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<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 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 "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 Latin-1.
509 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 · 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 · 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 · 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 · 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 · 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 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
556 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
558 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
560 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
562 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 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 · 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 · 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 · 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 · 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 · 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 · 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 · 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 · 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 · 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 · 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 use Foo;
654 print Foo->VERSION
656 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 While this might be too cumbersome to implement in event-based Pod
662 parsers, it is straightforward for parsers that return parse trees.
663 · Pod formatters, where feasible, are advised to avoid splitting
665 short verbatim paragraphs (under twelve lines, say) across pages.
666 · 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 · 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 · 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 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 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 · 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 · 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 · 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 · 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 · 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 · 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 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 · 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 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 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 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
789 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 "\x{221E}" => '\(in',
799 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 · 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 For example, this Pod text:
819 magic is enabled if you set C<$Currency> to 'E<euro>'.
821 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 A Pod formatter may also note, in a comment or warning, a list of
827 what unrenderable characters were encountered.
828 · 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 · 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 · 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 S<L</Autoloaded Functions>>
858 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 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
864 However, a misapplied space-to-NBSP replacement could (wrongly)
866 produce something equivalent to this:
867 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
869 ...which is almost definitely not going to work as a hyperlink
871 (assuming this formatter outputs a format supporting hypertext).
872 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 · 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 For example:
891 sigE<shy>action
893 manuE<shy>script
894 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
895 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 In practice, it is anticipated that this character will not be used
904 often, but formatters should either support it, or delete it.
905 · 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 · 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 · In parsing an L<...> code, Pod parsers must distinguish at least
926 four attributes:
927 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 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 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 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 Pod parsers may also note additional attributes including:
955 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 Sixth:
963 The raw original L<...> content, before text is split on "|",
964 "/", etc, and before E<...> codes are expanded.
965 (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 For example:
970 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 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 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 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 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 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 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 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 · 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 Pod processors must now treat "text|"-less links as follows:
1038 L<name> => L<name|name>
1040 L</section> => L<"section"|/section>
1041 L<name/section> => L<"section" in name|name/section>
1042 · Note that section names might contain markup. I.e., if a section
1044 starts with:
1045 =head2 About the C<-M> Operator
1047 or with:
1049 =item About the C<-M> Operator
1051 then a link to it would look like this:
1053 L<somedoc/About the C<-M> Operator>
1055 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 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1061 Operator</h1>
1062 ...
1064 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1066 Operator" in somedoc</a>
1067 · 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 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 · In a "L<text|...>" code, text may contain formatting codes for
1087 formatting or for E<...> escapes, as in:
1088 L<B<ummE<234>stuff>|...>
1090 For "L<...>" codes without a "name|" part, only "E<...>" and "Z<>"
1092 codes may occur. That is, authors should not use
1093 ""L<B<Foo::Bar>>"".
1094 Note, however, that formatting codes and Z<>'s can occur in any and
1096 all parts of an L<...> (i.e., in name, section, text, and url).
1097 Authors must not nest L<...> codes. For example, "L<The
1099 L<Foo::Bar> man page>" should be treated as an error.
1100 · Note that Pod authors may use formatting codes inside the "text"
1102 part of "L<text|name>" (and so on for L<text|/"sec">).
1103 In other words, this is valid:
1105 Go read L<the docs on C<$.>|perlvar/"$.">
1107 Some output formats that do allow rendering "L<...>" codes as
1109 hypertext, might not allow the link-text to be formatted; in that
1110 case, formatters will have to just ignore that formatting.
1111 · At time of writing, "L<name>" values are of two types: either the
1113 name of a Pod page like "L<Foo::Bar>" (which might be a real Perl
1114 module or program in an @INC / PATH directory, or a .pod file in
1115 those places); or the name of a Unix man page, like
1116 "L<crontab(5)>". In theory, "L<chmod>" in ambiguous between a Pod
1117 page called "chmod", or the Unix man page "chmod" (in whatever man-
1118 section). However, the presence of a string in parens, as in
1119 "crontab(5)", is sufficient to signal that what is being discussed
1120 is not a Pod page, and so is presumably a Unix man page. The
1121 distinction is of no importance to many Pod processors, but some
1122 processors that render to hypertext formats may need to distinguish
1123 them in order to know how to render a given "L<foo>" code.
1124 · Previous versions of perlpod allowed for a "L<section>" syntax (as
1126 in "L<Object Attributes>"), which was not easily distinguishable
1127 from "L<name>" syntax and for "L<"section">" which was only
1128 slightly less ambiguous. This syntax is no longer in the
1129 specification, and has been replaced by the "L</section>" syntax
1130 (where the slash was formerly optional). Pod parsers should
1131 tolerate the "L<"section">" syntax, for a while at least. The
1132 suggested heuristic for distinguishing "L<section>" from "L<name>"
1133 is that if it contains any whitespace, it's a section. Pod
1134 processors should warn about this being deprecated syntax.
1135
1137 "=over"..."=back" regions are used for various kinds of list-like
1138 structures. (I use the term "region" here simply as a collective term
1139 for everything from the "=over" to the matching "=back".)
1140 · The non-zero numeric indentlevel in "=over indentlevel" ...
1142 "=back" is used for giving the formatter a clue as to how many
1143 "spaces" (ems, or roughly equivalent units) it should tab over,
1144 although many formatters will have to convert this to an absolute
1145 measurement that may not exactly match with the size of spaces (or
1146 M's) in the document's base font. Other formatters may have to
1147 completely ignore the number. The lack of any explicit indentlevel
1148 parameter is equivalent to an indentlevel value of 4. Pod
1149 processors may complain if indentlevel is present but is not a
1150 positive number matching "m/\A(\d*\.)?\d+\z/".
1151 · Authors of Pod formatters are reminded that "=over" ... "=back" may
1153 map to several different constructs in your output format. For
1154 example, in converting Pod to (X)HTML, it can map to any of
1155 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1156 <blockquote>...</blockquote>. Similarly, "=item" can map to <li>
1157 or <dt>.
1158 · Each "=over" ... "=back" region should be one of the following:
1160 · An "=over" ... "=back" region containing only "=item *"
1162 commands, each followed by some number of ordinary/verbatim
1163 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1164 paragraphs, and "=begin"..."=end" regions.
1165 (Pod processors must tolerate a bare "=item" as if it were
1167 "=item *".) Whether "*" is rendered as a literal asterisk, an
1168 "o", or as some kind of real bullet character, is left up to
1169 the Pod formatter, and may depend on the level of nesting.
1170 · An "=over" ... "=back" region containing only
1172 "m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each group
1173 of them) followed by some number of ordinary/verbatim
1174 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1175 paragraphs, and/or "=begin"..."=end" codes. Note that the
1176 numbers must start at 1 in each section, and must proceed in
1177 order and without skipping numbers.
1178 (Pod processors must tolerate lines like "=item 1" as if they
1180 were "=item 1.", with the period.)
1181 · An "=over" ... "=back" region containing only "=item [text]"
1183 commands, each one (or each group of them) followed by some
1184 number of ordinary/verbatim paragraphs, other nested "=over"
1185 ... "=back" regions, or "=for..." paragraphs, and
1186 "=begin"..."=end" regions.
1187 The "=item [text]" paragraph should not match
1189 "m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor
1190 should it match just "m/\A=item\s*\z/".
1191 · An "=over" ... "=back" region containing no "=item" paragraphs
1193 at all, and containing only some number of ordinary/verbatim
1194 paragraphs, and possibly also some nested "=over" ... "=back"
1195 regions, "=for..." paragraphs, and "=begin"..."=end" regions.
1196 Such an itemless "=over" ... "=back" region in Pod is
1197 equivalent in meaning to a "<blockquote>...</blockquote>"
1198 element in HTML.
1199 Note that with all the above cases, you can determine which type of
1201 "=over" ... "=back" you have, by examining the first (non-"=cut",
1202 non-"=pod") Pod paragraph after the "=over" command.
1203 · Pod formatters must tolerate arbitrarily large amounts of text in
1205 the "=item text..." paragraph. In practice, most such paragraphs
1206 are short, as in:
1207 =item For cutting off our trade with all parts of the world
1209 But they may be arbitrarily long:
1211 =item For transporting us beyond seas to be tried for pretended
1213 offenses
1214 =item He is at this time transporting large armies of foreign
1216 mercenaries to complete the works of death, desolation and
1217 tyranny, already begun with circumstances of cruelty and perfidy
1218 scarcely paralleled in the most barbarous ages, and totally
1219 unworthy the head of a civilized nation.
1220 · Pod processors should tolerate "=item *" / "=item number" commands
1222 with no accompanying paragraph. The middle item is an example:
1223 =over
1225 =item 1
1227 Pick up dry cleaning.
1229 =item 2
1231 =item 3
1233 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1235 =back
1237 · No "=over" ... "=back" region can contain headings. Processors may
1239 treat such a heading as an error.
1240 · Note that an "=over" ... "=back" region should have some content.
1242 That is, authors should not have an empty region like this:
1243 =over
1245 =back
1247 Pod processors seeing such a contentless "=over" ... "=back"
1249 region, may ignore it, or may report it as an error.
1250 · Processors must tolerate an "=over" list that goes off the end of
1252 the document (i.e., which has no matching "=back"), but they may
1253 warn about such a list.
1254 · Authors of Pod formatters should note that this construct:
1256 =item Neque
1258 =item Porro
1260 =item Quisquam Est
1262 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1264 velit, sed quia non numquam eius modi tempora incidunt ut
1265 labore et dolore magnam aliquam quaerat voluptatem.
1266 =item Ut Enim
1268 is semantically ambiguous, in a way that makes formatting decisions
1270 a bit difficult. On the one hand, it could be mention of an item
1271 "Neque", mention of another item "Porro", and mention of another
1272 item "Quisquam Est", with just the last one requiring the
1273 explanatory paragraph "Qui dolorem ipsum quia dolor..."; and then
1274 an item "Ut Enim". In that case, you'd want to format it like so:
1275 Neque
1277 Porro
1279 Quisquam Est
1281 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1282 velit, sed quia non numquam eius modi tempora incidunt ut
1283 labore et dolore magnam aliquam quaerat voluptatem.
1284 Ut Enim
1286 But it could equally well be a discussion of three (related or
1288 equivalent) items, "Neque", "Porro", and "Quisquam Est", followed
1289 by a paragraph explaining them all, and then a new item "Ut Enim".
1290 In that case, you'd probably want to format it like so:
1291 Neque
1293 Porro
1294 Quisquam Est
1295 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1296 velit, sed quia non numquam eius modi tempora incidunt ut
1297 labore et dolore magnam aliquam quaerat voluptatem.
1298 Ut Enim
1300 But (for the foreseeable future), Pod does not provide any way for
1302 Pod authors to distinguish which grouping is meant by the above
1303 "=item"-cluster structure. So formatters should format it like so:
1304 Neque
1306 Porro
1308 Quisquam Est
1310 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1312 velit, sed quia non numquam eius modi tempora incidunt ut
1313 labore et dolore magnam aliquam quaerat voluptatem.
1314 Ut Enim
1316 That is, there should be (at least roughly) equal spacing between
1318 items as between paragraphs (although that spacing may well be less
1319 than the full height of a line of text). This leaves it to the
1320 reader to use (con)textual cues to figure out whether the "Qui
1321 dolorem ipsum..." paragraph applies to the "Quisquam Est" item or
1322 to all three items "Neque", "Porro", and "Quisquam Est". While not
1323 an ideal situation, this is preferable to providing formatting cues
1324 that may be actually contrary to the author's intent.
1325
1327 Data paragraphs are typically used for inlining non-Pod data that is to
1328 be used (typically passed through) when rendering the document to a
1329 specific format:
1330 =begin rtf
1332 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1334 =end rtf
1336 The exact same effect could, incidentally, be achieved with a single
1338 "=for" paragraph:
1339 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1341 (Although that is not formally a data paragraph, it has the same
1343 meaning as one, and Pod parsers may parse it as one.)
1344 Another example of a data paragraph:
1346 =begin html
1348 I like <em>PIE</em>!
1350 <hr>Especially pecan pie!
1352 =end html
1354 If these were ordinary paragraphs, the Pod parser would try to expand
1356 the "E</em>" (in the first paragraph) as a formatting code, just like
1357 "E<lt>" or "E<eacute>". But since this is in a "=begin
1358 identifier"..."=end identifier" region and the identifier "html"
1359 doesn't begin have a ":" prefix, the contents of this region are stored
1360 as data paragraphs, instead of being processed as ordinary paragraphs
1361 (or if they began with a spaces and/or tabs, as verbatim paragraphs).
1362 As a further example: At time of writing, no "biblio" identifier is
1364 supported, but suppose some processor were written to recognize it as a
1365 way of (say) denoting a bibliographic reference (necessarily containing
1366 formatting codes in ordinary paragraphs). The fact that "biblio"
1367 paragraphs were meant for ordinary processing would be indicated by
1368 prefacing each "biblio" identifier with a colon:
1369 =begin :biblio
1371 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1373 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1374 =end :biblio
1376 This would signal to the parser that paragraphs in this begin...end
1378 region are subject to normal handling as ordinary/verbatim paragraphs
1379 (while still tagged as meant only for processors that understand the
1380 "biblio" identifier). The same effect could be had with:
1381 =for :biblio
1383 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1384 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1385 The ":" on these identifiers means simply "process this stuff normally,
1387 even though the result will be for some special target". I suggest
1388 that parser APIs report "biblio" as the target identifier, but also
1389 report that it had a ":" prefix. (And similarly, with the above
1390 "html", report "html" as the target identifier, and note the lack of a
1391 ":" prefix.)
1392 Note that a "=begin identifier"..."=end identifier" region where
1394 identifier begins with a colon, can contain commands. For example:
1395 =begin :biblio
1397 Wirth's classic is available in several editions, including:
1399 =for comment
1401 hm, check abebooks.com for how much used copies cost.
1402 =over
1404 =item
1406 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1408 Teubner, Stuttgart. [Yes, it's in German.]
1409 =item
1411 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1413 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1414 =back
1416 =end :biblio
1418 Note, however, a "=begin identifier"..."=end identifier" region where
1420 identifier does not begin with a colon, should not directly contain
1421 "=head1" ... "=head4" commands, nor "=over", nor "=back", nor "=item".
1422 For example, this may be considered invalid:
1423 =begin somedata
1425 This is a data paragraph.
1427 =head1 Don't do this!
1429 This is a data paragraph too.
1431 =end somedata
1433 A Pod processor may signal that the above (specifically the "=head1"
1435 paragraph) is an error. Note, however, that the following should not
1436 be treated as an error:
1437 =begin somedata
1439 This is a data paragraph.
1441 =cut
1443 # Yup, this isn't Pod anymore.
1445 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1446 =pod
1448 This is a data paragraph too.
1450 =end somedata
1452 And this too is valid:
1454 =begin someformat
1456 This is a data paragraph.
1458 And this is a data paragraph.
1460 =begin someotherformat
1462 This is a data paragraph too.
1464 And this is a data paragraph too.
1466 =begin :yetanotherformat
1468 =head2 This is a command paragraph!
1470 This is an ordinary paragraph!
1472 And this is a verbatim paragraph!
1474 =end :yetanotherformat
1476 =end someotherformat
1478 Another data paragraph!
1480 =end someformat
1482 The contents of the above "=begin :yetanotherformat" ... "=end
1484 :yetanotherformat" region aren't data paragraphs, because the
1485 immediately containing region's identifier (":yetanotherformat") begins
1486 with a colon. In practice, most regions that contain data paragraphs
1487 will contain only data paragraphs; however, the above nesting is
1488 syntactically valid as Pod, even if it is rare. However, the handlers
1489 for some formats, like "html", will accept only data paragraphs, not
1490 nested regions; and they may complain if they see (targeted for them)
1491 nested regions, or commands, other than "=end", "=pod", and "=cut".
1492 Also consider this valid structure:
1494 =begin :biblio
1496 Wirth's classic is available in several editions, including:
1498 =over
1500 =item
1502 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1504 Teubner, Stuttgart. [Yes, it's in German.]
1505 =item
1507 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1509 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1510 =back
1512 Buy buy buy!
1514 =begin html
1516 <img src='wirth_spokesmodeling_book.png'>
1518 <hr>
1520 =end html
1522 Now now now!
1524 =end :biblio
1526 There, the "=begin html"..."=end html" region is nested inside the
1528 larger "=begin :biblio"..."=end :biblio" region. Note that the content
1529 of the "=begin html"..."=end html" region is data paragraph(s), because
1530 the immediately containing region's identifier ("html") doesn't begin
1531 with a colon.
1532 Pod parsers, when processing a series of data paragraphs one after
1534 another (within a single region), should consider them to be one large
1535 data paragraph that happens to contain blank lines. So the content of
1536 the above "=begin html"..."=end html" may be stored as two data
1537 paragraphs (one consisting of "<img
1538 src='wirth_spokesmodeling_book.png'>\n" and another consisting of
1539 "<hr>\n"), but should be stored as a single data paragraph (consisting
1540 of "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1541 Pod processors should tolerate empty "=begin something"..."=end
1543 something" regions, empty "=begin :something"..."=end :something"
1544 regions, and contentless "=for something" and "=for :something"
1545 paragraphs. I.e., these should be tolerated:
1546 =for html
1548 =begin html
1550 =end html
1552 =begin :biblio
1554 =end :biblio
1556 Incidentally, note that there's no easy way to express a data paragraph
1558 starting with something that looks like a command. Consider:
1559 =begin stuff
1561 =shazbot
1563 =end stuff
1565 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a
1567 data paragraph "=shazbot\n". However, you can express a data paragraph
1568 consisting of "=shazbot\n" using this code:
1569 =for stuff =shazbot
1571 The situation where this is necessary, is presumably quite rare.
1573 Note that =end commands must match the currently open =begin command.
1575 That is, they must properly nest. For example, this is valid:
1576 =begin outer
1578 X
1580 =begin inner
1582 Y
1584 =end inner
1586 Z
1588 =end outer
1590 while this is invalid:
1592 =begin outer
1594 X
1596 =begin inner
1598 Y
1600 =end outer
1602 Z
1604 =end inner
1606 This latter is improper because when the "=end outer" command is seen,
1608 the currently open region has the formatname "inner", not "outer". (It
1609 just happens that "outer" is the format name of a higher-up region.)
1610 This is an error. Processors must by default report this as an error,
1611 and may halt processing the document containing that error. A
1612 corollary of this is that regions cannot "overlap". That is, the latter
1613 block above does not represent a region called "outer" which contains X
1614 and Y, overlapping a region called "inner" which contains Y and Z. But
1615 because it is invalid (as all apparently overlapping regions would be),
1616 it doesn't represent that, or anything at all.
1617 Similarly, this is invalid:
1619 =begin thing
1621 =end hting
1623 This is an error because the region is opened by "thing", and the
1625 "=end" tries to close "hting" [sic].
1626 This is also invalid:
1628 =begin thing
1630 =end
1632 This is invalid because every "=end" command must have a formatname
1634 parameter.
1635
1637 perlpod, "PODs: Embedded Documentation" in perlsyn, podchecker
1638
1640 Sean M. Burke
1641perl v5.16.3 2013-03-04 PERLPODSPEC(1)