link
1249 "Perl.org|https://www.perl.org/" # original content
1251 Note that you can distinguish URL-links from anything else by the
1252 fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So
1253 C<LE<lt>http://www.perl.comE<gt>> is a URL, but
1254 C<LE<lt>HTTP::ResponseE<gt>> isn't.
1258 In case of LE<lt>...> codes with no "text|" part in them,
1259 older formatters have exhibited great variation in actually displaying
1260 the link or cross reference. For example, LE<lt>crontab(5)> would render
1261 as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage"
1262 or just "C<crontab(5)>".
1264 Pod processors must now treat "text|"-less links as follows:
1266 L<name> => L<name|name>
1267 L</section> => L<"section"|/section>
1268 L<name/section> => L<"section" in name|name/section>
1272 Note that section names might contain markup. I.e., if a section
1275 =head2 About the C<-M> Operator
1279 =item About the C<-M> Operator
1281 then a link to it would look like this:
1283 L<somedoc/About the C<-M> Operator>
1285 Formatters may choose to ignore the markup for purposes of resolving
1286 the link and use only the renderable characters in the section name,
1289 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1294 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1295 Operator" in somedoc</a>
1299 Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>>
1300 links from C<LE<lt>name/itemE<gt>> links (and their targets). These
1301 have been merged syntactically and semantically in the current
1302 specification, and I<section> can refer either to a "=headI<n> Heading
1303 Content" command or to a "=item Item Content" command. This
1304 specification does not specify what behavior should be in the case
1305 of a given document having several things all seeming to produce the
1306 same I<section> identifier (e.g., in HTML, several things all producing
1307 the same I<anchorname> in <a name="I<anchorname>">...</a>
1308 elements). Where Pod processors can control this behavior, they should
1309 use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the
1310 I<first> "Bar" section in Foo.
1312 But for some processors/formats this cannot be easily controlled; as
1313 with the HTML example, the behavior of multiple ambiguous
1314 <a name="I<anchorname>">...</a> is most easily just left up to
1319 In a C<LE<lt>text|...E<gt>> code, text may contain formatting codes
1320 for formatting or for EE<lt>...> escapes, as in:
1322 L<B<ummE<234>stuff>|...>
1324 For C<LE<lt>...E<gt>> codes without a "name|" part, only
1325 C<EE<lt>...E<gt>> and C<ZE<lt>E<gt>> codes may occur. That is,
1326 authors should not use "C<LE<lt>BE<lt>Foo::BarE<gt>E<gt>>".
1328 Note, however, that formatting codes and ZE<lt>>'s can occur in any
1329 and all parts of an LE<lt>...> (i.e., in I<name>, I<section>, I<text>,
1332 Authors must not nest LE<lt>...> codes. For example, "LE<lt>The
1333 LE<lt>Foo::Bar> man page>" should be treated as an error.
1337 Note that Pod authors may use formatting codes inside the "text"
1338 part of "LE<lt>text|name>" (and so on for LE<lt>text|/"sec">).
1340 In other words, this is valid:
1342 Go read L<the docs on C<$.>|perlvar/"$.">
1344 Some output formats that do allow rendering "LE<lt>...>" codes as
1345 hypertext, might not allow the link-text to be formatted; in
1346 that case, formatters will have to just ignore that formatting.
1350 At time of writing, C<LE<lt>nameE<gt>> values are of two types:
1351 either the name of a Pod page like C<LE<lt>Foo::BarE<gt>> (which
1352 might be a real Perl module or program in an @INC / PATH
1353 directory, or a .pod file in those places); or the name of a Unix
1354 man page, like C<LE<lt>crontab(5)E<gt>>. In theory, C<LE<lt>chmodE<gt>>
1355 is ambiguous between a Pod page called "chmod", or the Unix man page
1356 "chmod" (in whatever man-section). However, the presence of a string
1357 in parens, as in "crontab(5)", is sufficient to signal that what
1358 is being discussed is not a Pod page, and so is presumably a
1359 Unix man page. The distinction is of no importance to many
1360 Pod processors, but some processors that render to hypertext formats
1361 may need to distinguish them in order to know how to render a
1362 given C<LE<lt>fooE<gt>> code.
1366 Previous versions of perlpod allowed for a C<LE<lt>sectionE<gt>> syntax (as in
1367 C<LE<lt>Object AttributesE<gt>>), which was not easily distinguishable from
1368 C<LE<lt>nameE<gt>> syntax and for C<LE<lt>"section"E<gt>> which was only
1369 slightly less ambiguous. This syntax is no longer in the specification, and
1370 has been replaced by the C<LE<lt>/sectionE<gt>> syntax (where the slash was
1371 formerly optional). Pod parsers should tolerate the C<LE<lt>"section"E<gt>>
1372 syntax, for a while at least. The suggested heuristic for distinguishing
1373 C<LE<lt>sectionE<gt>> from C<LE<lt>nameE<gt>> is that if it contains any
1374 whitespace, it's a I<section>. Pod processors should warn about this being
1379 =head1 About =over...=back Regions
1381 "=over"..."=back" regions are used for various kinds of list-like
1382 structures. (I use the term "region" here simply as a collective
1383 term for everything from the "=over" to the matching "=back".)
1389 The non-zero numeric I<indentlevel> in "=over I<indentlevel>" ...
1390 "=back" is used for giving the formatter a clue as to how many
1391 "spaces" (ems, or roughly equivalent units) it should tab over,
1392 although many formatters will have to convert this to an absolute
1393 measurement that may not exactly match with the size of spaces (or M's)
1394 in the document's base font. Other formatters may have to completely
1395 ignore the number. The lack of any explicit I<indentlevel> parameter is
1396 equivalent to an I<indentlevel> value of 4. Pod processors may
1397 complain if I<indentlevel> is present but is not a positive number
1398 matching C<m/\A(\d*\.)?\d+\z/>.
1402 Authors of Pod formatters are reminded that "=over" ... "=back" may
1403 map to several different constructs in your output format. For
1404 example, in converting Pod to (X)HTML, it can map to any of
1405 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1406 <blockquote>...</blockquote>. Similarly, "=item" can map to <li> or
1411 Each "=over" ... "=back" region should be one of the following:
1417 An "=over" ... "=back" region containing only "=item *" commands,
1418 each followed by some number of ordinary/verbatim paragraphs, other
1419 nested "=over" ... "=back" regions, "=for..." paragraphs, and
1420 "=begin"..."=end" regions.
1422 (Pod processors must tolerate a bare "=item" as if it were "=item
1423 *".) Whether "*" is rendered as a literal asterisk, an "o", or as
1424 some kind of real bullet character, is left up to the Pod formatter,
1425 and may depend on the level of nesting.
1429 An "=over" ... "=back" region containing only
1430 C<m/\A=item\s+\d+\.?\s*\z/> paragraphs, each one (or each group of them)
1431 followed by some number of ordinary/verbatim paragraphs, other nested
1432 "=over" ... "=back" regions, "=for..." paragraphs, and/or
1433 "=begin"..."=end" codes. Note that the numbers must start at 1
1434 in each section, and must proceed in order and without skipping
1437 (Pod processors must tolerate lines like "=item 1" as if they were
1438 "=item 1.", with the period.)
1442 An "=over" ... "=back" region containing only "=item [text]"
1443 commands, each one (or each group of them) followed by some number of
1444 ordinary/verbatim paragraphs, other nested "=over" ... "=back"
1445 regions, or "=for..." paragraphs, and "=begin"..."=end" regions.
1447 The "=item [text]" paragraph should not match
1448 C<m/\A=item\s+\d+\.?\s*\z/> or C<m/\A=item\s+\*\s*\z/>, nor should it
1449 match just C<m/\A=item\s*\z/>.
1453 An "=over" ... "=back" region containing no "=item" paragraphs at
1454 all, and containing only some number of
1455 ordinary/verbatim paragraphs, and possibly also some nested "=over"
1456 ... "=back" regions, "=for..." paragraphs, and "=begin"..."=end"
1457 regions. Such an itemless "=over" ... "=back" region in Pod is
1458 equivalent in meaning to a "<blockquote>...</blockquote>" element in
1463 Note that with all the above cases, you can determine which type of
1464 "=over" ... "=back" you have, by examining the first (non-"=cut",
1465 non-"=pod") Pod paragraph after the "=over" command.
1469 Pod formatters I<must> tolerate arbitrarily large amounts of text
1470 in the "=item I<text...>" paragraph. In practice, most such
1471 paragraphs are short, as in:
1473 =item For cutting off our trade with all parts of the world
1475 But they may be arbitrarily long:
1477 =item For transporting us beyond seas to be tried for pretended
1480 =item He is at this time transporting large armies of foreign
1481 mercenaries to complete the works of death, desolation and
1482 tyranny, already begun with circumstances of cruelty and perfidy
1483 scarcely paralleled in the most barbarous ages, and totally
1484 unworthy the head of a civilized nation.
1488 Pod processors should tolerate "=item *" / "=item I<number>" commands
1489 with no accompanying paragraph. The middle item is an example:
1495 Pick up dry cleaning.
1501 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1507 No "=over" ... "=back" region can contain headings. Processors may
1508 treat such a heading as an error.
1512 Note that an "=over" ... "=back" region should have some
1513 content. That is, authors should not have an empty region like this:
1519 Pod processors seeing such a contentless "=over" ... "=back" region,
1520 may ignore it, or may report it as an error.
1524 Processors must tolerate an "=over" list that goes off the end of the
1525 document (i.e., which has no matching "=back"), but they may warn
1530 Authors of Pod formatters should note that this construct:
1538 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1539 velit, sed quia non numquam eius modi tempora incidunt ut
1540 labore et dolore magnam aliquam quaerat voluptatem.
1544 is semantically ambiguous, in a way that makes formatting decisions
1545 a bit difficult. On the one hand, it could be mention of an item
1546 "Neque", mention of another item "Porro", and mention of another
1547 item "Quisquam Est", with just the last one requiring the explanatory
1548 paragraph "Qui dolorem ipsum quia dolor..."; and then an item
1549 "Ut Enim". In that case, you'd want to format it like so:
1556 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1557 velit, sed quia non numquam eius modi tempora incidunt ut
1558 labore et dolore magnam aliquam quaerat voluptatem.
1562 But it could equally well be a discussion of three (related or equivalent)
1563 items, "Neque", "Porro", and "Quisquam Est", followed by a paragraph
1564 explaining them all, and then a new item "Ut Enim". In that case, you'd
1565 probably want to format it like so:
1570 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1571 velit, sed quia non numquam eius modi tempora incidunt ut
1572 labore et dolore magnam aliquam quaerat voluptatem.
1576 But (for the foreseeable future), Pod does not provide any way for Pod
1577 authors to distinguish which grouping is meant by the above
1578 "=item"-cluster structure. So formatters should format it like so:
1586 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1587 velit, sed quia non numquam eius modi tempora incidunt ut
1588 labore et dolore magnam aliquam quaerat voluptatem.
1592 That is, there should be (at least roughly) equal spacing between
1593 items as between paragraphs (although that spacing may well be less
1594 than the full height of a line of text). This leaves it to the reader
1595 to use (con)textual cues to figure out whether the "Qui dolorem
1596 ipsum..." paragraph applies to the "Quisquam Est" item or to all three
1597 items "Neque", "Porro", and "Quisquam Est". While not an ideal
1598 situation, this is preferable to providing formatting cues that may
1599 be actually contrary to the author's intent.
1605 =head1 About Data Paragraphs and "=begin/=end" Regions
1607 Data paragraphs are typically used for inlining non-Pod data that is
1608 to be used (typically passed through) when rendering the document to
1613 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1617 The exact same effect could, incidentally, be achieved with a single
1620 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1622 (Although that is not formally a data paragraph, it has the same
1623 meaning as one, and Pod parsers may parse it as one.)
1625 Another example of a data paragraph:
1629 I like <em>PIE</em>!
1631 <hr>Especially pecan pie!
1635 If these were ordinary paragraphs, the Pod parser would try to
1636 expand the "EE<lt>/em>" (in the first paragraph) as a formatting
1637 code, just like "EE<lt>lt>" or "EE<lt>eacute>". But since this
1638 is in a "=begin I<identifier>"..."=end I<identifier>" region I<and>
1639 the identifier "html" doesn't begin have a ":" prefix, the contents
1640 of this region are stored as data paragraphs, instead of being
1641 processed as ordinary paragraphs (or if they began with a spaces
1642 and/or tabs, as verbatim paragraphs).
1644 As a further example: At time of writing, no "biblio" identifier is
1645 supported, but suppose some processor were written to recognize it as
1646 a way of (say) denoting a bibliographic reference (necessarily
1647 containing formatting codes in ordinary paragraphs). The fact that
1648 "biblio" paragraphs were meant for ordinary processing would be
1649 indicated by prefacing each "biblio" identifier with a colon:
1653 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1654 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1658 This would signal to the parser that paragraphs in this begin...end
1659 region are subject to normal handling as ordinary/verbatim paragraphs
1660 (while still tagged as meant only for processors that understand the
1661 "biblio" identifier). The same effect could be had with:
1664 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1665 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1667 The ":" on these identifiers means simply "process this stuff
1668 normally, even though the result will be for some special target".
1669 I suggest that parser APIs report "biblio" as the target identifier,
1670 but also report that it had a ":" prefix. (And similarly, with the
1671 above "html", report "html" as the target identifier, and note the
1672 I<lack> of a ":" prefix.)
1674 Note that a "=begin I<identifier>"..."=end I<identifier>" region where
1675 I<identifier> begins with a colon, I<can> contain commands. For example:
1679 Wirth's classic is available in several editions, including:
1682 hm, check abebooks.com for how much used copies cost.
1688 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1689 Teubner, Stuttgart. [Yes, it's in German.]
1693 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1694 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1700 Note, however, a "=begin I<identifier>"..."=end I<identifier>"
1701 region where I<identifier> does I<not> begin with a colon, should not
1702 directly contain "=head1" ... "=head4" commands, nor "=over", nor "=back",
1703 nor "=item". For example, this may be considered invalid:
1707 This is a data paragraph.
1709 =head1 Don't do this!
1711 This is a data paragraph too.
1715 A Pod processor may signal that the above (specifically the "=head1"
1716 paragraph) is an error. Note, however, that the following should
1717 I<not> be treated as an error:
1721 This is a data paragraph.
1725 # Yup, this isn't Pod anymore.
1726 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1730 This is a data paragraph too.
1734 And this too is valid:
1738 This is a data paragraph.
1740 And this is a data paragraph.
1742 =begin someotherformat
1744 This is a data paragraph too.
1746 And this is a data paragraph too.
1748 =begin :yetanotherformat
1750 =head2 This is a command paragraph!
1752 This is an ordinary paragraph!
1754 And this is a verbatim paragraph!
1756 =end :yetanotherformat
1758 =end someotherformat
1760 Another data paragraph!
1764 The contents of the above "=begin :yetanotherformat" ...
1765 "=end :yetanotherformat" region I<aren't> data paragraphs, because
1766 the immediately containing region's identifier (":yetanotherformat")
1767 begins with a colon. In practice, most regions that contain
1768 data paragraphs will contain I<only> data paragraphs; however,
1769 the above nesting is syntactically valid as Pod, even if it is
1770 rare. However, the handlers for some formats, like "html",
1771 will accept only data paragraphs, not nested regions; and they may
1772 complain if they see (targeted for them) nested regions, or commands,
1773 other than "=end", "=pod", and "=cut".
1775 Also consider this valid structure:
1779 Wirth's classic is available in several editions, including:
1785 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1786 Teubner, Stuttgart. [Yes, it's in German.]
1790 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1791 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1799 <img src='wirth_spokesmodeling_book.png'>
1809 There, the "=begin html"..."=end html" region is nested inside
1810 the larger "=begin :biblio"..."=end :biblio" region. Note that the
1811 content of the "=begin html"..."=end html" region is data
1812 paragraph(s), because the immediately containing region's identifier
1813 ("html") I<doesn't> begin with a colon.
1815 Pod parsers, when processing a series of data paragraphs one
1816 after another (within a single region), should consider them to
1817 be one large data paragraph that happens to contain blank lines. So
1818 the content of the above "=begin html"..."=end html" I<may> be stored
1819 as two data paragraphs (one consisting of
1820 "<img src='wirth_spokesmodeling_book.png'>\n"
1821 and another consisting of "<hr>\n"), but I<should> be stored as
1822 a single data paragraph (consisting of
1823 "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1825 Pod processors should tolerate empty
1826 "=begin I<something>"..."=end I<something>" regions,
1827 empty "=begin :I<something>"..."=end :I<something>" regions, and
1828 contentless "=for I<something>" and "=for :I<something>"
1829 paragraphs. I.e., these should be tolerated:
1841 Incidentally, note that there's no easy way to express a data
1842 paragraph starting with something that looks like a command. Consider:
1850 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a data
1851 paragraph "=shazbot\n". However, you can express a data paragraph consisting
1852 of "=shazbot\n" using this code:
1856 The situation where this is necessary, is presumably quite rare.
1858 Note that =end commands must match the currently open =begin command. That
1859 is, they must properly nest. For example, this is valid:
1875 while this is invalid:
1891 This latter is improper because when the "=end outer" command is seen, the
1892 currently open region has the formatname "inner", not "outer". (It just
1893 happens that "outer" is the format name of a higher-up region.) This is
1894 an error. Processors must by default report this as an error, and may halt
1895 processing the document containing that error. A corollary of this is that
1896 regions cannot "overlap". That is, the latter block above does not represent
1897 a region called "outer" which contains X and Y, overlapping a region called
1898 "inner" which contains Y and Z. But because it is invalid (as all
1899 apparently overlapping regions would be), it doesn't represent that, or
1902 Similarly, this is invalid:
1908 This is an error because the region is opened by "thing", and the "=end"
1909 tries to close "hting" [sic].
1911 This is also invalid:
1917 This is invalid because every "=end" command must have a formatname
1922 L<perlpod>, L<perlsyn/"PODs: Embedded Documentation">,
https://perl5.git.perl.org / perl5.git / blob