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 <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 Within a Pod block, there are B<Pod paragraphs>. A Pod paragraph
102 consists of non-blank lines of text, separated by one or more blank
105 For purposes of Pod processing, there are four types of paragraphs in
112 A command paragraph (also called a "directive"). The first line of
113 this paragraph must match C<m/\A=[a-zA-Z]/>. Command paragraphs are
114 typically one line, as in:
120 But they may span several (non-blank) lines:
123 Hm, I wonder what it would look like if
124 you tried to write a BNF for Pod from this.
126 =head3 Dr. Strangelove, or: How I Learned to
127 Stop Worrying and Love the Bomb
129 I<Some> command paragraphs allow formatting codes in their content
130 (i.e., after the part that matches C<m/\A=[a-zA-Z]\S*\s*/>), as in:
132 =head1 Did You Remember to C<use strict;>?
134 In other words, the Pod processing handler for "head1" will apply the
135 same processing to "Did You Remember to CE<lt>use strict;>?" that it
136 would to an ordinary paragraph (i.e., formatting codes like
137 "CE<lt>...>") are parsed and presumably formatted appropriately, and
138 whitespace in the form of literal spaces and/or tabs is not
143 A B<verbatim paragraph>. The first line of this paragraph must be a
144 literal space or tab, and this paragraph must not be inside a "=begin
145 I<identifier>", ... "=end I<identifier>" sequence unless
146 "I<identifier>" begins with a colon (":"). That is, if a paragraph
147 starts with a literal space or tab, but I<is> inside a
148 "=begin I<identifier>", ... "=end I<identifier>" region, then it's
149 a data paragraph, unless "I<identifier>" begins with a colon.
151 Whitespace I<is> significant in verbatim paragraphs (although, in
152 processing, tabs are probably expanded).
156 An B<ordinary paragraph>. A paragraph is an ordinary paragraph
157 if its first line matches neither C<m/\A=[a-zA-Z]/> nor
158 C<m/\A[ \t]/>, I<and> if it's not inside a "=begin I<identifier>",
159 ... "=end I<identifier>" sequence unless "I<identifier>" begins with
164 A B<data paragraph>. This is a paragraph that I<is> inside a "=begin
165 I<identifier>" ... "=end I<identifier>" sequence where
166 "I<identifier>" does I<not> begin with a literal colon (":"). In
167 some sense, a data paragraph is not part of Pod at all (i.e.,
168 effectively it's "out-of-band"), since it's not subject to most kinds
169 of Pod parsing; but it is specified here, since Pod
170 parsers need to be able to call an event for it, or store it in some
171 form in a parse tree, or at least just parse I<around> it.
175 For example: consider the following paragraphs:
177 # <- that's the 0th column
187 Here, "=head1 Foo" and "=cut" are command paragraphs because the first
188 line of each matches C<m/\A=[a-zA-Z]/>. "I<[space][space]>$foo->bar"
189 is a verbatim paragraph, because its first line starts with a literal
190 whitespace character (and there's no "=begin"..."=end" region around).
192 The "=begin I<identifier>" ... "=end I<identifier>" commands stop
193 paragraphs that they surround from being parsed as ordinary or verbatim
194 paragraphs, if I<identifier> doesn't begin with a colon. This
195 is discussed in detail in the section
196 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
200 This section is intended to supplement and clarify the discussion in
201 L<perlpod/"Command Paragraph">. These are the currently recognized
206 =item "=head1", "=head2", "=head3", "=head4"
208 This command indicates that the text in the remainder of the paragraph
209 is a heading. That text may contain formatting codes. Examples:
211 =head1 Object Attributes
213 =head3 What B<Not> to Do!
217 This command indicates that this paragraph begins a Pod block. (If we
218 are already in the middle of a Pod block, this command has no effect at
219 all.) If there is any text in this command paragraph after "=pod",
220 it must be ignored. Examples:
224 This is a plain Pod paragraph.
226 =pod This text is ignored.
230 This command indicates that this line is the end of this previously
231 started Pod block. If there is any text after "=cut" on the line, it must be
236 =cut The documentation ends here.
239 # This is the first line of program text.
240 sub foo { # This is the second.
242 It is an error to try to I<start> a Pod block with a "=cut" command. In
243 that case, the Pod processor must halt parsing of the input file, and
244 must by default emit a warning.
248 This command indicates that this is the start of a list/indent
249 region. If there is any text following the "=over", it must consist
250 of only a nonzero positive numeral. The semantics of this numeral is
251 explained in the L</"About =over...=back Regions"> section, further
252 below. Formatting codes are not expanded. Examples:
262 This command indicates that an item in a list begins here. Formatting
263 codes are processed. The semantics of the (optional) text in the
264 remainder of this paragraph are
265 explained in the L</"About =over...=back Regions"> section, further
278 =item C<< $thing->stuff(I<dodad>) >>
280 =item For transporting us beyond seas to be tried for pretended
283 =item He is at this time transporting large armies of foreign
284 mercenaries to complete the works of death, desolation and
285 tyranny, already begun with circumstances of cruelty and perfidy
286 scarcely paralleled in the most barbarous ages, and totally
287 unworthy the head of a civilized nation.
291 This command indicates that this is the end of the region begun
292 by the most recent "=over" command. It permits no text after the
295 =item "=begin formatname"
297 =item "=begin formatname parameter"
299 This marks the following paragraphs (until the matching "=end
300 formatname") as being for some special kind of processing. Unless
301 "formatname" begins with a colon, the contained non-command
302 paragraphs are data paragraphs. But if "formatname" I<does> begin
303 with a colon, then non-command paragraphs are ordinary paragraphs
304 or data paragraphs. This is discussed in detail in the section
305 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
307 It is advised that formatnames match the regexp
308 C<m/\A:?[-a-zA-Z0-9_]+\z/>. Everything following whitespace after the
309 formatname is a parameter that may be used by the formatter when dealing
310 with this region. This parameter must not be repeated in the "=end"
311 paragraph. Implementors should anticipate future expansion in the
312 semantics and syntax of the first parameter to "=begin"/"=end"/"=for".
314 =item "=end formatname"
316 This marks the end of the region opened by the matching
317 "=begin formatname" region. If "formatname" is not the formatname
318 of the most recent open "=begin formatname" region, then this
319 is an error, and must generate an error message. This
320 is discussed in detail in the section
321 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
323 =item "=for formatname text..."
325 This is synonymous with:
333 That is, it creates a region consisting of a single paragraph; that
334 paragraph is to be treated as a normal paragraph if "formatname"
335 begins with a ":"; if "formatname" I<doesn't> begin with a colon,
336 then "text..." will constitute a data paragraph. There is no way
337 to use "=for formatname text..." to express "text..." as a verbatim
340 =item "=encoding encodingname"
342 This command, which should occur early in the document (at least
343 before any non-US-ASCII data!), declares that this document is
344 encoded in the encoding I<encodingname>, which must be
345 an encoding name that L<Encode> recognizes. (Encode's list
346 of supported encodings, in L<Encode::Supported>, is useful here.)
347 If the Pod parser cannot decode the declared encoding, it
348 should emit a warning and may abort parsing the document
351 A document having more than one "=encoding" line should be
352 considered an error. Pod processors may silently tolerate this if
353 the not-first "=encoding" lines are just duplicates of the
354 first one (e.g., if there's a "=encoding utf8" line, and later on
355 another "=encoding utf8" line). But Pod processors should complain if
356 there are contradictory "=encoding" lines in the same document
357 (e.g., if there is a "=encoding utf8" early in the document and
358 "=encoding big5" later). Pod processors that recognize BOMs
359 may also complain if they see an "=encoding" line
360 that contradicts the BOM (e.g., if a document with a UTF-16LE
361 BOM has an "=encoding shiftjis" line).
365 If a Pod processor sees any command other than the ones listed
366 above (like "=head", or "=haed1", or "=stuff", or "=cuttlefish",
367 or "=w123"), that processor must by default treat this as an
368 error. It must not process the paragraph beginning with that
369 command, must by default warn of this as an error, and may
370 abort the parse. A Pod parser may allow a way for particular
371 applications to add to the above list of known commands, and to
372 stipulate, for each additional command, whether formatting
373 codes should be processed.
375 Future versions of this specification may add additional
380 =head1 Pod Formatting Codes
382 (Note that in previous drafts of this document and of perlpod,
383 formatting codes were referred to as "interior sequences", and
384 this term may still be found in the documentation for Pod parsers,
385 and in error messages from Pod processors.)
387 There are two syntaxes for formatting codes:
393 A formatting code starts with a capital letter (just US-ASCII [A-Z])
394 followed by a "<", any number of characters, and ending with the first
395 matching ">". Examples:
397 That's what I<you> think!
399 What's C<dump()> for?
401 X<C<chmod> and C<unlink()> Under Different Operating Systems>
405 A formatting code starts with a capital letter (just US-ASCII [A-Z])
406 followed by two or more "<"'s, one or more whitespace characters,
407 any number of characters, one or more whitespace characters,
408 and ending with the first matching sequence of two or more ">"'s, where
409 the number of ">"'s equals the number of "<"'s in the opening of this
410 formatting code. Examples:
412 That's what I<< you >> think!
414 C<<< open(X, ">>thing.dat") || die $! >>>
418 With this syntax, the whitespace character(s) after the "CE<lt><<"
419 and before the ">>" (or whatever letter) are I<not> renderable. They
420 do not signify whitespace, are merely part of the formatting codes
421 themselves. That is, these are all synonymous:
433 Finally, the multiple-angle-bracket form does I<not> alter the interpretation
434 of nested formatting codes, meaning that the following four example lines are
435 identical in meaning:
437 B<example: C<$a E<lt>=E<gt> $b>>
439 B<example: C<< $a <=> $b >>>
441 B<example: C<< $a E<lt>=E<gt> $b >>>
443 B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
447 In parsing Pod, a notably tricky part is the correct parsing of
448 (potentially nested!) formatting codes. Implementors should
449 consult the code in the C<parse_text> routine in Pod::Parser as an
450 example of a correct implementation.
454 =item C<IE<lt>textE<gt>> -- italic text
456 See the brief discussion in L<perlpod/"Formatting Codes">.
458 =item C<BE<lt>textE<gt>> -- bold text
460 See the brief discussion in L<perlpod/"Formatting Codes">.
462 =item C<CE<lt>codeE<gt>> -- code text
464 See the brief discussion in L<perlpod/"Formatting Codes">.
466 =item C<FE<lt>filenameE<gt>> -- style for filenames
468 See the brief discussion in L<perlpod/"Formatting Codes">.
470 =item C<XE<lt>topic nameE<gt>> -- an index entry
472 See the brief discussion in L<perlpod/"Formatting Codes">.
474 This code is unusual in that most formatters completely discard
475 this code and its content. Other formatters will render it with
476 invisible codes that can be used in building an index of
477 the current document.
479 =item C<ZE<lt>E<gt>> -- a null (zero-effect) formatting code
481 Discussed briefly in L<perlpod/"Formatting Codes">.
483 This code is unusual is that it should have no content. That is,
484 a processor may complain if it sees C<ZE<lt>potatoesE<gt>>. Whether
485 or not it complains, the I<potatoes> text should ignored.
487 =item C<LE<lt>nameE<gt>> -- a hyperlink
489 The complicated syntaxes of this code are discussed at length in
490 L<perlpod/"Formatting Codes">, and implementation details are
491 discussed below, in L</"About LE<lt>...E<gt> Codes">. Parsing the
492 contents of LE<lt>content> is tricky. Notably, the content has to be
493 checked for whether it looks like a URL, or whether it has to be split
494 on literal "|" and/or "/" (in the right order!), and so on,
495 I<before> EE<lt>...> codes are resolved.
497 =item C<EE<lt>escapeE<gt>> -- a character escape
499 See L<perlpod/"Formatting Codes">, and several points in
500 L</Notes on Implementing Pod Processors>.
502 =item C<SE<lt>textE<gt>> -- text contains non-breaking spaces
504 This formatting code is syntactically simple, but semantically
505 complex. What it means is that each space in the printable
506 content of this code signifies a non-breaking space.
514 Both signify the monospace (c[ode] style) text consisting of
515 "$x", one space, "?", one space, ":", one space, "$z". The
516 difference is that in the latter, with the S code, those spaces
517 are not "normal" spaces, but instead are non-breaking spaces.
522 If a Pod processor sees any formatting code other than the ones
523 listed above (as in "NE<lt>...>", or "QE<lt>...>", etc.), that
524 processor must by default treat this as an error.
525 A Pod parser may allow a way for particular
526 applications to add to the above list of known formatting codes;
527 a Pod parser might even allow a way to stipulate, for each additional
528 command, whether it requires some form of special processing, as
531 Future versions of this specification may add additional
534 Historical note: A few older Pod processors would not see a ">" as
535 closing a "CE<lt>" code, if the ">" was immediately preceded by
536 a "-". This was so that this:
540 would parse as equivalent to this:
544 instead of as equivalent to a "C" formatting code containing
545 only "$foo-", and then a "bar>" outside the "C" formatting code. This
546 problem has since been solved by the addition of syntaxes like this:
550 Compliant parsers must not treat "->" as special.
552 Formatting codes absolutely cannot span paragraphs. If a code is
553 opened in one paragraph, and no closing code is found by the end of
554 that paragraph, the Pod parser must close that formatting code,
555 and should complain (as in "Unterminated I code in the paragraph
556 starting at line 123: 'Time objects are not...'"). So these
559 I<I told you not to do this!
561 Don't make me say it again!>
563 ...must I<not> be parsed as two paragraphs in italics (with the I
564 code starting in one paragraph and starting in another.) Instead,
565 the first paragraph should generate a warning, but that aside, the
566 above code must parse as if it were:
568 I<I told you not to do this!>
570 Don't make me say it again!E<gt>
572 (In SGMLish jargon, all Pod commands are like block-level
573 elements, whereas all Pod formatting codes are like inline-level
578 =head1 Notes on Implementing Pod Processors
580 The following is a long section of miscellaneous requirements
581 and suggestions to do with Pod processing.
587 Pod formatters should tolerate lines in verbatim blocks that are of
588 any length, even if that means having to break them (possibly several
589 times, for very long lines) to avoid text running off the side of the
590 page. Pod formatters may warn of such line-breaking. Such warnings
591 are particularly appropriate for lines are over 100 characters long, which
592 are usually not intentional.
596 Pod parsers must recognize I<all> of the three well-known newline
597 formats: CR, LF, and CRLF. See L<perlport|perlport>.
601 Pod parsers should accept input lines that are of any length.
605 Since Perl recognizes a Unicode Byte Order Mark at the start of files
606 as signaling that the file is Unicode encoded as in UTF-16 (whether
607 big-endian or little-endian) or UTF-8, Pod parsers should do the
608 same. Otherwise, the character encoding should be understood as
609 being UTF-8 if the first highbit byte sequence in the file seems
610 valid as a UTF-8 sequence, or otherwise as Latin-1.
612 Future versions of this specification may specify
613 how Pod can accept other encodings. Presumably treatment of other
614 encodings in Pod parsing would be as in XML parsing: whatever the
615 encoding declared by a particular Pod file, content is to be
616 stored in memory as Unicode characters.
620 The well known Unicode Byte Order Marks are as follows: if the
621 file begins with the two literal byte values 0xFE 0xFF, this is
622 the BOM for big-endian UTF-16. If the file begins with the two
623 literal byte value 0xFF 0xFE, this is the BOM for little-endian
624 UTF-16. If the file begins with the three literal byte values
625 0xEF 0xBB 0xBF, this is the BOM for UTF-8.
628 use bytes; print map sprintf(" 0x%02X", ord $_), split '', "\x{feff}";
632 If toke.c is modified to support UTF-32, add mention of those here.
636 A naive but often sufficient heuristic for testing the first highbit
637 byte-sequence in a BOM-less file (whether in code or in Pod!), to see
638 whether that sequence is valid as UTF-8 (RFC 2279) is to check whether
639 that the first byte in the sequence is in the range 0xC2 - 0xFD
640 I<and> whether the next byte is in the range
641 0x80 - 0xBF. If so, the parser may conclude that this file is in
642 UTF-8, and all highbit sequences in the file should be assumed to
643 be UTF-8. Otherwise the parser should treat the file as being
644 in Latin-1. (A better check is to pass a copy of the sequence to
645 L<utf8::decode()|utf8> which performs a full validity check on the
646 sequence and returns TRUE if it is valid UTF-8, FALSE otherwise. This
647 function is always pre-loaded, is fast because it is written in C, and
648 will only get called at most once, so you don't need to avoid it out of
649 performance concerns.)
650 In the unlikely circumstance that the first highbit
651 sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one
652 can cater to our heuristic (as well as any more intelligent heuristic)
653 by prefacing that line with a comment line containing a highbit
654 sequence that is clearly I<not> valid as UTF-8. A line consisting
655 of simply "#", an e-acute, and any non-highbit byte,
656 is sufficient to establish this file's encoding.
659 If/WHEN some brave soul makes these heuristics into a generic
660 text-file class (or PerlIO layer?), we can presumably delete
661 mention of these icky details from this file, and can instead
662 tell people to just use appropriate class/layer.
663 Auto-recognition of newline sequences would be another desirable
664 feature of such a class/layer.
668 "The probability that a string of characters
669 in any other encoding appears as valid UTF-8 is low" - RFC2279
673 This document's requirements and suggestions about encodings
674 do not apply to Pod processors running on non-ASCII platforms,
675 notably EBCDIC platforms.
679 Pod processors must treat a "=for [label] [content...]" paragraph as
680 meaning the same thing as a "=begin [label]" paragraph, content, and
681 an "=end [label]" paragraph. (The parser may conflate these two
682 constructs, or may leave them distinct, in the expectation that the
683 formatter will nevertheless treat them the same.)
687 When rendering Pod to a format that allows comments (i.e., to nearly
688 any format other than plaintext), a Pod formatter must insert comment
689 text identifying its name and version number, and the name and
690 version numbers of any modules it might be using to process the Pod.
693 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
695 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
697 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
699 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
701 Formatters may also insert additional comments, including: the
702 release date of the Pod formatter program, the contact address for
703 the author(s) of the formatter, the current time, the name of input
704 file, the formatting options in effect, version of Perl used, etc.
706 Formatters may also choose to note errors/warnings as comments,
707 besides or instead of emitting them otherwise (as in messages to
708 STDERR, or C<die>ing).
712 Pod parsers I<may> emit warnings or error messages ("Unknown E code
713 EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or
714 C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> allow
715 suppressing all such STDERR output, and instead allow an option for
716 reporting errors/warnings
717 in some other way, whether by triggering a callback, or noting errors
718 in some attribute of the document object, or some similarly unobtrusive
719 mechanism -- or even by appending a "Pod Errors" section to the end of
720 the parsed form of the document.
724 In cases of exceptionally aberrant documents, Pod parsers may abort the
725 parse. Even then, using C<die>ing/C<croak>ing is to be avoided; where
726 possible, the parser library may simply close the input file
727 and add text like "*** Formatting Aborted ***" to the end of the
728 (partial) in-memory document.
732 In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>)
733 are understood (i.e., I<not> verbatim paragraphs, but I<including>
734 ordinary paragraphs, and command paragraphs that produce renderable
735 text, like "=head1"), literal whitespace should generally be considered
736 "insignificant", in that one literal space has the same meaning as any
737 (nonzero) number of literal spaces, literal newlines, and literal tabs
738 (as long as this produces no blank lines, since those would terminate
739 the paragraph). Pod parsers should compact literal whitespace in each
740 processed paragraph, but may provide an option for overriding this
741 (since some processing tasks do not require it), or may follow
742 additional special rules (for example, specially treating
743 period-space-space or period-newline sequences).
747 Pod parsers should not, by default, try to coerce apostrophe (') and
748 quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to
749 turn backtick (`) into anything else but a single backtick character
750 (distinct from an open quote character!), nor "--" into anything but
751 two minus signs. They I<must never> do any of those things to text
752 in CE<lt>...> formatting codes, and never I<ever> to text in verbatim
757 When rendering Pod to a format that has two kinds of hyphens (-), one
758 that's a non-breaking hyphen, and another that's a breakable hyphen
759 (as in "object-oriented", which can be split across lines as
760 "object-", newline, "oriented"), formatters are encouraged to
761 generally translate "-" to non-breaking hyphen, but may apply
762 heuristics to convert some of these to breaking hyphens.
766 Pod formatters should make reasonable efforts to keep words of Perl
767 code from being broken across lines. For example, "Foo::Bar" in some
768 formatting systems is seen as eligible for being broken across lines
769 as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should
770 be avoided where possible, either by disabling all line-breaking in
771 mid-word, or by wrapping particular words with internal punctuation
772 in "don't break this across lines" codes (which in some formats may
773 not be a single code, but might be a matter of inserting non-breaking
774 zero-width spaces between every pair of characters in a word.)
778 Pod parsers should, by default, expand tabs in verbatim paragraphs as
779 they are processed, before passing them to the formatter or other
780 processor. Parsers may also allow an option for overriding this.
784 Pod parsers should, by default, remove newlines from the end of
785 ordinary and verbatim paragraphs before passing them to the
786 formatter. For example, while the paragraph you're reading now
787 could be considered, in Pod source, to end with (and contain)
788 the newline(s) that end it, it should be processed as ending with
789 (and containing) the period character that ends this sentence.
793 Pod parsers, when reporting errors, should make some effort to report
794 an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near
795 line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph
796 number ("Nested EE<lt>>'s in Paragraph #52 of Thing/Foo.pm!"). Where
797 this is problematic, the paragraph number should at least be
798 accompanied by an excerpt from the paragraph ("Nested EE<lt>>'s in
799 Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for
800 the CE<lt>interest rate> attribute...'").
804 Pod parsers, when processing a series of verbatim paragraphs one
805 after another, should consider them to be one large verbatim
806 paragraph that happens to contain blank lines. I.e., these two
807 lines, which have a blank line between them:
813 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
814 Foo->VERSION") before being passed to the formatter or other
815 processor. Parsers may also allow an option for overriding this.
817 While this might be too cumbersome to implement in event-based Pod
818 parsers, it is straightforward for parsers that return parse trees.
822 Pod formatters, where feasible, are advised to avoid splitting short
823 verbatim paragraphs (under twelve lines, say) across pages.
827 Pod parsers must treat a line with only spaces and/or tabs on it as a
828 "blank line" such as separates paragraphs. (Some older parsers
829 recognized only two adjacent newlines as a "blank line" but would not
830 recognize a newline, a space, and a newline, as a blank line. This
831 is noncompliant behavior.)
835 Authors of Pod formatters/processors should make every effort to
836 avoid writing their own Pod parser. There are already several in
837 CPAN, with a wide range of interface styles -- and one of them,
838 Pod::Parser, comes with modern versions of Perl.
842 Characters in Pod documents may be conveyed either as literals, or by
843 number in EE<lt>n> codes, or by an equivalent mnemonic, as in
844 EE<lt>eacute> which is exactly equivalent to EE<lt>233>.
846 Characters in the range 32-126 refer to those well known US-ASCII
847 characters (also defined there by Unicode, with the same meaning),
848 which all Pod formatters must render faithfully. Characters
849 in the ranges 0-31 and 127-159 should not be used (neither as
850 literals, nor as EE<lt>number> codes), except for the
851 literal byte-sequences for newline (13, 13 10, or 10), and tab (9).
853 Characters in the range 160-255 refer to Latin-1 characters (also
854 defined there by Unicode, with the same meaning). Characters above
855 255 should be understood to refer to Unicode characters.
860 that some formatters cannot reliably render characters outside 32-126;
861 and many are able to handle 32-126 and 160-255, but nothing above
866 Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for
867 less-than and greater-than, Pod parsers must understand "EE<lt>sol>"
868 for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar,
869 pipe). Pod parsers should also understand "EE<lt>lchevron>" and
870 "EE<lt>rchevron>" as legacy codes for characters 171 and 187, i.e.,
871 "left-pointing double angle quotation mark" = "left pointing
872 guillemet" and "right-pointing double angle quotation mark" = "right
873 pointing guillemet". (These look like little "<<" and ">>", and they
874 are now preferably expressed with the HTML/XHTML codes "EE<lt>laquo>"
879 Pod parsers should understand all "EE<lt>html>" codes as defined
880 in the entity declarations in the most recent XHTML specification at
881 C<www.W3.org>. Pod parsers must understand at least the entities
882 that define characters in the range 160-255 (Latin-1). Pod parsers,
883 when faced with some unknown "EE<lt>I<identifier>>" code,
884 shouldn't simply replace it with nullstring (by default, at least),
885 but may pass it through as a string consisting of the literal characters
886 E, less-than, I<identifier>, greater-than. Or Pod parsers may offer the
887 alternative option of processing such unknown
888 "EE<lt>I<identifier>>" codes by firing an event especially
889 for such codes, or by adding a special node-type to the in-memory
890 document tree. Such "EE<lt>I<identifier>>" may have special meaning
891 to some processors, or some processors may choose to add them to
892 a special error report.
896 Pod parsers must also support the XHTML codes "EE<lt>quot>" for
897 character 34 (doublequote, "), "EE<lt>amp>" for character 38
898 (ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, ').
902 Note that in all cases of "EE<lt>whatever>", I<whatever> (whether
903 an htmlname, or a number in any base) must consist only of
904 alphanumeric characters -- that is, I<whatever> must watch
905 C<m/\A\w+\z/>. So "EE<lt> 0 1 2 3 >" is invalid, because
906 it contains spaces, which aren't alphanumeric characters. This
907 presumably does not I<need> special treatment by a Pod processor;
908 " 0 1 2 3 " doesn't look like a number in any base, so it would
909 presumably be looked up in the table of HTML-like names. Since
910 there isn't (and cannot be) an HTML-like entity called " 0 1 2 3 ",
911 this will be treated as an error. However, Pod processors may
912 treat "EE<lt> 0 1 2 3 >" or "EE<lt>e-acute>" as I<syntactically>
913 invalid, potentially earning a different error message than the
914 error message (or warning, or event) generated by a merely unknown
915 (but theoretically valid) htmlname, as in "EE<lt>qacute>"
916 [sic]. However, Pod parsers are not required to make this
921 Note that EE<lt>number> I<must not> be interpreted as simply
922 "codepoint I<number> in the current/native character set". It always
923 means only "the character represented by codepoint I<number> in
924 Unicode." (This is identical to the semantics of &#I<number>; in XML.)
926 This will likely require many formatters to have tables mapping from
927 treatable Unicode codepoints (such as the "\xE9" for the e-acute
928 character) to the escape sequences or codes necessary for conveying
929 such sequences in the target output format. A converter to *roff
930 would, for example know that "\xE9" (whether conveyed literally, or via
931 a EE<lt>...> sequence) is to be conveyed as "e\\*'".
932 Similarly, a program rendering Pod in a Mac OS application window, would
933 presumably need to know that "\xE9" maps to codepoint 142 in MacRoman
934 encoding that (at time of writing) is native for Mac OS. Such
935 Unicode2whatever mappings are presumably already widely available for
936 common output formats. (Such mappings may be incomplete! Implementers
937 are not expected to bend over backwards in an attempt to render
938 Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any
939 of the other weird things that Unicode can encode.) And
940 if a Pod document uses a character not found in such a mapping, the
941 formatter should consider it an unrenderable character.
945 If, surprisingly, the implementor of a Pod formatter can't find a
946 satisfactory pre-existing table mapping from Unicode characters to
947 escapes in the target format (e.g., a decent table of Unicode
948 characters to *roff escapes), it will be necessary to build such a
949 table. If you are in this circumstance, you should begin with the
950 characters in the range 0x00A0 - 0x00FF, which is mostly the heavily
951 used accented characters. Then proceed (as patience permits and
952 fastidiousness compels) through the characters that the (X)HTML
953 standards groups judged important enough to merit mnemonics
954 for. These are declared in the (X)HTML specifications at the
955 www.W3.org site. At time of writing (September 2001), the most recent
956 entity declaration files are:
958 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
959 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
960 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
962 Then you can progress through any remaining notable Unicode characters
963 in the range 0x2000-0x204D (consult the character tables at
964 www.unicode.org), and whatever else strikes your fancy. For example,
965 in F<xhtml-symbol.ent>, there is the entry:
967 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
969 While the mapping "infin" to the character "\x{221E}" will (hopefully)
970 have been already handled by the Pod parser, the presence of the
971 character in this file means that it's reasonably important enough to
972 include in a formatter's table that maps from notable Unicode characters
973 to the codes necessary for rendering them. So for a Unicode-to-*roff
974 mapping, for example, this would merit the entry:
976 "\x{221E}" => '\(in',
978 It is eagerly hoped that in the future, increasing numbers of formats
979 (and formatters) will support Unicode characters directly (as (X)HTML
980 does with C<∞>, C<∞>, or C<∞>), reducing the need
981 for idiosyncratic mappings of Unicode-to-I<my_escapes>.
985 It is up to individual Pod formatter to display good judgement when
986 confronted with an unrenderable character (which is distinct from an
987 unknown EE<lt>thing> sequence that the parser couldn't resolve to
988 anything, renderable or not). It is good practice to map Latin letters
989 with diacritics (like "EE<lt>eacute>"/"EE<lt>233>") to the corresponding
990 unaccented US-ASCII letters (like a simple character 101, "e"), but
991 clearly this is often not feasible, and an unrenderable character may
992 be represented as "?", or the like. In attempting a sane fallback
993 (as from EE<lt>233> to "e"), Pod formatters may use the
994 %Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or
995 L<Text::Unidecode|Text::Unidecode>, if available.
997 For example, this Pod text:
999 magic is enabled if you set C<$Currency> to 'E<euro>'.
1002 "magic is enabled if you set C<$Currency> to 'I<?>'" or as
1003 "magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as
1004 "magic is enabled if you set C<$Currency> to '[x20AC]', etc.
1006 A Pod formatter may also note, in a comment or warning, a list of what
1007 unrenderable characters were encountered.
1011 EE<lt>...> may freely appear in any formatting code (other than
1012 in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The
1013 EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The
1014 EE<lt>euro>1,000,000 Solution|Million::Euros>".
1018 Some Pod formatters output to formats that implement non-breaking
1019 spaces as an individual character (which I'll call "NBSP"), and
1020 others output to formats that implement non-breaking spaces just as
1021 spaces wrapped in a "don't break this across lines" code. Note that
1022 at the level of Pod, both sorts of codes can occur: Pod can contain a
1023 NBSP character (whether as a literal, or as a "EE<lt>160>" or
1024 "EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo
1025 IE<lt>barE<gt> baz>" codes, where "mere spaces" (character 32) in
1026 such codes are taken to represent non-breaking spaces. Pod
1027 parsers should consider supporting the optional parsing of "SE<lt>foo
1028 IE<lt>barE<gt> baz>" as if it were
1029 "fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the
1030 optional parsing of groups of words joined by NBSP's as if each group
1031 were in a SE<lt>...> code, so that formatters may use the
1032 representation that maps best to what the output format demands.
1036 Some processors may find that the C<SE<lt>...E<gt>> code is easiest to
1037 implement by replacing each space in the parse tree under the content
1038 of the S, with an NBSP. But note: the replacement should apply I<not> to
1039 spaces in I<all> text, but I<only> to spaces in I<printable> text. (This
1040 distinction may or may not be evident in the particular tree/event
1041 model implemented by the Pod parser.) For example, consider this
1044 S<L</Autoloaded Functions>>
1046 This means that the space in the middle of the visible link text must
1047 not be broken across lines. In other words, it's the same as this:
1049 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
1051 However, a misapplied space-to-NBSP replacement could (wrongly)
1052 produce something equivalent to this:
1054 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
1056 ...which is almost definitely not going to work as a hyperlink (assuming
1057 this formatter outputs a format supporting hypertext).
1059 Formatters may choose to just not support the S format code,
1060 especially in cases where the output format simply has no NBSP
1061 character/code and no code for "don't break this stuff across lines".
1065 Besides the NBSP character discussed above, implementors are reminded
1066 of the existence of the other "special" character in Latin-1, the
1067 "soft hyphen" character, also known as "discretionary hyphen",
1068 i.e. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> =
1069 C<EE<lt>shyE<gt>>). This character expresses an optional hyphenation
1070 point. That is, it normally renders as nothing, but may render as a
1071 "-" if a formatter breaks the word at that point. Pod formatters
1072 should, as appropriate, do one of the following: 1) render this with
1073 a code with the same meaning (e.g., "\-" in RTF), 2) pass it through
1074 in the expectation that the formatter understands this character as
1075 such, or 3) delete it.
1081 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
1083 These signal to a formatter that if it is to hyphenate "sigaction"
1084 or "manuscript", then it should be done as
1085 "sig-I<[linebreak]>action" or "manu-I<[linebreak]>script"
1086 (and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> doesn't
1087 show up at all). And if it is
1088 to hyphenate "Jarkko" and/or "Hietaniemi", it can do
1089 so only at the points where there is a C<EE<lt>shyE<gt>> code.
1091 In practice, it is anticipated that this character will not be used
1092 often, but formatters should either support it, or delete it.
1096 If you think that you want to add a new command to Pod (like, say, a
1097 "=biblio" command), consider whether you could get the same
1098 effect with a for or begin/end sequence: "=for biblio ..." or "=begin
1099 biblio" ... "=end biblio". Pod processors that don't understand
1100 "=for biblio", etc, will simply ignore it, whereas they may complain
1101 loudly if they see "=biblio".
1105 Throughout this document, "Pod" has been the preferred spelling for
1106 the name of the documentation format. One may also use "POD" or
1107 "pod". For the documentation that is (typically) in the Pod
1108 format, you may use "pod", or "Pod", or "POD". Understanding these
1109 distinctions is useful; but obsessing over how to spell them, usually
1118 =head1 About LE<lt>...E<gt> Codes
1120 As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...>
1121 code is the most complex of the Pod formatting codes. The points below
1122 will hopefully clarify what it means and how processors should deal
1129 In parsing an LE<lt>...> code, Pod parsers must distinguish at least
1136 The link-text. If there is none, this must be undef. (E.g., in
1137 "LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions".
1138 In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no
1139 link text. Note that link text may contain formatting.)
1143 The possibly inferred link-text; i.e., if there was no real link
1144 text, then this is the text that we'll infer in its place. (E.g., for
1145 "LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".)
1149 The name or URL, or undef if none. (E.g., in "LE<lt>Perl
1150 Functions|perlfunc>", the name (also sometimes called the page)
1151 is "perlfunc". In "LE<lt>/CAVEATS>", the name is undef.)
1155 The section (AKA "item" in older perlpods), or undef if none. E.g.,
1156 in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "DESCRIPTION" is the section. (Note
1157 that this is not the same as a manpage section like the "5" in "man 5
1158 crontab". "Section Foo" in the Pod sense means the part of the text
1159 that's introduced by the heading or item whose text is "Foo".)
1163 Pod parsers may also note additional attributes including:
1169 A flag for whether item 3 (if present) is a URL (like
1170 "http://lists.perl.org" is), in which case there should be no section
1171 attribute; a Pod name (like "perldoc" and "Getopt::Std" are); or
1172 possibly a man page name (like "crontab(5)" is).
1176 The raw original LE<lt>...> content, before text is split on
1177 "|", "/", etc, and before EE<lt>...> codes are expanded.
1181 (The above were numbered only for concise reference below. It is not
1182 a requirement that these be passed as an actual list or array.)
1187 => undef, # link text
1188 "Foo::Bar", # possibly inferred link text
1191 'pod', # what sort of link
1192 "Foo::Bar" # original content
1194 L<Perlport's section on NL's|perlport/Newlines>
1195 => "Perlport's section on NL's", # link text
1196 "Perlport's section on NL's", # possibly inferred link text
1198 "Newlines", # section
1199 'pod', # what sort of link
1200 "Perlport's section on NL's|perlport/Newlines"
1203 L<perlport/Newlines>
1204 => undef, # link text
1205 '"Newlines" in perlport', # possibly inferred link text
1207 "Newlines", # section
1208 'pod', # what sort of link
1209 "perlport/Newlines" # original content
1211 L<crontab(5)/"DESCRIPTION">
1212 => undef, # link text
1213 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
1214 "crontab(5)", # name
1215 "DESCRIPTION", # section
1216 'man', # what sort of link
1217 'crontab(5)/"DESCRIPTION"' # original content
1219 L</Object Attributes>
1220 => undef, # link text
1221 '"Object Attributes"', # possibly inferred link text
1223 "Object Attributes", # section
1224 'pod', # what sort of link
1225 "/Object Attributes" # original content
1227 L<http://www.perl.org/>
1228 => undef, # link text
1229 "http://www.perl.org/", # possibly inferred link text
1230 "http://www.perl.org/", # name
1232 'url', # what sort of link
1233 "http://www.perl.org/" # original content
1235 L<Perl.org|http://www.perl.org/>
1236 => "Perl.org", # link text
1237 "http://www.perl.org/", # possibly inferred link text
1238 "http://www.perl.org/", # name
1240 'url', # what sort of link
1241 "Perl.org|http://www.perl.org/" # original content
1243 Note that you can distinguish URL-links from anything else by the
1244 fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So
1245 C<LE<lt>http://www.perl.comE<gt>> is a URL, but
1246 C<LE<lt>HTTP::ResponseE<gt>> isn't.
1250 In case of LE<lt>...> codes with no "text|" part in them,
1251 older formatters have exhibited great variation in actually displaying
1252 the link or cross reference. For example, LE<lt>crontab(5)> would render
1253 as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage"
1254 or just "C<crontab(5)>".
1256 Pod processors must now treat "text|"-less links as follows:
1258 L<name> => L<name|name>
1259 L</section> => L<"section"|/section>
1260 L<name/section> => L<"section" in name|name/section>
1264 Note that section names might contain markup. I.e., if a section
1267 =head2 About the C<-M> Operator
1271 =item About the C<-M> Operator
1273 then a link to it would look like this:
1275 L<somedoc/About the C<-M> Operator>
1277 Formatters may choose to ignore the markup for purposes of resolving
1278 the link and use only the renderable characters in the section name,
1281 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1286 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1287 Operator" in somedoc</a>
1291 Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>>
1292 links from C<LE<lt>name/itemE<gt>> links (and their targets). These
1293 have been merged syntactically and semantically in the current
1294 specification, and I<section> can refer either to a "=headI<n> Heading
1295 Content" command or to a "=item Item Content" command. This
1296 specification does not specify what behavior should be in the case
1297 of a given document having several things all seeming to produce the
1298 same I<section> identifier (e.g., in HTML, several things all producing
1299 the same I<anchorname> in <a name="I<anchorname>">...</a>
1300 elements). Where Pod processors can control this behavior, they should
1301 use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the
1302 I<first> "Bar" section in Foo.
1304 But for some processors/formats this cannot be easily controlled; as
1305 with the HTML example, the behavior of multiple ambiguous
1306 <a name="I<anchorname>">...</a> is most easily just left up to
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">,
https://perl5.git.perl.org / perl5.git / blob