1PERLPODSPEC(1) Perl Programmers Reference Guide PERLPODSPEC(1)
2
6 perlpodspec - Plain Old Documentation: format specification and notes
7
9 This document is detailed notes on the Pod markup language. Most
10 people will only have to read perlpod to know how to write in Pod, but
11 this document may answer some incidental questions to do with parsing
12 and rendering Pod.
13 In this document, "must" / "must not", "should" / "should not", and
15 "may" have their conventional (cf. RFC 2119) meanings: "X must do Y"
16 means that if X doesn't do Y, it's against this specification, and
17 should really be fixed. "X should do Y" means that it's recommended,
18 but X may fail to do Y, if there's a good reason. "X may do Y" is
19 merely a note that X can do Y at will (although it is up to the reader
20 to detect any connotation of "and I think it would be nice if X did Y"
21 versus "it wouldn't really bother me if X did Y").
22 Notably, when I say "the parser should do Y", the parser may fail to do
24 Y, if the calling application explicitly requests that the parser not
25 do Y. I often phrase this as "the parser should, by default, do Y."
26 This doesn't require the parser to provide an option for turning off
27 whatever feature Y is (like expanding tabs in verbatim paragraphs),
28 although it implicates that such an option may be provided.
29
31 Pod is embedded in files, typically Perl source files, although you can
32 write a file that's nothing but Pod.
33 A line in a file consists of zero or more non-newline characters,
35 terminated by either a newline or the end of the file.
36 A newline sequence is usually a platform-dependent concept, but Pod
38 parsers should understand it to mean any of CR (ASCII 13), LF (ASCII
39 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in addition
40 to any other system-specific meaning. The first CR/CRLF/LF sequence in
41 the file may be used as the basis for identifying the newline sequence
42 for parsing the rest of the file.
43 A blank line is a line consisting entirely of zero or more spaces
45 (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-
46 file. A non-blank line is a line containing one or more characters
47 other than space or tab (and terminated by a newline or end-of-file).
48 (Note: Many older Pod parsers did not accept a line consisting of
50 spaces/tabs and then a newline as a blank line. The only lines they
51 considered blank were lines consisting of no characters at all,
52 terminated by a newline.)
53 Whitespace is used in this document as a blanket term for spaces, tabs,
55 and newline sequences. (By itself, this term usually refers to literal
56 whitespace. That is, sequences of whitespace characters in Pod source,
57 as opposed to "E<32>", which is a formatting code that denotes a
58 whitespace character.)
59 A Pod parser is a module meant for parsing Pod (regardless of whether
61 this involves calling callbacks or building a parse tree or directly
62 formatting it). A Pod formatter (or Pod translator) is a module or
63 program that converts Pod to some other format (HTML, plaintext, TeX,
64 PostScript, RTF). A Pod processor might be a formatter or translator,
65 or might be a program that does something else with the Pod (like
66 counting words, scanning for index points, etc.).
67 Pod content is contained in Pod blocks. A Pod block starts with a line
69 that matches "m/\A=[a-zA-Z]/", and continues up to the next line that
70 matches "m/\A=cut/" or up to the end of the file if there is no
71 "m/\A=cut/" line.
72 Within a Pod block, there are Pod paragraphs. A Pod paragraph consists
74 of non-blank lines of text, separated by one or more blank lines.
75 For purposes of Pod processing, there are four types of paragraphs in a
77 Pod block:
78 · A command paragraph (also called a "directive"). The first line of
80 this paragraph must match "m/\A=[a-zA-Z]/". Command paragraphs are
81 typically one line, as in:
82 =head1 NOTES
84 =item *
86 But they may span several (non-blank) lines:
88 =for comment
90 Hm, I wonder what it would look like if
91 you tried to write a BNF for Pod from this.
92 =head3 Dr. Strangelove, or: How I Learned to
94 Stop Worrying and Love the Bomb
95 Some command paragraphs allow formatting codes in their content
97 (i.e., after the part that matches "m/\A=[a-zA-Z]\S*\s*/"), as in:
98 =head1 Did You Remember to C<use strict;>?
100 In other words, the Pod processing handler for "head1" will apply
102 the same processing to "Did You Remember to C<use strict;>?" that
103 it would to an ordinary paragraph (i.e., formatting codes like
104 "C<...>") are parsed and presumably formatted appropriately, and
105 whitespace in the form of literal spaces and/or tabs is not
106 significant.
107 · A verbatim paragraph. The first line of this paragraph must be a
109 literal space or tab, and this paragraph must not be inside a
110 "=begin identifier", ... "=end identifier" sequence unless
111 "identifier" begins with a colon (":"). That is, if a paragraph
112 starts with a literal space or tab, but is inside a "=begin
113 identifier", ... "=end identifier" region, then it's a data
114 paragraph, unless "identifier" begins with a colon.
115 Whitespace is significant in verbatim paragraphs (although, in
117 processing, tabs are probably expanded).
118 · An ordinary paragraph. A paragraph is an ordinary paragraph if its
120 first line matches neither "m/\A=[a-zA-Z]/" nor "m/\A[ \t]/", and
121 if it's not inside a "=begin identifier", ... "=end identifier"
122 sequence unless "identifier" begins with a colon (":").
123 · A data paragraph. This is a paragraph that is inside a "=begin
125 identifier" ... "=end identifier" sequence where "identifier" does
126 not begin with a literal colon (":"). In some sense, a data
127 paragraph is not part of Pod at all (i.e., effectively it's "out-
128 of-band"), since it's not subject to most kinds of Pod parsing; but
129 it is specified here, since Pod parsers need to be able to call an
130 event for it, or store it in some form in a parse tree, or at least
131 just parse around it.
132 For example: consider the following paragraphs:
134 # <- that's the 0th column
136 =head1 Foo
138 Stuff
140 $foo->bar
142 =cut
144 Here, "=head1 Foo" and "=cut" are command paragraphs because the first
146 line of each matches "m/\A=[a-zA-Z]/". "[space][space]$foo->bar" is a
147 verbatim paragraph, because its first line starts with a literal
148 whitespace character (and there's no "=begin"..."=end" region around).
149 The "=begin identifier" ... "=end identifier" commands stop paragraphs
151 that they surround from being parsed as ordinary or verbatim
152 paragraphs, if identifier doesn't begin with a colon. This is
153 discussed in detail in the section "About Data Paragraphs and
154 "=begin/=end" Regions".
155
157 This section is intended to supplement and clarify the discussion in
158 "Command Paragraph" in perlpod. These are the currently recognized Pod
159 commands:
160 "=head1", "=head2", "=head3", "=head4"
162 This command indicates that the text in the remainder of the
163 paragraph is a heading. That text may contain formatting codes.
164 Examples:
165 =head1 Object Attributes
167 =head3 What B<Not> to Do!
169 "=pod"
171 This command indicates that this paragraph begins a Pod block. (If
172 we are already in the middle of a Pod block, this command has no
173 effect at all.) If there is any text in this command paragraph
174 after "=pod", it must be ignored. Examples:
175 =pod
177 This is a plain Pod paragraph.
179 =pod This text is ignored.
181 "=cut"
183 This command indicates that this line is the end of this previously
184 started Pod block. If there is any text after "=cut" on the line,
185 it must be ignored. Examples:
186 =cut
188 =cut The documentation ends here.
190 =cut
192 # This is the first line of program text.
193 sub foo { # This is the second.
194 It is an error to try to start a Pod block with a "=cut" command.
196 In that case, the Pod processor must halt parsing of the input
197 file, and must by default emit a warning.
198 "=over"
200 This command indicates that this is the start of a list/indent
201 region. If there is any text following the "=over", it must
202 consist of only a nonzero positive numeral. The semantics of this
203 numeral is explained in the "About =over...=back Regions" section,
204 further below. Formatting codes are not expanded. Examples:
205 =over 3
207 =over 3.5
209 =over
211 "=item"
213 This command indicates that an item in a list begins here.
214 Formatting codes are processed. The semantics of the (optional)
215 text in the remainder of this paragraph are explained in the "About
216 =over...=back Regions" section, further below. Examples:
217 =item
219 =item *
221 =item *
223 =item 14
225 =item 3.
227 =item C<< $thing->stuff(I<dodad>) >>
229 =item For transporting us beyond seas to be tried for pretended
231 offenses
232 =item He is at this time transporting large armies of foreign
234 mercenaries to complete the works of death, desolation and
235 tyranny, already begun with circumstances of cruelty and perfidy
236 scarcely paralleled in the most barbarous ages, and totally
237 unworthy the head of a civilized nation.
238 "=back"
240 This command indicates that this is the end of the region begun by
241 the most recent "=over" command. It permits no text after the
242 "=back" command.
243 "=begin formatname"
245 "=begin formatname parameter"
246 This marks the following paragraphs (until the matching "=end
247 formatname") as being for some special kind of processing. Unless
248 "formatname" begins with a colon, the contained non-command
249 paragraphs are data paragraphs. But if "formatname" does begin
250 with a colon, then non-command paragraphs are ordinary paragraphs
251 or data paragraphs. This is discussed in detail in the section
252 "About Data Paragraphs and "=begin/=end" Regions".
253 It is advised that formatnames match the regexp
255 "m/\A:?[-a-zA-Z0-9_]+\z/". Everything following whitespace after
256 the formatname is a parameter that may be used by the formatter
257 when dealing with this region. This parameter must not be repeated
258 in the "=end" paragraph. Implementors should anticipate future
259 expansion in the semantics and syntax of the first parameter to
260 "=begin"/"=end"/"=for".
261 "=end formatname"
263 This marks the end of the region opened by the matching "=begin
264 formatname" region. If "formatname" is not the formatname of the
265 most recent open "=begin formatname" region, then this is an error,
266 and must generate an error message. This is discussed in detail in
267 the section "About Data Paragraphs and "=begin/=end" Regions".
268 "=for formatname text..."
270 This is synonymous with:
271 =begin formatname
273 text...
275 =end formatname
277 That is, it creates a region consisting of a single paragraph; that
279 paragraph is to be treated as a normal paragraph if "formatname"
280 begins with a ":"; if "formatname" doesn't begin with a colon, then
281 "text..." will constitute a data paragraph. There is no way to use
282 "=for formatname text..." to express "text..." as a verbatim
283 paragraph.
284 "=encoding encodingname"
286 This command, which should occur early in the document (at least
287 before any non-US-ASCII data!), declares that this document is
288 encoded in the encoding encodingname, which must be an encoding
289 name that Encode recognizes. (Encode's list of supported
290 encodings, in Encode::Supported, is useful here.) If the Pod
291 parser cannot decode the declared encoding, it should emit a
292 warning and may abort parsing the document altogether.
293 A document having more than one "=encoding" line should be
295 considered an error. Pod processors may silently tolerate this if
296 the not-first "=encoding" lines are just duplicates of the first
297 one (e.g., if there's a "=encoding utf8" line, and later on another
298 "=encoding utf8" line). But Pod processors should complain if
299 there are contradictory "=encoding" lines in the same document
300 (e.g., if there is a "=encoding utf8" early in the document and
301 "=encoding big5" later). Pod processors that recognize BOMs may
302 also complain if they see an "=encoding" line that contradicts the
303 BOM (e.g., if a document with a UTF-16LE BOM has an "=encoding
304 shiftjis" line).
305 If a Pod processor sees any command other than the ones listed above
307 (like "=head", or "=haed1", or "=stuff", or "=cuttlefish", or "=w123"),
308 that processor must by default treat this as an error. It must not
309 process the paragraph beginning with that command, must by default warn
310 of this as an error, and may abort the parse. A Pod parser may allow a
311 way for particular applications to add to the above list of known
312 commands, and to stipulate, for each additional command, whether
313 formatting codes should be processed.
314 Future versions of this specification may add additional commands.
316
318 (Note that in previous drafts of this document and of perlpod,
319 formatting codes were referred to as "interior sequences", and this
320 term may still be found in the documentation for Pod parsers, and in
321 error messages from Pod processors.)
322 There are two syntaxes for formatting codes:
324 · A formatting code starts with a capital letter (just US-ASCII
326 [A-Z]) followed by a "<", any number of characters, and ending with
327 the first matching ">". Examples:
328 That's what I<you> think!
330 What's C<CORE::dump()> for?
332 X<C<chmod> and C<unlink()> Under Different Operating Systems>
334 · A formatting code starts with a capital letter (just US-ASCII
336 [A-Z]) followed by two or more "<"'s, one or more whitespace
337 characters, any number of characters, one or more whitespace
338 characters, and ending with the first matching sequence of two or
339 more ">"'s, where the number of ">"'s equals the number of "<"'s in
340 the opening of this formatting code. Examples:
341 That's what I<< you >> think!
343 C<<< open(X, ">>thing.dat") || die $! >>>
345 B<< $foo->bar(); >>
347 With this syntax, the whitespace character(s) after the "C<<<" and
349 before the ">>>" (or whatever letter) are not renderable. They do
350 not signify whitespace, are merely part of the formatting codes
351 themselves. That is, these are all synonymous:
352 C<thing>
354 C<< thing >>
355 C<< thing >>
356 C<<< thing >>>
357 C<<<<
358 thing
359 >>>>
360 and so on.
362 Finally, the multiple-angle-bracket form does not alter the
364 interpretation of nested formatting codes, meaning that the
365 following four example lines are identical in meaning:
366 B<example: C<$a E<lt>=E<gt> $b>>
368 B<example: C<< $a <=> $b >>>
370 B<example: C<< $a E<lt>=E<gt> $b >>>
372 B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
374 In parsing Pod, a notably tricky part is the correct parsing of
376 (potentially nested!) formatting codes. Implementors should consult
377 the code in the "parse_text" routine in Pod::Parser as an example of a
378 correct implementation.
379 "I<text>" -- italic text
381 See the brief discussion in "Formatting Codes" in perlpod.
382 "B<text>" -- bold text
384 See the brief discussion in "Formatting Codes" in perlpod.
385 "C<code>" -- code text
387 See the brief discussion in "Formatting Codes" in perlpod.
388 "F<filename>" -- style for filenames
390 See the brief discussion in "Formatting Codes" in perlpod.
391 "X<topic name>" -- an index entry
393 See the brief discussion in "Formatting Codes" in perlpod.
394 This code is unusual in that most formatters completely discard
396 this code and its content. Other formatters will render it with
397 invisible codes that can be used in building an index of the
398 current document.
399 "Z<>" -- a null (zero-effect) formatting code
401 Discussed briefly in "Formatting Codes" in perlpod.
402 This code is unusual in that it should have no content. That is, a
404 processor may complain if it sees "Z<potatoes>". Whether or not it
405 complains, the potatoes text should ignored.
406 "L<name>" -- a hyperlink
408 The complicated syntaxes of this code are discussed at length in
409 "Formatting Codes" in perlpod, and implementation details are
410 discussed below, in "About L<...> Codes". Parsing the contents of
411 L<content> is tricky. Notably, the content has to be checked for
412 whether it looks like a URL, or whether it has to be split on
413 literal "|" and/or "/" (in the right order!), and so on, before
414 E<...> codes are resolved.
415 "E<escape>" -- a character escape
417 See "Formatting Codes" in perlpod, and several points in "Notes on
418 Implementing Pod Processors".
419 "S<text>" -- text contains non-breaking spaces
421 This formatting code is syntactically simple, but semantically
422 complex. What it means is that each space in the printable content
423 of this code signifies a non-breaking space.
424 Consider:
426 C<$x ? $y : $z>
428 S<C<$x ? $y : $z>>
430 Both signify the monospace (c[ode] style) text consisting of "$x",
432 one space, "?", one space, ":", one space, "$z". The difference is
433 that in the latter, with the S code, those spaces are not "normal"
434 spaces, but instead are non-breaking spaces.
435 If a Pod processor sees any formatting code other than the ones listed
437 above (as in "N<...>", or "Q<...>", etc.), that processor must by
438 default treat this as an error. A Pod parser may allow a way for
439 particular applications to add to the above list of known formatting
440 codes; a Pod parser might even allow a way to stipulate, for each
441 additional command, whether it requires some form of special
442 processing, as L<...> does.
443 Future versions of this specification may add additional formatting
445 codes.
446 Historical note: A few older Pod processors would not see a ">" as
448 closing a "C<" code, if the ">" was immediately preceded by a "-".
449 This was so that this:
450 C<$foo->bar>
452 would parse as equivalent to this:
454 C<$foo-E<gt>bar>
456 instead of as equivalent to a "C" formatting code containing only
458 "$foo-", and then a "bar>" outside the "C" formatting code. This
459 problem has since been solved by the addition of syntaxes like this:
460 C<< $foo->bar >>
462 Compliant parsers must not treat "->" as special.
464 Formatting codes absolutely cannot span paragraphs. If a code is
466 opened in one paragraph, and no closing code is found by the end of
467 that paragraph, the Pod parser must close that formatting code, and
468 should complain (as in "Unterminated I code in the paragraph starting
469 at line 123: 'Time objects are not...'"). So these two paragraphs:
470 I<I told you not to do this!
472 Don't make me say it again!>
474 ...must not be parsed as two paragraphs in italics (with the I code
476 starting in one paragraph and starting in another.) Instead, the first
477 paragraph should generate a warning, but that aside, the above code
478 must parse as if it were:
479 I<I told you not to do this!>
481 Don't make me say it again!E<gt>
483 (In SGMLish jargon, all Pod commands are like block-level elements,
485 whereas all Pod formatting codes are like inline-level elements.)
486
488 The following is a long section of miscellaneous requirements and
489 suggestions to do with Pod processing.
490 · Pod formatters should tolerate lines in verbatim blocks that are of
492 any length, even if that means having to break them (possibly
493 several times, for very long lines) to avoid text running off the
494 side of the page. Pod formatters may warn of such line-breaking.
495 Such warnings are particularly appropriate for lines are over 100
496 characters long, which are usually not intentional.
497 · Pod parsers must recognize all of the three well-known newline
499 formats: CR, LF, and CRLF. See perlport.
500 · Pod parsers should accept input lines that are of any length.
502 · Since Perl recognizes a Unicode Byte Order Mark at the start of
504 files as signaling that the file is Unicode encoded as in UTF-16
505 (whether big-endian or little-endian) or UTF-8, Pod parsers should
506 do the same. Otherwise, the character encoding should be
507 understood as being UTF-8 if the first highbit byte sequence in the
508 file seems valid as a UTF-8 sequence, or otherwise as CP-1252
509 (earlier versions of this specification used Latin-1 instead of
510 CP-1252).
511 Future versions of this specification may specify how Pod can
513 accept other encodings. Presumably treatment of other encodings in
514 Pod parsing would be as in XML parsing: whatever the encoding
515 declared by a particular Pod file, content is to be stored in
516 memory as Unicode characters.
517 · The well known Unicode Byte Order Marks are as follows: if the
519 file begins with the two literal byte values 0xFE 0xFF, this is the
520 BOM for big-endian UTF-16. If the file begins with the two literal
521 byte value 0xFF 0xFE, this is the BOM for little-endian UTF-16. On
522 an ASCII platform, if the file begins with the three literal byte
523 values 0xEF 0xBB 0xBF, this is the BOM for UTF-8. A mechanism
524 portable to EBCDIC platforms is to:
525 my $utf8_bom = "\x{FEFF}";
527 utf8::encode($utf8_bom);
528 · A naive, but often sufficient heuristic on ASCII platforms, for
530 testing the first highbit byte-sequence in a BOM-less file (whether
531 in code or in Pod!), to see whether that sequence is valid as UTF-8
532 (RFC 2279) is to check whether that the first byte in the sequence
533 is in the range 0xC2 - 0xFD and whether the next byte is in the
534 range 0x80 - 0xBF. If so, the parser may conclude that this file
535 is in UTF-8, and all highbit sequences in the file should be
536 assumed to be UTF-8. Otherwise the parser should treat the file as
537 being in CP-1252. (A better check, and which works on EBCDIC
538 platforms as well, is to pass a copy of the sequence to
539 utf8::decode() which performs a full validity check on the sequence
540 and returns TRUE if it is valid UTF-8, FALSE otherwise. This
541 function is always pre-loaded, is fast because it is written in C,
542 and will only get called at most once, so you don't need to avoid
543 it out of performance concerns.) In the unlikely circumstance that
544 the first highbit sequence in a truly non-UTF-8 file happens to
545 appear to be UTF-8, one can cater to our heuristic (as well as any
546 more intelligent heuristic) by prefacing that line with a comment
547 line containing a highbit sequence that is clearly not valid as
548 UTF-8. A line consisting of simply "#", an e-acute, and any non-
549 highbit byte, is sufficient to establish this file's encoding.
550 · Pod processors must treat a "=for [label] [content...]" paragraph
552 as meaning the same thing as a "=begin [label]" paragraph, content,
553 and an "=end [label]" paragraph. (The parser may conflate these
554 two constructs, or may leave them distinct, in the expectation that
555 the formatter will nevertheless treat them the same.)
556 · When rendering Pod to a format that allows comments (i.e., to
558 nearly any format other than plaintext), a Pod formatter must
559 insert comment text identifying its name and version number, and
560 the name and version numbers of any modules it might be using to
561 process the Pod. Minimal examples:
562 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
564 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
566 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
568 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
570 Formatters may also insert additional comments, including: the
572 release date of the Pod formatter program, the contact address for
573 the author(s) of the formatter, the current time, the name of input
574 file, the formatting options in effect, version of Perl used, etc.
575 Formatters may also choose to note errors/warnings as comments,
577 besides or instead of emitting them otherwise (as in messages to
578 STDERR, or "die"ing).
579 · Pod parsers may emit warnings or error messages ("Unknown E code
581 E<zslig>!") to STDERR (whether through printing to STDERR, or
582 "warn"ing/"carp"ing, or "die"ing/"croak"ing), but must allow
583 suppressing all such STDERR output, and instead allow an option for
584 reporting errors/warnings in some other way, whether by triggering
585 a callback, or noting errors in some attribute of the document
586 object, or some similarly unobtrusive mechanism -- or even by
587 appending a "Pod Errors" section to the end of the parsed form of
588 the document.
589 · In cases of exceptionally aberrant documents, Pod parsers may abort
591 the parse. Even then, using "die"ing/"croak"ing is to be avoided;
592 where possible, the parser library may simply close the input file
593 and add text like "*** Formatting Aborted ***" to the end of the
594 (partial) in-memory document.
595 · In paragraphs where formatting codes (like E<...>, B<...>) are
597 understood (i.e., not verbatim paragraphs, but including ordinary
598 paragraphs, and command paragraphs that produce renderable text,
599 like "=head1"), literal whitespace should generally be considered
600 "insignificant", in that one literal space has the same meaning as
601 any (nonzero) number of literal spaces, literal newlines, and
602 literal tabs (as long as this produces no blank lines, since those
603 would terminate the paragraph). Pod parsers should compact literal
604 whitespace in each processed paragraph, but may provide an option
605 for overriding this (since some processing tasks do not require
606 it), or may follow additional special rules (for example, specially
607 treating period-space-space or period-newline sequences).
608 · Pod parsers should not, by default, try to coerce apostrophe (')
610 and quote (") into smart quotes (little 9's, 66's, 99's, etc), nor
611 try to turn backtick (`) into anything else but a single backtick
612 character (distinct from an open quote character!), nor "--" into
613 anything but two minus signs. They must never do any of those
614 things to text in C<...> formatting codes, and never ever to text
615 in verbatim paragraphs.
616 · When rendering Pod to a format that has two kinds of hyphens (-),
618 one that's a non-breaking hyphen, and another that's a breakable
619 hyphen (as in "object-oriented", which can be split across lines as
620 "object-", newline, "oriented"), formatters are encouraged to
621 generally translate "-" to non-breaking hyphen, but may apply
622 heuristics to convert some of these to breaking hyphens.
623 · Pod formatters should make reasonable efforts to keep words of Perl
625 code from being broken across lines. For example, "Foo::Bar" in
626 some formatting systems is seen as eligible for being broken across
627 lines as "Foo::" newline "Bar" or even "Foo::-" newline "Bar".
628 This should be avoided where possible, either by disabling all
629 line-breaking in mid-word, or by wrapping particular words with
630 internal punctuation in "don't break this across lines" codes
631 (which in some formats may not be a single code, but might be a
632 matter of inserting non-breaking zero-width spaces between every
633 pair of characters in a word.)
634 · Pod parsers should, by default, expand tabs in verbatim paragraphs
636 as they are processed, before passing them to the formatter or
637 other processor. Parsers may also allow an option for overriding
638 this.
639 · Pod parsers should, by default, remove newlines from the end of
641 ordinary and verbatim paragraphs before passing them to the
642 formatter. For example, while the paragraph you're reading now
643 could be considered, in Pod source, to end with (and contain) the
644 newline(s) that end it, it should be processed as ending with (and
645 containing) the period character that ends this sentence.
646 · Pod parsers, when reporting errors, should make some effort to
648 report an approximate line number ("Nested E<>'s in Paragraph #52,
649 near line 633 of Thing/Foo.pm!"), instead of merely noting the
650 paragraph number ("Nested E<>'s in Paragraph #52 of
651 Thing/Foo.pm!"). Where this is problematic, the paragraph number
652 should at least be accompanied by an excerpt from the paragraph
653 ("Nested E<>'s in Paragraph #52 of Thing/Foo.pm, which begins
654 'Read/write accessor for the C<interest rate> attribute...'").
655 · Pod parsers, when processing a series of verbatim paragraphs one
657 after another, should consider them to be one large verbatim
658 paragraph that happens to contain blank lines. I.e., these two
659 lines, which have a blank line between them:
660 use Foo;
662 print Foo->VERSION
664 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
666 Foo->VERSION") before being passed to the formatter or other
667 processor. Parsers may also allow an option for overriding this.
668 While this might be too cumbersome to implement in event-based Pod
670 parsers, it is straightforward for parsers that return parse trees.
671 · Pod formatters, where feasible, are advised to avoid splitting
673 short verbatim paragraphs (under twelve lines, say) across pages.
674 · Pod parsers must treat a line with only spaces and/or tabs on it as
676 a "blank line" such as separates paragraphs. (Some older parsers
677 recognized only two adjacent newlines as a "blank line" but would
678 not recognize a newline, a space, and a newline, as a blank line.
679 This is noncompliant behavior.)
680 · Authors of Pod formatters/processors should make every effort to
682 avoid writing their own Pod parser. There are already several in
683 CPAN, with a wide range of interface styles -- and one of them,
684 Pod::Simple, comes with modern versions of Perl.
685 · Characters in Pod documents may be conveyed either as literals, or
687 by number in E<n> codes, or by an equivalent mnemonic, as in
688 E<eacute> which is exactly equivalent to E<233>. The numbers are
689 the Latin1/Unicode values, even on EBCDIC platforms.
690 When referring to characters by using a E<n> numeric code, numbers
692 in the range 32-126 refer to those well known US-ASCII characters
693 (also defined there by Unicode, with the same meaning), which all
694 Pod formatters must render faithfully. Characters whose E<>
695 numbers are in the ranges 0-31 and 127-159 should not be used
696 (neither as literals, nor as E<number> codes), except for the
697 literal byte-sequences for newline (ASCII 13, ASCII 13 10, or ASCII
698 10), and tab (ASCII 9).
699 Numbers in the range 160-255 refer to Latin-1 characters (also
701 defined there by Unicode, with the same meaning). Numbers above
702 255 should be understood to refer to Unicode characters.
703 · Be warned that some formatters cannot reliably render characters
705 outside 32-126; and many are able to handle 32-126 and 160-255, but
706 nothing above 255.
707 · Besides the well-known "E<lt>" and "E<gt>" codes for less-than and
709 greater-than, Pod parsers must understand "E<sol>" for "/"
710 (solidus, slash), and "E<verbar>" for "|" (vertical bar, pipe).
711 Pod parsers should also understand "E<lchevron>" and "E<rchevron>"
712 as legacy codes for characters 171 and 187, i.e., "left-pointing
713 double angle quotation mark" = "left pointing guillemet" and
714 "right-pointing double angle quotation mark" = "right pointing
715 guillemet". (These look like little "<<" and ">>", and they are
716 now preferably expressed with the HTML/XHTML codes "E<laquo>" and
717 "E<raquo>".)
718 · Pod parsers should understand all "E<html>" codes as defined in the
720 entity declarations in the most recent XHTML specification at
721 "www.W3.org". Pod parsers must understand at least the entities
722 that define characters in the range 160-255 (Latin-1). Pod
723 parsers, when faced with some unknown "E<identifier>" code,
724 shouldn't simply replace it with nullstring (by default, at least),
725 but may pass it through as a string consisting of the literal
726 characters E, less-than, identifier, greater-than. Or Pod parsers
727 may offer the alternative option of processing such unknown
728 "E<identifier>" codes by firing an event especially for such codes,
729 or by adding a special node-type to the in-memory document tree.
730 Such "E<identifier>" may have special meaning to some processors,
731 or some processors may choose to add them to a special error
732 report.
733 · Pod parsers must also support the XHTML codes "E<quot>" for
735 character 34 (doublequote, "), "E<amp>" for character 38
736 (ampersand, &), and "E<apos>" for character 39 (apostrophe, ').
737 · Note that in all cases of "E<whatever>", whatever (whether an
739 htmlname, or a number in any base) must consist only of
740 alphanumeric characters -- that is, whatever must match
741 "m/\A\w+\z/". So "E< 0 1 2 3 >" is invalid, because it contains
742 spaces, which aren't alphanumeric characters. This presumably does
743 not need special treatment by a Pod processor; " 0 1 2 3 " doesn't
744 look like a number in any base, so it would presumably be looked up
745 in the table of HTML-like names. Since there isn't (and cannot be)
746 an HTML-like entity called " 0 1 2 3 ", this will be treated as an
747 error. However, Pod processors may treat "E< 0 1 2 3 >" or
748 "E<e-acute>" as syntactically invalid, potentially earning a
749 different error message than the error message (or warning, or
750 event) generated by a merely unknown (but theoretically valid)
751 htmlname, as in "E<qacute>" [sic]. However, Pod parsers are not
752 required to make this distinction.
753 · Note that E<number> must not be interpreted as simply "codepoint
755 number in the current/native character set". It always means only
756 "the character represented by codepoint number in Unicode." (This
757 is identical to the semantics of &#number; in XML.)
758 This will likely require many formatters to have tables mapping
760 from treatable Unicode codepoints (such as the "\xE9" for the
761 e-acute character) to the escape sequences or codes necessary for
762 conveying such sequences in the target output format. A converter
763 to *roff would, for example know that "\xE9" (whether conveyed
764 literally, or via a E<...> sequence) is to be conveyed as "e\\*'".
765 Similarly, a program rendering Pod in a Mac OS application window,
766 would presumably need to know that "\xE9" maps to codepoint 142 in
767 MacRoman encoding that (at time of writing) is native for Mac OS.
768 Such Unicode2whatever mappings are presumably already widely
769 available for common output formats. (Such mappings may be
770 incomplete! Implementers are not expected to bend over backwards
771 in an attempt to render Cherokee syllabics, Etruscan runes,
772 Byzantine musical symbols, or any of the other weird things that
773 Unicode can encode.) And if a Pod document uses a character not
774 found in such a mapping, the formatter should consider it an
775 unrenderable character.
776 · If, surprisingly, the implementor of a Pod formatter can't find a
778 satisfactory pre-existing table mapping from Unicode characters to
779 escapes in the target format (e.g., a decent table of Unicode
780 characters to *roff escapes), it will be necessary to build such a
781 table. If you are in this circumstance, you should begin with the
782 characters in the range 0x00A0 - 0x00FF, which is mostly the
783 heavily used accented characters. Then proceed (as patience
784 permits and fastidiousness compels) through the characters that the
785 (X)HTML standards groups judged important enough to merit mnemonics
786 for. These are declared in the (X)HTML specifications at the
787 www.W3.org site. At time of writing (September 2001), the most
788 recent entity declaration files are:
789 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
791 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
792 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
793 Then you can progress through any remaining notable Unicode
795 characters in the range 0x2000-0x204D (consult the character tables
796 at www.unicode.org), and whatever else strikes your fancy. For
797 example, in xhtml-symbol.ent, there is the entry:
798 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
800 While the mapping "infin" to the character "\x{221E}" will
802 (hopefully) have been already handled by the Pod parser, the
803 presence of the character in this file means that it's reasonably
804 important enough to include in a formatter's table that maps from
805 notable Unicode characters to the codes necessary for rendering
806 them. So for a Unicode-to-*roff mapping, for example, this would
807 merit the entry:
808 "\x{221E}" => '\(in',
810 It is eagerly hoped that in the future, increasing numbers of
812 formats (and formatters) will support Unicode characters directly
813 (as (X)HTML does with "∞", "∞", or "∞"),
814 reducing the need for idiosyncratic mappings of
815 Unicode-to-my_escapes.
816 · It is up to individual Pod formatter to display good judgement when
818 confronted with an unrenderable character (which is distinct from
819 an unknown E<thing> sequence that the parser couldn't resolve to
820 anything, renderable or not). It is good practice to map Latin
821 letters with diacritics (like "E<eacute>"/"E<233>") to the
822 corresponding unaccented US-ASCII letters (like a simple character
823 101, "e"), but clearly this is often not feasible, and an
824 unrenderable character may be represented as "?", or the like. In
825 attempting a sane fallback (as from E<233> to "e"), Pod formatters
826 may use the %Latin1Code_to_fallback table in Pod::Escapes, or
827 Text::Unidecode, if available.
828 For example, this Pod text:
830 magic is enabled if you set C<$Currency> to 'E<euro>'.
832 may be rendered as: "magic is enabled if you set $Currency to '?'"
834 or as "magic is enabled if you set $Currency to '[euro]'", or as
835 "magic is enabled if you set $Currency to '[x20AC]', etc.
836 A Pod formatter may also note, in a comment or warning, a list of
838 what unrenderable characters were encountered.
839 · E<...> may freely appear in any formatting code (other than in
841 another E<...> or in an Z<>). That is, "X<The E<euro>1,000,000
842 Solution>" is valid, as is "L<The E<euro>1,000,000
843 Solution|Million::Euros>".
844 · Some Pod formatters output to formats that implement non-breaking
846 spaces as an individual character (which I'll call "NBSP"), and
847 others output to formats that implement non-breaking spaces just as
848 spaces wrapped in a "don't break this across lines" code. Note
849 that at the level of Pod, both sorts of codes can occur: Pod can
850 contain a NBSP character (whether as a literal, or as a "E<160>" or
851 "E<nbsp>" code); and Pod can contain "S<foo I<bar> baz>" codes,
852 where "mere spaces" (character 32) in such codes are taken to
853 represent non-breaking spaces. Pod parsers should consider
854 supporting the optional parsing of "S<foo I<bar> baz>" as if it
855 were "fooNBSPI<bar>NBSPbaz", and, going the other way, the optional
856 parsing of groups of words joined by NBSP's as if each group were
857 in a S<...> code, so that formatters may use the representation
858 that maps best to what the output format demands.
859 · Some processors may find that the "S<...>" code is easiest to
861 implement by replacing each space in the parse tree under the
862 content of the S, with an NBSP. But note: the replacement should
863 apply not to spaces in all text, but only to spaces in printable
864 text. (This distinction may or may not be evident in the
865 particular tree/event model implemented by the Pod parser.) For
866 example, consider this unusual case:
867 S<L</Autoloaded Functions>>
869 This means that the space in the middle of the visible link text
871 must not be broken across lines. In other words, it's the same as
872 this:
873 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
875 However, a misapplied space-to-NBSP replacement could (wrongly)
877 produce something equivalent to this:
878 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
880 ...which is almost definitely not going to work as a hyperlink
882 (assuming this formatter outputs a format supporting hypertext).
883 Formatters may choose to just not support the S format code,
885 especially in cases where the output format simply has no NBSP
886 character/code and no code for "don't break this stuff across
887 lines".
888 · Besides the NBSP character discussed above, implementors are
890 reminded of the existence of the other "special" character in
891 Latin-1, the "soft hyphen" character, also known as "discretionary
892 hyphen", i.e. "E<173>" = "E<0xAD>" = "E<shy>"). This character
893 expresses an optional hyphenation point. That is, it normally
894 renders as nothing, but may render as a "-" if a formatter breaks
895 the word at that point. Pod formatters should, as appropriate, do
896 one of the following: 1) render this with a code with the same
897 meaning (e.g., "\-" in RTF), 2) pass it through in the expectation
898 that the formatter understands this character as such, or 3) delete
899 it.
900 For example:
902 sigE<shy>action
904 manuE<shy>script
905 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
906 These signal to a formatter that if it is to hyphenate "sigaction"
908 or "manuscript", then it should be done as "sig-[linebreak]action"
909 or "manu-[linebreak]script" (and if it doesn't hyphenate it, then
910 the "E<shy>" doesn't show up at all). And if it is to hyphenate
911 "Jarkko" and/or "Hietaniemi", it can do so only at the points where
912 there is a "E<shy>" code.
913 In practice, it is anticipated that this character will not be used
915 often, but formatters should either support it, or delete it.
916 · If you think that you want to add a new command to Pod (like, say,
918 a "=biblio" command), consider whether you could get the same
919 effect with a for or begin/end sequence: "=for biblio ..." or
920 "=begin biblio" ... "=end biblio". Pod processors that don't
921 understand "=for biblio", etc, will simply ignore it, whereas they
922 may complain loudly if they see "=biblio".
923 · Throughout this document, "Pod" has been the preferred spelling for
925 the name of the documentation format. One may also use "POD" or
926 "pod". For the documentation that is (typically) in the Pod
927 format, you may use "pod", or "Pod", or "POD". Understanding these
928 distinctions is useful; but obsessing over how to spell them,
929 usually is not.
930
932 As you can tell from a glance at perlpod, the L<...> code is the most
933 complex of the Pod formatting codes. The points below will hopefully
934 clarify what it means and how processors should deal with it.
935 · In parsing an L<...> code, Pod parsers must distinguish at least
937 four attributes:
938 First:
940 The link-text. If there is none, this must be "undef". (E.g.,
941 in "L<Perl Functions|perlfunc>", the link-text is "Perl
942 Functions". In "L<Time::HiRes>" and even "L<|Time::HiRes>",
943 there is no link text. Note that link text may contain
944 formatting.)
945 Second:
947 The possibly inferred link-text; i.e., if there was no real
948 link text, then this is the text that we'll infer in its place.
949 (E.g., for "L<Getopt::Std>", the inferred link text is
950 "Getopt::Std".)
951 Third:
953 The name or URL, or "undef" if none. (E.g., in "L<Perl
954 Functions|perlfunc>", the name (also sometimes called the page)
955 is "perlfunc". In "L</CAVEATS>", the name is "undef".)
956 Fourth:
958 The section (AKA "item" in older perlpods), or "undef" if none.
959 E.g., in "L<Getopt::Std/DESCRIPTION>", "DESCRIPTION" is the
960 section. (Note that this is not the same as a manpage section
961 like the "5" in "man 5 crontab". "Section Foo" in the Pod
962 sense means the part of the text that's introduced by the
963 heading or item whose text is "Foo".)
964 Pod parsers may also note additional attributes including:
966 Fifth:
968 A flag for whether item 3 (if present) is a URL (like
969 "http://lists.perl.org" is), in which case there should be no
970 section attribute; a Pod name (like "perldoc" and "Getopt::Std"
971 are); or possibly a man page name (like "crontab(5)" is).
972 Sixth:
974 The raw original L<...> content, before text is split on "|",
975 "/", etc, and before E<...> codes are expanded.
976 (The above were numbered only for concise reference below. It is
978 not a requirement that these be passed as an actual list or array.)
979 For example:
981 L<Foo::Bar>
983 => undef, # link text
984 "Foo::Bar", # possibly inferred link text
985 "Foo::Bar", # name
986 undef, # section
987 'pod', # what sort of link
988 "Foo::Bar" # original content
989 L<Perlport's section on NL's|perlport/Newlines>
991 => "Perlport's section on NL's", # link text
992 "Perlport's section on NL's", # possibly inferred link text
993 "perlport", # name
994 "Newlines", # section
995 'pod', # what sort of link
996 "Perlport's section on NL's|perlport/Newlines"
997 # original content
998 L<perlport/Newlines>
1000 => undef, # link text
1001 '"Newlines" in perlport', # possibly inferred link text
1002 "perlport", # name
1003 "Newlines", # section
1004 'pod', # what sort of link
1005 "perlport/Newlines" # original content
1006 L<crontab(5)/"DESCRIPTION">
1008 => undef, # link text
1009 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
1010 "crontab(5)", # name
1011 "DESCRIPTION", # section
1012 'man', # what sort of link
1013 'crontab(5)/"DESCRIPTION"' # original content
1014 L</Object Attributes>
1016 => undef, # link text
1017 '"Object Attributes"', # possibly inferred link text
1018 undef, # name
1019 "Object Attributes", # section
1020 'pod', # what sort of link
1021 "/Object Attributes" # original content
1022 L<http://www.perl.org/>
1024 => undef, # link text
1025 "http://www.perl.org/", # possibly inferred link text
1026 "http://www.perl.org/", # name
1027 undef, # section
1028 'url', # what sort of link
1029 "http://www.perl.org/" # original content
1030 L<Perl.org|http://www.perl.org/>
1032 => "Perl.org", # link text
1033 "http://www.perl.org/", # possibly inferred link text
1034 "http://www.perl.org/", # name
1035 undef, # section
1036 'url', # what sort of link
1037 "Perl.org|http://www.perl.org/" # original content
1038 Note that you can distinguish URL-links from anything else by the
1040 fact that they match "m/\A\w+:[^:\s]\S*\z/". So
1041 "L<http://www.perl.com>" is a URL, but "L<HTTP::Response>" isn't.
1042 · In case of L<...> codes with no "text|" part in them, older
1044 formatters have exhibited great variation in actually displaying
1045 the link or cross reference. For example, L<crontab(5)> would
1046 render as "the crontab(5) manpage", or "in the crontab(5) manpage"
1047 or just "crontab(5)".
1048 Pod processors must now treat "text|"-less links as follows:
1050 L<name> => L<name|name>
1052 L</section> => L<"section"|/section>
1053 L<name/section> => L<"section" in name|name/section>
1054 · Note that section names might contain markup. I.e., if a section
1056 starts with:
1057 =head2 About the C<-M> Operator
1059 or with:
1061 =item About the C<-M> Operator
1063 then a link to it would look like this:
1065 L<somedoc/About the C<-M> Operator>
1067 Formatters may choose to ignore the markup for purposes of
1069 resolving the link and use only the renderable characters in the
1070 section name, as in:
1071 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1073 Operator</h1>
1074 ...
1076 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1078 Operator" in somedoc</a>
1079 · Previous versions of perlpod distinguished "L<name/"section">"
1081 links from "L<name/item>" links (and their targets). These have
1082 been merged syntactically and semantically in the current
1083 specification, and section can refer either to a "=headn Heading
1084 Content" command or to a "=item Item Content" command. This
1085 specification does not specify what behavior should be in the case
1086 of a given document having several things all seeming to produce
1087 the same section identifier (e.g., in HTML, several things all
1088 producing the same anchorname in <a name="anchorname">...</a>
1089 elements). Where Pod processors can control this behavior, they
1090 should use the first such anchor. That is, "L<Foo/Bar>" refers to
1091 the first "Bar" section in Foo.
1092 But for some processors/formats this cannot be easily controlled;
1094 as with the HTML example, the behavior of multiple ambiguous <a
1095 name="anchorname">...</a> is most easily just left up to browsers
1096 to decide.
1097 · In a "L<text|...>" code, text may contain formatting codes for
1099 formatting or for E<...> escapes, as in:
1100 L<B<ummE<234>stuff>|...>
1102 For "L<...>" codes without a "name|" part, only "E<...>" and "Z<>"
1104 codes may occur. That is, authors should not use
1105 ""L<B<Foo::Bar>>"".
1106 Note, however, that formatting codes and Z<>'s can occur in any and
1108 all parts of an L<...> (i.e., in name, section, text, and url).
1109 Authors must not nest L<...> codes. For example, "L<The
1111 L<Foo::Bar> man page>" should be treated as an error.
1112 · Note that Pod authors may use formatting codes inside the "text"
1114 part of "L<text|name>" (and so on for L<text|/"sec">).
1115 In other words, this is valid:
1117 Go read L<the docs on C<$.>|perlvar/"$.">
1119 Some output formats that do allow rendering "L<...>" codes as
1121 hypertext, might not allow the link-text to be formatted; in that
1122 case, formatters will have to just ignore that formatting.
1123 · At time of writing, "L<name>" values are of two types: either the
1125 name of a Pod page like "L<Foo::Bar>" (which might be a real Perl
1126 module or program in an @INC / PATH directory, or a .pod file in
1127 those places); or the name of a Unix man page, like
1128 "L<crontab(5)>". In theory, "L<chmod>" is ambiguous between a Pod
1129 page called "chmod", or the Unix man page "chmod" (in whatever man-
1130 section). However, the presence of a string in parens, as in
1131 "crontab(5)", is sufficient to signal that what is being discussed
1132 is not a Pod page, and so is presumably a Unix man page. The
1133 distinction is of no importance to many Pod processors, but some
1134 processors that render to hypertext formats may need to distinguish
1135 them in order to know how to render a given "L<foo>" code.
1136 · Previous versions of perlpod allowed for a "L<section>" syntax (as
1138 in "L<Object Attributes>"), which was not easily distinguishable
1139 from "L<name>" syntax and for "L<"section">" which was only
1140 slightly less ambiguous. This syntax is no longer in the
1141 specification, and has been replaced by the "L</section>" syntax
1142 (where the slash was formerly optional). Pod parsers should
1143 tolerate the "L<"section">" syntax, for a while at least. The
1144 suggested heuristic for distinguishing "L<section>" from "L<name>"
1145 is that if it contains any whitespace, it's a section. Pod
1146 processors should warn about this being deprecated syntax.
1147
1149 "=over"..."=back" regions are used for various kinds of list-like
1150 structures. (I use the term "region" here simply as a collective term
1151 for everything from the "=over" to the matching "=back".)
1152 · The non-zero numeric indentlevel in "=over indentlevel" ...
1154 "=back" is used for giving the formatter a clue as to how many
1155 "spaces" (ems, or roughly equivalent units) it should tab over,
1156 although many formatters will have to convert this to an absolute
1157 measurement that may not exactly match with the size of spaces (or
1158 M's) in the document's base font. Other formatters may have to
1159 completely ignore the number. The lack of any explicit indentlevel
1160 parameter is equivalent to an indentlevel value of 4. Pod
1161 processors may complain if indentlevel is present but is not a
1162 positive number matching "m/\A(\d*\.)?\d+\z/".
1163 · Authors of Pod formatters are reminded that "=over" ... "=back" may
1165 map to several different constructs in your output format. For
1166 example, in converting Pod to (X)HTML, it can map to any of
1167 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1168 <blockquote>...</blockquote>. Similarly, "=item" can map to <li>
1169 or <dt>.
1170 · Each "=over" ... "=back" region should be one of the following:
1172 · An "=over" ... "=back" region containing only "=item *"
1174 commands, each followed by some number of ordinary/verbatim
1175 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1176 paragraphs, and "=begin"..."=end" regions.
1177 (Pod processors must tolerate a bare "=item" as if it were
1179 "=item *".) Whether "*" is rendered as a literal asterisk, an
1180 "o", or as some kind of real bullet character, is left up to
1181 the Pod formatter, and may depend on the level of nesting.
1182 · An "=over" ... "=back" region containing only
1184 "m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each group
1185 of them) followed by some number of ordinary/verbatim
1186 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1187 paragraphs, and/or "=begin"..."=end" codes. Note that the
1188 numbers must start at 1 in each section, and must proceed in
1189 order and without skipping numbers.
1190 (Pod processors must tolerate lines like "=item 1" as if they
1192 were "=item 1.", with the period.)
1193 · An "=over" ... "=back" region containing only "=item [text]"
1195 commands, each one (or each group of them) followed by some
1196 number of ordinary/verbatim paragraphs, other nested "=over"
1197 ... "=back" regions, or "=for..." paragraphs, and
1198 "=begin"..."=end" regions.
1199 The "=item [text]" paragraph should not match
1201 "m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor
1202 should it match just "m/\A=item\s*\z/".
1203 · An "=over" ... "=back" region containing no "=item" paragraphs
1205 at all, and containing only some number of ordinary/verbatim
1206 paragraphs, and possibly also some nested "=over" ... "=back"
1207 regions, "=for..." paragraphs, and "=begin"..."=end" regions.
1208 Such an itemless "=over" ... "=back" region in Pod is
1209 equivalent in meaning to a "<blockquote>...</blockquote>"
1210 element in HTML.
1211 Note that with all the above cases, you can determine which type of
1213 "=over" ... "=back" you have, by examining the first (non-"=cut",
1214 non-"=pod") Pod paragraph after the "=over" command.
1215 · Pod formatters must tolerate arbitrarily large amounts of text in
1217 the "=item text..." paragraph. In practice, most such paragraphs
1218 are short, as in:
1219 =item For cutting off our trade with all parts of the world
1221 But they may be arbitrarily long:
1223 =item For transporting us beyond seas to be tried for pretended
1225 offenses
1226 =item He is at this time transporting large armies of foreign
1228 mercenaries to complete the works of death, desolation and
1229 tyranny, already begun with circumstances of cruelty and perfidy
1230 scarcely paralleled in the most barbarous ages, and totally
1231 unworthy the head of a civilized nation.
1232 · Pod processors should tolerate "=item *" / "=item number" commands
1234 with no accompanying paragraph. The middle item is an example:
1235 =over
1237 =item 1
1239 Pick up dry cleaning.
1241 =item 2
1243 =item 3
1245 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1247 =back
1249 · No "=over" ... "=back" region can contain headings. Processors may
1251 treat such a heading as an error.
1252 · Note that an "=over" ... "=back" region should have some content.
1254 That is, authors should not have an empty region like this:
1255 =over
1257 =back
1259 Pod processors seeing such a contentless "=over" ... "=back"
1261 region, may ignore it, or may report it as an error.
1262 · Processors must tolerate an "=over" list that goes off the end of
1264 the document (i.e., which has no matching "=back"), but they may
1265 warn about such a list.
1266 · Authors of Pod formatters should note that this construct:
1268 =item Neque
1270 =item Porro
1272 =item Quisquam Est
1274 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1276 velit, sed quia non numquam eius modi tempora incidunt ut
1277 labore et dolore magnam aliquam quaerat voluptatem.
1278 =item Ut Enim
1280 is semantically ambiguous, in a way that makes formatting decisions
1282 a bit difficult. On the one hand, it could be mention of an item
1283 "Neque", mention of another item "Porro", and mention of another
1284 item "Quisquam Est", with just the last one requiring the
1285 explanatory paragraph "Qui dolorem ipsum quia dolor..."; and then
1286 an item "Ut Enim". In that case, you'd want to format it like so:
1287 Neque
1289 Porro
1291 Quisquam Est
1293 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1294 velit, sed quia non numquam eius modi tempora incidunt ut
1295 labore et dolore magnam aliquam quaerat voluptatem.
1296 Ut Enim
1298 But it could equally well be a discussion of three (related or
1300 equivalent) items, "Neque", "Porro", and "Quisquam Est", followed
1301 by a paragraph explaining them all, and then a new item "Ut Enim".
1302 In that case, you'd probably want to format it like so:
1303 Neque
1305 Porro
1306 Quisquam Est
1307 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1308 velit, sed quia non numquam eius modi tempora incidunt ut
1309 labore et dolore magnam aliquam quaerat voluptatem.
1310 Ut Enim
1312 But (for the foreseeable future), Pod does not provide any way for
1314 Pod authors to distinguish which grouping is meant by the above
1315 "=item"-cluster structure. So formatters should format it like so:
1316 Neque
1318 Porro
1320 Quisquam Est
1322 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1324 velit, sed quia non numquam eius modi tempora incidunt ut
1325 labore et dolore magnam aliquam quaerat voluptatem.
1326 Ut Enim
1328 That is, there should be (at least roughly) equal spacing between
1330 items as between paragraphs (although that spacing may well be less
1331 than the full height of a line of text). This leaves it to the
1332 reader to use (con)textual cues to figure out whether the "Qui
1333 dolorem ipsum..." paragraph applies to the "Quisquam Est" item or
1334 to all three items "Neque", "Porro", and "Quisquam Est". While not
1335 an ideal situation, this is preferable to providing formatting cues
1336 that may be actually contrary to the author's intent.
1337
1339 Data paragraphs are typically used for inlining non-Pod data that is to
1340 be used (typically passed through) when rendering the document to a
1341 specific format:
1342 =begin rtf
1344 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1346 =end rtf
1348 The exact same effect could, incidentally, be achieved with a single
1350 "=for" paragraph:
1351 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1353 (Although that is not formally a data paragraph, it has the same
1355 meaning as one, and Pod parsers may parse it as one.)
1356 Another example of a data paragraph:
1358 =begin html
1360 I like <em>PIE</em>!
1362 <hr>Especially pecan pie!
1364 =end html
1366 If these were ordinary paragraphs, the Pod parser would try to expand
1368 the "E</em>" (in the first paragraph) as a formatting code, just like
1369 "E<lt>" or "E<eacute>". But since this is in a "=begin
1370 identifier"..."=end identifier" region and the identifier "html"
1371 doesn't begin have a ":" prefix, the contents of this region are stored
1372 as data paragraphs, instead of being processed as ordinary paragraphs
1373 (or if they began with a spaces and/or tabs, as verbatim paragraphs).
1374 As a further example: At time of writing, no "biblio" identifier is
1376 supported, but suppose some processor were written to recognize it as a
1377 way of (say) denoting a bibliographic reference (necessarily containing
1378 formatting codes in ordinary paragraphs). The fact that "biblio"
1379 paragraphs were meant for ordinary processing would be indicated by
1380 prefacing each "biblio" identifier with a colon:
1381 =begin :biblio
1383 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1385 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1386 =end :biblio
1388 This would signal to the parser that paragraphs in this begin...end
1390 region are subject to normal handling as ordinary/verbatim paragraphs
1391 (while still tagged as meant only for processors that understand the
1392 "biblio" identifier). The same effect could be had with:
1393 =for :biblio
1395 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1396 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1397 The ":" on these identifiers means simply "process this stuff normally,
1399 even though the result will be for some special target". I suggest
1400 that parser APIs report "biblio" as the target identifier, but also
1401 report that it had a ":" prefix. (And similarly, with the above
1402 "html", report "html" as the target identifier, and note the lack of a
1403 ":" prefix.)
1404 Note that a "=begin identifier"..."=end identifier" region where
1406 identifier begins with a colon, can contain commands. For example:
1407 =begin :biblio
1409 Wirth's classic is available in several editions, including:
1411 =for comment
1413 hm, check abebooks.com for how much used copies cost.
1414 =over
1416 =item
1418 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1420 Teubner, Stuttgart. [Yes, it's in German.]
1421 =item
1423 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1425 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1426 =back
1428 =end :biblio
1430 Note, however, a "=begin identifier"..."=end identifier" region where
1432 identifier does not begin with a colon, should not directly contain
1433 "=head1" ... "=head4" commands, nor "=over", nor "=back", nor "=item".
1434 For example, this may be considered invalid:
1435 =begin somedata
1437 This is a data paragraph.
1439 =head1 Don't do this!
1441 This is a data paragraph too.
1443 =end somedata
1445 A Pod processor may signal that the above (specifically the "=head1"
1447 paragraph) is an error. Note, however, that the following should not
1448 be treated as an error:
1449 =begin somedata
1451 This is a data paragraph.
1453 =cut
1455 # Yup, this isn't Pod anymore.
1457 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1458 =pod
1460 This is a data paragraph too.
1462 =end somedata
1464 And this too is valid:
1466 =begin someformat
1468 This is a data paragraph.
1470 And this is a data paragraph.
1472 =begin someotherformat
1474 This is a data paragraph too.
1476 And this is a data paragraph too.
1478 =begin :yetanotherformat
1480 =head2 This is a command paragraph!
1482 This is an ordinary paragraph!
1484 And this is a verbatim paragraph!
1486 =end :yetanotherformat
1488 =end someotherformat
1490 Another data paragraph!
1492 =end someformat
1494 The contents of the above "=begin :yetanotherformat" ... "=end
1496 :yetanotherformat" region aren't data paragraphs, because the
1497 immediately containing region's identifier (":yetanotherformat") begins
1498 with a colon. In practice, most regions that contain data paragraphs
1499 will contain only data paragraphs; however, the above nesting is
1500 syntactically valid as Pod, even if it is rare. However, the handlers
1501 for some formats, like "html", will accept only data paragraphs, not
1502 nested regions; and they may complain if they see (targeted for them)
1503 nested regions, or commands, other than "=end", "=pod", and "=cut".
1504 Also consider this valid structure:
1506 =begin :biblio
1508 Wirth's classic is available in several editions, including:
1510 =over
1512 =item
1514 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1516 Teubner, Stuttgart. [Yes, it's in German.]
1517 =item
1519 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1521 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1522 =back
1524 Buy buy buy!
1526 =begin html
1528 <img src='wirth_spokesmodeling_book.png'>
1530 <hr>
1532 =end html
1534 Now now now!
1536 =end :biblio
1538 There, the "=begin html"..."=end html" region is nested inside the
1540 larger "=begin :biblio"..."=end :biblio" region. Note that the content
1541 of the "=begin html"..."=end html" region is data paragraph(s), because
1542 the immediately containing region's identifier ("html") doesn't begin
1543 with a colon.
1544 Pod parsers, when processing a series of data paragraphs one after
1546 another (within a single region), should consider them to be one large
1547 data paragraph that happens to contain blank lines. So the content of
1548 the above "=begin html"..."=end html" may be stored as two data
1549 paragraphs (one consisting of "<img
1550 src='wirth_spokesmodeling_book.png'>\n" and another consisting of
1551 "<hr>\n"), but should be stored as a single data paragraph (consisting
1552 of "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1553 Pod processors should tolerate empty "=begin something"..."=end
1555 something" regions, empty "=begin :something"..."=end :something"
1556 regions, and contentless "=for something" and "=for :something"
1557 paragraphs. I.e., these should be tolerated:
1558 =for html
1560 =begin html
1562 =end html
1564 =begin :biblio
1566 =end :biblio
1568 Incidentally, note that there's no easy way to express a data paragraph
1570 starting with something that looks like a command. Consider:
1571 =begin stuff
1573 =shazbot
1575 =end stuff
1577 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a
1579 data paragraph "=shazbot\n". However, you can express a data paragraph
1580 consisting of "=shazbot\n" using this code:
1581 =for stuff =shazbot
1583 The situation where this is necessary, is presumably quite rare.
1585 Note that =end commands must match the currently open =begin command.
1587 That is, they must properly nest. For example, this is valid:
1588 =begin outer
1590 X
1592 =begin inner
1594 Y
1596 =end inner
1598 Z
1600 =end outer
1602 while this is invalid:
1604 =begin outer
1606 X
1608 =begin inner
1610 Y
1612 =end outer
1614 Z
1616 =end inner
1618 This latter is improper because when the "=end outer" command is seen,
1620 the currently open region has the formatname "inner", not "outer". (It
1621 just happens that "outer" is the format name of a higher-up region.)
1622 This is an error. Processors must by default report this as an error,
1623 and may halt processing the document containing that error. A
1624 corollary of this is that regions cannot "overlap". That is, the latter
1625 block above does not represent a region called "outer" which contains X
1626 and Y, overlapping a region called "inner" which contains Y and Z. But
1627 because it is invalid (as all apparently overlapping regions would be),
1628 it doesn't represent that, or anything at all.
1629 Similarly, this is invalid:
1631 =begin thing
1633 =end hting
1635 This is an error because the region is opened by "thing", and the
1637 "=end" tries to close "hting" [sic].
1638 This is also invalid:
1640 =begin thing
1642 =end
1644 This is invalid because every "=end" command must have a formatname
1646 parameter.
1647
1649 perlpod, "PODs: Embedded Documentation" in perlsyn, podchecker
1650
1652 Sean M. Burke
1653perl v5.30.1 2019-11-29 PERLPODSPEC(1)