4 perlpodspec - Plain Old Documentation: format specification and notes
8 This document is detailed notes on the Pod markup language. Most
9 people will only have to read L<perlpod|perlpod> to know how to write
10 in Pod, but this document may answer some incidental questions to do
11 with parsing and rendering Pod.
13 In this document, "must" / "must not", "should" /
14 "should not", and "may" have their conventional (cf. RFC 2119)
15 meanings: "X must do Y" means that if X doesn't do Y, it's against
16 this specification, and should really be fixed. "X should do Y"
17 means that it's recommended, but X may fail to do Y, if there's a
18 good reason. "X may do Y" is merely a note that X can do Y at
19 will (although it is up to the reader to detect any connotation of
20 "and I think it would be I<nice> if X did Y" versus "it wouldn't
21 really I<bother> me if X did Y").
23 Notably, when I say "the parser should do Y", the
24 parser may fail to do Y, if the calling application explicitly
25 requests that the parser I<not> do Y. I often phrase this as
26 "the parser should, by default, do Y." This doesn't I<require>
27 the parser to provide an option for turning off whatever
28 feature Y is (like expanding tabs in verbatim paragraphs), although
29 it implicates that such an option I<may> be provided.
31 =head1 Pod Definitions
33 Pod is embedded in files, typically Perl source files, although you
34 can write a file that's nothing but Pod.
36 A B<line> in a file consists of zero or more non-newline characters,
37 terminated by either a newline or the end of the file.
39 A B<newline sequence> is usually a platform-dependent concept, but
40 Pod parsers should understand it to mean any of CR (ASCII 13), LF
41 (ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in
42 addition to any other system-specific meaning. The first CR/CRLF/LF
43 sequence in the file may be used as the basis for identifying the
44 newline sequence for parsing the rest of the file.
46 A B<blank line> is a line consisting entirely of zero or more spaces
47 (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file.
48 A B<non-blank line> is a line containing one or more characters other
49 than space or tab (and terminated by a newline or end-of-file).
51 (I<Note:> Many older Pod parsers did not accept a line consisting of
52 spaces/tabs and then a newline as a blank line. The only lines they
53 considered blank were lines consisting of I<no characters at all>,
54 terminated by a newline.)
56 B<Whitespace> is used in this document as a blanket term for spaces,
57 tabs, and newline sequences. (By itself, this term usually refers
58 to literal whitespace. That is, sequences of whitespace characters
59 in Pod source, as opposed to "EE<lt>32>", which is a formatting
60 code that I<denotes> a whitespace character.)
62 A B<Pod parser> is a module meant for parsing Pod (regardless of
63 whether this involves calling callbacks or building a parse tree or
64 directly formatting it). A B<Pod formatter> (or B<Pod translator>)
65 is a module or program that converts Pod to some other format (HTML,
66 plaintext, TeX, PostScript, RTF). A B<Pod processor> might be a
67 formatter or translator, or might be a program that does something
68 else with the Pod (like counting words, scanning for index points,
71 Pod content is contained in B<Pod blocks>. A Pod block starts with a
72 line that matches <m/\A=[a-zA-Z]/>, and continues up to the next line
73 that matches C<m/\A=cut/> or up to the end of the file if there is
77 The current perlsyn says:
79 Note that pod translators should look at only paragraphs beginning
80 with a pod directive (it makes parsing easier), whereas the compiler
81 actually knows to look for pod escapes even in the middle of a
82 paragraph. This means that the following secret stuff will be ignored
83 by both the compiler and the translators.
86 warn "Neither POD nor CODE!?"
89 You probably shouldn't rely upon the warn() being podded out forever.
90 Not all pod translators are well-behaved in this regard, and perhaps
91 the compiler will become pickier.
93 I think that those paragraphs should just be removed; paragraph-based
94 parsing seems to have been largely abandoned, because of the hassle
95 with non-empty blank lines messing up what people meant by "paragraph".
96 Even if the "it makes parsing easier" bit were especially true,
97 it wouldn't be worth the confusion of having perl and pod2whatever
98 actually disagree on what can constitute a Pod block.
100 Within a Pod block, there are B<Pod paragraphs>. A Pod paragraph
101 consists of non-blank lines of text, separated by one or more blank
104 For purposes of Pod processing, there are four types of paragraphs in
111 A command paragraph (also called a "directive"). The first line of
112 this paragraph must match C<m/\A=[a-zA-Z]/>. Command paragraphs are
113 typically one line, as in:
119 But they may span several (non-blank) lines:
122 Hm, I wonder what it would look like if
123 you tried to write a BNF for Pod from this.
125 =head3 Dr. Strangelove, or: How I Learned to
126 Stop Worrying and Love the Bomb
128 I<Some> command paragraphs allow formatting codes in their content
129 (i.e., after the part that matches C<m/\A=[a-zA-Z]\S*\s*/>), as in:
131 =head1 Did You Remember to C<use strict;>?
133 In other words, the Pod processing handler for "head1" will apply the
134 same processing to "Did You Remember to CE<lt>use strict;>?" that it
135 would to an ordinary paragraph (i.e., formatting codes like
136 "CE<lt>...>") are parsed and presumably formatted appropriately, and
137 whitespace in the form of literal spaces and/or tabs is not
142 A B<verbatim paragraph>. The first line of this paragraph must be a
143 literal space or tab, and this paragraph must not be inside a "=begin
144 I<identifier>", ... "=end I<identifier>" sequence unless
145 "I<identifier>" begins with a colon (":"). That is, if a paragraph
146 starts with a literal space or tab, but I<is> inside a
147 "=begin I<identifier>", ... "=end I<identifier>" region, then it's
148 a data paragraph, unless "I<identifier>" begins with a colon.
150 Whitespace I<is> significant in verbatim paragraphs (although, in
151 processing, tabs are probably expanded).
155 An B<ordinary paragraph>. A paragraph is an ordinary paragraph
156 if its first line matches neither C<m/\A=[a-zA-Z]/> nor
157 C<m/\A[ \t]/>, I<and> if it's not inside a "=begin I<identifier>",
158 ... "=end I<identifier>" sequence unless "I<identifier>" begins with
163 A B<data paragraph>. This is a paragraph that I<is> inside a "=begin
164 I<identifier>" ... "=end I<identifier>" sequence where
165 "I<identifier>" does I<not> begin with a literal colon (":"). In
166 some sense, a data paragraph is not part of Pod at all (i.e.,
167 effectively it's "out-of-band"), since it's not subject to most kinds
168 of Pod parsing; but it is specified here, since Pod
169 parsers need to be able to call an event for it, or store it in some
170 form in a parse tree, or at least just parse I<around> it.
174 For example: consider the following paragraphs:
176 # <- that's the 0th column
186 Here, "=head1 Foo" and "=cut" are command paragraphs because the first
187 line of each matches C<m/\A=[a-zA-Z]/>. "I<[space][space]>$foo->bar"
188 is a verbatim paragraph, because its first line starts with a literal
189 whitespace character (and there's no "=begin"..."=end" region around).
191 The "=begin I<identifier>" ... "=end I<identifier>" commands stop
192 paragraphs that they surround from being parsed as ordinary or verbatim
193 paragraphs, if I<identifier> doesn't begin with a colon. This
194 is discussed in detail in the section
195 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
199 This section is intended to supplement and clarify the discussion in
200 L<perlpod/"Command Paragraph">. These are the currently recognized
205 =item "=head1", "=head2", "=head3", "=head4"
207 This command indicates that the text in the remainder of the paragraph
208 is a heading. That text may contain formatting codes. Examples:
210 =head1 Object Attributes
212 =head3 What B<Not> to Do!
216 This command indicates that this paragraph begins a Pod block. (If we
217 are already in the middle of a Pod block, this command has no effect at
218 all.) If there is any text in this command paragraph after "=pod",
219 it must be ignored. Examples:
223 This is a plain Pod paragraph.
225 =pod This text is ignored.
229 This command indicates that this line is the end of this previously
230 started Pod block. If there is any text after "=cut" on the line, it must be
235 =cut The documentation ends here.
238 # This is the first line of program text.
239 sub foo { # This is the second.
241 It is an error to try to I<start> a Pod block with a "=cut" command. In
242 that case, the Pod processor must halt parsing of the input file, and
243 must by default emit a warning.
247 This command indicates that this is the start of a list/indent
248 region. If there is any text following the "=over", it must consist
249 of only a nonzero positive numeral. The semantics of this numeral is
250 explained in the L</"About =over...=back Regions"> section, further
251 below. Formatting codes are not expanded. Examples:
261 This command indicates that an item in a list begins here. Formatting
262 codes are processed. The semantics of the (optional) text in the
263 remainder of this paragraph are
264 explained in the L</"About =over...=back Regions"> section, further
277 =item C<< $thing->stuff(I<dodad>) >>
279 =item For transporting us beyond seas to be tried for pretended
282 =item He is at this time transporting large armies of foreign
283 mercenaries to complete the works of death, desolation and
284 tyranny, already begun with circumstances of cruelty and perfidy
285 scarcely paralleled in the most barbarous ages, and totally
286 unworthy the head of a civilized nation.
290 This command indicates that this is the end of the region begun
291 by the most recent "=over" command. It permits no text after the
294 =item "=begin formatname"
296 =item "=begin formatname parameter"
298 This marks the following paragraphs (until the matching "=end
299 formatname") as being for some special kind of processing. Unless
300 "formatname" begins with a colon, the contained non-command
301 paragraphs are data paragraphs. But if "formatname" I<does> begin
302 with a colon, then non-command paragraphs are ordinary paragraphs
303 or data paragraphs. This is discussed in detail in the section
304 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
306 It is advised that formatnames match the regexp
307 C<m/\A:?[−a−zA−Z0−9_]+\z/>. Everything following whitespace after the
308 formatname is a parameter that may be used by the formatter when dealing
309 with this region. This parameter must not be repeated in the "=end"
310 paragraph. Implementors should anticipate future expansion in the
311 semantics and syntax of the first parameter to "=begin"/"=end"/"=for".
313 =item "=end formatname"
315 This marks the end of the region opened by the matching
316 "=begin formatname" region. If "formatname" is not the formatname
317 of the most recent open "=begin formatname" region, then this
318 is an error, and must generate an error message. This
319 is discussed in detail in the section
320 L</About Data Paragraphs and "=beginE<sol>=end" Regions>.
322 =item "=for formatname text..."
324 This is synonymous with:
332 That is, it creates a region consisting of a single paragraph; that
333 paragraph is to be treated as a normal paragraph if "formatname"
334 begins with a ":"; if "formatname" I<doesn't> begin with a colon,
335 then "text..." will constitute a data paragraph. There is no way
336 to use "=for formatname text..." to express "text..." as a verbatim
339 =item "=encoding encodingname"
341 This command, which should occur early in the document (at least
342 before any non-US-ASCII data!), declares that this document is
343 encoded in the encoding I<encodingname>, which must be
344 an encoding name that L<Encode> recognizes. (Encode's list
345 of supported encodings, in L<Encode::Supported>, is useful here.)
346 If the Pod parser cannot decode the declared encoding, it
347 should emit a warning and may abort parsing the document
350 A document having more than one "=encoding" line should be
351 considered an error. Pod processors may silently tolerate this if
352 the not-first "=encoding" lines are just duplicates of the
353 first one (e.g., if there's a "=encoding utf8" line, and later on
354 another "=encoding utf8" line). But Pod processors should complain if
355 there are contradictory "=encoding" lines in the same document
356 (e.g., if there is a "=encoding utf8" early in the document and
357 "=encoding big5" later). Pod processors that recognize BOMs
358 may also complain if they see an "=encoding" line
359 that contradicts the BOM (e.g., if a document with a UTF-16LE
360 BOM has an "=encoding shiftjis" line).
364 If a Pod processor sees any command other than the ones listed
365 above (like "=head", or "=haed1", or "=stuff", or "=cuttlefish",
366 or "=w123"), that processor must by default treat this as an
367 error. It must not process the paragraph beginning with that
368 command, must by default warn of this as an error, and may
369 abort the parse. A Pod parser may allow a way for particular
370 applications to add to the above list of known commands, and to
371 stipulate, for each additional command, whether formatting
372 codes should be processed.
374 Future versions of this specification may add additional
379 =head1 Pod Formatting Codes
381 (Note that in previous drafts of this document and of perlpod,
382 formatting codes were referred to as "interior sequences", and
383 this term may still be found in the documentation for Pod parsers,
384 and in error messages from Pod processors.)
386 There are two syntaxes for formatting codes:
392 A formatting code starts with a capital letter (just US-ASCII [A-Z])
393 followed by a "<", any number of characters, and ending with the first
394 matching ">". Examples:
396 That's what I<you> think!
398 What's C<dump()> for?
400 X<C<chmod> and C<unlink()> Under Different Operating Systems>
404 A formatting code starts with a capital letter (just US-ASCII [A-Z])
405 followed by two or more "<"'s, one or more whitespace characters,
406 any number of characters, one or more whitespace characters,
407 and ending with the first matching sequence of two or more ">"'s, where
408 the number of ">"'s equals the number of "<"'s in the opening of this
409 formatting code. Examples:
411 That's what I<< you >> think!
413 C<<< open(X, ">>thing.dat") || die $! >>>
417 With this syntax, the whitespace character(s) after the "CE<lt><<"
418 and before the ">>" (or whatever letter) are I<not> renderable. They
419 do not signify whitespace, are merely part of the formatting codes
420 themselves. That is, these are all synonymous:
434 In parsing Pod, a notably tricky part is the correct parsing of
435 (potentially nested!) formatting codes. Implementors should
436 consult the code in the C<parse_text> routine in Pod::Parser as an
437 example of a correct implementation.
441 =item C<IE<lt>textE<gt>> -- italic text
443 See the brief discussion in L<perlpod/"Formatting Codes">.
445 =item C<BE<lt>textE<gt>> -- bold text
447 See the brief discussion in L<perlpod/"Formatting Codes">.
449 =item C<CE<lt>codeE<gt>> -- code text
451 See the brief discussion in L<perlpod/"Formatting Codes">.
453 =item C<FE<lt>filenameE<gt>> -- style for filenames
455 See the brief discussion in L<perlpod/"Formatting Codes">.
457 =item C<XE<lt>topic nameE<gt>> -- an index entry
459 See the brief discussion in L<perlpod/"Formatting Codes">.
461 This code is unusual in that most formatters completely discard
462 this code and its content. Other formatters will render it with
463 invisible codes that can be used in building an index of
464 the current document.
466 =item C<ZE<lt>E<gt>> -- a null (zero-effect) formatting code
468 Discussed briefly in L<perlpod/"Formatting Codes">.
470 This code is unusual is that it should have no content. That is,
471 a processor may complain if it sees C<ZE<lt>potatoesE<gt>>. Whether
472 or not it complains, the I<potatoes> text should ignored.
474 =item C<LE<lt>nameE<gt>> -- a hyperlink
476 The complicated syntaxes of this code are discussed at length in
477 L<perlpod/"Formatting Codes">, and implementation details are
478 discussed below, in L</"About LE<lt>...E<gt> Codes">. Parsing the
479 contents of LE<lt>content> is tricky. Notably, the content has to be
480 checked for whether it looks like a URL, or whether it has to be split
481 on literal "|" and/or "/" (in the right order!), and so on,
482 I<before> EE<lt>...> codes are resolved.
484 =item C<EE<lt>escapeE<gt>> -- a character escape
486 See L<perlpod/"Formatting Codes">, and several points in
487 L</Notes on Implementing Pod Processors>.
489 =item C<SE<lt>textE<gt>> -- text contains non-breaking spaces
491 This formatting code is syntactically simple, but semantically
492 complex. What it means is that each space in the printable
493 content of this code signifies a non-breaking space.
501 Both signify the monospace (c[ode] style) text consisting of
502 "$x", one space, "?", one space, ":", one space, "$z". The
503 difference is that in the latter, with the S code, those spaces
504 are not "normal" spaces, but instead are non-breaking spaces.
509 If a Pod processor sees any formatting code other than the ones
510 listed above (as in "NE<lt>...>", or "QE<lt>...>", etc.), that
511 processor must by default treat this as an error.
512 A Pod parser may allow a way for particular
513 applications to add to the above list of known formatting codes;
514 a Pod parser might even allow a way to stipulate, for each additional
515 command, whether it requires some form of special processing, as
518 Future versions of this specification may add additional
521 Historical note: A few older Pod processors would not see a ">" as
522 closing a "CE<lt>" code, if the ">" was immediately preceded by
523 a "-". This was so that this:
527 would parse as equivalent to this:
531 instead of as equivalent to a "C" formatting code containing
532 only "$foo-", and then a "bar>" outside the "C" formatting code. This
533 problem has since been solved by the addition of syntaxes like this:
537 Compliant parsers must not treat "->" as special.
539 Formatting codes absolutely cannot span paragraphs. If a code is
540 opened in one paragraph, and no closing code is found by the end of
541 that paragraph, the Pod parser must close that formatting code,
542 and should complain (as in "Unterminated I code in the paragraph
543 starting at line 123: 'Time objects are not...'"). So these
546 I<I told you not to do this!
548 Don't make me say it again!>
550 ...must I<not> be parsed as two paragraphs in italics (with the I
551 code starting in one paragraph and starting in another.) Instead,
552 the first paragraph should generate a warning, but that aside, the
553 above code must parse as if it were:
555 I<I told you not to do this!>
557 Don't make me say it again!E<gt>
559 (In SGMLish jargon, all Pod commands are like block-level
560 elements, whereas all Pod formatting codes are like inline-level
565 =head1 Notes on Implementing Pod Processors
567 The following is a long section of miscellaneous requirements
568 and suggestions to do with Pod processing.
574 Pod formatters should tolerate lines in verbatim blocks that are of
575 any length, even if that means having to break them (possibly several
576 times, for very long lines) to avoid text running off the side of the
577 page. Pod formatters may warn of such line-breaking. Such warnings
578 are particularly appropriate for lines are over 100 characters long, which
579 are usually not intentional.
583 Pod parsers must recognize I<all> of the three well-known newline
584 formats: CR, LF, and CRLF. See L<perlport|perlport>.
588 Pod parsers should accept input lines that are of any length.
592 Since Perl recognizes a Unicode Byte Order Mark at the start of files
593 as signaling that the file is Unicode encoded as in UTF-16 (whether
594 big-endian or little-endian) or UTF-8, Pod parsers should do the
595 same. Otherwise, the character encoding should be understood as
596 being UTF-8 if the first highbit byte sequence in the file seems
597 valid as a UTF-8 sequence, or otherwise as Latin-1.
599 Future versions of this specification may specify
600 how Pod can accept other encodings. Presumably treatment of other
601 encodings in Pod parsing would be as in XML parsing: whatever the
602 encoding declared by a particular Pod file, content is to be
603 stored in memory as Unicode characters.
607 The well known Unicode Byte Order Marks are as follows: if the
608 file begins with the two literal byte values 0xFE 0xFF, this is
609 the BOM for big-endian UTF-16. If the file begins with the two
610 literal byte value 0xFF 0xFE, this is the BOM for little-endian
611 UTF-16. If the file begins with the three literal byte values
612 0xEF 0xBB 0xBF, this is the BOM for UTF-8.
615 use bytes; print map sprintf(" 0x%02X", ord $_), split '', "\x{feff}";
619 If toke.c is modified to support UTF-32, add mention of those here.
623 A naive but sufficient heuristic for testing the first highbit
624 byte-sequence in a BOM-less file (whether in code or in Pod!), to see
625 whether that sequence is valid as UTF-8 (RFC 2279) is to check whether
626 that the first byte in the sequence is in the range 0xC0 - 0xFD
627 I<and> whether the next byte is in the range
628 0x80 - 0xBF. If so, the parser may conclude that this file is in
629 UTF-8, and all highbit sequences in the file should be assumed to
630 be UTF-8. Otherwise the parser should treat the file as being
631 in Latin-1. In the unlikely circumstance that the first highbit
632 sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one
633 can cater to our heuristic (as well as any more intelligent heuristic)
634 by prefacing that line with a comment line containing a highbit
635 sequence that is clearly I<not> valid as UTF-8. A line consisting
636 of simply "#", an e-acute, and any non-highbit byte,
637 is sufficient to establish this file's encoding.
640 If/WHEN some brave soul makes these heuristics into a generic
641 text-file class (or PerlIO layer?), we can presumably delete
642 mention of these icky details from this file, and can instead
643 tell people to just use appropriate class/layer.
644 Auto-recognition of newline sequences would be another desirable
645 feature of such a class/layer.
649 "The probability that a string of characters
650 in any other encoding appears as valid UTF-8 is low" - RFC2279
654 This document's requirements and suggestions about encodings
655 do not apply to Pod processors running on non-ASCII platforms,
656 notably EBCDIC platforms.
660 Pod processors must treat a "=for [label] [content...]" paragraph as
661 meaning the same thing as a "=begin [label]" paragraph, content, and
662 an "=end [label]" paragraph. (The parser may conflate these two
663 constructs, or may leave them distinct, in the expectation that the
664 formatter will nevertheless treat them the same.)
668 When rendering Pod to a format that allows comments (i.e., to nearly
669 any format other than plaintext), a Pod formatter must insert comment
670 text identifying its name and version number, and the name and
671 version numbers of any modules it might be using to process the Pod.
674 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
676 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
678 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
680 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
682 Formatters may also insert additional comments, including: the
683 release date of the Pod formatter program, the contact address for
684 the author(s) of the formatter, the current time, the name of input
685 file, the formatting options in effect, version of Perl used, etc.
687 Formatters may also choose to note errors/warnings as comments,
688 besides or instead of emitting them otherwise (as in messages to
689 STDERR, or C<die>ing).
693 Pod parsers I<may> emit warnings or error messages ("Unknown E code
694 EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or
695 C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> allow
696 suppressing all such STDERR output, and instead allow an option for
697 reporting errors/warnings
698 in some other way, whether by triggering a callback, or noting errors
699 in some attribute of the document object, or some similarly unobtrusive
700 mechanism -- or even by appending a "Pod Errors" section to the end of
701 the parsed form of the document.
705 In cases of exceptionally aberrant documents, Pod parsers may abort the
706 parse. Even then, using C<die>ing/C<croak>ing is to be avoided; where
707 possible, the parser library may simply close the input file
708 and add text like "*** Formatting Aborted ***" to the end of the
709 (partial) in-memory document.
713 In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>)
714 are understood (i.e., I<not> verbatim paragraphs, but I<including>
715 ordinary paragraphs, and command paragraphs that produce renderable
716 text, like "=head1"), literal whitespace should generally be considered
717 "insignificant", in that one literal space has the same meaning as any
718 (nonzero) number of literal spaces, literal newlines, and literal tabs
719 (as long as this produces no blank lines, since those would terminate
720 the paragraph). Pod parsers should compact literal whitespace in each
721 processed paragraph, but may provide an option for overriding this
722 (since some processing tasks do not require it), or may follow
723 additional special rules (for example, specially treating
724 period-space-space or period-newline sequences).
728 Pod parsers should not, by default, try to coerce apostrophe (') and
729 quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to
730 turn backtick (`) into anything else but a single backtick character
731 (distinct from an open quote character!), nor "--" into anything but
732 two minus signs. They I<must never> do any of those things to text
733 in CE<lt>...> formatting codes, and never I<ever> to text in verbatim
738 When rendering Pod to a format that has two kinds of hyphens (-), one
739 that's a non-breaking hyphen, and another that's a breakable hyphen
740 (as in "object-oriented", which can be split across lines as
741 "object-", newline, "oriented"), formatters are encouraged to
742 generally translate "-" to non-breaking hyphen, but may apply
743 heuristics to convert some of these to breaking hyphens.
747 Pod formatters should make reasonable efforts to keep words of Perl
748 code from being broken across lines. For example, "Foo::Bar" in some
749 formatting systems is seen as eligible for being broken across lines
750 as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should
751 be avoided where possible, either by disabling all line-breaking in
752 mid-word, or by wrapping particular words with internal punctuation
753 in "don't break this across lines" codes (which in some formats may
754 not be a single code, but might be a matter of inserting non-breaking
755 zero-width spaces between every pair of characters in a word.)
759 Pod parsers should, by default, expand tabs in verbatim paragraphs as
760 they are processed, before passing them to the formatter or other
761 processor. Parsers may also allow an option for overriding this.
765 Pod parsers should, by default, remove newlines from the end of
766 ordinary and verbatim paragraphs before passing them to the
767 formatter. For example, while the paragraph you're reading now
768 could be considered, in Pod source, to end with (and contain)
769 the newline(s) that end it, it should be processed as ending with
770 (and containing) the period character that ends this sentence.
774 Pod parsers, when reporting errors, should make some effort to report
775 an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near
776 line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph
777 number ("Nested EE<lt>>'s in Paragraph #52 of Thing/Foo.pm!"). Where
778 this is problematic, the paragraph number should at least be
779 accompanied by an excerpt from the paragraph ("Nested EE<lt>>'s in
780 Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for
781 the CE<lt>interest rate> attribute...'").
785 Pod parsers, when processing a series of verbatim paragraphs one
786 after another, should consider them to be one large verbatim
787 paragraph that happens to contain blank lines. I.e., these two
788 lines, which have a blank line between them:
794 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
795 Foo->VERSION") before being passed to the formatter or other
796 processor. Parsers may also allow an option for overriding this.
798 While this might be too cumbersome to implement in event-based Pod
799 parsers, it is straightforward for parsers that return parse trees.
803 Pod formatters, where feasible, are advised to avoid splitting short
804 verbatim paragraphs (under twelve lines, say) across pages.
808 Pod parsers must treat a line with only spaces and/or tabs on it as a
809 "blank line" such as separates paragraphs. (Some older parsers
810 recognized only two adjacent newlines as a "blank line" but would not
811 recognize a newline, a space, and a newline, as a blank line. This
812 is noncompliant behavior.)
816 Authors of Pod formatters/processors should make every effort to
817 avoid writing their own Pod parser. There are already several in
818 CPAN, with a wide range of interface styles -- and one of them,
819 Pod::Parser, comes with modern versions of Perl.
823 Characters in Pod documents may be conveyed either as literals, or by
824 number in EE<lt>n> codes, or by an equivalent mnemonic, as in
825 EE<lt>eacute> which is exactly equivalent to EE<lt>233>.
827 Characters in the range 32-126 refer to those well known US-ASCII
828 characters (also defined there by Unicode, with the same meaning),
829 which all Pod formatters must render faithfully. Characters
830 in the ranges 0-31 and 127-159 should not be used (neither as
831 literals, nor as EE<lt>number> codes), except for the
832 literal byte-sequences for newline (13, 13 10, or 10), and tab (9).
834 Characters in the range 160-255 refer to Latin-1 characters (also
835 defined there by Unicode, with the same meaning). Characters above
836 255 should be understood to refer to Unicode characters.
841 that some formatters cannot reliably render characters outside 32-126;
842 and many are able to handle 32-126 and 160-255, but nothing above
847 Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for
848 less-than and greater-than, Pod parsers must understand "EE<lt>sol>"
849 for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar,
850 pipe). Pod parsers should also understand "EE<lt>lchevron>" and
851 "EE<lt>rchevron>" as legacy codes for characters 171 and 187, i.e.,
852 "left-pointing double angle quotation mark" = "left pointing
853 guillemet" and "right-pointing double angle quotation mark" = "right
854 pointing guillemet". (These look like little "<<" and ">>", and they
855 are now preferably expressed with the HTML/XHTML codes "EE<lt>laquo>"
860 Pod parsers should understand all "EE<lt>html>" codes as defined
861 in the entity declarations in the most recent XHTML specification at
862 C<www.W3.org>. Pod parsers must understand at least the entities
863 that define characters in the range 160-255 (Latin-1). Pod parsers,
864 when faced with some unknown "EE<lt>I<identifier>>" code,
865 shouldn't simply replace it with nullstring (by default, at least),
866 but may pass it through as a string consisting of the literal characters
867 E, less-than, I<identifier>, greater-than. Or Pod parsers may offer the
868 alternative option of processing such unknown
869 "EE<lt>I<identifier>>" codes by firing an event especially
870 for such codes, or by adding a special node-type to the in-memory
871 document tree. Such "EE<lt>I<identifier>>" may have special meaning
872 to some processors, or some processors may choose to add them to
873 a special error report.
877 Pod parsers must also support the XHTML codes "EE<lt>quot>" for
878 character 34 (doublequote, "), "EE<lt>amp>" for character 38
879 (ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, ').
883 Note that in all cases of "EE<lt>whatever>", I<whatever> (whether
884 an htmlname, or a number in any base) must consist only of
885 alphanumeric characters -- that is, I<whatever> must watch
886 C<m/\A\w+\z/>. So "EE<lt> 0 1 2 3 >" is invalid, because
887 it contains spaces, which aren't alphanumeric characters. This
888 presumably does not I<need> special treatment by a Pod processor;
889 " 0 1 2 3 " doesn't look like a number in any base, so it would
890 presumably be looked up in the table of HTML-like names. Since
891 there isn't (and cannot be) an HTML-like entity called " 0 1 2 3 ",
892 this will be treated as an error. However, Pod processors may
893 treat "EE<lt> 0 1 2 3 >" or "EE<lt>e-acute>" as I<syntactically>
894 invalid, potentially earning a different error message than the
895 error message (or warning, or event) generated by a merely unknown
896 (but theoretically valid) htmlname, as in "EE<lt>qacute>"
897 [sic]. However, Pod parsers are not required to make this
902 Note that EE<lt>number> I<must not> be interpreted as simply
903 "codepoint I<number> in the current/native character set". It always
904 means only "the character represented by codepoint I<number> in
905 Unicode." (This is identical to the semantics of &#I<number>; in XML.)
907 This will likely require many formatters to have tables mapping from
908 treatable Unicode codepoints (such as the "\xE9" for the e-acute
909 character) to the escape sequences or codes necessary for conveying
910 such sequences in the target output format. A converter to *roff
911 would, for example know that "\xE9" (whether conveyed literally, or via
912 a EE<lt>...> sequence) is to be conveyed as "e\\*'".
913 Similarly, a program rendering Pod in a Mac OS application window, would
914 presumably need to know that "\xE9" maps to codepoint 142 in MacRoman
915 encoding that (at time of writing) is native for Mac OS. Such
916 Unicode2whatever mappings are presumably already widely available for
917 common output formats. (Such mappings may be incomplete! Implementers
918 are not expected to bend over backwards in an attempt to render
919 Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any
920 of the other weird things that Unicode can encode.) And
921 if a Pod document uses a character not found in such a mapping, the
922 formatter should consider it an unrenderable character.
926 If, surprisingly, the implementor of a Pod formatter can't find a
927 satisfactory pre-existing table mapping from Unicode characters to
928 escapes in the target format (e.g., a decent table of Unicode
929 characters to *roff escapes), it will be necessary to build such a
930 table. If you are in this circumstance, you should begin with the
931 characters in the range 0x00A0 - 0x00FF, which is mostly the heavily
932 used accented characters. Then proceed (as patience permits and
933 fastidiousness compels) through the characters that the (X)HTML
934 standards groups judged important enough to merit mnemonics
935 for. These are declared in the (X)HTML specifications at the
936 www.W3.org site. At time of writing (September 2001), the most recent
937 entity declaration files are:
939 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
940 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
941 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
943 Then you can progress through any remaining notable Unicode characters
944 in the range 0x2000-0x204D (consult the character tables at
945 www.unicode.org), and whatever else strikes your fancy. For example,
946 in F<xhtml-symbol.ent>, there is the entry:
948 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
950 While the mapping "infin" to the character "\x{221E}" will (hopefully)
951 have been already handled by the Pod parser, the presence of the
952 character in this file means that it's reasonably important enough to
953 include in a formatter's table that maps from notable Unicode characters
954 to the codes necessary for rendering them. So for a Unicode-to-*roff
955 mapping, for example, this would merit the entry:
957 "\x{221E}" => '\(in',
959 It is eagerly hoped that in the future, increasing numbers of formats
960 (and formatters) will support Unicode characters directly (as (X)HTML
961 does with C<∞>, C<∞>, or C<∞>), reducing the need
962 for idiosyncratic mappings of Unicode-to-I<my_escapes>.
966 It is up to individual Pod formatter to display good judgement when
967 confronted with an unrenderable character (which is distinct from an
968 unknown EE<lt>thing> sequence that the parser couldn't resolve to
969 anything, renderable or not). It is good practice to map Latin letters
970 with diacritics (like "EE<lt>eacute>"/"EE<lt>233>") to the corresponding
971 unaccented US-ASCII letters (like a simple character 101, "e"), but
972 clearly this is often not feasible, and an unrenderable character may
973 be represented as "?", or the like. In attempting a sane fallback
974 (as from EE<lt>233> to "e"), Pod formatters may use the
975 %Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or
976 L<Text::Unidecode|Text::Unidecode>, if available.
978 For example, this Pod text:
980 magic is enabled if you set C<$Currency> to 'E<euro>'.
983 "magic is enabled if you set C<$Currency> to 'I<?>'" or as
984 "magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as
985 "magic is enabled if you set C<$Currency> to '[x20AC]', etc.
987 A Pod formatter may also note, in a comment or warning, a list of what
988 unrenderable characters were encountered.
992 EE<lt>...> may freely appear in any formatting code (other than
993 in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The
994 EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The
995 EE<lt>euro>1,000,000 Solution|Million::Euros>".
999 Some Pod formatters output to formats that implement non-breaking
1000 spaces as an individual character (which I'll call "NBSP"), and
1001 others output to formats that implement non-breaking spaces just as
1002 spaces wrapped in a "don't break this across lines" code. Note that
1003 at the level of Pod, both sorts of codes can occur: Pod can contain a
1004 NBSP character (whether as a literal, or as a "EE<lt>160>" or
1005 "EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo
1006 IE<lt>barE<gt> baz>" codes, where "mere spaces" (character 32) in
1007 such codes are taken to represent non-breaking spaces. Pod
1008 parsers should consider supporting the optional parsing of "SE<lt>foo
1009 IE<lt>barE<gt> baz>" as if it were
1010 "fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the
1011 optional parsing of groups of words joined by NBSP's as if each group
1012 were in a SE<lt>...> code, so that formatters may use the
1013 representation that maps best to what the output format demands.
1017 Some processors may find that the C<SE<lt>...E<gt>> code is easiest to
1018 implement by replacing each space in the parse tree under the content
1019 of the S, with an NBSP. But note: the replacement should apply I<not> to
1020 spaces in I<all> text, but I<only> to spaces in I<printable> text. (This
1021 distinction may or may not be evident in the particular tree/event
1022 model implemented by the Pod parser.) For example, consider this
1025 S<L</Autoloaded Functions>>
1027 This means that the space in the middle of the visible link text must
1028 not be broken across lines. In other words, it's the same as this:
1030 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
1032 However, a misapplied space-to-NBSP replacement could (wrongly)
1033 produce something equivalent to this:
1035 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
1037 ...which is almost definitely not going to work as a hyperlink (assuming
1038 this formatter outputs a format supporting hypertext).
1040 Formatters may choose to just not support the S format code,
1041 especially in cases where the output format simply has no NBSP
1042 character/code and no code for "don't break this stuff across lines".
1046 Besides the NBSP character discussed above, implementors are reminded
1047 of the existence of the other "special" character in Latin-1, the
1048 "soft hyphen" character, also known as "discretionary hyphen",
1049 i.e. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> =
1050 C<EE<lt>shyE<gt>>). This character expresses an optional hyphenation
1051 point. That is, it normally renders as nothing, but may render as a
1052 "-" if a formatter breaks the word at that point. Pod formatters
1053 should, as appropriate, do one of the following: 1) render this with
1054 a code with the same meaning (e.g., "\-" in RTF), 2) pass it through
1055 in the expectation that the formatter understands this character as
1056 such, or 3) delete it.
1062 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
1064 These signal to a formatter that if it is to hyphenate "sigaction"
1065 or "manuscript", then it should be done as
1066 "sig-I<[linebreak]>action" or "manu-I<[linebreak]>script"
1067 (and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> doesn't
1068 show up at all). And if it is
1069 to hyphenate "Jarkko" and/or "Hietaniemi", it can do
1070 so only at the points where there is a C<EE<lt>shyE<gt>> code.
1072 In practice, it is anticipated that this character will not be used
1073 often, but formatters should either support it, or delete it.
1077 If you think that you want to add a new command to Pod (like, say, a
1078 "=biblio" command), consider whether you could get the same
1079 effect with a for or begin/end sequence: "=for biblio ..." or "=begin
1080 biblio" ... "=end biblio". Pod processors that don't understand
1081 "=for biblio", etc, will simply ignore it, whereas they may complain
1082 loudly if they see "=biblio".
1086 Throughout this document, "Pod" has been the preferred spelling for
1087 the name of the documentation format. One may also use "POD" or
1088 "pod". For the documentation that is (typically) in the Pod
1089 format, you may use "pod", or "Pod", or "POD". Understanding these
1090 distinctions is useful; but obsessing over how to spell them, usually
1099 =head1 About LE<lt>...E<gt> Codes
1101 As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...>
1102 code is the most complex of the Pod formatting codes. The points below
1103 will hopefully clarify what it means and how processors should deal
1110 In parsing an LE<lt>...> code, Pod parsers must distinguish at least
1117 The link-text. If there is none, this must be undef. (E.g., in
1118 "LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions".
1119 In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no
1120 link text. Note that link text may contain formatting.)
1124 The possibly inferred link-text; i.e., if there was no real link
1125 text, then this is the text that we'll infer in its place. (E.g., for
1126 "LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".)
1130 The name or URL, or undef if none. (E.g., in "LE<lt>Perl
1131 Functions|perlfunc>", the name (also sometimes called the page)
1132 is "perlfunc". In "LE<lt>/CAVEATS>", the name is undef.)
1136 The section (AKA "item" in older perlpods), or undef if none. E.g.,
1137 in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "DESCRIPTION" is the section. (Note
1138 that this is not the same as a manpage section like the "5" in "man 5
1139 crontab". "Section Foo" in the Pod sense means the part of the text
1140 that's introduced by the heading or item whose text is "Foo".)
1144 Pod parsers may also note additional attributes including:
1150 A flag for whether item 3 (if present) is a URL (like
1151 "http://lists.perl.org" is), in which case there should be no section
1152 attribute; a Pod name (like "perldoc" and "Getopt::Std" are); or
1153 possibly a man page name (like "crontab(5)" is).
1157 The raw original LE<lt>...> content, before text is split on
1158 "|", "/", etc, and before EE<lt>...> codes are expanded.
1162 (The above were numbered only for concise reference below. It is not
1163 a requirement that these be passed as an actual list or array.)
1168 => undef, # link text
1169 "Foo::Bar", # possibly inferred link text
1172 'pod', # what sort of link
1173 "Foo::Bar" # original content
1175 L<Perlport's section on NL's|perlport/Newlines>
1176 => "Perlport's section on NL's", # link text
1177 "Perlport's section on NL's", # possibly inferred link text
1179 "Newlines", # section
1180 'pod', # what sort of link
1181 "Perlport's section on NL's|perlport/Newlines" # orig. content
1183 L<perlport/Newlines>
1184 => undef, # link text
1185 '"Newlines" in perlport', # possibly inferred link text
1187 "Newlines", # section
1188 'pod', # what sort of link
1189 "perlport/Newlines" # original content
1191 L<crontab(5)/"DESCRIPTION">
1192 => undef, # link text
1193 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
1194 "crontab(5)", # name
1195 "DESCRIPTION", # section
1196 'man', # what sort of link
1197 'crontab(5)/"DESCRIPTION"' # original content
1199 L</Object Attributes>
1200 => undef, # link text
1201 '"Object Attributes"', # possibly inferred link text
1203 "Object Attributes", # section
1204 'pod', # what sort of link
1205 "/Object Attributes" # original content
1207 L<http://www.perl.org/>
1208 => undef, # link text
1209 "http://www.perl.org/", # possibly inferred link text
1210 "http://www.perl.org/", # name
1212 'url', # what sort of link
1213 "http://www.perl.org/" # original content
1215 L<Perl.org|http://www.perl.org/>
1216 => "Perl.org", # link text
1217 "http://www.perl.org/", # possibly inferred link text
1218 "http://www.perl.org/", # name
1220 'url', # what sort of link
1221 "Perl.org|http://www.perl.org/" # original content
1223 Note that you can distinguish URL-links from anything else by the
1224 fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So
1225 C<LE<lt>http://www.perl.comE<gt>> is a URL, but
1226 C<LE<lt>HTTP::ResponseE<gt>> isn't.
1230 In case of LE<lt>...> codes with no "text|" part in them,
1231 older formatters have exhibited great variation in actually displaying
1232 the link or cross reference. For example, LE<lt>crontab(5)> would render
1233 as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage"
1234 or just "C<crontab(5)>".
1236 Pod processors must now treat "text|"-less links as follows:
1238 L<name> => L<name|name>
1239 L</section> => L<"section"|/section>
1240 L<name/section> => L<"section" in name|name/section>
1244 Note that section names might contain markup. I.e., if a section
1247 =head2 About the C<-M> Operator
1251 =item About the C<-M> Operator
1253 then a link to it would look like this:
1255 L<somedoc/About the C<-M> Operator>
1257 Formatters may choose to ignore the markup for purposes of resolving
1258 the link and use only the renderable characters in the section name,
1261 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1266 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1267 Operator" in somedoc</a>
1271 Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>>
1272 links from C<LE<lt>name/itemE<gt>> links (and their targets). These
1273 have been merged syntactically and semantically in the current
1274 specification, and I<section> can refer either to a "=headI<n> Heading
1275 Content" command or to a "=item Item Content" command. This
1276 specification does not specify what behavior should be in the case
1277 of a given document having several things all seeming to produce the
1278 same I<section> identifier (e.g., in HTML, several things all producing
1279 the same I<anchorname> in <a name="I<anchorname>">...</a>
1280 elements). Where Pod processors can control this behavior, they should
1281 use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the
1282 I<first> "Bar" section in Foo.
1284 But for some processors/formats this cannot be easily controlled; as
1285 with the HTML example, the behavior of multiple ambiguous
1286 <a name="I<anchorname>">...</a> is most easily just left up to
1291 Authors wanting to link to a particular (absolute) URL, must do so
1292 only with "LE<lt>scheme:...>" codes (like
1293 LE<lt>http://www.perl.org>), and must not attempt "LE<lt>Some Site
1294 Name|scheme:...>" codes. This restriction avoids many problems
1295 in parsing and rendering LE<lt>...> codes.
1299 In a C<LE<lt>text|...E<gt>> code, text may contain formatting codes
1300 for formatting or for EE<lt>...> escapes, as in:
1302 L<B<ummE<234>stuff>|...>
1304 For C<LE<lt>...E<gt>> codes without a "name|" part, only
1305 C<EE<lt>...E<gt>> and C<ZE<lt>E<gt>> codes may occur. That is,
1306 authors should not use "C<LE<lt>BE<lt>Foo::BarE<gt>E<gt>>".
1308 Note, however, that formatting codes and ZE<lt>>'s can occur in any
1309 and all parts of an LE<lt>...> (i.e., in I<name>, I<section>, I<text>,
1312 Authors must not nest LE<lt>...> codes. For example, "LE<lt>The
1313 LE<lt>Foo::Bar> man page>" should be treated as an error.
1317 Note that Pod authors may use formatting codes inside the "text"
1318 part of "LE<lt>text|name>" (and so on for LE<lt>text|/"sec">).
1320 In other words, this is valid:
1322 Go read L<the docs on C<$.>|perlvar/"$.">
1324 Some output formats that do allow rendering "LE<lt>...>" codes as
1325 hypertext, might not allow the link-text to be formatted; in
1326 that case, formatters will have to just ignore that formatting.
1330 At time of writing, C<LE<lt>nameE<gt>> values are of two types:
1331 either the name of a Pod page like C<LE<lt>Foo::BarE<gt>> (which
1332 might be a real Perl module or program in an @INC / PATH
1333 directory, or a .pod file in those places); or the name of a Unix
1334 man page, like C<LE<lt>crontab(5)E<gt>>. In theory, C<LE<lt>chmodE<gt>>
1335 in ambiguous between a Pod page called "chmod", or the Unix man page
1336 "chmod" (in whatever man-section). However, the presence of a string
1337 in parens, as in "crontab(5)", is sufficient to signal that what
1338 is being discussed is not a Pod page, and so is presumably a
1339 Unix man page. The distinction is of no importance to many
1340 Pod processors, but some processors that render to hypertext formats
1341 may need to distinguish them in order to know how to render a
1342 given C<LE<lt>fooE<gt>> code.
1346 Previous versions of perlpod allowed for a C<LE<lt>sectionE<gt>> syntax (as in
1347 C<LE<lt>Object AttributesE<gt>>), which was not easily distinguishable from
1348 C<LE<lt>nameE<gt>> syntax and for C<LE<lt>"section"E<gt>> which was only
1349 slightly less ambiguous. This syntax is no longer in the specification, and
1350 has been replaced by the C<LE<lt>/sectionE<gt>> syntax (where the slash was
1351 formerly optional). Pod parsers should tolerate the C<LE<lt>"section"E<gt>>
1352 syntax, for a while at least. The suggested heuristic for distinguishing
1353 C<LE<lt>sectionE<gt>> from C<LE<lt>nameE<gt>> is that if it contains any
1354 whitespace, it's a I<section>. Pod processors should warn about this being
1359 =head1 About =over...=back Regions
1361 "=over"..."=back" regions are used for various kinds of list-like
1362 structures. (I use the term "region" here simply as a collective
1363 term for everything from the "=over" to the matching "=back".)
1369 The non-zero numeric I<indentlevel> in "=over I<indentlevel>" ...
1370 "=back" is used for giving the formatter a clue as to how many
1371 "spaces" (ems, or roughly equivalent units) it should tab over,
1372 although many formatters will have to convert this to an absolute
1373 measurement that may not exactly match with the size of spaces (or M's)
1374 in the document's base font. Other formatters may have to completely
1375 ignore the number. The lack of any explicit I<indentlevel> parameter is
1376 equivalent to an I<indentlevel> value of 4. Pod processors may
1377 complain if I<indentlevel> is present but is not a positive number
1378 matching C<m/\A(\d*\.)?\d+\z/>.
1382 Authors of Pod formatters are reminded that "=over" ... "=back" may
1383 map to several different constructs in your output format. For
1384 example, in converting Pod to (X)HTML, it can map to any of
1385 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1386 <blockquote>...</blockquote>. Similarly, "=item" can map to <li> or
1391 Each "=over" ... "=back" region should be one of the following:
1397 An "=over" ... "=back" region containing only "=item *" commands,
1398 each followed by some number of ordinary/verbatim paragraphs, other
1399 nested "=over" ... "=back" regions, "=for..." paragraphs, and
1400 "=begin"..."=end" regions.
1402 (Pod processors must tolerate a bare "=item" as if it were "=item
1403 *".) Whether "*" is rendered as a literal asterisk, an "o", or as
1404 some kind of real bullet character, is left up to the Pod formatter,
1405 and may depend on the level of nesting.
1409 An "=over" ... "=back" region containing only
1410 C<m/\A=item\s+\d+\.?\s*\z/> paragraphs, each one (or each group of them)
1411 followed by some number of ordinary/verbatim paragraphs, other nested
1412 "=over" ... "=back" regions, "=for..." paragraphs, and/or
1413 "=begin"..."=end" codes. Note that the numbers must start at 1
1414 in each section, and must proceed in order and without skipping
1417 (Pod processors must tolerate lines like "=item 1" as if they were
1418 "=item 1.", with the period.)
1422 An "=over" ... "=back" region containing only "=item [text]"
1423 commands, each one (or each group of them) followed by some number of
1424 ordinary/verbatim paragraphs, other nested "=over" ... "=back"
1425 regions, or "=for..." paragraphs, and "=begin"..."=end" regions.
1427 The "=item [text]" paragraph should not match
1428 C<m/\A=item\s+\d+\.?\s*\z/> or C<m/\A=item\s+\*\s*\z/>, nor should it
1429 match just C<m/\A=item\s*\z/>.
1433 An "=over" ... "=back" region containing no "=item" paragraphs at
1434 all, and containing only some number of
1435 ordinary/verbatim paragraphs, and possibly also some nested "=over"
1436 ... "=back" regions, "=for..." paragraphs, and "=begin"..."=end"
1437 regions. Such an itemless "=over" ... "=back" region in Pod is
1438 equivalent in meaning to a "<blockquote>...</blockquote>" element in
1443 Note that with all the above cases, you can determine which type of
1444 "=over" ... "=back" you have, by examining the first (non-"=cut",
1445 non-"=pod") Pod paragraph after the "=over" command.
1449 Pod formatters I<must> tolerate arbitrarily large amounts of text
1450 in the "=item I<text...>" paragraph. In practice, most such
1451 paragraphs are short, as in:
1453 =item For cutting off our trade with all parts of the world
1455 But they may be arbitrarily long:
1457 =item For transporting us beyond seas to be tried for pretended
1460 =item He is at this time transporting large armies of foreign
1461 mercenaries to complete the works of death, desolation and
1462 tyranny, already begun with circumstances of cruelty and perfidy
1463 scarcely paralleled in the most barbarous ages, and totally
1464 unworthy the head of a civilized nation.
1468 Pod processors should tolerate "=item *" / "=item I<number>" commands
1469 with no accompanying paragraph. The middle item is an example:
1475 Pick up dry cleaning.
1481 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1487 No "=over" ... "=back" region can contain headings. Processors may
1488 treat such a heading as an error.
1492 Note that an "=over" ... "=back" region should have some
1493 content. That is, authors should not have an empty region like this:
1499 Pod processors seeing such a contentless "=over" ... "=back" region,
1500 may ignore it, or may report it as an error.
1504 Processors must tolerate an "=over" list that goes off the end of the
1505 document (i.e., which has no matching "=back"), but they may warn
1510 Authors of Pod formatters should note that this construct:
1518 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1519 velit, sed quia non numquam eius modi tempora incidunt ut
1520 labore et dolore magnam aliquam quaerat voluptatem.
1524 is semantically ambiguous, in a way that makes formatting decisions
1525 a bit difficult. On the one hand, it could be mention of an item
1526 "Neque", mention of another item "Porro", and mention of another
1527 item "Quisquam Est", with just the last one requiring the explanatory
1528 paragraph "Qui dolorem ipsum quia dolor..."; and then an item
1529 "Ut Enim". In that case, you'd want to format it like so:
1536 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1537 velit, sed quia non numquam eius modi tempora incidunt ut
1538 labore et dolore magnam aliquam quaerat voluptatem.
1542 But it could equally well be a discussion of three (related or equivalent)
1543 items, "Neque", "Porro", and "Quisquam Est", followed by a paragraph
1544 explaining them all, and then a new item "Ut Enim". In that case, you'd
1545 probably want to format it like so:
1550 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1551 velit, sed quia non numquam eius modi tempora incidunt ut
1552 labore et dolore magnam aliquam quaerat voluptatem.
1556 But (for the foreseeable future), Pod does not provide any way for Pod
1557 authors to distinguish which grouping is meant by the above
1558 "=item"-cluster structure. So formatters should format it like so:
1566 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1567 velit, sed quia non numquam eius modi tempora incidunt ut
1568 labore et dolore magnam aliquam quaerat voluptatem.
1572 That is, there should be (at least roughly) equal spacing between
1573 items as between paragraphs (although that spacing may well be less
1574 than the full height of a line of text). This leaves it to the reader
1575 to use (con)textual cues to figure out whether the "Qui dolorem
1576 ipsum..." paragraph applies to the "Quisquam Est" item or to all three
1577 items "Neque", "Porro", and "Quisquam Est". While not an ideal
1578 situation, this is preferable to providing formatting cues that may
1579 be actually contrary to the author's intent.
1585 =head1 About Data Paragraphs and "=begin/=end" Regions
1587 Data paragraphs are typically used for inlining non-Pod data that is
1588 to be used (typically passed through) when rendering the document to
1593 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1597 The exact same effect could, incidentally, be achieved with a single
1600 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1602 (Although that is not formally a data paragraph, it has the same
1603 meaning as one, and Pod parsers may parse it as one.)
1605 Another example of a data paragraph:
1609 I like <em>PIE</em>!
1611 <hr>Especially pecan pie!
1615 If these were ordinary paragraphs, the Pod parser would try to
1616 expand the "EE<lt>/em>" (in the first paragraph) as a formatting
1617 code, just like "EE<lt>lt>" or "EE<lt>eacute>". But since this
1618 is in a "=begin I<identifier>"..."=end I<identifier>" region I<and>
1619 the identifier "html" doesn't begin have a ":" prefix, the contents
1620 of this region are stored as data paragraphs, instead of being
1621 processed as ordinary paragraphs (or if they began with a spaces
1622 and/or tabs, as verbatim paragraphs).
1624 As a further example: At time of writing, no "biblio" identifier is
1625 supported, but suppose some processor were written to recognize it as
1626 a way of (say) denoting a bibliographic reference (necessarily
1627 containing formatting codes in ordinary paragraphs). The fact that
1628 "biblio" paragraphs were meant for ordinary processing would be
1629 indicated by prefacing each "biblio" identifier with a colon:
1633 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1634 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1638 This would signal to the parser that paragraphs in this begin...end
1639 region are subject to normal handling as ordinary/verbatim paragraphs
1640 (while still tagged as meant only for processors that understand the
1641 "biblio" identifier). The same effect could be had with:
1644 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1645 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1647 The ":" on these identifiers means simply "process this stuff
1648 normally, even though the result will be for some special target".
1649 I suggest that parser APIs report "biblio" as the target identifier,
1650 but also report that it had a ":" prefix. (And similarly, with the
1651 above "html", report "html" as the target identifier, and note the
1652 I<lack> of a ":" prefix.)
1654 Note that a "=begin I<identifier>"..."=end I<identifier>" region where
1655 I<identifier> begins with a colon, I<can> contain commands. For example:
1659 Wirth's classic is available in several editions, including:
1662 hm, check abebooks.com for how much used copies cost.
1668 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1669 Teubner, Stuttgart. [Yes, it's in German.]
1673 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1674 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1680 Note, however, a "=begin I<identifier>"..."=end I<identifier>"
1681 region where I<identifier> does I<not> begin with a colon, should not
1682 directly contain "=head1" ... "=head4" commands, nor "=over", nor "=back",
1683 nor "=item". For example, this may be considered invalid:
1687 This is a data paragraph.
1689 =head1 Don't do this!
1691 This is a data paragraph too.
1695 A Pod processor may signal that the above (specifically the "=head1"
1696 paragraph) is an error. Note, however, that the following should
1697 I<not> be treated as an error:
1701 This is a data paragraph.
1705 # Yup, this isn't Pod anymore.
1706 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1710 This is a data paragraph too.
1714 And this too is valid:
1718 This is a data paragraph.
1720 And this is a data paragraph.
1722 =begin someotherformat
1724 This is a data paragraph too.
1726 And this is a data paragraph too.
1728 =begin :yetanotherformat
1730 =head2 This is a command paragraph!
1732 This is an ordinary paragraph!
1734 And this is a verbatim paragraph!
1736 =end :yetanotherformat
1738 =end someotherformat
1740 Another data paragraph!
1744 The contents of the above "=begin :yetanotherformat" ...
1745 "=end :yetanotherformat" region I<aren't> data paragraphs, because
1746 the immediately containing region's identifier (":yetanotherformat")
1747 begins with a colon. In practice, most regions that contain
1748 data paragraphs will contain I<only> data paragraphs; however,
1749 the above nesting is syntactically valid as Pod, even if it is
1750 rare. However, the handlers for some formats, like "html",
1751 will accept only data paragraphs, not nested regions; and they may
1752 complain if they see (targeted for them) nested regions, or commands,
1753 other than "=end", "=pod", and "=cut".
1755 Also consider this valid structure:
1759 Wirth's classic is available in several editions, including:
1765 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1766 Teubner, Stuttgart. [Yes, it's in German.]
1770 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1771 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1779 <img src='wirth_spokesmodeling_book.png'>
1789 There, the "=begin html"..."=end html" region is nested inside
1790 the larger "=begin :biblio"..."=end :biblio" region. Note that the
1791 content of the "=begin html"..."=end html" region is data
1792 paragraph(s), because the immediately containing region's identifier
1793 ("html") I<doesn't> begin with a colon.
1795 Pod parsers, when processing a series of data paragraphs one
1796 after another (within a single region), should consider them to
1797 be one large data paragraph that happens to contain blank lines. So
1798 the content of the above "=begin html"..."=end html" I<may> be stored
1799 as two data paragraphs (one consisting of
1800 "<img src='wirth_spokesmodeling_book.png'>\n"
1801 and another consisting of "<hr>\n"), but I<should> be stored as
1802 a single data paragraph (consisting of
1803 "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1805 Pod processors should tolerate empty
1806 "=begin I<something>"..."=end I<something>" regions,
1807 empty "=begin :I<something>"..."=end :I<something>" regions, and
1808 contentless "=for I<something>" and "=for :I<something>"
1809 paragraphs. I.e., these should be tolerated:
1821 Incidentally, note that there's no easy way to express a data
1822 paragraph starting with something that looks like a command. Consider:
1830 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a data
1831 paragraph "=shazbot\n". However, you can express a data paragraph consisting
1832 of "=shazbot\n" using this code:
1836 The situation where this is necessary, is presumably quite rare.
1838 Note that =end commands must match the currently open =begin command. That
1839 is, they must properly nest. For example, this is valid:
1855 while this is invalid:
1871 This latter is improper because when the "=end outer" command is seen, the
1872 currently open region has the formatname "inner", not "outer". (It just
1873 happens that "outer" is the format name of a higher-up region.) This is
1874 an error. Processors must by default report this as an error, and may halt
1875 processing the document containing that error. A corollary of this is that
1876 regions cannot "overlap". That is, the latter block above does not represent
1877 a region called "outer" which contains X and Y, overlapping a region called
1878 "inner" which contains Y and Z. But because it is invalid (as all
1879 apparently overlapping regions would be), it doesn't represent that, or
1882 Similarly, this is invalid:
1888 This is an error because the region is opened by "thing", and the "=end"
1889 tries to close "hting" [sic].
1891 This is also invalid:
1897 This is invalid because every "=end" command must have a formatname
1902 L<perlpod>, L<perlsyn/"PODs: Embedded Documentation">,