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 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 0xC0 - 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. In the unlikely circumstance that the first highbit
645 sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one
646 can cater to our heuristic (as well as any more intelligent heuristic)
647 by prefacing that line with a comment line containing a highbit
648 sequence that is clearly I<not> valid as UTF-8. A line consisting
649 of simply "#", an e-acute, and any non-highbit byte,
650 is sufficient to establish this file's encoding.
653 If/WHEN some brave soul makes these heuristics into a generic
654 text-file class (or PerlIO layer?), we can presumably delete
655 mention of these icky details from this file, and can instead
656 tell people to just use appropriate class/layer.
657 Auto-recognition of newline sequences would be another desirable
658 feature of such a class/layer.
662 "The probability that a string of characters
663 in any other encoding appears as valid UTF-8 is low" - RFC2279
667 This document's requirements and suggestions about encodings
668 do not apply to Pod processors running on non-ASCII platforms,
669 notably EBCDIC platforms.
673 Pod processors must treat a "=for [label] [content...]" paragraph as
674 meaning the same thing as a "=begin [label]" paragraph, content, and
675 an "=end [label]" paragraph. (The parser may conflate these two
676 constructs, or may leave them distinct, in the expectation that the
677 formatter will nevertheless treat them the same.)
681 When rendering Pod to a format that allows comments (i.e., to nearly
682 any format other than plaintext), a Pod formatter must insert comment
683 text identifying its name and version number, and the name and
684 version numbers of any modules it might be using to process the Pod.
687 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
689 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
691 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
693 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
695 Formatters may also insert additional comments, including: the
696 release date of the Pod formatter program, the contact address for
697 the author(s) of the formatter, the current time, the name of input
698 file, the formatting options in effect, version of Perl used, etc.
700 Formatters may also choose to note errors/warnings as comments,
701 besides or instead of emitting them otherwise (as in messages to
702 STDERR, or C<die>ing).
706 Pod parsers I<may> emit warnings or error messages ("Unknown E code
707 EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or
708 C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> allow
709 suppressing all such STDERR output, and instead allow an option for
710 reporting errors/warnings
711 in some other way, whether by triggering a callback, or noting errors
712 in some attribute of the document object, or some similarly unobtrusive
713 mechanism -- or even by appending a "Pod Errors" section to the end of
714 the parsed form of the document.
718 In cases of exceptionally aberrant documents, Pod parsers may abort the
719 parse. Even then, using C<die>ing/C<croak>ing is to be avoided; where
720 possible, the parser library may simply close the input file
721 and add text like "*** Formatting Aborted ***" to the end of the
722 (partial) in-memory document.
726 In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>)
727 are understood (i.e., I<not> verbatim paragraphs, but I<including>
728 ordinary paragraphs, and command paragraphs that produce renderable
729 text, like "=head1"), literal whitespace should generally be considered
730 "insignificant", in that one literal space has the same meaning as any
731 (nonzero) number of literal spaces, literal newlines, and literal tabs
732 (as long as this produces no blank lines, since those would terminate
733 the paragraph). Pod parsers should compact literal whitespace in each
734 processed paragraph, but may provide an option for overriding this
735 (since some processing tasks do not require it), or may follow
736 additional special rules (for example, specially treating
737 period-space-space or period-newline sequences).
741 Pod parsers should not, by default, try to coerce apostrophe (') and
742 quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to
743 turn backtick (`) into anything else but a single backtick character
744 (distinct from an open quote character!), nor "--" into anything but
745 two minus signs. They I<must never> do any of those things to text
746 in CE<lt>...> formatting codes, and never I<ever> to text in verbatim
751 When rendering Pod to a format that has two kinds of hyphens (-), one
752 that's a non-breaking hyphen, and another that's a breakable hyphen
753 (as in "object-oriented", which can be split across lines as
754 "object-", newline, "oriented"), formatters are encouraged to
755 generally translate "-" to non-breaking hyphen, but may apply
756 heuristics to convert some of these to breaking hyphens.
760 Pod formatters should make reasonable efforts to keep words of Perl
761 code from being broken across lines. For example, "Foo::Bar" in some
762 formatting systems is seen as eligible for being broken across lines
763 as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should
764 be avoided where possible, either by disabling all line-breaking in
765 mid-word, or by wrapping particular words with internal punctuation
766 in "don't break this across lines" codes (which in some formats may
767 not be a single code, but might be a matter of inserting non-breaking
768 zero-width spaces between every pair of characters in a word.)
772 Pod parsers should, by default, expand tabs in verbatim paragraphs as
773 they are processed, before passing them to the formatter or other
774 processor. Parsers may also allow an option for overriding this.
778 Pod parsers should, by default, remove newlines from the end of
779 ordinary and verbatim paragraphs before passing them to the
780 formatter. For example, while the paragraph you're reading now
781 could be considered, in Pod source, to end with (and contain)
782 the newline(s) that end it, it should be processed as ending with
783 (and containing) the period character that ends this sentence.
787 Pod parsers, when reporting errors, should make some effort to report
788 an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near
789 line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph
790 number ("Nested EE<lt>>'s in Paragraph #52 of Thing/Foo.pm!"). Where
791 this is problematic, the paragraph number should at least be
792 accompanied by an excerpt from the paragraph ("Nested EE<lt>>'s in
793 Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for
794 the CE<lt>interest rate> attribute...'").
798 Pod parsers, when processing a series of verbatim paragraphs one
799 after another, should consider them to be one large verbatim
800 paragraph that happens to contain blank lines. I.e., these two
801 lines, which have a blank line between them:
807 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
808 Foo->VERSION") before being passed to the formatter or other
809 processor. Parsers may also allow an option for overriding this.
811 While this might be too cumbersome to implement in event-based Pod
812 parsers, it is straightforward for parsers that return parse trees.
816 Pod formatters, where feasible, are advised to avoid splitting short
817 verbatim paragraphs (under twelve lines, say) across pages.
821 Pod parsers must treat a line with only spaces and/or tabs on it as a
822 "blank line" such as separates paragraphs. (Some older parsers
823 recognized only two adjacent newlines as a "blank line" but would not
824 recognize a newline, a space, and a newline, as a blank line. This
825 is noncompliant behavior.)
829 Authors of Pod formatters/processors should make every effort to
830 avoid writing their own Pod parser. There are already several in
831 CPAN, with a wide range of interface styles -- and one of them,
832 Pod::Parser, comes with modern versions of Perl.
836 Characters in Pod documents may be conveyed either as literals, or by
837 number in EE<lt>n> codes, or by an equivalent mnemonic, as in
838 EE<lt>eacute> which is exactly equivalent to EE<lt>233>.
840 Characters in the range 32-126 refer to those well known US-ASCII
841 characters (also defined there by Unicode, with the same meaning),
842 which all Pod formatters must render faithfully. Characters
843 in the ranges 0-31 and 127-159 should not be used (neither as
844 literals, nor as EE<lt>number> codes), except for the
845 literal byte-sequences for newline (13, 13 10, or 10), and tab (9).
847 Characters in the range 160-255 refer to Latin-1 characters (also
848 defined there by Unicode, with the same meaning). Characters above
849 255 should be understood to refer to Unicode characters.
854 that some formatters cannot reliably render characters outside 32-126;
855 and many are able to handle 32-126 and 160-255, but nothing above
860 Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for
861 less-than and greater-than, Pod parsers must understand "EE<lt>sol>"
862 for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar,
863 pipe). Pod parsers should also understand "EE<lt>lchevron>" and
864 "EE<lt>rchevron>" as legacy codes for characters 171 and 187, i.e.,
865 "left-pointing double angle quotation mark" = "left pointing
866 guillemet" and "right-pointing double angle quotation mark" = "right
867 pointing guillemet". (These look like little "<<" and ">>", and they
868 are now preferably expressed with the HTML/XHTML codes "EE<lt>laquo>"
873 Pod parsers should understand all "EE<lt>html>" codes as defined
874 in the entity declarations in the most recent XHTML specification at
875 C<www.W3.org>. Pod parsers must understand at least the entities
876 that define characters in the range 160-255 (Latin-1). Pod parsers,
877 when faced with some unknown "EE<lt>I<identifier>>" code,
878 shouldn't simply replace it with nullstring (by default, at least),
879 but may pass it through as a string consisting of the literal characters
880 E, less-than, I<identifier>, greater-than. Or Pod parsers may offer the
881 alternative option of processing such unknown
882 "EE<lt>I<identifier>>" codes by firing an event especially
883 for such codes, or by adding a special node-type to the in-memory
884 document tree. Such "EE<lt>I<identifier>>" may have special meaning
885 to some processors, or some processors may choose to add them to
886 a special error report.
890 Pod parsers must also support the XHTML codes "EE<lt>quot>" for
891 character 34 (doublequote, "), "EE<lt>amp>" for character 38
892 (ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, ').
896 Note that in all cases of "EE<lt>whatever>", I<whatever> (whether
897 an htmlname, or a number in any base) must consist only of
898 alphanumeric characters -- that is, I<whatever> must watch
899 C<m/\A\w+\z/>. So "EE<lt> 0 1 2 3 >" is invalid, because
900 it contains spaces, which aren't alphanumeric characters. This
901 presumably does not I<need> special treatment by a Pod processor;
902 " 0 1 2 3 " doesn't look like a number in any base, so it would
903 presumably be looked up in the table of HTML-like names. Since
904 there isn't (and cannot be) an HTML-like entity called " 0 1 2 3 ",
905 this will be treated as an error. However, Pod processors may
906 treat "EE<lt> 0 1 2 3 >" or "EE<lt>e-acute>" as I<syntactically>
907 invalid, potentially earning a different error message than the
908 error message (or warning, or event) generated by a merely unknown
909 (but theoretically valid) htmlname, as in "EE<lt>qacute>"
910 [sic]. However, Pod parsers are not required to make this
915 Note that EE<lt>number> I<must not> be interpreted as simply
916 "codepoint I<number> in the current/native character set". It always
917 means only "the character represented by codepoint I<number> in
918 Unicode." (This is identical to the semantics of &#I<number>; in XML.)
920 This will likely require many formatters to have tables mapping from
921 treatable Unicode codepoints (such as the "\xE9" for the e-acute
922 character) to the escape sequences or codes necessary for conveying
923 such sequences in the target output format. A converter to *roff
924 would, for example know that "\xE9" (whether conveyed literally, or via
925 a EE<lt>...> sequence) is to be conveyed as "e\\*'".
926 Similarly, a program rendering Pod in a Mac OS application window, would
927 presumably need to know that "\xE9" maps to codepoint 142 in MacRoman
928 encoding that (at time of writing) is native for Mac OS. Such
929 Unicode2whatever mappings are presumably already widely available for
930 common output formats. (Such mappings may be incomplete! Implementers
931 are not expected to bend over backwards in an attempt to render
932 Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any
933 of the other weird things that Unicode can encode.) And
934 if a Pod document uses a character not found in such a mapping, the
935 formatter should consider it an unrenderable character.
939 If, surprisingly, the implementor of a Pod formatter can't find a
940 satisfactory pre-existing table mapping from Unicode characters to
941 escapes in the target format (e.g., a decent table of Unicode
942 characters to *roff escapes), it will be necessary to build such a
943 table. If you are in this circumstance, you should begin with the
944 characters in the range 0x00A0 - 0x00FF, which is mostly the heavily
945 used accented characters. Then proceed (as patience permits and
946 fastidiousness compels) through the characters that the (X)HTML
947 standards groups judged important enough to merit mnemonics
948 for. These are declared in the (X)HTML specifications at the
949 www.W3.org site. At time of writing (September 2001), the most recent
950 entity declaration files are:
952 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
953 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
954 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
956 Then you can progress through any remaining notable Unicode characters
957 in the range 0x2000-0x204D (consult the character tables at
958 www.unicode.org), and whatever else strikes your fancy. For example,
959 in F<xhtml-symbol.ent>, there is the entry:
961 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
963 While the mapping "infin" to the character "\x{221E}" will (hopefully)
964 have been already handled by the Pod parser, the presence of the
965 character in this file means that it's reasonably important enough to
966 include in a formatter's table that maps from notable Unicode characters
967 to the codes necessary for rendering them. So for a Unicode-to-*roff
968 mapping, for example, this would merit the entry:
970 "\x{221E}" => '\(in',
972 It is eagerly hoped that in the future, increasing numbers of formats
973 (and formatters) will support Unicode characters directly (as (X)HTML
974 does with C<∞>, C<∞>, or C<∞>), reducing the need
975 for idiosyncratic mappings of Unicode-to-I<my_escapes>.
979 It is up to individual Pod formatter to display good judgement when
980 confronted with an unrenderable character (which is distinct from an
981 unknown EE<lt>thing> sequence that the parser couldn't resolve to
982 anything, renderable or not). It is good practice to map Latin letters
983 with diacritics (like "EE<lt>eacute>"/"EE<lt>233>") to the corresponding
984 unaccented US-ASCII letters (like a simple character 101, "e"), but
985 clearly this is often not feasible, and an unrenderable character may
986 be represented as "?", or the like. In attempting a sane fallback
987 (as from EE<lt>233> to "e"), Pod formatters may use the
988 %Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or
989 L<Text::Unidecode|Text::Unidecode>, if available.
991 For example, this Pod text:
993 magic is enabled if you set C<$Currency> to 'E<euro>'.
996 "magic is enabled if you set C<$Currency> to 'I<?>'" or as
997 "magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as
998 "magic is enabled if you set C<$Currency> to '[x20AC]', etc.
1000 A Pod formatter may also note, in a comment or warning, a list of what
1001 unrenderable characters were encountered.
1005 EE<lt>...> may freely appear in any formatting code (other than
1006 in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The
1007 EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The
1008 EE<lt>euro>1,000,000 Solution|Million::Euros>".
1012 Some Pod formatters output to formats that implement non-breaking
1013 spaces as an individual character (which I'll call "NBSP"), and
1014 others output to formats that implement non-breaking spaces just as
1015 spaces wrapped in a "don't break this across lines" code. Note that
1016 at the level of Pod, both sorts of codes can occur: Pod can contain a
1017 NBSP character (whether as a literal, or as a "EE<lt>160>" or
1018 "EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo
1019 IE<lt>barE<gt> baz>" codes, where "mere spaces" (character 32) in
1020 such codes are taken to represent non-breaking spaces. Pod
1021 parsers should consider supporting the optional parsing of "SE<lt>foo
1022 IE<lt>barE<gt> baz>" as if it were
1023 "fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the
1024 optional parsing of groups of words joined by NBSP's as if each group
1025 were in a SE<lt>...> code, so that formatters may use the
1026 representation that maps best to what the output format demands.
1030 Some processors may find that the C<SE<lt>...E<gt>> code is easiest to
1031 implement by replacing each space in the parse tree under the content
1032 of the S, with an NBSP. But note: the replacement should apply I<not> to
1033 spaces in I<all> text, but I<only> to spaces in I<printable> text. (This
1034 distinction may or may not be evident in the particular tree/event
1035 model implemented by the Pod parser.) For example, consider this
1038 S<L</Autoloaded Functions>>
1040 This means that the space in the middle of the visible link text must
1041 not be broken across lines. In other words, it's the same as this:
1043 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
1045 However, a misapplied space-to-NBSP replacement could (wrongly)
1046 produce something equivalent to this:
1048 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
1050 ...which is almost definitely not going to work as a hyperlink (assuming
1051 this formatter outputs a format supporting hypertext).
1053 Formatters may choose to just not support the S format code,
1054 especially in cases where the output format simply has no NBSP
1055 character/code and no code for "don't break this stuff across lines".
1059 Besides the NBSP character discussed above, implementors are reminded
1060 of the existence of the other "special" character in Latin-1, the
1061 "soft hyphen" character, also known as "discretionary hyphen",
1062 i.e. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> =
1063 C<EE<lt>shyE<gt>>). This character expresses an optional hyphenation
1064 point. That is, it normally renders as nothing, but may render as a
1065 "-" if a formatter breaks the word at that point. Pod formatters
1066 should, as appropriate, do one of the following: 1) render this with
1067 a code with the same meaning (e.g., "\-" in RTF), 2) pass it through
1068 in the expectation that the formatter understands this character as
1069 such, or 3) delete it.
1075 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
1077 These signal to a formatter that if it is to hyphenate "sigaction"
1078 or "manuscript", then it should be done as
1079 "sig-I<[linebreak]>action" or "manu-I<[linebreak]>script"
1080 (and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> doesn't
1081 show up at all). And if it is
1082 to hyphenate "Jarkko" and/or "Hietaniemi", it can do
1083 so only at the points where there is a C<EE<lt>shyE<gt>> code.
1085 In practice, it is anticipated that this character will not be used
1086 often, but formatters should either support it, or delete it.
1090 If you think that you want to add a new command to Pod (like, say, a
1091 "=biblio" command), consider whether you could get the same
1092 effect with a for or begin/end sequence: "=for biblio ..." or "=begin
1093 biblio" ... "=end biblio". Pod processors that don't understand
1094 "=for biblio", etc, will simply ignore it, whereas they may complain
1095 loudly if they see "=biblio".
1099 Throughout this document, "Pod" has been the preferred spelling for
1100 the name of the documentation format. One may also use "POD" or
1101 "pod". For the documentation that is (typically) in the Pod
1102 format, you may use "pod", or "Pod", or "POD". Understanding these
1103 distinctions is useful; but obsessing over how to spell them, usually
1112 =head1 About LE<lt>...E<gt> Codes
1114 As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...>
1115 code is the most complex of the Pod formatting codes. The points below
1116 will hopefully clarify what it means and how processors should deal
1123 In parsing an LE<lt>...> code, Pod parsers must distinguish at least
1130 The link-text. If there is none, this must be undef. (E.g., in
1131 "LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions".
1132 In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no
1133 link text. Note that link text may contain formatting.)
1137 The possibly inferred link-text; i.e., if there was no real link
1138 text, then this is the text that we'll infer in its place. (E.g., for
1139 "LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".)
1143 The name or URL, or undef if none. (E.g., in "LE<lt>Perl
1144 Functions|perlfunc>", the name (also sometimes called the page)
1145 is "perlfunc". In "LE<lt>/CAVEATS>", the name is undef.)
1149 The section (AKA "item" in older perlpods), or undef if none. E.g.,
1150 in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "DESCRIPTION" is the section. (Note
1151 that this is not the same as a manpage section like the "5" in "man 5
1152 crontab". "Section Foo" in the Pod sense means the part of the text
1153 that's introduced by the heading or item whose text is "Foo".)
1157 Pod parsers may also note additional attributes including:
1163 A flag for whether item 3 (if present) is a URL (like
1164 "http://lists.perl.org" is), in which case there should be no section
1165 attribute; a Pod name (like "perldoc" and "Getopt::Std" are); or
1166 possibly a man page name (like "crontab(5)" is).
1170 The raw original LE<lt>...> content, before text is split on
1171 "|", "/", etc, and before EE<lt>...> codes are expanded.
1175 (The above were numbered only for concise reference below. It is not
1176 a requirement that these be passed as an actual list or array.)
1181 => undef, # link text
1182 "Foo::Bar", # possibly inferred link text
1185 'pod', # what sort of link
1186 "Foo::Bar" # original content
1188 L<Perlport's section on NL's|perlport/Newlines>
1189 => "Perlport's section on NL's", # link text
1190 "Perlport's section on NL's", # possibly inferred link text
1192 "Newlines", # section
1193 'pod', # what sort of link
1194 "Perlport's section on NL's|perlport/Newlines"
1197 L<perlport/Newlines>
1198 => undef, # link text
1199 '"Newlines" in perlport', # possibly inferred link text
1201 "Newlines", # section
1202 'pod', # what sort of link
1203 "perlport/Newlines" # original content
1205 L<crontab(5)/"DESCRIPTION">
1206 => undef, # link text
1207 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
1208 "crontab(5)", # name
1209 "DESCRIPTION", # section
1210 'man', # what sort of link
1211 'crontab(5)/"DESCRIPTION"' # original content
1213 L</Object Attributes>
1214 => undef, # link text
1215 '"Object Attributes"', # possibly inferred link text
1217 "Object Attributes", # section
1218 'pod', # what sort of link
1219 "/Object Attributes" # original content
1221 L<http://www.perl.org/>
1222 => undef, # link text
1223 "http://www.perl.org/", # possibly inferred link text
1224 "http://www.perl.org/", # name
1226 'url', # what sort of link
1227 "http://www.perl.org/" # original content
1229 L<Perl.org|http://www.perl.org/>
1230 => "Perl.org", # link text
1231 "http://www.perl.org/", # possibly inferred link text
1232 "http://www.perl.org/", # name
1234 'url', # what sort of link
1235 "Perl.org|http://www.perl.org/" # original content
1237 Note that you can distinguish URL-links from anything else by the
1238 fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So
1239 C<LE<lt>http://www.perl.comE<gt>> is a URL, but
1240 C<LE<lt>HTTP::ResponseE<gt>> isn't.
1244 In case of LE<lt>...> codes with no "text|" part in them,
1245 older formatters have exhibited great variation in actually displaying
1246 the link or cross reference. For example, LE<lt>crontab(5)> would render
1247 as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage"
1248 or just "C<crontab(5)>".
1250 Pod processors must now treat "text|"-less links as follows:
1252 L<name> => L<name|name>
1253 L</section> => L<"section"|/section>
1254 L<name/section> => L<"section" in name|name/section>
1258 Note that section names might contain markup. I.e., if a section
1261 =head2 About the C<-M> Operator
1265 =item About the C<-M> Operator
1267 then a link to it would look like this:
1269 L<somedoc/About the C<-M> Operator>
1271 Formatters may choose to ignore the markup for purposes of resolving
1272 the link and use only the renderable characters in the section name,
1275 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1280 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1281 Operator" in somedoc</a>
1285 Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>>
1286 links from C<LE<lt>name/itemE<gt>> links (and their targets). These
1287 have been merged syntactically and semantically in the current
1288 specification, and I<section> can refer either to a "=headI<n> Heading
1289 Content" command or to a "=item Item Content" command. This
1290 specification does not specify what behavior should be in the case
1291 of a given document having several things all seeming to produce the
1292 same I<section> identifier (e.g., in HTML, several things all producing
1293 the same I<anchorname> in <a name="I<anchorname>">...</a>
1294 elements). Where Pod processors can control this behavior, they should
1295 use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the
1296 I<first> "Bar" section in Foo.
1298 But for some processors/formats this cannot be easily controlled; as
1299 with the HTML example, the behavior of multiple ambiguous
1300 <a name="I<anchorname>">...</a> is most easily just left up to
1305 In a C<LE<lt>text|...E<gt>> code, text may contain formatting codes
1306 for formatting or for EE<lt>...> escapes, as in:
1308 L<B<ummE<234>stuff>|...>
1310 For C<LE<lt>...E<gt>> codes without a "name|" part, only
1311 C<EE<lt>...E<gt>> and C<ZE<lt>E<gt>> codes may occur. That is,
1312 authors should not use "C<LE<lt>BE<lt>Foo::BarE<gt>E<gt>>".
1314 Note, however, that formatting codes and ZE<lt>>'s can occur in any
1315 and all parts of an LE<lt>...> (i.e., in I<name>, I<section>, I<text>,
1318 Authors must not nest LE<lt>...> codes. For example, "LE<lt>The
1319 LE<lt>Foo::Bar> man page>" should be treated as an error.
1323 Note that Pod authors may use formatting codes inside the "text"
1324 part of "LE<lt>text|name>" (and so on for LE<lt>text|/"sec">).
1326 In other words, this is valid:
1328 Go read L<the docs on C<$.>|perlvar/"$.">
1330 Some output formats that do allow rendering "LE<lt>...>" codes as
1331 hypertext, might not allow the link-text to be formatted; in
1332 that case, formatters will have to just ignore that formatting.
1336 At time of writing, C<LE<lt>nameE<gt>> values are of two types:
1337 either the name of a Pod page like C<LE<lt>Foo::BarE<gt>> (which
1338 might be a real Perl module or program in an @INC / PATH
1339 directory, or a .pod file in those places); or the name of a Unix
1340 man page, like C<LE<lt>crontab(5)E<gt>>. In theory, C<LE<lt>chmodE<gt>>
1341 in ambiguous between a Pod page called "chmod", or the Unix man page
1342 "chmod" (in whatever man-section). However, the presence of a string
1343 in parens, as in "crontab(5)", is sufficient to signal that what
1344 is being discussed is not a Pod page, and so is presumably a
1345 Unix man page. The distinction is of no importance to many
1346 Pod processors, but some processors that render to hypertext formats
1347 may need to distinguish them in order to know how to render a
1348 given C<LE<lt>fooE<gt>> code.
1352 Previous versions of perlpod allowed for a C<LE<lt>sectionE<gt>> syntax (as in
1353 C<LE<lt>Object AttributesE<gt>>), which was not easily distinguishable from
1354 C<LE<lt>nameE<gt>> syntax and for C<LE<lt>"section"E<gt>> which was only
1355 slightly less ambiguous. This syntax is no longer in the specification, and
1356 has been replaced by the C<LE<lt>/sectionE<gt>> syntax (where the slash was
1357 formerly optional). Pod parsers should tolerate the C<LE<lt>"section"E<gt>>
1358 syntax, for a while at least. The suggested heuristic for distinguishing
1359 C<LE<lt>sectionE<gt>> from C<LE<lt>nameE<gt>> is that if it contains any
1360 whitespace, it's a I<section>. Pod processors should warn about this being
1365 =head1 About =over...=back Regions
1367 "=over"..."=back" regions are used for various kinds of list-like
1368 structures. (I use the term "region" here simply as a collective
1369 term for everything from the "=over" to the matching "=back".)
1375 The non-zero numeric I<indentlevel> in "=over I<indentlevel>" ...
1376 "=back" is used for giving the formatter a clue as to how many
1377 "spaces" (ems, or roughly equivalent units) it should tab over,
1378 although many formatters will have to convert this to an absolute
1379 measurement that may not exactly match with the size of spaces (or M's)
1380 in the document's base font. Other formatters may have to completely
1381 ignore the number. The lack of any explicit I<indentlevel> parameter is
1382 equivalent to an I<indentlevel> value of 4. Pod processors may
1383 complain if I<indentlevel> is present but is not a positive number
1384 matching C<m/\A(\d*\.)?\d+\z/>.
1388 Authors of Pod formatters are reminded that "=over" ... "=back" may
1389 map to several different constructs in your output format. For
1390 example, in converting Pod to (X)HTML, it can map to any of
1391 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1392 <blockquote>...</blockquote>. Similarly, "=item" can map to <li> or
1397 Each "=over" ... "=back" region should be one of the following:
1403 An "=over" ... "=back" region containing only "=item *" commands,
1404 each followed by some number of ordinary/verbatim paragraphs, other
1405 nested "=over" ... "=back" regions, "=for..." paragraphs, and
1406 "=begin"..."=end" regions.
1408 (Pod processors must tolerate a bare "=item" as if it were "=item
1409 *".) Whether "*" is rendered as a literal asterisk, an "o", or as
1410 some kind of real bullet character, is left up to the Pod formatter,
1411 and may depend on the level of nesting.
1415 An "=over" ... "=back" region containing only
1416 C<m/\A=item\s+\d+\.?\s*\z/> paragraphs, each one (or each group of them)
1417 followed by some number of ordinary/verbatim paragraphs, other nested
1418 "=over" ... "=back" regions, "=for..." paragraphs, and/or
1419 "=begin"..."=end" codes. Note that the numbers must start at 1
1420 in each section, and must proceed in order and without skipping
1423 (Pod processors must tolerate lines like "=item 1" as if they were
1424 "=item 1.", with the period.)
1428 An "=over" ... "=back" region containing only "=item [text]"
1429 commands, each one (or each group of them) followed by some number of
1430 ordinary/verbatim paragraphs, other nested "=over" ... "=back"
1431 regions, or "=for..." paragraphs, and "=begin"..."=end" regions.
1433 The "=item [text]" paragraph should not match
1434 C<m/\A=item\s+\d+\.?\s*\z/> or C<m/\A=item\s+\*\s*\z/>, nor should it
1435 match just C<m/\A=item\s*\z/>.
1439 An "=over" ... "=back" region containing no "=item" paragraphs at
1440 all, and containing only some number of
1441 ordinary/verbatim paragraphs, and possibly also some nested "=over"
1442 ... "=back" regions, "=for..." paragraphs, and "=begin"..."=end"
1443 regions. Such an itemless "=over" ... "=back" region in Pod is
1444 equivalent in meaning to a "<blockquote>...</blockquote>" element in
1449 Note that with all the above cases, you can determine which type of
1450 "=over" ... "=back" you have, by examining the first (non-"=cut",
1451 non-"=pod") Pod paragraph after the "=over" command.
1455 Pod formatters I<must> tolerate arbitrarily large amounts of text
1456 in the "=item I<text...>" paragraph. In practice, most such
1457 paragraphs are short, as in:
1459 =item For cutting off our trade with all parts of the world
1461 But they may be arbitrarily long:
1463 =item For transporting us beyond seas to be tried for pretended
1466 =item He is at this time transporting large armies of foreign
1467 mercenaries to complete the works of death, desolation and
1468 tyranny, already begun with circumstances of cruelty and perfidy
1469 scarcely paralleled in the most barbarous ages, and totally
1470 unworthy the head of a civilized nation.
1474 Pod processors should tolerate "=item *" / "=item I<number>" commands
1475 with no accompanying paragraph. The middle item is an example:
1481 Pick up dry cleaning.
1487 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1493 No "=over" ... "=back" region can contain headings. Processors may
1494 treat such a heading as an error.
1498 Note that an "=over" ... "=back" region should have some
1499 content. That is, authors should not have an empty region like this:
1505 Pod processors seeing such a contentless "=over" ... "=back" region,
1506 may ignore it, or may report it as an error.
1510 Processors must tolerate an "=over" list that goes off the end of the
1511 document (i.e., which has no matching "=back"), but they may warn
1516 Authors of Pod formatters should note that this construct:
1524 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1525 velit, sed quia non numquam eius modi tempora incidunt ut
1526 labore et dolore magnam aliquam quaerat voluptatem.
1530 is semantically ambiguous, in a way that makes formatting decisions
1531 a bit difficult. On the one hand, it could be mention of an item
1532 "Neque", mention of another item "Porro", and mention of another
1533 item "Quisquam Est", with just the last one requiring the explanatory
1534 paragraph "Qui dolorem ipsum quia dolor..."; and then an item
1535 "Ut Enim". In that case, you'd want to format it like so:
1542 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1543 velit, sed quia non numquam eius modi tempora incidunt ut
1544 labore et dolore magnam aliquam quaerat voluptatem.
1548 But it could equally well be a discussion of three (related or equivalent)
1549 items, "Neque", "Porro", and "Quisquam Est", followed by a paragraph
1550 explaining them all, and then a new item "Ut Enim". In that case, you'd
1551 probably want to format it like so:
1556 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1557 velit, sed quia non numquam eius modi tempora incidunt ut
1558 labore et dolore magnam aliquam quaerat voluptatem.
1562 But (for the foreseeable future), Pod does not provide any way for Pod
1563 authors to distinguish which grouping is meant by the above
1564 "=item"-cluster structure. So formatters should format it like so:
1572 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1573 velit, sed quia non numquam eius modi tempora incidunt ut
1574 labore et dolore magnam aliquam quaerat voluptatem.
1578 That is, there should be (at least roughly) equal spacing between
1579 items as between paragraphs (although that spacing may well be less
1580 than the full height of a line of text). This leaves it to the reader
1581 to use (con)textual cues to figure out whether the "Qui dolorem
1582 ipsum..." paragraph applies to the "Quisquam Est" item or to all three
1583 items "Neque", "Porro", and "Quisquam Est". While not an ideal
1584 situation, this is preferable to providing formatting cues that may
1585 be actually contrary to the author's intent.
1591 =head1 About Data Paragraphs and "=begin/=end" Regions
1593 Data paragraphs are typically used for inlining non-Pod data that is
1594 to be used (typically passed through) when rendering the document to
1599 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1603 The exact same effect could, incidentally, be achieved with a single
1606 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1608 (Although that is not formally a data paragraph, it has the same
1609 meaning as one, and Pod parsers may parse it as one.)
1611 Another example of a data paragraph:
1615 I like <em>PIE</em>!
1617 <hr>Especially pecan pie!
1621 If these were ordinary paragraphs, the Pod parser would try to
1622 expand the "EE<lt>/em>" (in the first paragraph) as a formatting
1623 code, just like "EE<lt>lt>" or "EE<lt>eacute>". But since this
1624 is in a "=begin I<identifier>"..."=end I<identifier>" region I<and>
1625 the identifier "html" doesn't begin have a ":" prefix, the contents
1626 of this region are stored as data paragraphs, instead of being
1627 processed as ordinary paragraphs (or if they began with a spaces
1628 and/or tabs, as verbatim paragraphs).
1630 As a further example: At time of writing, no "biblio" identifier is
1631 supported, but suppose some processor were written to recognize it as
1632 a way of (say) denoting a bibliographic reference (necessarily
1633 containing formatting codes in ordinary paragraphs). The fact that
1634 "biblio" paragraphs were meant for ordinary processing would be
1635 indicated by prefacing each "biblio" identifier with a colon:
1639 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1640 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1644 This would signal to the parser that paragraphs in this begin...end
1645 region are subject to normal handling as ordinary/verbatim paragraphs
1646 (while still tagged as meant only for processors that understand the
1647 "biblio" identifier). The same effect could be had with:
1650 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1651 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1653 The ":" on these identifiers means simply "process this stuff
1654 normally, even though the result will be for some special target".
1655 I suggest that parser APIs report "biblio" as the target identifier,
1656 but also report that it had a ":" prefix. (And similarly, with the
1657 above "html", report "html" as the target identifier, and note the
1658 I<lack> of a ":" prefix.)
1660 Note that a "=begin I<identifier>"..."=end I<identifier>" region where
1661 I<identifier> begins with a colon, I<can> contain commands. For example:
1665 Wirth's classic is available in several editions, including:
1668 hm, check abebooks.com for how much used copies cost.
1674 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1675 Teubner, Stuttgart. [Yes, it's in German.]
1679 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1680 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1686 Note, however, a "=begin I<identifier>"..."=end I<identifier>"
1687 region where I<identifier> does I<not> begin with a colon, should not
1688 directly contain "=head1" ... "=head4" commands, nor "=over", nor "=back",
1689 nor "=item". For example, this may be considered invalid:
1693 This is a data paragraph.
1695 =head1 Don't do this!
1697 This is a data paragraph too.
1701 A Pod processor may signal that the above (specifically the "=head1"
1702 paragraph) is an error. Note, however, that the following should
1703 I<not> be treated as an error:
1707 This is a data paragraph.
1711 # Yup, this isn't Pod anymore.
1712 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1716 This is a data paragraph too.
1720 And this too is valid:
1724 This is a data paragraph.
1726 And this is a data paragraph.
1728 =begin someotherformat
1730 This is a data paragraph too.
1732 And this is a data paragraph too.
1734 =begin :yetanotherformat
1736 =head2 This is a command paragraph!
1738 This is an ordinary paragraph!
1740 And this is a verbatim paragraph!
1742 =end :yetanotherformat
1744 =end someotherformat
1746 Another data paragraph!
1750 The contents of the above "=begin :yetanotherformat" ...
1751 "=end :yetanotherformat" region I<aren't> data paragraphs, because
1752 the immediately containing region's identifier (":yetanotherformat")
1753 begins with a colon. In practice, most regions that contain
1754 data paragraphs will contain I<only> data paragraphs; however,
1755 the above nesting is syntactically valid as Pod, even if it is
1756 rare. However, the handlers for some formats, like "html",
1757 will accept only data paragraphs, not nested regions; and they may
1758 complain if they see (targeted for them) nested regions, or commands,
1759 other than "=end", "=pod", and "=cut".
1761 Also consider this valid structure:
1765 Wirth's classic is available in several editions, including:
1771 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1772 Teubner, Stuttgart. [Yes, it's in German.]
1776 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1777 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1785 <img src='wirth_spokesmodeling_book.png'>
1795 There, the "=begin html"..."=end html" region is nested inside
1796 the larger "=begin :biblio"..."=end :biblio" region. Note that the
1797 content of the "=begin html"..."=end html" region is data
1798 paragraph(s), because the immediately containing region's identifier
1799 ("html") I<doesn't> begin with a colon.
1801 Pod parsers, when processing a series of data paragraphs one
1802 after another (within a single region), should consider them to
1803 be one large data paragraph that happens to contain blank lines. So
1804 the content of the above "=begin html"..."=end html" I<may> be stored
1805 as two data paragraphs (one consisting of
1806 "<img src='wirth_spokesmodeling_book.png'>\n"
1807 and another consisting of "<hr>\n"), but I<should> be stored as
1808 a single data paragraph (consisting of
1809 "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1811 Pod processors should tolerate empty
1812 "=begin I<something>"..."=end I<something>" regions,
1813 empty "=begin :I<something>"..."=end :I<something>" regions, and
1814 contentless "=for I<something>" and "=for :I<something>"
1815 paragraphs. I.e., these should be tolerated:
1827 Incidentally, note that there's no easy way to express a data
1828 paragraph starting with something that looks like a command. Consider:
1836 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a data
1837 paragraph "=shazbot\n". However, you can express a data paragraph consisting
1838 of "=shazbot\n" using this code:
1842 The situation where this is necessary, is presumably quite rare.
1844 Note that =end commands must match the currently open =begin command. That
1845 is, they must properly nest. For example, this is valid:
1861 while this is invalid:
1877 This latter is improper because when the "=end outer" command is seen, the
1878 currently open region has the formatname "inner", not "outer". (It just
1879 happens that "outer" is the format name of a higher-up region.) This is
1880 an error. Processors must by default report this as an error, and may halt
1881 processing the document containing that error. A corollary of this is that
1882 regions cannot "overlap". That is, the latter block above does not represent
1883 a region called "outer" which contains X and Y, overlapping a region called
1884 "inner" which contains Y and Z. But because it is invalid (as all
1885 apparently overlapping regions would be), it doesn't represent that, or
1888 Similarly, this is invalid:
1894 This is an error because the region is opened by "thing", and the "=end"
1895 tries to close "hting" [sic].
1897 This is also invalid:
1903 This is invalid because every "=end" command must have a formatname
1908 L<perlpod>, L<perlsyn/"PODs: Embedded Documentation">,
https://perl5.git.perl.org / perl5.git / blob