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
32 can 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:?[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 "=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 In parsing Pod, a notably tricky part is the correct parsing of
364 (potentially nested!) formatting codes. Implementors should consult
365 the code in the "parse_text" routine in Pod::Parser as an example of a
366 correct implementation.
367 "I<text>" -- italic text
369 See the brief discussion in "Formatting Codes" in perlpod.
370 "B<text>" -- bold text
372 See the brief discussion in "Formatting Codes" in perlpod.
373 "C<code>" -- code text
375 See the brief discussion in "Formatting Codes" in perlpod.
376 "F<filename>" -- style for filenames
378 See the brief discussion in "Formatting Codes" in perlpod.
379 "X<topic name>" -- an index entry
381 See the brief discussion in "Formatting Codes" in perlpod.
382 This code is unusual in that most formatters completely discard
384 this code and its content. Other formatters will render it with
385 invisible codes that can be used in building an index of the
386 current document.
387 "Z<>" -- a null (zero-effect) formatting code
389 Discussed briefly in "Formatting Codes" in perlpod.
390 This code is unusual is that it should have no content. That is, a
392 processor may complain if it sees "Z<potatoes>". Whether or not it
393 complains, the potatoes text should ignored.
394 "L<name>" -- a hyperlink
396 The complicated syntaxes of this code are discussed at length in
397 "Formatting Codes" in perlpod, and implementation details are
398 discussed below, in "About L<...> Codes". Parsing the contents of
399 L<content> is tricky. Notably, the content has to be checked for
400 whether it looks like a URL, or whether it has to be split on
401 literal "|" and/or "/" (in the right order!), and so on, before
402 E<...> codes are resolved.
403 "E<escape>" -- a character escape
405 See "Formatting Codes" in perlpod, and several points in "Notes on
406 Implementing Pod Processors".
407 "S<text>" -- text contains non-breaking spaces
409 This formatting code is syntactically simple, but semantically
410 complex. What it means is that each space in the printable content
411 of this code signifies a non-breaking space.
412 Consider:
414 C<$x ? $y : $z>
416 S<C<$x ? $y : $z>>
418 Both signify the monospace (c[ode] style) text consisting of "$x",
420 one space, "?", one space, ":", one space, "$z". The difference is
421 that in the latter, with the S code, those spaces are not "normal"
422 spaces, but instead are non-breaking spaces.
423 If a Pod processor sees any formatting code other than the ones listed
425 above (as in "N<...>", or "Q<...>", etc.), that processor must by
426 default treat this as an error. A Pod parser may allow a way for
427 particular applications to add to the above list of known formatting
428 codes; a Pod parser might even allow a way to stipulate, for each
429 additional command, whether it requires some form of special
430 processing, as L<...> does.
431 Future versions of this specification may add additional formatting
433 codes.
434 Historical note: A few older Pod processors would not see a ">" as
436 closing a "C<" code, if the ">" was immediately preceded by a "-".
437 This was so that this:
438 C<$foo->bar>
440 would parse as equivalent to this:
442 C<$foo-E<gt>bar>
444 instead of as equivalent to a "C" formatting code containing only
446 "$foo-", and then a "bar>" outside the "C" formatting code. This
447 problem has since been solved by the addition of syntaxes like this:
448 C<< $foo->bar >>
450 Compliant parsers must not treat "->" as special.
452 Formatting codes absolutely cannot span paragraphs. If a code is
454 opened in one paragraph, and no closing code is found by the end of
455 that paragraph, the Pod parser must close that formatting code, and
456 should complain (as in "Unterminated I code in the paragraph starting
457 at line 123: 'Time objects are not...'"). So these two paragraphs:
458 I<I told you not to do this!
460 Don't make me say it again!>
462 ...must not be parsed as two paragraphs in italics (with the I code
464 starting in one paragraph and starting in another.) Instead, the first
465 paragraph should generate a warning, but that aside, the above code
466 must parse as if it were:
467 I<I told you not to do this!>
469 Don't make me say it again!E<gt>
471 (In SGMLish jargon, all Pod commands are like block-level elements,
473 whereas all Pod formatting codes are like inline-level elements.)
474
476 The following is a long section of miscellaneous requirements and
477 suggestions to do with Pod processing.
478 · Pod formatters should tolerate lines in verbatim blocks that are of
480 any length, even if that means having to break them (possibly
481 several times, for very long lines) to avoid text running off the
482 side of the page. Pod formatters may warn of such line-breaking.
483 Such warnings are particularly appropriate for lines are over 100
484 characters long, which are usually not intentional.
485 · Pod parsers must recognize all of the three well-known newline
487 formats: CR, LF, and CRLF. See perlport.
488 · Pod parsers should accept input lines that are of any length.
490 · Since Perl recognizes a Unicode Byte Order Mark at the start of
492 files as signaling that the file is Unicode encoded as in UTF-16
493 (whether big-endian or little-endian) or UTF-8, Pod parsers should
494 do the same. Otherwise, the character encoding should be
495 understood as being UTF-8 if the first highbit byte sequence in the
496 file seems valid as a UTF-8 sequence, or otherwise as Latin-1.
497 Future versions of this specification may specify how Pod can
499 accept other encodings. Presumably treatment of other encodings in
500 Pod parsing would be as in XML parsing: whatever the encoding
501 declared by a particular Pod file, content is to be stored in
502 memory as Unicode characters.
503 · The well known Unicode Byte Order Marks are as follows: if the
505 file begins with the two literal byte values 0xFE 0xFF, this is the
506 BOM for big-endian UTF-16. If the file begins with the two literal
507 byte value 0xFF 0xFE, this is the BOM for little-endian UTF-16. If
508 the file begins with the three literal byte values 0xEF 0xBB 0xBF,
509 this is the BOM for UTF-8.
510 · A naive but sufficient heuristic for testing the first highbit
512 byte-sequence in a BOM-less file (whether in code or in Pod!), to
513 see whether that sequence is valid as UTF-8 (RFC 2279) is to check
514 whether that the first byte in the sequence is in the range 0xC0 -
515 0xFD and whether the next byte is in the range 0x80 - 0xBF. If so,
516 the parser may conclude that this file is in UTF-8, and all highbit
517 sequences in the file should be assumed to be UTF-8. Otherwise the
518 parser should treat the file as being in Latin-1. In the unlikely
519 circumstance that the first highbit sequence in a truly non-UTF-8
520 file happens to appear to be UTF-8, one can cater to our heuristic
521 (as well as any more intelligent heuristic) by prefacing that line
522 with a comment line containing a highbit sequence that is clearly
523 not valid as UTF-8. A line consisting of simply "#", an e-acute,
524 and any non-highbit byte, is sufficient to establish this file's
525 encoding.
526 · This document's requirements and suggestions about encodings do not
528 apply to Pod processors running on non-ASCII platforms, notably
529 EBCDIC platforms.
530 · Pod processors must treat a "=for [label] [content...]" paragraph
532 as meaning the same thing as a "=begin [label]" paragraph, content,
533 and an "=end [label]" paragraph. (The parser may conflate these
534 two constructs, or may leave them distinct, in the expectation that
535 the formatter will nevertheless treat them the same.)
536 · When rendering Pod to a format that allows comments (i.e., to
538 nearly any format other than plaintext), a Pod formatter must
539 insert comment text identifying its name and version number, and
540 the name and version numbers of any modules it might be using to
541 process the Pod. Minimal examples:
542 %% POD::Pod2PS v3.14159, using POD::Parser v1.92
544 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
546 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
548 .\" Pod::Man version 3.14159, using POD::Parser version 1.92
550 Formatters may also insert additional comments, including: the
552 release date of the Pod formatter program, the contact address for
553 the author(s) of the formatter, the current time, the name of input
554 file, the formatting options in effect, version of Perl used, etc.
555 Formatters may also choose to note errors/warnings as comments,
557 besides or instead of emitting them otherwise (as in messages to
558 STDERR, or "die"ing).
559 · Pod parsers may emit warnings or error messages ("Unknown E code
561 E<zslig>!") to STDERR (whether through printing to STDERR, or
562 "warn"ing/"carp"ing, or "die"ing/"croak"ing), but must allow
563 suppressing all such STDERR output, and instead allow an option for
564 reporting errors/warnings in some other way, whether by triggering
565 a callback, or noting errors in some attribute of the document
566 object, or some similarly unobtrusive mechanism -- or even by
567 appending a "Pod Errors" section to the end of the parsed form of
568 the document.
569 · In cases of exceptionally aberrant documents, Pod parsers may abort
571 the parse. Even then, using "die"ing/"croak"ing is to be avoided;
572 where possible, the parser library may simply close the input file
573 and add text like "*** Formatting Aborted ***" to the end of the
574 (partial) in-memory document.
575 · In paragraphs where formatting codes (like E<...>, B<...>) are
577 understood (i.e., not verbatim paragraphs, but including ordinary
578 paragraphs, and command paragraphs that produce renderable text,
579 like "=head1"), literal whitespace should generally be considered
580 "insignificant", in that one literal space has the same meaning as
581 any (nonzero) number of literal spaces, literal newlines, and
582 literal tabs (as long as this produces no blank lines, since those
583 would terminate the paragraph). Pod parsers should compact literal
584 whitespace in each processed paragraph, but may provide an option
585 for overriding this (since some processing tasks do not require
586 it), or may follow additional special rules (for example, specially
587 treating period-space-space or period-newline sequences).
588 · Pod parsers should not, by default, try to coerce apostrophe (')
590 and quote (") into smart quotes (little 9's, 66's, 99's, etc), nor
591 try to turn backtick (`) into anything else but a single backtick
592 character (distinct from an open quote character!), nor "--" into
593 anything but two minus signs. They must never do any of those
594 things to text in C<...> formatting codes, and never ever to text
595 in verbatim paragraphs.
596 · When rendering Pod to a format that has two kinds of hyphens (-),
598 one that's a non-breaking hyphen, and another that's a breakable
599 hyphen (as in "object-oriented", which can be split across lines as
600 "object-", newline, "oriented"), formatters are encouraged to
601 generally translate "-" to non-breaking hyphen, but may apply
602 heuristics to convert some of these to breaking hyphens.
603 · Pod formatters should make reasonable efforts to keep words of Perl
605 code from being broken across lines. For example, "Foo::Bar" in
606 some formatting systems is seen as eligible for being broken across
607 lines as "Foo::" newline "Bar" or even "Foo::-" newline "Bar".
608 This should be avoided where possible, either by disabling all
609 line-breaking in mid-word, or by wrapping particular words with
610 internal punctuation in "don't break this across lines" codes
611 (which in some formats may not be a single code, but might be a
612 matter of inserting non-breaking zero-width spaces between every
613 pair of characters in a word.)
614 · Pod parsers should, by default, expand tabs in verbatim paragraphs
616 as they are processed, before passing them to the formatter or
617 other processor. Parsers may also allow an option for overriding
618 this.
619 · Pod parsers should, by default, remove newlines from the end of
621 ordinary and verbatim paragraphs before passing them to the
622 formatter. For example, while the paragraph you're reading now
623 could be considered, in Pod source, to end with (and contain) the
624 newline(s) that end it, it should be processed as ending with (and
625 containing) the period character that ends this sentence.
626 · Pod parsers, when reporting errors, should make some effort to
628 report an approximate line number ("Nested E<>'s in Paragraph #52,
629 near line 633 of Thing/Foo.pm!"), instead of merely noting the
630 paragraph number ("Nested E<>'s in Paragraph #52 of
631 Thing/Foo.pm!"). Where this is problematic, the paragraph number
632 should at least be accompanied by an excerpt from the paragraph
633 ("Nested E<>'s in Paragraph #52 of Thing/Foo.pm, which begins
634 'Read/write accessor for the C<interest rate> attribute...'").
635 · Pod parsers, when processing a series of verbatim paragraphs one
637 after another, should consider them to be one large verbatim
638 paragraph that happens to contain blank lines. I.e., these two
639 lines, which have a blank line between them:
640 use Foo;
642 print Foo->VERSION
644 should be unified into one paragraph ("\tuse Foo;\n\n\tprint
646 Foo->VERSION") before being passed to the formatter or other
647 processor. Parsers may also allow an option for overriding this.
648 While this might be too cumbersome to implement in event-based Pod
650 parsers, it is straightforward for parsers that return parse trees.
651 · Pod formatters, where feasible, are advised to avoid splitting
653 short verbatim paragraphs (under twelve lines, say) across pages.
654 · Pod parsers must treat a line with only spaces and/or tabs on it as
656 a "blank line" such as separates paragraphs. (Some older parsers
657 recognized only two adjacent newlines as a "blank line" but would
658 not recognize a newline, a space, and a newline, as a blank line.
659 This is noncompliant behavior.)
660 · Authors of Pod formatters/processors should make every effort to
662 avoid writing their own Pod parser. There are already several in
663 CPAN, with a wide range of interface styles -- and one of them,
664 Pod::Parser, comes with modern versions of Perl.
665 · Characters in Pod documents may be conveyed either as literals, or
667 by number in E<n> codes, or by an equivalent mnemonic, as in
668 E<eacute> which is exactly equivalent to E<233>.
669 Characters in the range 32-126 refer to those well known US-ASCII
671 characters (also defined there by Unicode, with the same meaning),
672 which all Pod formatters must render faithfully. Characters in the
673 ranges 0-31 and 127-159 should not be used (neither as literals,
674 nor as E<number> codes), except for the literal byte-sequences for
675 newline (13, 13 10, or 10), and tab (9).
676 Characters in the range 160-255 refer to Latin-1 characters (also
678 defined there by Unicode, with the same meaning). Characters above
679 255 should be understood to refer to Unicode characters.
680 · Be warned that some formatters cannot reliably render characters
682 outside 32-126; and many are able to handle 32-126 and 160-255, but
683 nothing above 255.
684 · Besides the well-known "E<lt>" and "E<gt>" codes for less-than and
686 greater-than, Pod parsers must understand "E<sol>" for "/"
687 (solidus, slash), and "E<verbar>" for "|" (vertical bar, pipe).
688 Pod parsers should also understand "E<lchevron>" and "E<rchevron>"
689 as legacy codes for characters 171 and 187, i.e., "left-pointing
690 double angle quotation mark" = "left pointing guillemet" and
691 "right-pointing double angle quotation mark" = "right pointing
692 guillemet". (These look like little "<<" and ">>", and they are
693 now preferably expressed with the HTML/XHTML codes "E<laquo>" and
694 "E<raquo>".)
695 · Pod parsers should understand all "E<html>" codes as defined in the
697 entity declarations in the most recent XHTML specification at
698 "www.W3.org". Pod parsers must understand at least the entities
699 that define characters in the range 160-255 (Latin-1). Pod
700 parsers, when faced with some unknown "E<identifier>" code,
701 shouldn't simply replace it with nullstring (by default, at least),
702 but may pass it through as a string consisting of the literal
703 characters E, less-than, identifier, greater-than. Or Pod parsers
704 may offer the alternative option of processing such unknown
705 "E<identifier>" codes by firing an event especially for such codes,
706 or by adding a special node-type to the in-memory document tree.
707 Such "E<identifier>" may have special meaning to some processors,
708 or some processors may choose to add them to a special error
709 report.
710 · Pod parsers must also support the XHTML codes "E<quot>" for
712 character 34 (doublequote, "), "E<amp>" for character 38
713 (ampersand, &), and "E<apos>" for character 39 (apostrophe, ').
714 · Note that in all cases of "E<whatever>", whatever (whether an
716 htmlname, or a number in any base) must consist only of
717 alphanumeric characters -- that is, whatever must watch
718 "m/\A\w+\z/". So "E< 0 1 2 3 >" is invalid, because it contains
719 spaces, which aren't alphanumeric characters. This presumably does
720 not need special treatment by a Pod processor; " 0 1 2 3 " doesn't
721 look like a number in any base, so it would presumably be looked up
722 in the table of HTML-like names. Since there isn't (and cannot be)
723 an HTML-like entity called " 0 1 2 3 ", this will be treated as an
724 error. However, Pod processors may treat "E< 0 1 2 3 >" or
725 "E<e-acute>" as syntactically invalid, potentially earning a
726 different error message than the error message (or warning, or
727 event) generated by a merely unknown (but theoretically valid)
728 htmlname, as in "E<qacute>" [sic]. However, Pod parsers are not
729 required to make this distinction.
730 · Note that E<number> must not be interpreted as simply "codepoint
732 number in the current/native character set". It always means only
733 "the character represented by codepoint number in Unicode." (This
734 is identical to the semantics of &#number; in XML.)
735 This will likely require many formatters to have tables mapping
737 from treatable Unicode codepoints (such as the "\xE9" for the
738 e-acute character) to the escape sequences or codes necessary for
739 conveying such sequences in the target output format. A converter
740 to *roff would, for example know that "\xE9" (whether conveyed
741 literally, or via a E<...> sequence) is to be conveyed as "e\\*'".
742 Similarly, a program rendering Pod in a Mac OS application window,
743 would presumably need to know that "\xE9" maps to codepoint 142 in
744 MacRoman encoding that (at time of writing) is native for Mac OS.
745 Such Unicode2whatever mappings are presumably already widely
746 available for common output formats. (Such mappings may be
747 incomplete! Implementers are not expected to bend over backwards
748 in an attempt to render Cherokee syllabics, Etruscan runes,
749 Byzantine musical symbols, or any of the other weird things that
750 Unicode can encode.) And if a Pod document uses a character not
751 found in such a mapping, the formatter should consider it an
752 unrenderable character.
753 · If, surprisingly, the implementor of a Pod formatter can't find a
755 satisfactory pre-existing table mapping from Unicode characters to
756 escapes in the target format (e.g., a decent table of Unicode
757 characters to *roff escapes), it will be necessary to build such a
758 table. If you are in this circumstance, you should begin with the
759 characters in the range 0x00A0 - 0x00FF, which is mostly the
760 heavily used accented characters. Then proceed (as patience
761 permits and fastidiousness compels) through the characters that the
762 (X)HTML standards groups judged important enough to merit mnemonics
763 for. These are declared in the (X)HTML specifications at the
764 www.W3.org site. At time of writing (September 2001), the most
765 recent entity declaration files are:
766 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
768 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
769 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
770 Then you can progress through any remaining notable Unicode
772 characters in the range 0x2000-0x204D (consult the character tables
773 at www.unicode.org), and whatever else strikes your fancy. For
774 example, in xhtml-symbol.ent, there is the entry:
775 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
777 While the mapping "infin" to the character "\x{221E}" will
779 (hopefully) have been already handled by the Pod parser, the
780 presence of the character in this file means that it's reasonably
781 important enough to include in a formatter's table that maps from
782 notable Unicode characters to the codes necessary for rendering
783 them. So for a Unicode-to-*roff mapping, for example, this would
784 merit the entry:
785 "\x{221E}" => '\(in',
787 It is eagerly hoped that in the future, increasing numbers of
789 formats (and formatters) will support Unicode characters directly
790 (as (X)HTML does with "∞", "∞", or "∞"),
791 reducing the need for idiosyncratic mappings of
792 Unicode-to-my_escapes.
793 · It is up to individual Pod formatter to display good judgement when
795 confronted with an unrenderable character (which is distinct from
796 an unknown E<thing> sequence that the parser couldn't resolve to
797 anything, renderable or not). It is good practice to map Latin
798 letters with diacritics (like "E<eacute>"/"E<233>") to the
799 corresponding unaccented US-ASCII letters (like a simple character
800 101, "e"), but clearly this is often not feasible, and an
801 unrenderable character may be represented as "?", or the like. In
802 attempting a sane fallback (as from E<233> to "e"), Pod formatters
803 may use the %Latin1Code_to_fallback table in Pod::Escapes, or
804 Text::Unidecode, if available.
805 For example, this Pod text:
807 magic is enabled if you set C<$Currency> to 'E<euro>'.
809 may be rendered as: "magic is enabled if you set $Currency to '?'"
811 or as "magic is enabled if you set $Currency to '[euro]'", or as
812 "magic is enabled if you set $Currency to '[x20AC]', etc.
813 A Pod formatter may also note, in a comment or warning, a list of
815 what unrenderable characters were encountered.
816 · E<...> may freely appear in any formatting code (other than in
818 another E<...> or in an Z<>). That is, "X<The E<euro>1,000,000
819 Solution>" is valid, as is "L<The E<euro>1,000,000
820 Solution|Million::Euros>".
821 · Some Pod formatters output to formats that implement non-breaking
823 spaces as an individual character (which I'll call "NBSP"), and
824 others output to formats that implement non-breaking spaces just as
825 spaces wrapped in a "don't break this across lines" code. Note
826 that at the level of Pod, both sorts of codes can occur: Pod can
827 contain a NBSP character (whether as a literal, or as a "E<160>" or
828 "E<nbsp>" code); and Pod can contain "S<foo I<bar> baz>" codes,
829 where "mere spaces" (character 32) in such codes are taken to
830 represent non-breaking spaces. Pod parsers should consider
831 supporting the optional parsing of "S<foo I<bar> baz>" as if it
832 were "fooNBSPI<bar>NBSPbaz", and, going the other way, the optional
833 parsing of groups of words joined by NBSP's as if each group were
834 in a S<...> code, so that formatters may use the representation
835 that maps best to what the output format demands.
836 · Some processors may find that the "S<...>" code is easiest to
838 implement by replacing each space in the parse tree under the
839 content of the S, with an NBSP. But note: the replacement should
840 apply not to spaces in all text, but only to spaces in printable
841 text. (This distinction may or may not be evident in the
842 particular tree/event model implemented by the Pod parser.) For
843 example, consider this unusual case:
844 S<L</Autoloaded Functions>>
846 This means that the space in the middle of the visible link text
848 must not be broken across lines. In other words, it's the same as
849 this:
850 L<"AutoloadedE<160>Functions"/Autoloaded Functions>
852 However, a misapplied space-to-NBSP replacement could (wrongly)
854 produce something equivalent to this:
855 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
857 ...which is almost definitely not going to work as a hyperlink
859 (assuming this formatter outputs a format supporting hypertext).
860 Formatters may choose to just not support the S format code,
862 especially in cases where the output format simply has no NBSP
863 character/code and no code for "don't break this stuff across
864 lines".
865 · Besides the NBSP character discussed above, implementors are
867 reminded of the existence of the other "special" character in
868 Latin-1, the "soft hyphen" character, also known as "discretionary
869 hyphen", i.e. "E<173>" = "E<0xAD>" = "E<shy>"). This character
870 expresses an optional hyphenation point. That is, it normally
871 renders as nothing, but may render as a "-" if a formatter breaks
872 the word at that point. Pod formatters should, as appropriate, do
873 one of the following: 1) render this with a code with the same
874 meaning (e.g., "\-" in RTF), 2) pass it through in the expectation
875 that the formatter understands this character as such, or 3) delete
876 it.
877 For example:
879 sigE<shy>action
881 manuE<shy>script
882 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
883 These signal to a formatter that if it is to hyphenate "sigaction"
885 or "manuscript", then it should be done as "sig-[linebreak]action"
886 or "manu-[linebreak]script" (and if it doesn't hyphenate it, then
887 the "E<shy>" doesn't show up at all). And if it is to hyphenate
888 "Jarkko" and/or "Hietaniemi", it can do so only at the points where
889 there is a "E<shy>" code.
890 In practice, it is anticipated that this character will not be used
892 often, but formatters should either support it, or delete it.
893 · If you think that you want to add a new command to Pod (like, say,
895 a "=biblio" command), consider whether you could get the same
896 effect with a for or begin/end sequence: "=for biblio ..." or
897 "=begin biblio" ... "=end biblio". Pod processors that don't
898 understand "=for biblio", etc, will simply ignore it, whereas they
899 may complain loudly if they see "=biblio".
900 · Throughout this document, "Pod" has been the preferred spelling for
902 the name of the documentation format. One may also use "POD" or
903 "pod". For the documentation that is (typically) in the Pod
904 format, you may use "pod", or "Pod", or "POD". Understanding these
905 distinctions is useful; but obsessing over how to spell them,
906 usually is not.
907
909 As you can tell from a glance at perlpod, the L<...> code is the most
910 complex of the Pod formatting codes. The points below will hopefully
911 clarify what it means and how processors should deal with it.
912 · In parsing an L<...> code, Pod parsers must distinguish at least
914 four attributes:
915 First:
917 The link-text. If there is none, this must be undef. (E.g.,
918 in "L<Perl Functions|perlfunc>", the link-text is "Perl
919 Functions". In "L<Time::HiRes>" and even "L<|Time::HiRes>",
920 there is no link text. Note that link text may contain
921 formatting.)
922 Second:
924 The possibly inferred link-text -- i.e., if there was no real
925 link text, then this is the text that we'll infer in its place.
926 (E.g., for "L<Getopt::Std>", the inferred link text is
927 "Getopt::Std".)
928 Third:
930 The name or URL, or undef if none. (E.g., in "L<Perl
931 Functions|perlfunc>", the name -- also sometimes called the
932 page -- is "perlfunc". In "L</CAVEATS>", the name is undef.)
933 Fourth:
935 The section (AKA "item" in older perlpods), or undef if none.
936 E.g., in "L<Getopt::Std/DESCRIPTION>", "DESCRIPTION" is the
937 section. (Note that this is not the same as a manpage section
938 like the "5" in "man 5 crontab". "Section Foo" in the Pod
939 sense means the part of the text that's introduced by the
940 heading or item whose text is "Foo".)
941 Pod parsers may also note additional attributes including:
943 Fifth:
945 A flag for whether item 3 (if present) is a URL (like
946 "http://lists.perl.org" is), in which case there should be no
947 section attribute; a Pod name (like "perldoc" and "Getopt::Std"
948 are); or possibly a man page name (like "crontab(5)" is).
949 Sixth:
951 The raw original L<...> content, before text is split on "|",
952 "/", etc, and before E<...> codes are expanded.
953 (The above were numbered only for concise reference below. It is
955 not a requirement that these be passed as an actual list or array.)
956 For example:
958 L<Foo::Bar>
960 => undef, # link text
961 "Foo::Bar", # possibly inferred link text
962 "Foo::Bar", # name
963 undef, # section
964 'pod', # what sort of link
965 "Foo::Bar" # original content
966 L<Perlport's section on NL's|perlport/Newlines>
968 => "Perlport's section on NL's", # link text
969 "Perlport's section on NL's", # possibly inferred link text
970 "perlport", # name
971 "Newlines", # section
972 'pod', # what sort of link
973 "Perlport's section on NL's|perlport/Newlines" # orig. content
974 L<perlport/Newlines>
976 => undef, # link text
977 '"Newlines" in perlport', # possibly inferred link text
978 "perlport", # name
979 "Newlines", # section
980 'pod', # what sort of link
981 "perlport/Newlines" # original content
982 L<crontab(5)/"DESCRIPTION">
984 => undef, # link text
985 '"DESCRIPTION" in crontab(5)', # possibly inferred link text
986 "crontab(5)", # name
987 "DESCRIPTION", # section
988 'man', # what sort of link
989 'crontab(5)/"DESCRIPTION"' # original content
990 L</Object Attributes>
992 => undef, # link text
993 '"Object Attributes"', # possibly inferred link text
994 undef, # name
995 "Object Attributes", # section
996 'pod', # what sort of link
997 "/Object Attributes" # original content
998 L<http://www.perl.org/>
1000 => undef, # link text
1001 "http://www.perl.org/", # possibly inferred link text
1002 "http://www.perl.org/", # name
1003 undef, # section
1004 'url', # what sort of link
1005 "http://www.perl.org/" # original content
1006 L<Perl.org|http://www.perl.org/>
1008 => "Perl.org", # link text
1009 "http://www.perl.org/", # possibly inferred link text
1010 "http://www.perl.org/", # name
1011 undef, # section
1012 'url', # what sort of link
1013 "Perl.org|http://www.perl.org/" # original content
1014 Note that you can distinguish URL-links from anything else by the
1016 fact that they match "m/\A\w+:[^:\s]\S*\z/". So
1017 "L<http://www.perl.com>" is a URL, but "L<HTTP::Response>" isn't.
1018 · In case of L<...> codes with no "text|" part in them, older
1020 formatters have exhibited great variation in actually displaying
1021 the link or cross reference. For example, L<crontab(5)> would
1022 render as "the crontab(5) manpage", or "in the crontab(5) manpage"
1023 or just "crontab(5)".
1024 Pod processors must now treat "text|"-less links as follows:
1026 L<name> => L<name|name>
1028 L</section> => L<"section"|/section>
1029 L<name/section> => L<"section" in name|name/section>
1030 · Note that section names might contain markup. I.e., if a section
1032 starts with:
1033 =head2 About the C<-M> Operator
1035 or with:
1037 =item About the C<-M> Operator
1039 then a link to it would look like this:
1041 L<somedoc/About the C<-M> Operator>
1043 Formatters may choose to ignore the markup for purposes of
1045 resolving the link and use only the renderable characters in the
1046 section name, as in:
1047 <h1><a name="About_the_-M_Operator">About the <code>-M</code>
1049 Operator</h1>
1050 ...
1052 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
1054 Operator" in somedoc</a>
1055 · Previous versions of perlpod distinguished "L<name/"section">"
1057 links from "L<name/item>" links (and their targets). These have
1058 been merged syntactically and semantically in the current
1059 specification, and section can refer either to a "=headn Heading
1060 Content" command or to a "=item Item Content" command. This
1061 specification does not specify what behavior should be in the case
1062 of a given document having several things all seeming to produce
1063 the same section identifier (e.g., in HTML, several things all
1064 producing the same anchorname in <a name="anchorname">...</a>
1065 elements). Where Pod processors can control this behavior, they
1066 should use the first such anchor. That is, "L<Foo/Bar>" refers to
1067 the first "Bar" section in Foo.
1068 But for some processors/formats this cannot be easily controlled;
1070 as with the HTML example, the behavior of multiple ambiguous <a
1071 name="anchorname">...</a> is most easily just left up to browsers
1072 to decide.
1073 · Authors wanting to link to a particular (absolute) URL, must do so
1075 only with "L<scheme:...>" codes (like L<http://www.perl.org>), and
1076 must not attempt "L<Some Site Name|scheme:...>" codes. This
1077 restriction avoids many problems in parsing and rendering L<...>
1078 codes.
1079 · In a "L<text|...>" code, text may contain formatting codes for
1081 formatting or for E<...> escapes, as in:
1082 L<B<ummE<234>stuff>|...>
1084 For "L<...>" codes without a "name|" part, only "E<...>" and "Z<>"
1086 codes may occur -- no other formatting codes. That is, authors
1087 should not use ""L<B<Foo::Bar>>"".
1088 Note, however, that formatting codes and Z<>'s can occur in any and
1090 all parts of an L<...> (i.e., in name, section, text, and url).
1091 Authors must not nest L<...> codes. For example, "L<The
1093 L<Foo::Bar> man page>" should be treated as an error.
1094 · Note that Pod authors may use formatting codes inside the "text"
1096 part of "L<text|name>" (and so on for L<text|/"sec">).
1097 In other words, this is valid:
1099 Go read L<the docs on C<$.>|perlvar/"$.">
1101 Some output formats that do allow rendering "L<...>" codes as
1103 hypertext, might not allow the link-text to be formatted; in that
1104 case, formatters will have to just ignore that formatting.
1105 · At time of writing, "L<name>" values are of two types: either the
1107 name of a Pod page like "L<Foo::Bar>" (which might be a real Perl
1108 module or program in an @INC / PATH directory, or a .pod file in
1109 those places); or the name of a UNIX man page, like
1110 "L<crontab(5)>". In theory, "L<chmod>" in ambiguous between a Pod
1111 page called "chmod", or the Unix man page "chmod" (in whatever man-
1112 section). However, the presence of a string in parens, as in
1113 "crontab(5)", is sufficient to signal that what is being discussed
1114 is not a Pod page, and so is presumably a UNIX man page. The
1115 distinction is of no importance to many Pod processors, but some
1116 processors that render to hypertext formats may need to distinguish
1117 them in order to know how to render a given "L<foo>" code.
1118 · Previous versions of perlpod allowed for a "L<section>" syntax (as
1120 in "L<Object Attributes>"), which was not easily distinguishable
1121 from "L<name>" syntax and for "L<"section">" which was only
1122 slightly less ambiguous. This syntax is no longer in the
1123 specification, and has been replaced by the "L</section>" syntax
1124 (where the slash was formerly optional). Pod parsers should
1125 tolerate the "L<"section">" syntax, for a while at least. The
1126 suggested heuristic for distinguishing "L<section>" from "L<name>"
1127 is that if it contains any whitespace, it's a section. Pod
1128 processors should warn about this being deprecated syntax.
1129
1131 "=over"..."=back" regions are used for various kinds of list-like
1132 structures. (I use the term "region" here simply as a collective term
1133 for everything from the "=over" to the matching "=back".)
1134 · The non-zero numeric indentlevel in "=over indentlevel" ...
1136 "=back" is used for giving the formatter a clue as to how many
1137 "spaces" (ems, or roughly equivalent units) it should tab over,
1138 although many formatters will have to convert this to an absolute
1139 measurement that may not exactly match with the size of spaces (or
1140 M's) in the document's base font. Other formatters may have to
1141 completely ignore the number. The lack of any explicit indentlevel
1142 parameter is equivalent to an indentlevel value of 4. Pod
1143 processors may complain if indentlevel is present but is not a
1144 positive number matching "m/\A(\d*\.)?\d+\z/".
1145 · Authors of Pod formatters are reminded that "=over" ... "=back" may
1147 map to several different constructs in your output format. For
1148 example, in converting Pod to (X)HTML, it can map to any of
1149 <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
1150 <blockquote>...</blockquote>. Similarly, "=item" can map to <li>
1151 or <dt>.
1152 · Each "=over" ... "=back" region should be one of the following:
1154 · An "=over" ... "=back" region containing only "=item *"
1156 commands, each followed by some number of ordinary/verbatim
1157 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1158 paragraphs, and "=begin"..."=end" regions.
1159 (Pod processors must tolerate a bare "=item" as if it were
1161 "=item *".) Whether "*" is rendered as a literal asterisk, an
1162 "o", or as some kind of real bullet character, is left up to
1163 the Pod formatter, and may depend on the level of nesting.
1164 · An "=over" ... "=back" region containing only
1166 "m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each group
1167 of them) followed by some number of ordinary/verbatim
1168 paragraphs, other nested "=over" ... "=back" regions, "=for..."
1169 paragraphs, and/or "=begin"..."=end" codes. Note that the
1170 numbers must start at 1 in each section, and must proceed in
1171 order and without skipping numbers.
1172 (Pod processors must tolerate lines like "=item 1" as if they
1174 were "=item 1.", with the period.)
1175 · An "=over" ... "=back" region containing only "=item [text]"
1177 commands, each one (or each group of them) followed by some
1178 number of ordinary/verbatim paragraphs, other nested "=over"
1179 ... "=back" regions, or "=for..." paragraphs, and
1180 "=begin"..."=end" regions.
1181 The "=item [text]" paragraph should not match
1183 "m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor
1184 should it match just "m/\A=item\s*\z/".
1185 · An "=over" ... "=back" region containing no "=item" paragraphs
1187 at all, and containing only some number of ordinary/verbatim
1188 paragraphs, and possibly also some nested "=over" ... "=back"
1189 regions, "=for..." paragraphs, and "=begin"..."=end" regions.
1190 Such an itemless "=over" ... "=back" region in Pod is
1191 equivalent in meaning to a "<blockquote>...</blockquote>"
1192 element in HTML.
1193 Note that with all the above cases, you can determine which type of
1195 "=over" ... "=back" you have, by examining the first (non-"=cut",
1196 non-"=pod") Pod paragraph after the "=over" command.
1197 · Pod formatters must tolerate arbitrarily large amounts of text in
1199 the "=item text..." paragraph. In practice, most such paragraphs
1200 are short, as in:
1201 =item For cutting off our trade with all parts of the world
1203 But they may be arbitrarily long:
1205 =item For transporting us beyond seas to be tried for pretended
1207 offenses
1208 =item He is at this time transporting large armies of foreign
1210 mercenaries to complete the works of death, desolation and
1211 tyranny, already begun with circumstances of cruelty and perfidy
1212 scarcely paralleled in the most barbarous ages, and totally
1213 unworthy the head of a civilized nation.
1214 · Pod processors should tolerate "=item *" / "=item number" commands
1216 with no accompanying paragraph. The middle item is an example:
1217 =over
1219 =item 1
1221 Pick up dry cleaning.
1223 =item 2
1225 =item 3
1227 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
1229 =back
1231 · No "=over" ... "=back" region can contain headings. Processors may
1233 treat such a heading as an error.
1234 · Note that an "=over" ... "=back" region should have some content.
1236 That is, authors should not have an empty region like this:
1237 =over
1239 =back
1241 Pod processors seeing such a contentless "=over" ... "=back"
1243 region, may ignore it, or may report it as an error.
1244 · Processors must tolerate an "=over" list that goes off the end of
1246 the document (i.e., which has no matching "=back"), but they may
1247 warn about such a list.
1248 · Authors of Pod formatters should note that this construct:
1250 =item Neque
1252 =item Porro
1254 =item Quisquam Est
1256 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1258 velit, sed quia non numquam eius modi tempora incidunt ut
1259 labore et dolore magnam aliquam quaerat voluptatem.
1260 =item Ut Enim
1262 is semantically ambiguous, in a way that makes formatting decisions
1264 a bit difficult. On the one hand, it could be mention of an item
1265 "Neque", mention of another item "Porro", and mention of another
1266 item "Quisquam Est", with just the last one requiring the
1267 explanatory paragraph "Qui dolorem ipsum quia dolor..."; and then
1268 an item "Ut Enim". In that case, you'd want to format it like so:
1269 Neque
1271 Porro
1273 Quisquam Est
1275 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1276 velit, sed quia non numquam eius modi tempora incidunt ut
1277 labore et dolore magnam aliquam quaerat voluptatem.
1278 Ut Enim
1280 But it could equally well be a discussion of three (related or
1282 equivalent) items, "Neque", "Porro", and "Quisquam Est", followed
1283 by a paragraph explaining them all, and then a new item "Ut Enim".
1284 In that case, you'd probably want to format it like so:
1285 Neque
1287 Porro
1288 Quisquam Est
1289 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1290 velit, sed quia non numquam eius modi tempora incidunt ut
1291 labore et dolore magnam aliquam quaerat voluptatem.
1292 Ut Enim
1294 But (for the foreseeable future), Pod does not provide any way for
1296 Pod authors to distinguish which grouping is meant by the above
1297 "=item"-cluster structure. So formatters should format it like so:
1298 Neque
1300 Porro
1302 Quisquam Est
1304 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
1306 velit, sed quia non numquam eius modi tempora incidunt ut
1307 labore et dolore magnam aliquam quaerat voluptatem.
1308 Ut Enim
1310 That is, there should be (at least roughly) equal spacing between
1312 items as between paragraphs (although that spacing may well be less
1313 than the full height of a line of text). This leaves it to the
1314 reader to use (con)textual cues to figure out whether the "Qui
1315 dolorem ipsum..." paragraph applies to the "Quisquam Est" item or
1316 to all three items "Neque", "Porro", and "Quisquam Est". While not
1317 an ideal situation, this is preferable to providing formatting cues
1318 that may be actually contrary to the author's intent.
1319
1321 Data paragraphs are typically used for inlining non-Pod data that is to
1322 be used (typically passed through) when rendering the document to a
1323 specific format:
1324 =begin rtf
1326 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1328 =end rtf
1330 The exact same effect could, incidentally, be achieved with a single
1332 "=for" paragraph:
1333 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
1335 (Although that is not formally a data paragraph, it has the same
1337 meaning as one, and Pod parsers may parse it as one.)
1338 Another example of a data paragraph:
1340 =begin html
1342 I like <em>PIE</em>!
1344 <hr>Especially pecan pie!
1346 =end html
1348 If these were ordinary paragraphs, the Pod parser would try to expand
1350 the "E</em>" (in the first paragraph) as a formatting code, just like
1351 "E<lt>" or "E<eacute>". But since this is in a "=begin
1352 identifier"..."=end identifier" region and the identifier "html"
1353 doesn't begin have a ":" prefix, the contents of this region are stored
1354 as data paragraphs, instead of being processed as ordinary paragraphs
1355 (or if they began with a spaces and/or tabs, as verbatim paragraphs).
1356 As a further example: At time of writing, no "biblio" identifier is
1358 supported, but suppose some processor were written to recognize it as a
1359 way of (say) denoting a bibliographic reference (necessarily containing
1360 formatting codes in ordinary paragraphs). The fact that "biblio"
1361 paragraphs were meant for ordinary processing would be indicated by
1362 prefacing each "biblio" identifier with a colon:
1363 =begin :biblio
1365 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1367 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1368 =end :biblio
1370 This would signal to the parser that paragraphs in this begin...end
1372 region are subject to normal handling as ordinary/verbatim paragraphs
1373 (while still tagged as meant only for processors that understand the
1374 "biblio" identifier). The same effect could be had with:
1375 =for :biblio
1377 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1378 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1379 The ":" on these identifiers means simply "process this stuff normally,
1381 even though the result will be for some special target". I suggest
1382 that parser APIs report "biblio" as the target identifier, but also
1383 report that it had a ":" prefix. (And similarly, with the above
1384 "html", report "html" as the target identifier, and note the lack of a
1385 ":" prefix.)
1386 Note that a "=begin identifier"..."=end identifier" region where
1388 identifier begins with a colon, can contain commands. For example:
1389 =begin :biblio
1391 Wirth's classic is available in several editions, including:
1393 =for comment
1395 hm, check abebooks.com for how much used copies cost.
1396 =over
1398 =item
1400 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1402 Teubner, Stuttgart. [Yes, it's in German.]
1403 =item
1405 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1407 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1408 =back
1410 =end :biblio
1412 Note, however, a "=begin identifier"..."=end identifier" region where
1414 identifier does not begin with a colon, should not directly contain
1415 "=head1" ... "=head4" commands, nor "=over", nor "=back", nor "=item".
1416 For example, this may be considered invalid:
1417 =begin somedata
1419 This is a data paragraph.
1421 =head1 Don't do this!
1423 This is a data paragraph too.
1425 =end somedata
1427 A Pod processor may signal that the above (specifically the "=head1"
1429 paragraph) is an error. Note, however, that the following should not
1430 be treated as an error:
1431 =begin somedata
1433 This is a data paragraph.
1435 =cut
1437 # Yup, this isn't Pod anymore.
1439 sub excl { (rand() > .5) ? "hoo!" : "hah!" }
1440 =pod
1442 This is a data paragraph too.
1444 =end somedata
1446 And this too is valid:
1448 =begin someformat
1450 This is a data paragraph.
1452 And this is a data paragraph.
1454 =begin someotherformat
1456 This is a data paragraph too.
1458 And this is a data paragraph too.
1460 =begin :yetanotherformat
1462 =head2 This is a command paragraph!
1464 This is an ordinary paragraph!
1466 And this is a verbatim paragraph!
1468 =end :yetanotherformat
1470 =end someotherformat
1472 Another data paragraph!
1474 =end someformat
1476 The contents of the above "=begin :yetanotherformat" ... "=end
1478 :yetanotherformat" region aren't data paragraphs, because the
1479 immediately containing region's identifier (":yetanotherformat") begins
1480 with a colon. In practice, most regions that contain data paragraphs
1481 will contain only data paragraphs; however, the above nesting is
1482 syntactically valid as Pod, even if it is rare. However, the handlers
1483 for some formats, like "html", will accept only data paragraphs, not
1484 nested regions; and they may complain if they see (targeted for them)
1485 nested regions, or commands, other than "=end", "=pod", and "=cut".
1486 Also consider this valid structure:
1488 =begin :biblio
1490 Wirth's classic is available in several editions, including:
1492 =over
1494 =item
1496 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
1498 Teubner, Stuttgart. [Yes, it's in German.]
1499 =item
1501 Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
1503 Programs.> Prentice-Hall, Englewood Cliffs, NJ.
1504 =back
1506 Buy buy buy!
1508 =begin html
1510 <img src='wirth_spokesmodeling_book.png'>
1512 <hr>
1514 =end html
1516 Now now now!
1518 =end :biblio
1520 There, the "=begin html"..."=end html" region is nested inside the
1522 larger "=begin :biblio"..."=end :biblio" region. Note that the content
1523 of the "=begin html"..."=end html" region is data paragraph(s), because
1524 the immediately containing region's identifier ("html") doesn't begin
1525 with a colon.
1526 Pod parsers, when processing a series of data paragraphs one after
1528 another (within a single region), should consider them to be one large
1529 data paragraph that happens to contain blank lines. So the content of
1530 the above "=begin html"..."=end html" may be stored as two data
1531 paragraphs (one consisting of "<img
1532 src='wirth_spokesmodeling_book.png'>\n" and another consisting of
1533 "<hr>\n"), but should be stored as a single data paragraph (consisting
1534 of "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
1535 Pod processors should tolerate empty "=begin something"..."=end
1537 something" regions, empty "=begin :something"..."=end :something"
1538 regions, and contentless "=for something" and "=for :something"
1539 paragraphs. I.e., these should be tolerated:
1540 =for html
1542 =begin html
1544 =end html
1546 =begin :biblio
1548 =end :biblio
1550 Incidentally, note that there's no easy way to express a data paragraph
1552 starting with something that looks like a command. Consider:
1553 =begin stuff
1555 =shazbot
1557 =end stuff
1559 There, "=shazbot" will be parsed as a Pod command "shazbot", not as a
1561 data paragraph "=shazbot\n". However, you can express a data paragraph
1562 consisting of "=shazbot\n" using this code:
1563 =for stuff =shazbot
1565 The situation where this is necessary, is presumably quite rare.
1567 Note that =end commands must match the currently open =begin command.
1569 That is, they must properly nest. For example, this is valid:
1570 =begin outer
1572 X
1574 =begin inner
1576 Y
1578 =end inner
1580 Z
1582 =end outer
1584 while this is invalid:
1586 =begin outer
1588 X
1590 =begin inner
1592 Y
1594 =end outer
1596 Z
1598 =end inner
1600 This latter is improper because when the "=end outer" command is seen,
1602 the currently open region has the formatname "inner", not "outer". (It
1603 just happens that "outer" is the format name of a higher-up region.)
1604 This is an error. Processors must by default report this as an error,
1605 and may halt processing the document containing that error. A
1606 corollary of this is that regions cannot "overlap" -- i.e., the latter
1607 block above does not represent a region called "outer" which contains X
1608 and Y, overlapping a region called "inner" which contains Y and Z. But
1609 because it is invalid (as all apparently overlapping regions would be),
1610 it doesn't represent that, or anything at all.
1611 Similarly, this is invalid:
1613 =begin thing
1615 =end hting
1617 This is an error because the region is opened by "thing", and the
1619 "=end" tries to close "hting" [sic].
1620 This is also invalid:
1622 =begin thing
1624 =end
1626 This is invalid because every "=end" command must have a formatname
1628 parameter.
1629
1631 perlpod, "PODs: Embedded Documentation" in perlsyn, podchecker
1632
1634 Sean M. Burke
1635perl v5.10.1 2017-03-22 PERLPODSPEC(1)