4 perlpodspec - Plain Old Documentation: format specification and notes
8 This document is detailed notes on the Pod markup language. Most
9 people will only have to read L<perlpod|perlpod> to know how to write
10 in Pod, but this document may answer some incidental questions to do
11 with parsing and rendering Pod.
13 In this document, "must" / "must not", "should" /
14 "should not", and "may" have their conventional (cf. RFC 2119)
15 meanings: "X must do Y" means that if X doesn't do Y, it's against
16 this specification, and should really be fixed. "X should do Y"
17 means that it's recommended, but X may fail to do Y, if there's a
18 good reason. "X may do Y" is merely a note that X can do Y at
19 will (although it is up to the reader to detect any connotation of
20 "and I think it would be I<nice> if X did Y" versus "it wouldn't
21 really I<bother> me if X did Y").
23 Notably, when I say "the parser should do Y", the
24 parser may fail to do Y, if the calling application explicitly
25 requests that the parser I<not> do Y. I often phrase this as
26 "the parser should, by default, do Y." This doesn't I<require>
27 the parser to provide an option for turning off whatever
28 feature Y is (like expanding tabs in verbatim paragraphs), although
29 it implicates that such an option I<may> be provided.
31 =head1 Pod Definitions
33 Pod is embedded in files, typically Perl source files, although you
34 can write a file that's nothing but Pod.
36 A B<line> in a file consists of zero or more non-newline characters,
37 terminated by either a newline or the end of the file.
39 A B<newline sequence> is usually a platform-dependent concept, but
40 Pod parsers should understand it to mean any of CR (ASCII 13), LF
41 (ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in
42 addition to any other system-specific meaning. The first CR/CRLF/LF
43 sequence in the file may be used as the basis for identifying the
44 newline sequence for parsing the rest of the file.
46 A B<blank line> is a line consisting entirely of zero or more spaces
47 (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file.
48 A B<non-blank line> is a line containing one or more characters other
49 than space or tab (and terminated by a newline or end-of-file).
51 (I<Note:> Many older Pod parsers did not accept a line consisting of
52 spaces/tabs and then a newline as a blank line. The only lines they
53 considered blank were lines consisting of I<no characters at all>,
54 terminated by a newline.)
56 B<Whitespace> is used in this document as a blanket term for spaces,
57 tabs, and newline sequences. (By itself, this term usually refers
58 to literal whitespace. That is, sequences of whitespace characters
59 in Pod source, as opposed to "EE<lt>32>", which is a formatting
60 code that I<denotes> a whitespace character.)
62 A B<Pod parser> is a module meant for parsing Pod (regardless of
63 whether this involves calling callbacks or building a parse tree or
64 directly formatting it). A B<Pod formatter> (or B<Pod translator>)
65 is a module or program that converts Pod to some other format (HTML,
66 plaintext, TeX, PostScript, RTF). A B<Pod processor> might be a
67 formatter or translator, or might be a program that does something
68 else with the Pod (like counting words, scanning for index points,
71 Pod content is contained in B<Pod blocks>. A Pod block starts with a
72 line that matches <m/\A=[a-zA-Z]/>, and continues up to the next line
73 that matches C<m/\A=cut/> or up to the end of the file if there is
77 The current perlsyn says:
79 Note that pod translators should look at only paragraphs beginning
80 with a pod directive (it makes parsing easier), whereas the compiler
81 actually knows to look for pod escapes even in the middle of a
82 paragraph. This means that the following secret stuff will be ignored
83 by both the compiler and the translators.
86 warn "Neither POD nor CODE!?"
89 You probably shouldn't rely upon the warn() being podded out forever.
90 Not all pod translators are well-behaved in this regard, and perhaps
91 the compiler will become pickier.
93 I think that those paragraphs should just be removed; paragraph-based
94 parsing seems to have been largely abandoned, because of the hassle
95 with non-empty blank lines messing up what people meant by "paragraph".
96 Even if the "it makes parsing easier" bit were especially true,
97 it wouldn't be worth the confusion of having perl and pod2whatever
98 actually disagree on what can constitute a Pod block.
100 Within a Pod block, there are B<Pod paragraphs>. A Pod paragraph
101 consists of non-blank lines of text, separated by one or more blank
104 For purposes of Pod processing, there are four types of paragraphs in
111 A command paragraph (also called a "directive"). The first line of
112 this paragraph must match C<m/\A=[a-zA-Z]/>. Command paragraphs are
113 typically one line, as in:
119 But they may span several (non-blank) lines:
122 Hm, I wonder what it would look like if
123 you tried to write a BNF for Pod from this.
125 =head3 Dr. Strangelove, or: How I Learned to
126 Stop Worrying and Love the Bomb
128 I<Some> command paragraphs allow formatting codes in their content
129 (i.e., after the part that matches C<m/\A=[a-zA-Z]\S*\s*/>), as in:
131 =head1 Did You Remember to C<use strict;>?
133 In other words, the Pod processing handler for "head1" will apply the
134 same processing to "Did You Remember to CE<lt>use strict;>?" that it
135 would to an ordinary paragraph (i.e., formatting codes like
136 "CE<lt>...>") are parsed and presumably formatted appropriately, and
137 whitespace in the form of literal spaces and/or tabs is not
142 A B<verbatim paragraph>. The first line of this paragraph must be a
143 literal space or tab, and this paragraph must not be inside a "=begin
144 I<identifier>", ... "=end I<identifier>" sequence unless
145 "I<identifier>" begins with a colon (":"). That is, if a paragraph
146 starts with a literal space or tab, but I<is> inside a
147 "=begin I<identifier>", ... "=end I<identifier>" region, then it's
148 a data paragraph, unless "I<identifier>" begins with a colon.
150 Whitespace I<is> significant in verbatim paragraphs (although, in
151 processing, tabs are probably expanded).
155 An B<ordinary paragraph>. A paragraph is an ordinary paragraph
156 if its first line matches neither C<m/\A=[a-zA-Z]/> nor
157 C<m/\A[ \t]/>, I<and> if it's not inside a "=begin I<identifier>",
158 ... "=end I<identifier>" sequence unless "I<identifier>" begins with
163 A B<data paragraph>. This is a paragraph that I<is> inside a "=begin
164 I<identifier>" ... "=end I<identifier>" sequence where
165 "I<identifier>" does I<not> begin with a literal colon (":"). In
166 some sense, a data paragraph is not part of Pod at all (i.e.,
167 effectively it's "out-of-band"), since it's not subject to most kinds
168 of Pod parsing; but it is specified here, since Pod
169 parsers need to be able to call an event for it, or store it in some
170 form in a parse tree, or at least just parse I<around> it.
174 For example: consider the following paragraphs:
176 # <- that's the 0th column
186 Here, "=head1 Foo" and "=cut" are command paragraphs because the first
187 line of each matches C<m/\A=[a-zA-Z]/>. "I<[space][space]>$foo->bar"
188 is a verbatim paragraph, because its first line starts with a literal
189 whitespace character (and there's no "=begin"..."=end" region around).
191 The "=begin I<identifier>" ... "=end I<identifier>" commands stop
192 paragraphs that they surround from being parsed as ordinary or verbatim
193 paragraphs, if I<identifier> doesn't begin with a colon. This
194 is discussed in detail in the section
195 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
199 This section is intended to supplement and clarify the discussion in
200 L<perlpod/"Command Paragraph">. These are the currently recognized
205 =item "=head1", "=head2", "=head3", "=head4"
207 This command indicates that the text in the remainder of the paragraph
208 is a heading. That text may contain formatting codes. Examples:
210 =head1 Object Attributes
212 =head3 What B<Not> to Do!
216 This command indicates that this paragraph begins a Pod block. (If we
217 are already in the middle of a Pod block, this command has no effect at
218 all.) If there is any text in this command paragraph after "=pod",
219 it must be ignored. Examples:
223 This is a plain Pod paragraph.
225 =pod This text is ignored.
229 This command indicates that this line is the end of this previously
230 started Pod block. If there is any text after "=cut" on the line, it must be
235 =cut The documentation ends here.
238 # This is the first line of program text.
239 sub foo { # This is the second.
241 It is an error to try to I<start> a Pod block with a "=cut" command. In
242 that case, the Pod processor must halt parsing of the input file, and
243 must by default emit a warning.
247 This command indicates that this is the start of a list/indent
248 region. If there is any text following the "=over", it must consist
249 of only a nonzero positive numeral. The semantics of this numeral is
250 explained in the L</"About =over...=back Regions"> section, further
251 below. Formatting codes are not expanded. Examples:
261 This command indicates that an item in a list begins here. Formatting
262 codes are processed. The semantics of the (optional) text in the
263 remainder of this paragraph are
264 explained in the L</"About =over...=back Regions"> section, further
277 =item C<< $thing->stuff(I<dodad>) >>
279 =item For transporting us beyond seas to be tried for pretended
282 =item He is at this time transporting large armies of foreign
283 mercenaries to complete the works of death, desolation and
284 tyranny, already begun with circumstances of cruelty and perfidy
285 scarcely paralleled in the most barbarous ages, and totally
286 unworthy the head of a civilized nation.
290 This command indicates that this is the end of the region begun
291 by the most recent "=over" command. It permits no text after the
294 =item "=begin formatname"
296 =item "=begin formatname parameter"
298 This marks the following paragraphs (until the matching "=end
299 formatname") as being for some special kind of processing. Unless
300 "formatname" begins with a colon, the contained non-command
301 paragraphs are data paragraphs. But if "formatname" I<does> begin
302 with a colon, then non-command paragraphs are ordinary paragraphs
303 or data paragraphs. This is discussed in detail in the section
304 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
306 It is advised that formatnames match the regexp
307 C<m/\A:?[−a−zA−Z0−9_]+\z/>. Everything following whitespace after the
308 formatname is a parameter that may be used by the formatter when dealing
309 with this region. This parameter must not be repeated in the "=end"
310 paragraph. Implementors should anticipate future expansion in the
311 semantics and syntax of the first parameter to "=begin"/"=end"/"=for".
313 =item "=end formatname"
315 This marks the end of the region opened by the matching
316 "=begin formatname" region. If "formatname" is not the formatname
317 of the most recent open "=begin formatname" region, then this
318 is an error, and must generate an error message. This
319 is discussed in detail in the section
320 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
322 =item "=for formatname text..."
324 This is synonymous with:
332 That is, it creates a region consisting of a single paragraph; that
333 paragraph is to be treated as a normal paragraph if "formatname"
334 begins with a ":"; if "formatname" I<doesn't> begin with a colon,
335 then "text..." will constitute a data paragraph. There is no way
336 to use "=for formatname text..." to express "text..." as a verbatim
339 =item "=encoding encodingname"
341 This command, which should occur early in the document (at least
342 before any non-US-ASCII data!), declares that this document is
343 encoded in the encoding I<encodingname>, which must be
344 an encoding name that L<Encode> recognizes. (Encode's list
345 of supported encodings, in L<Encode::Supported>, is useful here.)
346 If the Pod parser cannot decode the declared encoding, it
347 should emit a warning and may abort parsing the document
350 A document having more than one "=encoding" line should be
351 considered an error. Pod processors may silently tolerate this if
352 the not-first "=encoding" lines are just duplicates of the
353 first one (e.g., if there's a "=encoding utf8" line, and later on
354 another "=encoding utf8" line). But Pod processors should complain if
355 there are contradictory "=encoding" lines in the same document
356 (e.g., if there is a "=encoding utf8" early in the document and
357 "=encoding big5" later). Pod processors that recognize BOMs
358 may also complain if they see an "=encoding" line
359 that contradicts the BOM (e.g., if a document with a UTF-16LE
360 BOM has an "=encoding shiftjis" line).
364 If a Pod processor sees any command other than the ones listed
365 above (like "=head", or "=haed1", or "=stuff", or "=cuttlefish",
366 or "=w123"), that processor must by default treat this as an
367 error. It must not process the paragraph beginning with that
368 command, must by default warn of this as an error, and may
369 abort the parse. A Pod parser may allow a way for particular
370 applications to add to the above list of known commands, and to
371 stipulate, for each additional command, whether formatting
372 codes should be processed.
374 Future versions of this specification may add additional
379 =head1 Pod Formatting Codes
381 (Note that in previous drafts of this document and of perlpod,
382 formatting codes were referred to as "interior sequences", and
383 this term may still be found in the documentation for Pod parsers,
384 and in error messages from Pod processors.)
386 There are two syntaxes for formatting codes:
392 A formatting code starts with a capital letter (just US-ASCII [A-Z])
393 followed by a "<", any number of characters, and ending with the first
394 matching ">". Examples:
396 That's what I<you> think!
398 What's C<dump()> for?
400 X<C<chmod> and C<unlink()> Under Different Operating Systems>
404 A formatting code starts with a capital letter (just US-ASCII [A-Z])
405 followed by two or more "<"'s, one or more whitespace characters,
406 any number of characters, one or more whitespace characters,
407 and ending with the first matching sequence of two or more ">"'s, where
408 the number of ">"'s equals the number of "<"'s in the opening of this
409 formatting code. Examples:
411 That's what I<< you >> think!
413 C<<< open(X, ">>thing.dat") || die $! >>>
417 With this syntax, the whitespace character(s) after the "CE<lt><<"
418 and before the ">>" (or whatever letter) are I<not> renderable. They
419 do not signify whitespace, are merely part of the formatting codes
420 themselves. That is, these are all synonymous:
432 Finally, the multiple-angle-bracket form does I<not> alter the interpretation
433 of nested formatting codes, meaning that the following four example lines are
434 identical in meaning:
436 B<example: C<$a E<lt>=E<gt> $b>>
438 B<example: C<< $a <=> $b >>>
440 B<example: C<< $a E<lt>=E<gt> $b >>>
442 B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
446 In parsing Pod, a notably tricky part is the correct parsing of
447 (potentially nested!) formatting codes. Implementors should
448 consult the code in the C<parse_text> routine in Pod::Parser as an
449 example of a correct implementation.
453 =item C<IE<lt>textE<gt>> -- italic text
455 See the brief discussion in L<perlpod/"Formatting Codes">.
457 =item C<BE<lt>textE<gt>> -- bold text
459 See the brief discussion in L<perlpod/"Formatting Codes">.
461 =item C<CE<lt>codeE<gt>> -- code text
463 See the brief discussion in L<perlpod/"Formatting Codes">.
465 =item C<FE<lt>filenameE<gt>> -- style for filenames
467 See the brief discussion in L<perlpod/"Formatting Codes">.
469 =item C<XE<lt>topic nameE<gt>> -- an index entry
471 See the brief discussion in L<perlpod/"Formatting Codes">.
473 This code is unusual in that most formatters completely discard
474 this code and its content. Other formatters will render it with
475 invisible codes that can be used in building an index of
476 the current document.
478 =item C<ZE<lt>E<gt>> -- a null (zero-effect) formatting code
480 Discussed briefly in L<perlpod/"Formatting Codes">.
482 This code is unusual is that it should have no content. That is,
483 a processor may complain if it sees C<ZE<lt>potatoesE<gt>>. Whether
484 or not it complains, the I<potatoes> text should ignored.
486 =item C<LE<lt>nameE<gt>> -- a hyperlink
488 The complicated syntaxes of this code are discussed at length in
489 L<perlpod/"Formatting Codes">, and implementation details are
490 discussed below, in L</"About LE<lt>...E<gt> Codes">. Parsing the
491 contents of LE<lt>content> is tricky. Notably, the content has to be
492 checked for whether it looks like a URL, or whether it has to be split
493 on literal "|" and/or "/" (in the right order!), and so on,
494 I<before> EE<lt>...> codes are resolved.
496 =item C<EE<lt>escapeE<gt>> -- a character escape
498 See L<perlpod/"Formatting Codes">, and several points in
499 L</Notes on Implementing Pod Processors>.
501 =item C<SE<lt>textE<gt>> -- text contains non-breaking spaces
503 This formatting code is syntactically simple, but semantically
504 complex. What it means is that each space in the printable
505 content of this code signifies a non-breaking space.
513 Both signify the monospace (c[ode] style) text consisting of
514 "$x", one space, "?", one space, ":", one space, "$z". The
515 difference is that in the latter, with the S code, those spaces
516 are not "normal" spaces, but instead are non-breaking spaces.
521 If a Pod processor sees any formatting code other than the ones
522 listed above (as in "NE<lt>...>", or "QE<lt>...>", etc.), that
523 processor must by default treat this as an error.
524 A Pod parser may allow a way for particular
525 applications to add to the above list of known formatting codes;
526 a Pod parser might even allow a way to stipulate, for each additional
527 command, whether it requires some form of special processing, as
530 Future versions of this specification may add additional
533 Historical note: A few older Pod processors would not see a ">" as
534 closing a "CE<lt>" code, if the ">" was immediately preceded by
535 a "-". This was so that this:
539 would parse as equivalent to this:
543 instead of as equivalent to a "C" formatting code containing
544 only "$foo-", and then a "bar>" outside the "C" formatting code. This
545 problem has since been solved by the addition of syntaxes like this:
549 Compliant parsers must not treat "->" as special.
551 Formatting codes absolutely cannot span paragraphs. If a code is
552 opened in one paragraph, and no closing code is found by the end of
553 that paragraph, the Pod parser must close that formatting code,
554 and should complain (as in "Unterminated I code in the paragraph
555 starting at line 123: 'Time objects are not...'"). So these
558 I<I told you not to do this!
560 Don't make me say it again!>
562 ...must I<not> be parsed as two paragraphs in italics (with the I
563 code starting in one paragraph and starting in another.) Instead,
564 the first paragraph should generate a warning, but that aside, the
565 above code must parse as if it were:
567 I<I told you not to do this!>
569 Don't make me say it again!E<gt>
571 (In SGMLish jargon, all Pod commands are like block-level
572 elements, whereas all Pod formatting codes are like inline-level
577 =head1 Notes on Implementing Pod Processors
579 The following is a long section of miscellaneous requirements
580 and suggestions to do with Pod processing.
586 Pod formatters should tolerate lines in verbatim blocks that are of
587 any length, even if that means having to break them (possibly several
588 times, for very long lines) to avoid text running off the side of the
589 page. Pod formatters may warn of such line-breaking. Such warnings
590 are particularly appropriate for lines are over 100 characters long, which
591 are usually not intentional.
595 Pod parsers must recognize I<all> of the three well-known newline
596 formats: CR, LF, and CRLF. See L<perlport|perlport>.
600 Pod parsers should accept input lines that are of any length.
604 Since Perl recognizes a Unicode Byte Order Mark at the start of files
605 as signaling that the file is Unicode encoded as in UTF-16 (whether
606 big-endian or little-endian) or UTF-8, Pod parsers should do the
607 same. Otherwise, the character encoding should be understood as
608 being UTF-8 if the first highbit byte sequence in the file seems
609 valid as a UTF-8 sequence, or otherwise as Latin-1.
611 Future versions of this specification may specify
612 how Pod can accept other encodings. Presumably treatment of other
613 encodings in Pod parsing would be as in XML parsing: whatever the
614 encoding declared by a particular Pod file, content is to be
615 stored in memory as Unicode characters.
619 The well known Unicode Byte Order Marks are as follows: if the
620 file begins with the two literal byte values 0xFE 0xFF, this is
621 the BOM for big-endian UTF-16. If the file begins with the two
622 literal byte value 0xFF 0xFE, this is the BOM for little-endian
623 UTF-16. If the file begins with the three literal byte values
624 0xEF 0xBB 0xBF, this is the BOM for UTF-8.
627 use bytes; print map sprintf(" 0x%02X", ord $_), split '', "\x{feff}";
631 If toke.c is modified to support UTF-32, add mention of those here.
635 A naive but sufficient heuristic for testing the first highbit
636 byte-sequence in a BOM-less file (whether in code or in Pod!), to see
637 whether that sequence is valid as UTF-8 (RFC 2279) is to check whether
638 that the first byte in the sequence is in the range 0xC0 - 0xFD
639 I<and> whether the next byte is in the range
640 0x80 - 0xBF. If so, the parser may conclude that this file is in
641 UTF-8, and all highbit sequences in the file should be assumed to
642 be UTF-8. Otherwise the parser should treat the file as being
643 in Latin-1. In the unlikely circumstance that the first highbit
644 sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one
645 can cater to our heuristic (as well as any more intelligent heuristic)
646 by prefacing that line with a comment line containing a highbit
647 sequence that is clearly I<not> valid as UTF-8. A line consisting
648 of simply "#", an e-acute, and any non-highbit byte,
649 is sufficient to establish this file's encoding.
652 If/WHEN some brave soul makes these heuristics into a generic
653 text-file class (or PerlIO layer?), we can presumably delete
654 mention of these icky details from this file, and can instead
655 tell people to just use appropriate class/layer.
656 Auto-recognition of newline sequences would be another desirable
657 feature of such a class/layer.
661 "The probability that a string of characters
662 in any other encoding appears as valid UTF-8 is low" - RFC2279
666 This document's requirements and suggestions about encodings
667 do not apply to Pod processors running on non-ASCII platforms,
668 notably EBCDIC platforms.
672 Pod processors must treat a "=for [label] [content...]" paragraph as
673 meaning the same thing as a "=begin [label]" paragraph, content, and
674 an "=end [label]" paragraph. (The parser may conflate these two
675 constructs, or may leave them distinct, in the expectation that the
676 formatter will nevertheless treat them the same.)
680 When rendering Pod to a format that allows comments (i.e., to nearly
681 any format other than plaintext), a Pod formatter must insert comment
682 text identifying its name and version number, and the name and
683 version numbers of any modules it might be using to process the Pod.
686 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
688 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
690 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
692 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
694 Formatters may also insert additional comments, including: the
695 release date of the Pod formatter program, the contact address for
696 the author(s) of the formatter, the current time, the name of input
697 file, the formatting options in effect, version of Perl used, etc.
699 Formatters may also choose to note errors/warnings as comments,
700 besides or instead of emitting them otherwise (as in messages to
701 STDERR, or C<die>ing).
705 Pod parsers I<may> emit warnings or error messages ("Unknown E code
706 EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or
707 C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> allow
708 suppressing all such STDERR output, and instead allow an option for
709 reporting errors/warnings
710 in some other way, whether by triggering a callback, or noting errors
711 in some attribute of the document object, or some similarly unobtrusive
712 mechanism -- or even by appending a "Pod Errors" section to the end of
713 the parsed form of the document.
717 In cases of exceptionally aberrant documents, Pod parsers may abort the
718 parse. Even then, using C<die>ing/C<croak>ing is to be avoided; where
719 possible, the parser library may simply close the input file
720 and add text like "*** Formatting Aborted ***" to the end of the
721 (partial) in-memory document.
725 In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>)
726 are understood (i.e., I<not> verbatim paragraphs, but I<including>
727 ordinary paragraphs, and command paragraphs that produce renderable
728 text, like "=head1"), literal whitespace should generally be considered
729 "insignificant", in that one literal space has the same meaning as any
730 (nonzero) number of literal spaces, literal newlines, and literal tabs
731 (as long as this produces no blank lines, since those would terminate
732 the paragraph). Pod parsers should compact literal whitespace in each
733 processed paragraph, but may provide an option for overriding this
734 (since some processing tasks do not require it), or may follow
735 additional special rules (for example, specially treating
736 period-space-space or period-newline sequences).
740 Pod parsers should not, by default, try to coerce apostrophe (') and
741 quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to
742 turn backtick (`) into anything else but a single backtick character
743 (distinct from an open quote character!), nor "--" into anything but
744 two minus signs. They I<must never> do any of those things to text
745 in CE<lt>...> formatting codes, and never I<ever> to text in verbatim
750 When rendering Pod to a format that has two kinds of hyphens (-), one
751 that's a non-breaking hyphen, and another that's a breakable hyphen
752 (as in "object-oriented", which can be split across lines as
753 "object-", newline, "oriented"), formatters are encouraged to
754 generally translate "-" to non-breaking hyphen, but may apply
755 heuristics to convert some of these to breaking hyphens.
759 Pod formatters should make reasonable efforts to keep words of Perl
760 code from being broken across lines. For example, "Foo::Bar" in some
761 formatting systems is seen as eligible for being broken across lines
762 as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should
763 be avoided where possible, either by disabling all line-breaking in
764 mid-word, or by wrapping particular words with internal punctuation
765 in "don't break this across lines" codes (which in some formats may
766 not be a single code, but might be a matter of inserting non-breaking
767 zero-width spaces between every pair of characters in a word.)
771 Pod parsers should, by default, expand tabs in verbatim paragraphs as
772 they are processed, before passing them to the formatter or other
773 processor. Parsers may also allow an option for overriding this.
777 Pod parsers should, by default, remove newlines from the end of
778 ordinary and verbatim paragraphs before passing them to the
779 formatter. For example, while the paragraph you're reading now
780 could be considered, in Pod source, to end with (and contain)
781 the newline(s) that end it, it should be processed as ending with
782 (and containing) the period character that ends this sentence.
786 Pod parsers, when reporting errors, should make some effort to report
787 an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near
788 line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph
789 number ("Nested EE<lt>>'s in Paragraph #52 of Thing/Foo.pm!"). Where
790 this is problematic, the paragraph number should at least be
791 accompanied by an excerpt from the paragraph ("Nested EE<lt>>'s in
792 Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for
793 the CE<lt>interest rate> attribute...'").
797 Pod parsers, when processing a series of verbatim paragraphs one
798 after another, should consider them to be one large verbatim
799 paragraph that happens to contain blank lines. I.e., these two
800 lines, which have a blank line between them:
806 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
807 Foo->VERSION") before being passed to the formatter or other
808 processor. Parsers may also allow an option for overriding this.
810 While this might be too cumbersome to implement in event-based Pod
811 parsers, it is straightforward for parsers that return parse trees.
815 Pod formatters, where feasible, are advised to avoid splitting short
816 verbatim paragraphs (under twelve lines, say) across pages.
820 Pod parsers must treat a line with only spaces and/or tabs on it as a
821 "blank line" such as separates paragraphs. (Some older parsers
822 recognized only two adjacent newlines as a "blank line" but would not
823 recognize a newline, a space, and a newline, as a blank line. This
824 is noncompliant behavior.)
828 Authors of Pod formatters/processors should make every effort to
829 avoid writing their own Pod parser. There are already several in
830 CPAN, with a wide range of interface styles -- and one of them,
831 Pod::Parser, comes with modern versions of Perl.
835 Characters in Pod documents may be conveyed either as literals, or by
836 number in EE<lt>n> codes, or by an equivalent mnemonic, as in
837 EE<lt>eacute> which is exactly equivalent to EE<lt>233>.
839 Characters in the range 32-126 refer to those well known US-ASCII
840 characters (also defined there by Unicode, with the same meaning),
841 which all Pod formatters must render faithfully. Characters
842 in the ranges 0-31 and 127-159 should not be used (neither as
843 literals, nor as EE<lt>number> codes), except for the
844 literal byte-sequences for newline (13, 13 10, or 10), and tab (9).
846 Characters in the range 160-255 refer to Latin-1 characters (also
847 defined there by Unicode, with the same meaning). Characters above
848 255 should be understood to refer to Unicode characters.
853 that some formatters cannot reliably render characters outside 32-126;
854 and many are able to handle 32-126 and 160-255, but nothing above
859 Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for
860 less-than and greater-than, Pod parsers must understand "EE<lt>sol>"
861 for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar,
862 pipe). Pod parsers should also understand "EE<lt>lchevron>" and
863 "EE<lt>rchevron>" as legacy codes for characters 171 and 187, i.e.,
864 "left-pointing double angle quotation mark" = "left pointing
865 guillemet" and "right-pointing double angle quotation mark" = "right
866 pointing guillemet". (These look like little "<<" and ">>", and they
867 are now preferably expressed with the HTML/XHTML codes "EE<lt>laquo>"
872 Pod parsers should understand all "EE<lt>html>" codes as defined
873 in the entity declarations in the most recent XHTML specification at
874 C<www.W3.org>. Pod parsers must understand at least the entities
875 that define characters in the range 160-255 (Latin-1). Pod parsers,
876 when faced with some unknown "EE<lt>I<identifier>>" code,
877 shouldn't simply replace it with nullstring (by default, at least),
878 but may pass it through as a string consisting of the literal characters
879 E, less-than, I<identifier>, greater-than. Or Pod parsers may offer the
880 alternative option of processing such unknown
881 "EE<lt>I<identifier>>" codes by firing an event especially
882 for such codes, or by adding a special node-type to the in-memory
883 document tree. Such "EE<lt>I<identifier>>" may have special meaning
884 to some processors, or some processors may choose to add them to
885 a special error report.
889 Pod parsers must also support the XHTML codes "EE<lt>quot>" for
890 character 34 (doublequote, "), "EE<lt>amp>" for character 38
891 (ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, ').
895 Note that in all cases of "EE<lt>whatever>", I<whatever> (whether
896 an htmlname, or a number in any base) must consist only of
897 alphanumeric characters -- that is, I<whatever> must watch
898 C<m/\A\w+\z/>. So "EE<lt> 0 1 2 3 >" is invalid, because
899 it contains spaces, which aren't alphanumeric characters. This
900 presumably does not I<need> special treatment by a Pod processor;
901 " 0 1 2 3 " doesn't look like a number in any base, so it would
902 presumably be looked up in the table of HTML-like names. Since
903 there isn't (and cannot be) an HTML-like entity called " 0 1 2 3 ",
904 this will be treated as an error. However, Pod processors may
905 treat "EE<lt> 0 1 2 3 >" or "EE<lt>e-acute>" as I<syntactically>
906 invalid, potentially earning a different error message than the
907 error message (or warning, or event) generated by a merely unknown
908 (but theoretically valid) htmlname, as in "EE<lt>qacute>"
909 [sic]. However, Pod parsers are not required to make this
914 Note that EE<lt>number> I<must not> be interpreted as simply
915 "codepoint I<number> in the current/native character set". It always
916 means only "the character represented by codepoint I<number> in
917 Unicode." (This is identical to the semantics of &#I<number>; in XML.)
919 This will likely require many formatters to have tables mapping from
920 treatable Unicode codepoints (such as the "\xE9" for the e-acute
921 character) to the escape sequences or codes necessary for conveying
922 such sequences in the target output format. A converter to *roff
923 would, for example know that "\xE9" (whether conveyed literally, or via
924 a EE<lt>...> sequence) is to be conveyed as "e\\*'".
925 Similarly, a program rendering Pod in a Mac OS application window, would
926 presumably need to know that "\xE9" maps to codepoint 142 in MacRoman
927 encoding that (at time of writing) is native for Mac OS. Such
928 Unicode2whatever mappings are presumably already widely available for
929 common output formats. (Such mappings may be incomplete! Implementers
930 are not expected to bend over backwards in an attempt to render
931 Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any
932 of the other weird things that Unicode can encode.) And
933 if a Pod document uses a character not found in such a mapping, the
934 formatter should consider it an unrenderable character.
938 If, surprisingly, the implementor of a Pod formatter can't find a
939 satisfactory pre-existing table mapping from Unicode characters to
940 escapes in the target format (e.g., a decent table of Unicode
941 characters to *roff escapes), it will be necessary to build such a
942 table. If you are in this circumstance, you should begin with the
943 characters in the range 0x00A0 - 0x00FF, which is mostly the heavily
944 used accented characters. Then proceed (as patience permits and
945 fastidiousness compels) through the characters that the (X)HTML
946 standards groups judged important enough to merit mnemonics
947 for. These are declared in the (X)HTML specifications at the
948 www.W3.org site. At time of writing (September 2001), the most recent
949 entity declaration files are:
951 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
952 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
953 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
955 Then you can progress through any remaining notable Unicode characters
956 in the range 0x2000-0x204D (consult the character tables at
957 www.unicode.org), and whatever else strikes your fancy. For example,
958 in F<xhtml-symbol.ent>, there is the entry:
960 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
962 While the mapping "infin" to the character "\x{221E}" will (hopefully)
963 have been already handled by the Pod parser, the presence of the
964 character in this file means that it's reasonably important enough to
965 include in a formatter's table that maps from notable Unicode characters
966 to the codes necessary for rendering them. So for a Unicode-to-*roff
967 mapping, for example, this would merit the entry:
969 "\x{221E}" => '\(in',
971 It is eagerly hoped that in the future, increasing numbers of formats
972 (and formatters) will support Unicode characters directly (as (X)HTML
973 does with C<∞>, C<∞>, or C<∞>), reducing the need
974 for idiosyncratic mappings of Unicode-to-I<my_escapes>.
978 It is up to individual Pod formatter to display good judgement when
979 confronted with an unrenderable character (which is distinct from an
980 unknown EE<lt>thing> sequence that the parser couldn't resolve to
981 anything, renderable or not). It is good practice to map Latin letters
982 with diacritics (like "EE<lt>eacute>"/"EE<lt>233>") to the corresponding
983 unaccented US-ASCII letters (like a simple character 101, "e"), but
984 clearly this is often not feasible, and an unrenderable character may
985 be represented as "?", or the like. In attempting a sane fallback
986 (as from EE<lt>233> to "e"), Pod formatters may use the
987 %Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or
988 L<Text::Unidecode|Text::Unidecode>, if available.
990 For example, this Pod text:
992 magic is enabled if you set C<$Currency> to 'E<euro>'.
995 "magic is enabled if you set C<$Currency> to 'I<?>'" or as
996 "magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as
997 "magic is enabled if you set C<$Currency> to '[x20AC]', etc.
999 A Pod formatter may also note, in a comment or warning, a list of what
1000 unrenderable characters were encountered.
1004 EE<lt>...> may freely appear in any formatting code (other than
1005 in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The
1006 EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The
1007 EE<lt>euro>1,000,000 Solution|Million::Euros>".
1011 Some Pod formatters output to formats that implement non-breaking
1012 spaces as an individual character (which I'll call "NBSP"), and
1013 others output to formats that implement non-breaking spaces just as
1014 spaces wrapped in a "don't break this across lines" code. Note that
1015 at the level of Pod, both sorts of codes can occur: Pod can contain a
1016 NBSP character (whether as a literal, or as a "EE<lt>160>" or
1017 "EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo
1018 IE<lt>barE<gt> baz>" codes, where "mere spaces" (character 32) in
1019 such codes are taken to represent non-breaking spaces. Pod
1020 parsers should consider supporting the optional parsing of "SE<lt>foo
1021 IE<lt>barE<gt> baz>" as if it were
1022 "fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the
1023 optional parsing of groups of words joined by NBSP's as if each group
1024 were in a SE<lt>...> code, so that formatters may use the
1025 representation that maps best to what the output format demands.
1029 Some processors may find that the C<SE<lt>...E<gt>> code is easiest to
1030 implement by replacing each space in the parse tree under the content
1031 of the S, with an NBSP. But note: the replacement should apply I<not> to
1032 spaces in I<all> text, but I<only> to spaces in I<printable> text. (This
1033 distinction may or may not be evident in the particular tree/event
1034 model implemented by the Pod parser.) For example, consider this
1037 S<L</Autoloaded Functions>>
1039 This means that the space in the middle of the visible link text must
1040 not be broken across lines. In other words, it's the same as this:
1042 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
1044 However, a misapplied space-to-NBSP replacement could (wrongly)
1045 produce something equivalent to this:
1047 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
1049 ...which is almost definitely not going to work as a hyperlink (assuming
1050 this formatter outputs a format supporting hypertext).
1052 Formatters may choose to just not support the S format code,
1053 especially in cases where the output format simply has no NBSP
1054 character/code and no code for "don't break this stuff across lines".
1058 Besides the NBSP character discussed above, implementors are reminded
1059 of the existence of the other "special" character in Latin-1, the
1060 "soft hyphen" character, also known as "discretionary hyphen",
1061 i.e. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> =
1062 C<EE<lt>shyE<gt>>). This character expresses an optional hyphenation
1063 point. That is, it normally renders as nothing, but may render as a
1064 "-" if a formatter breaks the word at that point. Pod formatters
1065 should, as appropriate, do one of the following: 1) render this with
1066 a code with the same meaning (e.g., "\-" in RTF), 2) pass it through
1067 in the expectation that the formatter understands this character as
1068 such, or 3) delete it.
1074 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
1076 These signal to a formatter that if it is to hyphenate "sigaction"
1077 or "manuscript", then it should be done as
1078 "sig-I<[linebreak]>action" or "manu-I<[linebreak]>script"
1079 (and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> doesn't
1080 show up at all). And if it is
1081 to hyphenate "Jarkko" and/or "Hietaniemi", it can do
1082 so only at the points where there is a C<EE<lt>shyE<gt>> code.
1084 In practice, it is anticipated that this character will not be used
1085 often, but formatters should either support it, or delete it.
1089 If you think that you want to add a new command to Pod (like, say, a
1090 "=biblio" command), consider whether you could get the same
1091 effect with a for or begin/end sequence: "=for biblio ..." or "=begin
1092 biblio" ... "=end biblio". Pod processors that don't understand
1093 "=for biblio", etc, will simply ignore it, whereas they may complain
1094 loudly if they see "=biblio".
1098 Throughout this document, "Pod" has been the preferred spelling for
1099 the name of the documentation format. One may also use "POD" or
1100 "pod". For the documentation that is (typically) in the Pod
1101 format, you may use "pod", or "Pod", or "POD". Understanding these
1102 distinctions is useful; but obsessing over how to spell them, usually
1111 =head1 About LE<lt>...E<gt> Codes
1113 As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...>
1114 code is the most complex of the Pod formatting codes. The points below
1115 will hopefully clarify what it means and how processors should deal
1122 In parsing an LE<lt>...> code, Pod parsers must distinguish at least
1129 The link-text. If there is none, this must be undef. (E.g., in
1130 "LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions".
1131 In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no
1132 link text. Note that link text may contain formatting.)
1136 The possibly inferred link-text; i.e., if there was no real link
1137 text, then this is the text that we'll infer in its place. (E.g., for
1138 "LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".)
1142 The name or URL, or undef if none. (E.g., in "LE<lt>Perl
1143 Functions|perlfunc>", the name (also sometimes called the page)
1144 is "perlfunc". In "LE<lt>/CAVEATS>", the name is undef.)
1148 The section (AKA "item" in older perlpods), or undef if none. E.g.,
1149 in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "DESCRIPTION" is the section. (Note
1150 that this is not the same as a manpage section like the "5" in "man 5
1151 crontab". "Section Foo" in the Pod sense means the part of the text
1152 that's introduced by the heading or item whose text is "Foo".)
1156 Pod parsers may also note additional attributes including:
1162 A flag for whether item 3 (if present) is a URL (like
1163 "http://lists.perl.org" is), in which case there should be no section
1164 attribute; a Pod name (like "perldoc" and "Getopt::Std" are); or
1165 possibly a man page name (like "crontab(5)" is).
1169 The raw original LE<lt>...> content, before text is split on
1170 "|", "/", etc, and before EE<lt>...> codes are expanded.
1174 (The above were numbered only for concise reference below. It is not
1175 a requirement that these be passed as an actual list or array.)
1180 => undef, # link text
1181 "Foo::Bar", # possibly inferred link text
1184 'pod', # what sort of link
1185 "Foo::Bar" # original content
1187 L<Perlport's section on NL's|perlport/Newlines>
1188 => "Perlport's section on NL's", # link text
1189 "Perlport's section on NL's", # possibly inferred link text
1191 "Newlines", # section
1192 'pod', # what sort of link
1193 "Perlport's section on NL's|perlport/Newlines" # orig. content
1195 L<perlport/Newlines>
1196 => undef, # link text
1197 '"Newlines" in perlport', # possibly inferred link text
1199 "Newlines", # section
1200 'pod', # what sort of link
1201 "perlport/Newlines" # original content
1203 L<crontab(5)/"DESCRIPTION">
1204 => undef, # link text
1205 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
1206 "crontab(5)", # name
1207 "DESCRIPTION", # section
1208 'man', # what sort of link
1209 'crontab(5)/"DESCRIPTION"' # original content
1211 L</Object Attributes>
1212 => undef, # link text
1213 '"Object Attributes"', # possibly inferred link text
1215 "Object Attributes", # section
1216 'pod', # what sort of link
1217 "/Object Attributes" # original content
1219 L<http://www.perl.org/>
1220 => undef, # link text
1221 "http://www.perl.org/", # possibly inferred link text
1222 "http://www.perl.org/", # name
1224 'url', # what sort of link
1225 "http://www.perl.org/" # original content
1227 L<Perl.org|http://www.perl.org/>
1228 => "Perl.org", # link text
1229 "http://www.perl.org/", # possibly inferred link text
1230 "http://www.perl.org/", # name
1232 'url', # what sort of link
1233 "Perl.org|http://www.perl.org/" # original content
1235 Note that you can distinguish URL-links from anything else by the
1236 fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So
1237 C<LE<lt>http://www.perl.comE<gt>> is a URL, but
1238 C<LE<lt>HTTP::ResponseE<gt>> isn't.
1242 In case of LE<lt>...> codes with no "text|" part in them,
1243 older formatters have exhibited great variation in actually displaying
1244 the link or cross reference. For example, LE<lt>crontab(5)> would render
1245 as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage"
1246 or just "C<crontab(5)>".
1248 Pod processors must now treat "text|"-less links as follows:
1250 L<name> => L<name|name>
1251 L</section> => L<"section"|/section>
1252 L<name/section> => L<"section" in name|name/section>
1256 Note that section names might contain markup. I.e., if a section
1259 =head2 About the C<-M> Operator
1263 =item About the C<-M> Operator
1265 then a link to it would look like this:
1267 L<somedoc/About the C<-M> Operator>
1269 Formatters may choose to ignore the markup for purposes of resolving
1270 the link and use only the renderable characters in the section name,
1273 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1278 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1279 Operator" in somedoc</a>
1283 Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>>
1284 links from C<LE<lt>name/itemE<gt>> links (and their targets). These
1285 have been merged syntactically and semantically in the current
1286 specification, and I<section> can refer either to a "=headI<n> Heading
1287 Content" command or to a "=item Item Content" command. This
1288 specification does not specify what behavior should be in the case
1289 of a given document having several things all seeming to produce the
1290 same I<section> identifier (e.g., in HTML, several things all producing
1291 the same I<anchorname> in <a name="I<anchorname>">...</a>
1292 elements). Where Pod processors can control this behavior, they should
1293 use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the
1294 I<first> "Bar" section in Foo.
1296 But for some processors/formats this cannot be easily controlled; as
1297 with the HTML example, the behavior of multiple ambiguous
1298 <a name="I<anchorname>">...</a> is most easily just left up to
1303 Authors wanting to link to a particular (absolute) URL, must do so
1304 only with "LE<lt>scheme:...>" codes (like
1305 LE<lt>http://www.perl.org>), and must not attempt "LE<lt>Some Site
1306 Name|scheme:...>" codes. This restriction avoids many problems
1307 in parsing and rendering LE<lt>...> codes.
1311 In a C<LE<lt>text|...E<gt>> code, text may contain formatting codes
1312 for formatting or for EE<lt>...> escapes, as in:
1314 L<B<ummE<234>stuff>|...>
1316 For C<LE<lt>...E<gt>> codes without a "name|" part, only
1317 C<EE<lt>...E<gt>> and C<ZE<lt>E<gt>> codes may occur. That is,
1318 authors should not use "C<LE<lt>BE<lt>Foo::BarE<gt>E<gt>>".
1320 Note, however, that formatting codes and ZE<lt>>'s can occur in any
1321 and all parts of an LE<lt>...> (i.e., in I<name>, I<section>, I<text>,
1324 Authors must not nest LE<lt>...> codes. For example, "LE<lt>The
1325 LE<lt>Foo::Bar> man page>" should be treated as an error.
1329 Note that Pod authors may use formatting codes inside the "text"
1330 part of "LE<lt>text|name>" (and so on for LE<lt>text|/"sec">).
1332 In other words, this is valid:
1334 Go read L<the docs on C<$.>|perlvar/"$.">
1336 Some output formats that do allow rendering "LE<lt>...>" codes as
1337 hypertext, might not allow the link-text to be formatted; in
1338 that case, formatters will have to just ignore that formatting.
1342 At time of writing, C<LE<lt>nameE<gt>> values are of two types:
1343 either the name of a Pod page like C<LE<lt>Foo::BarE<gt>> (which
1344 might be a real Perl module or program in an @INC / PATH
1345 directory, or a .pod file in those places); or the name of a Unix
1346 man page, like C<LE<lt>crontab(5)E<gt>>. In theory, C<LE<lt>chmodE<gt>>
1347 in ambiguous between a Pod page called "chmod", or the Unix man page
1348 "chmod" (in whatever man-section). However, the presence of a string
1349 in parens, as in "crontab(5)", is sufficient to signal that what
1350 is being discussed is not a Pod page, and so is presumably a
1351 Unix man page. The distinction is of no importance to many
1352 Pod processors, but some processors that render to hypertext formats
1353 may need to distinguish them in order to know how to render a
1354 given C<LE<lt>fooE<gt>> code.
1358 Previous versions of perlpod allowed for a C<LE<lt>sectionE<gt>> syntax (as in
1359 C<LE<lt>Object AttributesE<gt>>), which was not easily distinguishable from
1360 C<LE<lt>nameE<gt>> syntax and for C<LE<lt>"section"E<gt>> which was only
1361 slightly less ambiguous. This syntax is no longer in the specification, and
1362 has been replaced by the C<LE<lt>/sectionE<gt>> syntax (where the slash was
1363 formerly optional). Pod parsers should tolerate the C<LE<lt>"section"E<gt>>
1364 syntax, for a while at least. The suggested heuristic for distinguishing
1365 C<LE<lt>sectionE<gt>> from C<LE<lt>nameE<gt>> is that if it contains any
1366 whitespace, it's a I<section>. Pod processors should warn about this being
1371 =head1 About =over...=back Regions
1373 "=over"..."=back" regions are used for various kinds of list-like
1374 structures. (I use the term "region" here simply as a collective
1375 term for everything from the "=over" to the matching "=back".)
1381 The non-zero numeric I<indentlevel> in "=over I<indentlevel>" ...
1382 "=back" is used for giving the formatter a clue as to how many
1383 "spaces" (ems, or roughly equivalent units) it should tab over,
1384 although many formatters will have to convert this to an absolute
1385 measurement that may not exactly match with the size of spaces (or M's)
1386 in the document's base font. Other formatters may have to completely
1387 ignore the number. The lack of any explicit I<indentlevel> parameter is
1388 equivalent to an I<indentlevel> value of 4. Pod processors may
1389 complain if I<indentlevel> is present but is not a positive number
1390 matching C<m/\A(\d*\.)?\d+\z/>.
1394 Authors of Pod formatters are reminded that "=over" ... "=back" may
1395 map to several different constructs in your output format. For
1396 example, in converting Pod to (X)HTML, it can map to any of
1397 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1398 <blockquote>...</blockquote>. Similarly, "=item" can map to <li> or
1403 Each "=over" ... "=back" region should be one of the following:
1409 An "=over" ... "=back" region containing only "=item *" commands,
1410 each followed by some number of ordinary/verbatim paragraphs, other
1411 nested "=over" ... "=back" regions, "=for..." paragraphs, and
1412 "=begin"..."=end" regions.
1414 (Pod processors must tolerate a bare "=item" as if it were "=item
1415 *".) Whether "*" is rendered as a literal asterisk, an "o", or as
1416 some kind of real bullet character, is left up to the Pod formatter,
1417 and may depend on the level of nesting.
1421 An "=over" ... "=back" region containing only
1422 C<m/\A=item\s+\d+\.?\s*\z/> paragraphs, each one (or each group of them)
1423 followed by some number of ordinary/verbatim paragraphs, other nested
1424 "=over" ... "=back" regions, "=for..." paragraphs, and/or
1425 "=begin"..."=end" codes. Note that the numbers must start at 1
1426 in each section, and must proceed in order and without skipping
1429 (Pod processors must tolerate lines like "=item 1" as if they were
1430 "=item 1.", with the period.)
1434 An "=over" ... "=back" region containing only "=item [text]"
1435 commands, each one (or each group of them) followed by some number of
1436 ordinary/verbatim paragraphs, other nested "=over" ... "=back"
1437 regions, or "=for..." paragraphs, and "=begin"..."=end" regions.
1439 The "=item [text]" paragraph should not match
1440 C<m/\A=item\s+\d+\.?\s*\z/> or C<m/\A=item\s+\*\s*\z/>, nor should it
1441 match just C<m/\A=item\s*\z/>.
1445 An "=over" ... "=back" region containing no "=item" paragraphs at
1446 all, and containing only some number of
1447 ordinary/verbatim paragraphs, and possibly also some nested "=over"
1448 ... "=back" regions, "=for..." paragraphs, and "=begin"..."=end"
1449 regions. Such an itemless "=over" ... "=back" region in Pod is
1450 equivalent in meaning to a "<blockquote>...</blockquote>" element in
1455 Note that with all the above cases, you can determine which type of
1456 "=over" ... "=back" you have, by examining the first (non-"=cut",
1457 non-"=pod") Pod paragraph after the "=over" command.
1461 Pod formatters I<must> tolerate arbitrarily large amounts of text
1462 in the "=item I<text...>" paragraph. In practice, most such
1463 paragraphs are short, as in:
1465 =item For cutting off our trade with all parts of the world
1467 But they may be arbitrarily long:
1469 =item For transporting us beyond seas to be tried for pretended
1472 =item He is at this time transporting large armies of foreign
1473 mercenaries to complete the works of death, desolation and
1474 tyranny, already begun with circumstances of cruelty and perfidy
1475 scarcely paralleled in the most barbarous ages, and totally
1476 unworthy the head of a civilized nation.
1480 Pod processors should tolerate "=item *" / "=item I<number>" commands
1481 with no accompanying paragraph. The middle item is an example:
1487 Pick up dry cleaning.
1493 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1499 No "=over" ... "=back" region can contain headings. Processors may
1500 treat such a heading as an error.
1504 Note that an "=over" ... "=back" region should have some
1505 content. That is, authors should not have an empty region like this:
1511 Pod processors seeing such a contentless "=over" ... "=back" region,
1512 may ignore it, or may report it as an error.
1516 Processors must tolerate an "=over" list that goes off the end of the
1517 document (i.e., which has no matching "=back"), but they may warn
1522 Authors of Pod formatters should note that this construct:
1530 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1531 velit, sed quia non numquam eius modi tempora incidunt ut
1532 labore et dolore magnam aliquam quaerat voluptatem.
1536 is semantically ambiguous, in a way that makes formatting decisions
1537 a bit difficult. On the one hand, it could be mention of an item
1538 "Neque", mention of another item "Porro", and mention of another
1539 item "Quisquam Est", with just the last one requiring the explanatory
1540 paragraph "Qui dolorem ipsum quia dolor..."; and then an item
1541 "Ut Enim". In that case, you'd want to format it like so:
1548 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1549 velit, sed quia non numquam eius modi tempora incidunt ut
1550 labore et dolore magnam aliquam quaerat voluptatem.
1554 But it could equally well be a discussion of three (related or equivalent)
1555 items, "Neque", "Porro", and "Quisquam Est", followed by a paragraph
1556 explaining them all, and then a new item "Ut Enim". In that case, you'd
1557 probably want to format it like so:
1562 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1563 velit, sed quia non numquam eius modi tempora incidunt ut
1564 labore et dolore magnam aliquam quaerat voluptatem.
1568 But (for the foreseeable future), Pod does not provide any way for Pod
1569 authors to distinguish which grouping is meant by the above
1570 "=item"-cluster structure. So formatters should format it like so:
1578 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1579 velit, sed quia non numquam eius modi tempora incidunt ut
1580 labore et dolore magnam aliquam quaerat voluptatem.
1584 That is, there should be (at least roughly) equal spacing between
1585 items as between paragraphs (although that spacing may well be less
1586 than the full height of a line of text). This leaves it to the reader
1587 to use (con)textual cues to figure out whether the "Qui dolorem
1588 ipsum..." paragraph applies to the "Quisquam Est" item or to all three
1589 items "Neque", "Porro", and "Quisquam Est". While not an ideal
1590 situation, this is preferable to providing formatting cues that may
1591 be actually contrary to the author's intent.
1597 =head1 About Data Paragraphs and "=begin/=end" Regions
1599 Data paragraphs are typically used for inlining non-Pod data that is
1600 to be used (typically passed through) when rendering the document to
1605 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1609 The exact same effect could, incidentally, be achieved with a single
1612 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1614 (Although that is not formally a data paragraph, it has the same
1615 meaning as one, and Pod parsers may parse it as one.)
1617 Another example of a data paragraph:
1621 I like <em>PIE</em>!
1623 <hr>Especially pecan pie!
1627 If these were ordinary paragraphs, the Pod parser would try to
1628 expand the "EE<lt>/em>" (in the first paragraph) as a formatting
1629 code, just like "EE<lt>lt>" or "EE<lt>eacute>". But since this
1630 is in a "=begin I<identifier>"..."=end I<identifier>" region I<and>
1631 the identifier "html" doesn't begin have a ":" prefix, the contents
1632 of this region are stored as data paragraphs, instead of being
1633 processed as ordinary paragraphs (or if they began with a spaces
1634 and/or tabs, as verbatim paragraphs).
1636 As a further example: At time of writing, no "biblio" identifier is
1637 supported, but suppose some processor were written to recognize it as
1638 a way of (say) denoting a bibliographic reference (necessarily
1639 containing formatting codes in ordinary paragraphs). The fact that
1640 "biblio" paragraphs were meant for ordinary processing would be
1641 indicated by prefacing each "biblio" identifier with a colon:
1645 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1646 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1650 This would signal to the parser that paragraphs in this begin...end
1651 region are subject to normal handling as ordinary/verbatim paragraphs
1652 (while still tagged as meant only for processors that understand the
1653 "biblio" identifier). The same effect could be had with:
1656 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1657 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1659 The ":" on these identifiers means simply "process this stuff
1660 normally, even though the result will be for some special target".
1661 I suggest that parser APIs report "biblio" as the target identifier,
1662 but also report that it had a ":" prefix. (And similarly, with the
1663 above "html", report "html" as the target identifier, and note the
1664 I<lack> of a ":" prefix.)
1666 Note that a "=begin I<identifier>"..."=end I<identifier>" region where
1667 I<identifier> begins with a colon, I<can> contain commands. For example:
1671 Wirth's classic is available in several editions, including:
1674 hm, check abebooks.com for how much used copies cost.
1680 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1681 Teubner, Stuttgart. [Yes, it's in German.]
1685 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1686 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1692 Note, however, a "=begin I<identifier>"..."=end I<identifier>"
1693 region where I<identifier> does I<not> begin with a colon, should not
1694 directly contain "=head1" ... "=head4" commands, nor "=over", nor "=back",
1695 nor "=item". For example, this may be considered invalid:
1699 This is a data paragraph.
1701 =head1 Don't do this!
1703 This is a data paragraph too.
1707 A Pod processor may signal that the above (specifically the "=head1"
1708 paragraph) is an error. Note, however, that the following should
1709 I<not> be treated as an error:
1713 This is a data paragraph.
1717 # Yup, this isn't Pod anymore.
1718 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1722 This is a data paragraph too.
1726 And this too is valid:
1730 This is a data paragraph.
1732 And this is a data paragraph.
1734 =begin someotherformat
1736 This is a data paragraph too.
1738 And this is a data paragraph too.
1740 =begin :yetanotherformat
1742 =head2 This is a command paragraph!
1744 This is an ordinary paragraph!
1746 And this is a verbatim paragraph!
1748 =end :yetanotherformat
1750 =end someotherformat
1752 Another data paragraph!
1756 The contents of the above "=begin :yetanotherformat" ...
1757 "=end :yetanotherformat" region I<aren't> data paragraphs, because
1758 the immediately containing region's identifier (":yetanotherformat")
1759 begins with a colon. In practice, most regions that contain
1760 data paragraphs will contain I<only> data paragraphs; however,
1761 the above nesting is syntactically valid as Pod, even if it is
1762 rare. However, the handlers for some formats, like "html",
1763 will accept only data paragraphs, not nested regions; and they may
1764 complain if they see (targeted for them) nested regions, or commands,
1765 other than "=end", "=pod", and "=cut".
1767 Also consider this valid structure:
1771 Wirth's classic is available in several editions, including:
1777 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1778 Teubner, Stuttgart. [Yes, it's in German.]
1782 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1783 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1791 <img src='wirth_spokesmodeling_book.png'>
1801 There, the "=begin html"..."=end html" region is nested inside
1802 the larger "=begin :biblio"..."=end :biblio" region. Note that the
1803 content of the "=begin html"..."=end html" region is data
1804 paragraph(s), because the immediately containing region's identifier
1805 ("html") I<doesn't> begin with a colon.
1807 Pod parsers, when processing a series of data paragraphs one
1808 after another (within a single region), should consider them to
1809 be one large data paragraph that happens to contain blank lines. So
1810 the content of the above "=begin html"..."=end html" I<may> be stored
1811 as two data paragraphs (one consisting of
1812 "<img src='wirth_spokesmodeling_book.png'>\n"
1813 and another consisting of "<hr>\n"), but I<should> be stored as
1814 a single data paragraph (consisting of
1815 "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1817 Pod processors should tolerate empty
1818 "=begin I<something>"..."=end I<something>" regions,
1819 empty "=begin :I<something>"..."=end :I<something>" regions, and
1820 contentless "=for I<something>" and "=for :I<something>"
1821 paragraphs. I.e., these should be tolerated:
1833 Incidentally, note that there's no easy way to express a data
1834 paragraph starting with something that looks like a command. Consider:
1842 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a data
1843 paragraph "=shazbot\n". However, you can express a data paragraph consisting
1844 of "=shazbot\n" using this code:
1848 The situation where this is necessary, is presumably quite rare.
1850 Note that =end commands must match the currently open =begin command. That
1851 is, they must properly nest. For example, this is valid:
1867 while this is invalid:
1883 This latter is improper because when the "=end outer" command is seen, the
1884 currently open region has the formatname "inner", not "outer". (It just
1885 happens that "outer" is the format name of a higher-up region.) This is
1886 an error. Processors must by default report this as an error, and may halt
1887 processing the document containing that error. A corollary of this is that
1888 regions cannot "overlap". That is, the latter block above does not represent
1889 a region called "outer" which contains X and Y, overlapping a region called
1890 "inner" which contains Y and Z. But because it is invalid (as all
1891 apparently overlapping regions would be), it doesn't represent that, or
1894 Similarly, this is invalid:
1900 This is an error because the region is opened by "thing", and the "=end"
1901 tries to close "hting" [sic].
1903 This is also invalid:
1909 This is invalid because every "=end" command must have a formatname
1914 L<perlpod>, L<perlsyn/"PODs: Embedded Documentation">,