1PERLPODSPEC(1) Perl Programmers Reference Guide PERLPODSPEC(1)
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3
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6 perlpodspec - Plain Old Documentation: format specification and notes
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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
14 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
23 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
34 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
37 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
44 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
49 (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
54 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
60 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
68 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
73 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
76 For purposes of Pod processing, there are four types of paragraphs in a
77 Pod block:
78
79 · 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
83 =head1 NOTES
84
85 =item *
86
87 But they may span several (non-blank) lines:
88
89 =for comment
90 Hm, I wonder what it would look like if
91 you tried to write a BNF for Pod from this.
92
93 =head3 Dr. Strangelove, or: How I Learned to
94 Stop Worrying and Love the Bomb
95
96 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
99 =head1 Did You Remember to C<use strict;>?
100
101 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
108 · 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
116 Whitespace is significant in verbatim paragraphs (although, in
117 processing, tabs are probably expanded).
118
119 · 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
124 · 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
133 For example: consider the following paragraphs:
134
135 # <- that's the 0th column
136
137 =head1 Foo
138
139 Stuff
140
141 $foo->bar
142
143 =cut
144
145 Here, "=head1 Foo" and "=cut" are command paragraphs because the first
146 line of each matches "m/\A=[a-zA-Z]/". "[space][space]$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
150 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
161 "=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
166 =head1 Object Attributes
167
168 =head3 What B<Not> to Do!
169
170 "=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
176 =pod
177
178 This is a plain Pod paragraph.
179
180 =pod This text is ignored.
181
182 "=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
187 =cut
188
189 =cut The documentation ends here.
190
191 =cut
192 # This is the first line of program text.
193 sub foo { # This is the second.
194
195 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
199 "=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
206 =over 3
207
208 =over 3.5
209
210 =over
211
212 "=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
218 =item
219
220 =item *
221
222 =item *
223
224 =item 14
225
226 =item 3.
227
228 =item C<< $thing->stuff(I<dodad>) >>
229
230 =item For transporting us beyond seas to be tried for pretended
231 offenses
232
233 =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
239 "=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
244 "=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
254 It is advised that formatnames match the regexp
255 "m/\A:?[aXXaaXXzAaXXZ0aXX9_]+\z/". Everything following whitespace
256 after the formatname is a parameter that may be used by the
257 formatter when dealing with this region. This parameter must not
258 be repeated in the "=end" paragraph. Implementors should
259 anticipate future expansion in the semantics and syntax of the
260 first parameter to "=begin"/"=end"/"=for".
261
262 "=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
269 "=for formatname text..."
270 This is synonymous with:
271
272 =begin formatname
273
274 text...
275
276 =end formatname
277
278 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
285 "=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
294 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
306 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
315 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
323 There are two syntaxes for formatting codes:
324
325 · 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
329 That's what I<you> think!
330
331 What's C<dump()> for?
332
333 X<C<chmod> and C<unlink()> Under Different Operating Systems>
334
335 · 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
342 That's what I<< you >> think!
343
344 C<<< open(X, ">>thing.dat") || die $! >>>
345
346 B<< $foo->bar(); >>
347
348 With this syntax, the whitespace character(s) after the "C<<<" and
349 before the ">>" (or whatever letter) are not renderable. They do
350 not signify whitespace, are merely part of the formatting codes
351 themselves. That is, these are all synonymous:
352
353 C<thing>
354 C<< thing >>
355 C<< thing >>
356 C<<< thing >>>
357 C<<<<
358 thing
359 >>>>
360
361 and so on.
362
363 Finally, the multiple-angle-bracket form does not alter the
364 interpretation of nested formatting codes, meaning that the
365 following four example lines are identical in meaning:
366
367 B<example: C<$a E<lt>=E<gt> $b>>
368
369 B<example: C<< $a <=> $b >>>
370
371 B<example: C<< $a E<lt>=E<gt> $b >>>
372
373 B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
374
375 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
380 "I<text>" -- italic text
381 See the brief discussion in "Formatting Codes" in perlpod.
382
383 "B<text>" -- bold text
384 See the brief discussion in "Formatting Codes" in perlpod.
385
386 "C<code>" -- code text
387 See the brief discussion in "Formatting Codes" in perlpod.
388
389 "F<filename>" -- style for filenames
390 See the brief discussion in "Formatting Codes" in perlpod.
391
392 "X<topic name>" -- an index entry
393 See the brief discussion in "Formatting Codes" in perlpod.
394
395 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
400 "Z<>" -- a null (zero-effect) formatting code
401 Discussed briefly in "Formatting Codes" in perlpod.
402
403 This code is unusual is 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
407 "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
416 "E<escape>" -- a character escape
417 See "Formatting Codes" in perlpod, and several points in "Notes on
418 Implementing Pod Processors".
419
420 "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
425 Consider:
426
427 C<$x ? $y : $z>
428
429 S<C<$x ? $y : $z>>
430
431 Both signify the monospace (c[ode] style) text consisting of "$x",
432 one space, "?", one space, ":", one space, "$z". The difference is
433 that in the latter, with the S code, those spaces are not "normal"
434 spaces, but instead are non-breaking spaces.
435
436 If a Pod processor sees any formatting code other than the ones listed
437 above (as in "N<...>", or "Q<...>", etc.), that processor must by
438 default treat this as an error. A Pod parser may allow a way for
439 particular applications to add to the above list of known formatting
440 codes; a Pod parser might even allow a way to stipulate, for each
441 additional command, whether it requires some form of special
442 processing, as L<...> does.
443
444 Future versions of this specification may add additional formatting
445 codes.
446
447 Historical note: A few older Pod processors would not see a ">" as
448 closing a "C<" code, if the ">" was immediately preceded by a "-".
449 This was so that this:
450
451 C<$foo->bar>
452
453 would parse as equivalent to this:
454
455 C<$foo-E<gt>bar>
456
457 instead of as equivalent to a "C" formatting code containing only
458 "$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
461 C<< $foo->bar >>
462
463 Compliant parsers must not treat "->" as special.
464
465 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
471 I<I told you not to do this!
472
473 Don't make me say it again!>
474
475 ...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
480 I<I told you not to do this!>
481
482 Don't make me say it again!E<gt>
483
484 (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
491 · 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
498 · Pod parsers must recognize all of the three well-known newline
499 formats: CR, LF, and CRLF. See perlport.
500
501 · Pod parsers should accept input lines that are of any length.
502
503 · 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 Latin-1.
509
510 Future versions of this specification may specify how Pod can
511 accept other encodings. Presumably treatment of other encodings in
512 Pod parsing would be as in XML parsing: whatever the encoding
513 declared by a particular Pod file, content is to be stored in
514 memory as Unicode characters.
515
516 · The well known Unicode Byte Order Marks are as follows: if the
517 file begins with the two literal byte values 0xFE 0xFF, this is the
518 BOM for big-endian UTF-16. If the file begins with the two literal
519 byte value 0xFF 0xFE, this is the BOM for little-endian UTF-16. If
520 the file begins with the three literal byte values 0xEF 0xBB 0xBF,
521 this is the BOM for UTF-8.
522
523 · A naive but sufficient heuristic for testing the first highbit
524 byte-sequence in a BOM-less file (whether in code or in Pod!), to
525 see whether that sequence is valid as UTF-8 (RFC 2279) is to check
526 whether that the first byte in the sequence is in the range 0xC0 -
527 0xFD and whether the next byte is in the range 0x80 - 0xBF. If so,
528 the parser may conclude that this file is in UTF-8, and all highbit
529 sequences in the file should be assumed to be UTF-8. Otherwise the
530 parser should treat the file as being in Latin-1. In the unlikely
531 circumstance that the first highbit sequence in a truly non-UTF-8
532 file happens to appear to be UTF-8, one can cater to our heuristic
533 (as well as any more intelligent heuristic) by prefacing that line
534 with a comment line containing a highbit sequence that is clearly
535 not valid as UTF-8. A line consisting of simply "#", an e-acute,
536 and any non-highbit byte, is sufficient to establish this file's
537 encoding.
538
539 · This document's requirements and suggestions about encodings do not
540 apply to Pod processors running on non-ASCII platforms, notably
541 EBCDIC platforms.
542
543 · Pod processors must treat a "=for [label] [content...]" paragraph
544 as meaning the same thing as a "=begin [label]" paragraph, content,
545 and an "=end [label]" paragraph. (The parser may conflate these
546 two constructs, or may leave them distinct, in the expectation that
547 the formatter will nevertheless treat them the same.)
548
549 · When rendering Pod to a format that allows comments (i.e., to
550 nearly any format other than plaintext), a Pod formatter must
551 insert comment text identifying its name and version number, and
552 the name and version numbers of any modules it might be using to
553 process the Pod. Minimal examples:
554
555 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
556
557 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
558
559 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
560
561 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
562
563 Formatters may also insert additional comments, including: the
564 release date of the Pod formatter program, the contact address for
565 the author(s) of the formatter, the current time, the name of input
566 file, the formatting options in effect, version of Perl used, etc.
567
568 Formatters may also choose to note errors/warnings as comments,
569 besides or instead of emitting them otherwise (as in messages to
570 STDERR, or "die"ing).
571
572 · Pod parsers may emit warnings or error messages ("Unknown E code
573 E<zslig>!") to STDERR (whether through printing to STDERR, or
574 "warn"ing/"carp"ing, or "die"ing/"croak"ing), but must allow
575 suppressing all such STDERR output, and instead allow an option for
576 reporting errors/warnings in some other way, whether by triggering
577 a callback, or noting errors in some attribute of the document
578 object, or some similarly unobtrusive mechanism -- or even by
579 appending a "Pod Errors" section to the end of the parsed form of
580 the document.
581
582 · In cases of exceptionally aberrant documents, Pod parsers may abort
583 the parse. Even then, using "die"ing/"croak"ing is to be avoided;
584 where possible, the parser library may simply close the input file
585 and add text like "*** Formatting Aborted ***" to the end of the
586 (partial) in-memory document.
587
588 · In paragraphs where formatting codes (like E<...>, B<...>) are
589 understood (i.e., not verbatim paragraphs, but including ordinary
590 paragraphs, and command paragraphs that produce renderable text,
591 like "=head1"), literal whitespace should generally be considered
592 "insignificant", in that one literal space has the same meaning as
593 any (nonzero) number of literal spaces, literal newlines, and
594 literal tabs (as long as this produces no blank lines, since those
595 would terminate the paragraph). Pod parsers should compact literal
596 whitespace in each processed paragraph, but may provide an option
597 for overriding this (since some processing tasks do not require
598 it), or may follow additional special rules (for example, specially
599 treating period-space-space or period-newline sequences).
600
601 · Pod parsers should not, by default, try to coerce apostrophe (')
602 and quote (") into smart quotes (little 9's, 66's, 99's, etc), nor
603 try to turn backtick (`) into anything else but a single backtick
604 character (distinct from an open quote character!), nor "--" into
605 anything but two minus signs. They must never do any of those
606 things to text in C<...> formatting codes, and never ever to text
607 in verbatim paragraphs.
608
609 · When rendering Pod to a format that has two kinds of hyphens (-),
610 one that's a non-breaking hyphen, and another that's a breakable
611 hyphen (as in "object-oriented", which can be split across lines as
612 "object-", newline, "oriented"), formatters are encouraged to
613 generally translate "-" to non-breaking hyphen, but may apply
614 heuristics to convert some of these to breaking hyphens.
615
616 · Pod formatters should make reasonable efforts to keep words of Perl
617 code from being broken across lines. For example, "Foo::Bar" in
618 some formatting systems is seen as eligible for being broken across
619 lines as "Foo::" newline "Bar" or even "Foo::-" newline "Bar".
620 This should be avoided where possible, either by disabling all
621 line-breaking in mid-word, or by wrapping particular words with
622 internal punctuation in "don't break this across lines" codes
623 (which in some formats may not be a single code, but might be a
624 matter of inserting non-breaking zero-width spaces between every
625 pair of characters in a word.)
626
627 · Pod parsers should, by default, expand tabs in verbatim paragraphs
628 as they are processed, before passing them to the formatter or
629 other processor. Parsers may also allow an option for overriding
630 this.
631
632 · Pod parsers should, by default, remove newlines from the end of
633 ordinary and verbatim paragraphs before passing them to the
634 formatter. For example, while the paragraph you're reading now
635 could be considered, in Pod source, to end with (and contain) the
636 newline(s) that end it, it should be processed as ending with (and
637 containing) the period character that ends this sentence.
638
639 · Pod parsers, when reporting errors, should make some effort to
640 report an approximate line number ("Nested E<>'s in Paragraph #52,
641 near line 633 of Thing/Foo.pm!"), instead of merely noting the
642 paragraph number ("Nested E<>'s in Paragraph #52 of
643 Thing/Foo.pm!"). Where this is problematic, the paragraph number
644 should at least be accompanied by an excerpt from the paragraph
645 ("Nested E<>'s in Paragraph #52 of Thing/Foo.pm, which begins
646 'Read/write accessor for the C<interest rate> attribute...'").
647
648 · Pod parsers, when processing a series of verbatim paragraphs one
649 after another, should consider them to be one large verbatim
650 paragraph that happens to contain blank lines. I.e., these two
651 lines, which have a blank line between them:
652
653 use Foo;
654
655 print Foo->VERSION
656
657 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
658 Foo->VERSION") before being passed to the formatter or other
659 processor. Parsers may also allow an option for overriding this.
660
661 While this might be too cumbersome to implement in event-based Pod
662 parsers, it is straightforward for parsers that return parse trees.
663
664 · Pod formatters, where feasible, are advised to avoid splitting
665 short verbatim paragraphs (under twelve lines, say) across pages.
666
667 · Pod parsers must treat a line with only spaces and/or tabs on it as
668 a "blank line" such as separates paragraphs. (Some older parsers
669 recognized only two adjacent newlines as a "blank line" but would
670 not recognize a newline, a space, and a newline, as a blank line.
671 This is noncompliant behavior.)
672
673 · Authors of Pod formatters/processors should make every effort to
674 avoid writing their own Pod parser. There are already several in
675 CPAN, with a wide range of interface styles -- and one of them,
676 Pod::Parser, comes with modern versions of Perl.
677
678 · Characters in Pod documents may be conveyed either as literals, or
679 by number in E<n> codes, or by an equivalent mnemonic, as in
680 E<eacute> which is exactly equivalent to E<233>.
681
682 Characters in the range 32-126 refer to those well known US-ASCII
683 characters (also defined there by Unicode, with the same meaning),
684 which all Pod formatters must render faithfully. Characters in the
685 ranges 0-31 and 127-159 should not be used (neither as literals,
686 nor as E<number> codes), except for the literal byte-sequences for
687 newline (13, 13 10, or 10), and tab (9).
688
689 Characters in the range 160-255 refer to Latin-1 characters (also
690 defined there by Unicode, with the same meaning). Characters above
691 255 should be understood to refer to Unicode characters.
692
693 · Be warned that some formatters cannot reliably render characters
694 outside 32-126; and many are able to handle 32-126 and 160-255, but
695 nothing above 255.
696
697 · Besides the well-known "E<lt>" and "E<gt>" codes for less-than and
698 greater-than, Pod parsers must understand "E<sol>" for "/"
699 (solidus, slash), and "E<verbar>" for "|" (vertical bar, pipe).
700 Pod parsers should also understand "E<lchevron>" and "E<rchevron>"
701 as legacy codes for characters 171 and 187, i.e., "left-pointing
702 double angle quotation mark" = "left pointing guillemet" and
703 "right-pointing double angle quotation mark" = "right pointing
704 guillemet". (These look like little "<<" and ">>", and they are
705 now preferably expressed with the HTML/XHTML codes "E<laquo>" and
706 "E<raquo>".)
707
708 · Pod parsers should understand all "E<html>" codes as defined in the
709 entity declarations in the most recent XHTML specification at
710 "www.W3.org". Pod parsers must understand at least the entities
711 that define characters in the range 160-255 (Latin-1). Pod
712 parsers, when faced with some unknown "E<identifier>" code,
713 shouldn't simply replace it with nullstring (by default, at least),
714 but may pass it through as a string consisting of the literal
715 characters E, less-than, identifier, greater-than. Or Pod parsers
716 may offer the alternative option of processing such unknown
717 "E<identifier>" codes by firing an event especially for such codes,
718 or by adding a special node-type to the in-memory document tree.
719 Such "E<identifier>" may have special meaning to some processors,
720 or some processors may choose to add them to a special error
721 report.
722
723 · Pod parsers must also support the XHTML codes "E<quot>" for
724 character 34 (doublequote, "), "E<amp>" for character 38
725 (ampersand, &), and "E<apos>" for character 39 (apostrophe, ').
726
727 · Note that in all cases of "E<whatever>", whatever (whether an
728 htmlname, or a number in any base) must consist only of
729 alphanumeric characters -- that is, whatever must watch
730 "m/\A\w+\z/". So "E< 0 1 2 3 >" is invalid, because it contains
731 spaces, which aren't alphanumeric characters. This presumably does
732 not need special treatment by a Pod processor; " 0 1 2 3 " doesn't
733 look like a number in any base, so it would presumably be looked up
734 in the table of HTML-like names. Since there isn't (and cannot be)
735 an HTML-like entity called " 0 1 2 3 ", this will be treated as an
736 error. However, Pod processors may treat "E< 0 1 2 3 >" or
737 "E<e-acute>" as syntactically invalid, potentially earning a
738 different error message than the error message (or warning, or
739 event) generated by a merely unknown (but theoretically valid)
740 htmlname, as in "E<qacute>" [sic]. However, Pod parsers are not
741 required to make this distinction.
742
743 · Note that E<number> must not be interpreted as simply "codepoint
744 number in the current/native character set". It always means only
745 "the character represented by codepoint number in Unicode." (This
746 is identical to the semantics of &#number; in XML.)
747
748 This will likely require many formatters to have tables mapping
749 from treatable Unicode codepoints (such as the "\xE9" for the
750 e-acute character) to the escape sequences or codes necessary for
751 conveying such sequences in the target output format. A converter
752 to *roff would, for example know that "\xE9" (whether conveyed
753 literally, or via a E<...> sequence) is to be conveyed as "e\\*'".
754 Similarly, a program rendering Pod in a Mac OS application window,
755 would presumably need to know that "\xE9" maps to codepoint 142 in
756 MacRoman encoding that (at time of writing) is native for Mac OS.
757 Such Unicode2whatever mappings are presumably already widely
758 available for common output formats. (Such mappings may be
759 incomplete! Implementers are not expected to bend over backwards
760 in an attempt to render Cherokee syllabics, Etruscan runes,
761 Byzantine musical symbols, or any of the other weird things that
762 Unicode can encode.) And if a Pod document uses a character not
763 found in such a mapping, the formatter should consider it an
764 unrenderable character.
765
766 · If, surprisingly, the implementor of a Pod formatter can't find a
767 satisfactory pre-existing table mapping from Unicode characters to
768 escapes in the target format (e.g., a decent table of Unicode
769 characters to *roff escapes), it will be necessary to build such a
770 table. If you are in this circumstance, you should begin with the
771 characters in the range 0x00A0 - 0x00FF, which is mostly the
772 heavily used accented characters. Then proceed (as patience
773 permits and fastidiousness compels) through the characters that the
774 (X)HTML standards groups judged important enough to merit mnemonics
775 for. These are declared in the (X)HTML specifications at the
776 www.W3.org site. At time of writing (September 2001), the most
777 recent entity declaration files are:
778
779 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
780 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
781 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
782
783 Then you can progress through any remaining notable Unicode
784 characters in the range 0x2000-0x204D (consult the character tables
785 at www.unicode.org), and whatever else strikes your fancy. For
786 example, in xhtml-symbol.ent, there is the entry:
787
788 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
789
790 While the mapping "infin" to the character "\x{221E}" will
791 (hopefully) have been already handled by the Pod parser, the
792 presence of the character in this file means that it's reasonably
793 important enough to include in a formatter's table that maps from
794 notable Unicode characters to the codes necessary for rendering
795 them. So for a Unicode-to-*roff mapping, for example, this would
796 merit the entry:
797
798 "\x{221E}" => '\(in',
799
800 It is eagerly hoped that in the future, increasing numbers of
801 formats (and formatters) will support Unicode characters directly
802 (as (X)HTML does with "∞", "∞", or "∞"),
803 reducing the need for idiosyncratic mappings of
804 Unicode-to-my_escapes.
805
806 · It is up to individual Pod formatter to display good judgement when
807 confronted with an unrenderable character (which is distinct from
808 an unknown E<thing> sequence that the parser couldn't resolve to
809 anything, renderable or not). It is good practice to map Latin
810 letters with diacritics (like "E<eacute>"/"E<233>") to the
811 corresponding unaccented US-ASCII letters (like a simple character
812 101, "e"), but clearly this is often not feasible, and an
813 unrenderable character may be represented as "?", or the like. In
814 attempting a sane fallback (as from E<233> to "e"), Pod formatters
815 may use the %Latin1Code_to_fallback table in Pod::Escapes, or
816 Text::Unidecode, if available.
817
818 For example, this Pod text:
819
820 magic is enabled if you set C<$Currency> to 'E<euro>'.
821
822 may be rendered as: "magic is enabled if you set $Currency to '?'"
823 or as "magic is enabled if you set $Currency to '[euro]'", or as
824 "magic is enabled if you set $Currency to '[x20AC]', etc.
825
826 A Pod formatter may also note, in a comment or warning, a list of
827 what unrenderable characters were encountered.
828
829 · E<...> may freely appear in any formatting code (other than in
830 another E<...> or in an Z<>). That is, "X<The E<euro>1,000,000
831 Solution>" is valid, as is "L<The E<euro>1,000,000
832 Solution|Million::Euros>".
833
834 · Some Pod formatters output to formats that implement non-breaking
835 spaces as an individual character (which I'll call "NBSP"), and
836 others output to formats that implement non-breaking spaces just as
837 spaces wrapped in a "don't break this across lines" code. Note
838 that at the level of Pod, both sorts of codes can occur: Pod can
839 contain a NBSP character (whether as a literal, or as a "E<160>" or
840 "E<nbsp>" code); and Pod can contain "S<foo I<bar> baz>" codes,
841 where "mere spaces" (character 32) in such codes are taken to
842 represent non-breaking spaces. Pod parsers should consider
843 supporting the optional parsing of "S<foo I<bar> baz>" as if it
844 were "fooNBSPI<bar>NBSPbaz", and, going the other way, the optional
845 parsing of groups of words joined by NBSP's as if each group were
846 in a S<...> code, so that formatters may use the representation
847 that maps best to what the output format demands.
848
849 · Some processors may find that the "S<...>" code is easiest to
850 implement by replacing each space in the parse tree under the
851 content of the S, with an NBSP. But note: the replacement should
852 apply not to spaces in all text, but only to spaces in printable
853 text. (This distinction may or may not be evident in the
854 particular tree/event model implemented by the Pod parser.) For
855 example, consider this unusual case:
856
857 S<L</Autoloaded Functions>>
858
859 This means that the space in the middle of the visible link text
860 must not be broken across lines. In other words, it's the same as
861 this:
862
863 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
864
865 However, a misapplied space-to-NBSP replacement could (wrongly)
866 produce something equivalent to this:
867
868 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
869
870 ...which is almost definitely not going to work as a hyperlink
871 (assuming this formatter outputs a format supporting hypertext).
872
873 Formatters may choose to just not support the S format code,
874 especially in cases where the output format simply has no NBSP
875 character/code and no code for "don't break this stuff across
876 lines".
877
878 · Besides the NBSP character discussed above, implementors are
879 reminded of the existence of the other "special" character in
880 Latin-1, the "soft hyphen" character, also known as "discretionary
881 hyphen", i.e. "E<173>" = "E<0xAD>" = "E<shy>"). This character
882 expresses an optional hyphenation point. That is, it normally
883 renders as nothing, but may render as a "-" if a formatter breaks
884 the word at that point. Pod formatters should, as appropriate, do
885 one of the following: 1) render this with a code with the same
886 meaning (e.g., "\-" in RTF), 2) pass it through in the expectation
887 that the formatter understands this character as such, or 3) delete
888 it.
889
890 For example:
891
892 sigE<shy>action
893 manuE<shy>script
894 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
895
896 These signal to a formatter that if it is to hyphenate "sigaction"
897 or "manuscript", then it should be done as "sig-[linebreak]action"
898 or "manu-[linebreak]script" (and if it doesn't hyphenate it, then
899 the "E<shy>" doesn't show up at all). And if it is to hyphenate
900 "Jarkko" and/or "Hietaniemi", it can do so only at the points where
901 there is a "E<shy>" code.
902
903 In practice, it is anticipated that this character will not be used
904 often, but formatters should either support it, or delete it.
905
906 · If you think that you want to add a new command to Pod (like, say,
907 a "=biblio" command), consider whether you could get the same
908 effect with a for or begin/end sequence: "=for biblio ..." or
909 "=begin biblio" ... "=end biblio". Pod processors that don't
910 understand "=for biblio", etc, will simply ignore it, whereas they
911 may complain loudly if they see "=biblio".
912
913 · Throughout this document, "Pod" has been the preferred spelling for
914 the name of the documentation format. One may also use "POD" or
915 "pod". For the documentation that is (typically) in the Pod
916 format, you may use "pod", or "Pod", or "POD". Understanding these
917 distinctions is useful; but obsessing over how to spell them,
918 usually is not.
919
921 As you can tell from a glance at perlpod, the L<...> code is the most
922 complex of the Pod formatting codes. The points below will hopefully
923 clarify what it means and how processors should deal with it.
924
925 · In parsing an L<...> code, Pod parsers must distinguish at least
926 four attributes:
927
928 First:
929 The link-text. If there is none, this must be undef. (E.g.,
930 in "L<Perl Functions|perlfunc>", the link-text is "Perl
931 Functions". In "L<Time::HiRes>" and even "L<|Time::HiRes>",
932 there is no link text. Note that link text may contain
933 formatting.)
934
935 Second:
936 The possibly inferred link-text; i.e., if there was no real
937 link text, then this is the text that we'll infer in its place.
938 (E.g., for "L<Getopt::Std>", the inferred link text is
939 "Getopt::Std".)
940
941 Third:
942 The name or URL, or undef if none. (E.g., in "L<Perl
943 Functions|perlfunc>", the name (also sometimes called the page)
944 is "perlfunc". In "L</CAVEATS>", the name is undef.)
945
946 Fourth:
947 The section (AKA "item" in older perlpods), or undef if none.
948 E.g., in "L<Getopt::Std/DESCRIPTION>", "DESCRIPTION" is the
949 section. (Note that this is not the same as a manpage section
950 like the "5" in "man 5 crontab". "Section Foo" in the Pod
951 sense means the part of the text that's introduced by the
952 heading or item whose text is "Foo".)
953
954 Pod parsers may also note additional attributes including:
955
956 Fifth:
957 A flag for whether item 3 (if present) is a URL (like
958 "http://lists.perl.org" is), in which case there should be no
959 section attribute; a Pod name (like "perldoc" and "Getopt::Std"
960 are); or possibly a man page name (like "crontab(5)" is).
961
962 Sixth:
963 The raw original L<...> content, before text is split on "|",
964 "/", etc, and before E<...> codes are expanded.
965
966 (The above were numbered only for concise reference below. It is
967 not a requirement that these be passed as an actual list or array.)
968
969 For example:
970
971 L<Foo::Bar>
972 => undef, # link text
973 "Foo::Bar", # possibly inferred link text
974 "Foo::Bar", # name
975 undef, # section
976 'pod', # what sort of link
977 "Foo::Bar" # original content
978
979 L<Perlport's section on NL's|perlport/Newlines>
980 => "Perlport's section on NL's", # link text
981 "Perlport's section on NL's", # possibly inferred link text
982 "perlport", # name
983 "Newlines", # section
984 'pod', # what sort of link
985 "Perlport's section on NL's|perlport/Newlines" # orig. content
986
987 L<perlport/Newlines>
988 => undef, # link text
989 '"Newlines" in perlport', # possibly inferred link text
990 "perlport", # name
991 "Newlines", # section
992 'pod', # what sort of link
993 "perlport/Newlines" # original content
994
995 L<crontab(5)/"DESCRIPTION">
996 => undef, # link text
997 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
998 "crontab(5)", # name
999 "DESCRIPTION", # section
1000 'man', # what sort of link
1001 'crontab(5)/"DESCRIPTION"' # original content
1002
1003 L</Object Attributes>
1004 => undef, # link text
1005 '"Object Attributes"', # possibly inferred link text
1006 undef, # name
1007 "Object Attributes", # section
1008 'pod', # what sort of link
1009 "/Object Attributes" # original content
1010
1011 L<http://www.perl.org/>
1012 => undef, # link text
1013 "http://www.perl.org/", # possibly inferred link text
1014 "http://www.perl.org/", # name
1015 undef, # section
1016 'url', # what sort of link
1017 "http://www.perl.org/" # original content
1018
1019 L<Perl.org|http://www.perl.org/>
1020 => "Perl.org", # link text
1021 "http://www.perl.org/", # possibly inferred link text
1022 "http://www.perl.org/", # name
1023 undef, # section
1024 'url', # what sort of link
1025 "Perl.org|http://www.perl.org/" # original content
1026
1027 Note that you can distinguish URL-links from anything else by the
1028 fact that they match "m/\A\w+:[^:\s]\S*\z/". So
1029 "L<http://www.perl.com>" is a URL, but "L<HTTP::Response>" isn't.
1030
1031 · In case of L<...> codes with no "text|" part in them, older
1032 formatters have exhibited great variation in actually displaying
1033 the link or cross reference. For example, L<crontab(5)> would
1034 render as "the crontab(5) manpage", or "in the crontab(5) manpage"
1035 or just "crontab(5)".
1036
1037 Pod processors must now treat "text|"-less links as follows:
1038
1039 L<name> => L<name|name>
1040 L</section> => L<"section"|/section>
1041 L<name/section> => L<"section" in name|name/section>
1042
1043 · Note that section names might contain markup. I.e., if a section
1044 starts with:
1045
1046 =head2 About the C<-M> Operator
1047
1048 or with:
1049
1050 =item About the C<-M> Operator
1051
1052 then a link to it would look like this:
1053
1054 L<somedoc/About the C<-M> Operator>
1055
1056 Formatters may choose to ignore the markup for purposes of
1057 resolving the link and use only the renderable characters in the
1058 section name, as in:
1059
1060 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1061 Operator</h1>
1062
1063 ...
1064
1065 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1066 Operator" in somedoc</a>
1067
1068 · Previous versions of perlpod distinguished "L<name/"section">"
1069 links from "L<name/item>" links (and their targets). These have
1070 been merged syntactically and semantically in the current
1071 specification, and section can refer either to a "=headn Heading
1072 Content" command or to a "=item Item Content" command. This
1073 specification does not specify what behavior should be in the case
1074 of a given document having several things all seeming to produce
1075 the same section identifier (e.g., in HTML, several things all
1076 producing the same anchorname in <a name="anchorname">...</a>
1077 elements). Where Pod processors can control this behavior, they
1078 should use the first such anchor. That is, "L<Foo/Bar>" refers to
1079 the first "Bar" section in Foo.
1080
1081 But for some processors/formats this cannot be easily controlled;
1082 as with the HTML example, the behavior of multiple ambiguous <a
1083 name="anchorname">...</a> is most easily just left up to browsers
1084 to decide.
1085
1086 · Authors wanting to link to a particular (absolute) URL, must do so
1087 only with "L<scheme:...>" codes (like L<http://www.perl.org>), and
1088 must not attempt "L<Some Site Name|scheme:...>" codes. This
1089 restriction avoids many problems in parsing and rendering L<...>
1090 codes.
1091
1092 · In a "L<text|...>" code, text may contain formatting codes for
1093 formatting or for E<...> escapes, as in:
1094
1095 L<B<ummE<234>stuff>|...>
1096
1097 For "L<...>" codes without a "name|" part, only "E<...>" and "Z<>"
1098 codes may occur. That is, authors should not use
1099 ""L<B<Foo::Bar>>"".
1100
1101 Note, however, that formatting codes and Z<>'s can occur in any and
1102 all parts of an L<...> (i.e., in name, section, text, and url).
1103
1104 Authors must not nest L<...> codes. For example, "L<The
1105 L<Foo::Bar> man page>" should be treated as an error.
1106
1107 · Note that Pod authors may use formatting codes inside the "text"
1108 part of "L<text|name>" (and so on for L<text|/"sec">).
1109
1110 In other words, this is valid:
1111
1112 Go read L<the docs on C<$.>|perlvar/"$.">
1113
1114 Some output formats that do allow rendering "L<...>" codes as
1115 hypertext, might not allow the link-text to be formatted; in that
1116 case, formatters will have to just ignore that formatting.
1117
1118 · At time of writing, "L<name>" values are of two types: either the
1119 name of a Pod page like "L<Foo::Bar>" (which might be a real Perl
1120 module or program in an @INC / PATH directory, or a .pod file in
1121 those places); or the name of a Unix man page, like
1122 "L<crontab(5)>". In theory, "L<chmod>" in ambiguous between a Pod
1123 page called "chmod", or the Unix man page "chmod" (in whatever man-
1124 section). However, the presence of a string in parens, as in
1125 "crontab(5)", is sufficient to signal that what is being discussed
1126 is not a Pod page, and so is presumably a Unix man page. The
1127 distinction is of no importance to many Pod processors, but some
1128 processors that render to hypertext formats may need to distinguish
1129 them in order to know how to render a given "L<foo>" code.
1130
1131 · Previous versions of perlpod allowed for a "L<section>" syntax (as
1132 in "L<Object Attributes>"), which was not easily distinguishable
1133 from "L<name>" syntax and for "L<"section">" which was only
1134 slightly less ambiguous. This syntax is no longer in the
1135 specification, and has been replaced by the "L</section>" syntax
1136 (where the slash was formerly optional). Pod parsers should
1137 tolerate the "L<"section">" syntax, for a while at least. The
1138 suggested heuristic for distinguishing "L<section>" from "L<name>"
1139 is that if it contains any whitespace, it's a section. Pod
1140 processors should warn about this being deprecated syntax.
1141
1143 "=over"..."=back" regions are used for various kinds of list-like
1144 structures. (I use the term "region" here simply as a collective term
1145 for everything from the "=over" to the matching "=back".)
1146
1147 · The non-zero numeric indentlevel in "=over indentlevel" ...
1148 "=back" is used for giving the formatter a clue as to how many
1149 "spaces" (ems, or roughly equivalent units) it should tab over,
1150 although many formatters will have to convert this to an absolute
1151 measurement that may not exactly match with the size of spaces (or
1152 M's) in the document's base font. Other formatters may have to
1153 completely ignore the number. The lack of any explicit indentlevel
1154 parameter is equivalent to an indentlevel value of 4. Pod
1155 processors may complain if indentlevel is present but is not a
1156 positive number matching "m/\A(\d*\.)?\d+\z/".
1157
1158 · Authors of Pod formatters are reminded that "=over" ... "=back" may
1159 map to several different constructs in your output format. For
1160 example, in converting Pod to (X)HTML, it can map to any of
1161 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1162 <blockquote>...</blockquote>. Similarly, "=item" can map to <li>
1163 or <dt>.
1164
1165 · Each "=over" ... "=back" region should be one of the following:
1166
1167 · An "=over" ... "=back" region containing only "=item *"
1168 commands, each followed by some number of ordinary/verbatim
1169 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1170 paragraphs, and "=begin"..."=end" regions.
1171
1172 (Pod processors must tolerate a bare "=item" as if it were
1173 "=item *".) Whether "*" is rendered as a literal asterisk, an
1174 "o", or as some kind of real bullet character, is left up to
1175 the Pod formatter, and may depend on the level of nesting.
1176
1177 · An "=over" ... "=back" region containing only
1178 "m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each group
1179 of them) followed by some number of ordinary/verbatim
1180 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1181 paragraphs, and/or "=begin"..."=end" codes. Note that the
1182 numbers must start at 1 in each section, and must proceed in
1183 order and without skipping numbers.
1184
1185 (Pod processors must tolerate lines like "=item 1" as if they
1186 were "=item 1.", with the period.)
1187
1188 · An "=over" ... "=back" region containing only "=item [text]"
1189 commands, each one (or each group of them) followed by some
1190 number of ordinary/verbatim paragraphs, other nested "=over"
1191 ... "=back" regions, or "=for..." paragraphs, and
1192 "=begin"..."=end" regions.
1193
1194 The "=item [text]" paragraph should not match
1195 "m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor
1196 should it match just "m/\A=item\s*\z/".
1197
1198 · An "=over" ... "=back" region containing no "=item" paragraphs
1199 at all, and containing only some number of ordinary/verbatim
1200 paragraphs, and possibly also some nested "=over" ... "=back"
1201 regions, "=for..." paragraphs, and "=begin"..."=end" regions.
1202 Such an itemless "=over" ... "=back" region in Pod is
1203 equivalent in meaning to a "<blockquote>...</blockquote>"
1204 element in HTML.
1205
1206 Note that with all the above cases, you can determine which type of
1207 "=over" ... "=back" you have, by examining the first (non-"=cut",
1208 non-"=pod") Pod paragraph after the "=over" command.
1209
1210 · Pod formatters must tolerate arbitrarily large amounts of text in
1211 the "=item text..." paragraph. In practice, most such paragraphs
1212 are short, as in:
1213
1214 =item For cutting off our trade with all parts of the world
1215
1216 But they may be arbitrarily long:
1217
1218 =item For transporting us beyond seas to be tried for pretended
1219 offenses
1220
1221 =item He is at this time transporting large armies of foreign
1222 mercenaries to complete the works of death, desolation and
1223 tyranny, already begun with circumstances of cruelty and perfidy
1224 scarcely paralleled in the most barbarous ages, and totally
1225 unworthy the head of a civilized nation.
1226
1227 · Pod processors should tolerate "=item *" / "=item number" commands
1228 with no accompanying paragraph. The middle item is an example:
1229
1230 =over
1231
1232 =item 1
1233
1234 Pick up dry cleaning.
1235
1236 =item 2
1237
1238 =item 3
1239
1240 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1241
1242 =back
1243
1244 · No "=over" ... "=back" region can contain headings. Processors may
1245 treat such a heading as an error.
1246
1247 · Note that an "=over" ... "=back" region should have some content.
1248 That is, authors should not have an empty region like this:
1249
1250 =over
1251
1252 =back
1253
1254 Pod processors seeing such a contentless "=over" ... "=back"
1255 region, may ignore it, or may report it as an error.
1256
1257 · Processors must tolerate an "=over" list that goes off the end of
1258 the document (i.e., which has no matching "=back"), but they may
1259 warn about such a list.
1260
1261 · Authors of Pod formatters should note that this construct:
1262
1263 =item Neque
1264
1265 =item Porro
1266
1267 =item Quisquam Est
1268
1269 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1270 velit, sed quia non numquam eius modi tempora incidunt ut
1271 labore et dolore magnam aliquam quaerat voluptatem.
1272
1273 =item Ut Enim
1274
1275 is semantically ambiguous, in a way that makes formatting decisions
1276 a bit difficult. On the one hand, it could be mention of an item
1277 "Neque", mention of another item "Porro", and mention of another
1278 item "Quisquam Est", with just the last one requiring the
1279 explanatory paragraph "Qui dolorem ipsum quia dolor..."; and then
1280 an item "Ut Enim". In that case, you'd want to format it like so:
1281
1282 Neque
1283
1284 Porro
1285
1286 Quisquam Est
1287 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1288 velit, sed quia non numquam eius modi tempora incidunt ut
1289 labore et dolore magnam aliquam quaerat voluptatem.
1290
1291 Ut Enim
1292
1293 But it could equally well be a discussion of three (related or
1294 equivalent) items, "Neque", "Porro", and "Quisquam Est", followed
1295 by a paragraph explaining them all, and then a new item "Ut Enim".
1296 In that case, you'd probably want to format it like so:
1297
1298 Neque
1299 Porro
1300 Quisquam Est
1301 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1302 velit, sed quia non numquam eius modi tempora incidunt ut
1303 labore et dolore magnam aliquam quaerat voluptatem.
1304
1305 Ut Enim
1306
1307 But (for the foreseeable future), Pod does not provide any way for
1308 Pod authors to distinguish which grouping is meant by the above
1309 "=item"-cluster structure. So formatters should format it like so:
1310
1311 Neque
1312
1313 Porro
1314
1315 Quisquam Est
1316
1317 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1318 velit, sed quia non numquam eius modi tempora incidunt ut
1319 labore et dolore magnam aliquam quaerat voluptatem.
1320
1321 Ut Enim
1322
1323 That is, there should be (at least roughly) equal spacing between
1324 items as between paragraphs (although that spacing may well be less
1325 than the full height of a line of text). This leaves it to the
1326 reader to use (con)textual cues to figure out whether the "Qui
1327 dolorem ipsum..." paragraph applies to the "Quisquam Est" item or
1328 to all three items "Neque", "Porro", and "Quisquam Est". While not
1329 an ideal situation, this is preferable to providing formatting cues
1330 that may be actually contrary to the author's intent.
1331
1333 Data paragraphs are typically used for inlining non-Pod data that is to
1334 be used (typically passed through) when rendering the document to a
1335 specific format:
1336
1337 =begin rtf
1338
1339 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1340
1341 =end rtf
1342
1343 The exact same effect could, incidentally, be achieved with a single
1344 "=for" paragraph:
1345
1346 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1347
1348 (Although that is not formally a data paragraph, it has the same
1349 meaning as one, and Pod parsers may parse it as one.)
1350
1351 Another example of a data paragraph:
1352
1353 =begin html
1354
1355 I like <em>PIE</em>!
1356
1357 <hr>Especially pecan pie!
1358
1359 =end html
1360
1361 If these were ordinary paragraphs, the Pod parser would try to expand
1362 the "E</em>" (in the first paragraph) as a formatting code, just like
1363 "E<lt>" or "E<eacute>". But since this is in a "=begin
1364 identifier"..."=end identifier" region and the identifier "html"
1365 doesn't begin have a ":" prefix, the contents of this region are stored
1366 as data paragraphs, instead of being processed as ordinary paragraphs
1367 (or if they began with a spaces and/or tabs, as verbatim paragraphs).
1368
1369 As a further example: At time of writing, no "biblio" identifier is
1370 supported, but suppose some processor were written to recognize it as a
1371 way of (say) denoting a bibliographic reference (necessarily containing
1372 formatting codes in ordinary paragraphs). The fact that "biblio"
1373 paragraphs were meant for ordinary processing would be indicated by
1374 prefacing each "biblio" identifier with a colon:
1375
1376 =begin :biblio
1377
1378 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1379 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1380
1381 =end :biblio
1382
1383 This would signal to the parser that paragraphs in this begin...end
1384 region are subject to normal handling as ordinary/verbatim paragraphs
1385 (while still tagged as meant only for processors that understand the
1386 "biblio" identifier). The same effect could be had with:
1387
1388 =for :biblio
1389 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1390 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1391
1392 The ":" on these identifiers means simply "process this stuff normally,
1393 even though the result will be for some special target". I suggest
1394 that parser APIs report "biblio" as the target identifier, but also
1395 report that it had a ":" prefix. (And similarly, with the above
1396 "html", report "html" as the target identifier, and note the lack of a
1397 ":" prefix.)
1398
1399 Note that a "=begin identifier"..."=end identifier" region where
1400 identifier begins with a colon, can contain commands. For example:
1401
1402 =begin :biblio
1403
1404 Wirth's classic is available in several editions, including:
1405
1406 =for comment
1407 hm, check abebooks.com for how much used copies cost.
1408
1409 =over
1410
1411 =item
1412
1413 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1414 Teubner, Stuttgart. [Yes, it's in German.]
1415
1416 =item
1417
1418 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1419 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1420
1421 =back
1422
1423 =end :biblio
1424
1425 Note, however, a "=begin identifier"..."=end identifier" region where
1426 identifier does not begin with a colon, should not directly contain
1427 "=head1" ... "=head4" commands, nor "=over", nor "=back", nor "=item".
1428 For example, this may be considered invalid:
1429
1430 =begin somedata
1431
1432 This is a data paragraph.
1433
1434 =head1 Don't do this!
1435
1436 This is a data paragraph too.
1437
1438 =end somedata
1439
1440 A Pod processor may signal that the above (specifically the "=head1"
1441 paragraph) is an error. Note, however, that the following should not
1442 be treated as an error:
1443
1444 =begin somedata
1445
1446 This is a data paragraph.
1447
1448 =cut
1449
1450 # Yup, this isn't Pod anymore.
1451 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1452
1453 =pod
1454
1455 This is a data paragraph too.
1456
1457 =end somedata
1458
1459 And this too is valid:
1460
1461 =begin someformat
1462
1463 This is a data paragraph.
1464
1465 And this is a data paragraph.
1466
1467 =begin someotherformat
1468
1469 This is a data paragraph too.
1470
1471 And this is a data paragraph too.
1472
1473 =begin :yetanotherformat
1474
1475 =head2 This is a command paragraph!
1476
1477 This is an ordinary paragraph!
1478
1479 And this is a verbatim paragraph!
1480
1481 =end :yetanotherformat
1482
1483 =end someotherformat
1484
1485 Another data paragraph!
1486
1487 =end someformat
1488
1489 The contents of the above "=begin :yetanotherformat" ... "=end
1490 :yetanotherformat" region aren't data paragraphs, because the
1491 immediately containing region's identifier (":yetanotherformat") begins
1492 with a colon. In practice, most regions that contain data paragraphs
1493 will contain only data paragraphs; however, the above nesting is
1494 syntactically valid as Pod, even if it is rare. However, the handlers
1495 for some formats, like "html", will accept only data paragraphs, not
1496 nested regions; and they may complain if they see (targeted for them)
1497 nested regions, or commands, other than "=end", "=pod", and "=cut".
1498
1499 Also consider this valid structure:
1500
1501 =begin :biblio
1502
1503 Wirth's classic is available in several editions, including:
1504
1505 =over
1506
1507 =item
1508
1509 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1510 Teubner, Stuttgart. [Yes, it's in German.]
1511
1512 =item
1513
1514 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1515 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1516
1517 =back
1518
1519 Buy buy buy!
1520
1521 =begin html
1522
1523 <img src='wirth_spokesmodeling_book.png'>
1524
1525 <hr>
1526
1527 =end html
1528
1529 Now now now!
1530
1531 =end :biblio
1532
1533 There, the "=begin html"..."=end html" region is nested inside the
1534 larger "=begin :biblio"..."=end :biblio" region. Note that the content
1535 of the "=begin html"..."=end html" region is data paragraph(s), because
1536 the immediately containing region's identifier ("html") doesn't begin
1537 with a colon.
1538
1539 Pod parsers, when processing a series of data paragraphs one after
1540 another (within a single region), should consider them to be one large
1541 data paragraph that happens to contain blank lines. So the content of
1542 the above "=begin html"..."=end html" may be stored as two data
1543 paragraphs (one consisting of "<img
1544 src='wirth_spokesmodeling_book.png'>\n" and another consisting of
1545 "<hr>\n"), but should be stored as a single data paragraph (consisting
1546 of "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1547
1548 Pod processors should tolerate empty "=begin something"..."=end
1549 something" regions, empty "=begin :something"..."=end :something"
1550 regions, and contentless "=for something" and "=for :something"
1551 paragraphs. I.e., these should be tolerated:
1552
1553 =for html
1554
1555 =begin html
1556
1557 =end html
1558
1559 =begin :biblio
1560
1561 =end :biblio
1562
1563 Incidentally, note that there's no easy way to express a data paragraph
1564 starting with something that looks like a command. Consider:
1565
1566 =begin stuff
1567
1568 =shazbot
1569
1570 =end stuff
1571
1572 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a
1573 data paragraph "=shazbot\n". However, you can express a data paragraph
1574 consisting of "=shazbot\n" using this code:
1575
1576 =for stuff =shazbot
1577
1578 The situation where this is necessary, is presumably quite rare.
1579
1580 Note that =end commands must match the currently open =begin command.
1581 That is, they must properly nest. For example, this is valid:
1582
1583 =begin outer
1584
1585 X
1586
1587 =begin inner
1588
1589 Y
1590
1591 =end inner
1592
1593 Z
1594
1595 =end outer
1596
1597 while this is invalid:
1598
1599 =begin outer
1600
1601 X
1602
1603 =begin inner
1604
1605 Y
1606
1607 =end outer
1608
1609 Z
1610
1611 =end inner
1612
1613 This latter is improper because when the "=end outer" command is seen,
1614 the currently open region has the formatname "inner", not "outer". (It
1615 just happens that "outer" is the format name of a higher-up region.)
1616 This is an error. Processors must by default report this as an error,
1617 and may halt processing the document containing that error. A
1618 corollary of this is that regions cannot "overlap". That is, the latter
1619 block above does not represent a region called "outer" which contains X
1620 and Y, overlapping a region called "inner" which contains Y and Z. But
1621 because it is invalid (as all apparently overlapping regions would be),
1622 it doesn't represent that, or anything at all.
1623
1624 Similarly, this is invalid:
1625
1626 =begin thing
1627
1628 =end hting
1629
1630 This is an error because the region is opened by "thing", and the
1631 "=end" tries to close "hting" [sic].
1632
1633 This is also invalid:
1634
1635 =begin thing
1636
1637 =end
1638
1639 This is invalid because every "=end" command must have a formatname
1640 parameter.
1641
1643 perlpod, "PODs: Embedded Documentation" in perlsyn, podchecker
1644
1646 Sean M. Burke
1647
1648
1649
1650perl v5.12.4 2011-06-07 PERLPODSPEC(1)