1=encoding utf8 2 3=head1 NAME 4 5perlpodspec - Plain Old Documentation: format specification and notes 6 7=head1 DESCRIPTION 8 9This document is detailed notes on the Pod markup language. Most 10people will only have to read L<perlpod|perlpod> to know how to write 11in Pod, but this document may answer some incidental questions to do 12with parsing and rendering Pod. 13 14In this document, "must" / "must not", "should" / 15"should not", and "may" have their conventional (cf. RFC 2119) 16meanings: "X must do Y" means that if X doesn't do Y, it's against 17this specification, and should really be fixed. "X should do Y" 18means that it's recommended, but X may fail to do Y, if there's a 19good reason. "X may do Y" is merely a note that X can do Y at 20will (although it is up to the reader to detect any connotation of 21"and I think it would be I<nice> if X did Y" versus "it wouldn't 22really I<bother> me if X did Y"). 23 24Notably, when I say "the parser should do Y", the 25parser may fail to do Y, if the calling application explicitly 26requests that the parser I<not> do Y. I often phrase this as 27"the parser should, by default, do Y." This doesn't I<require> 28the parser to provide an option for turning off whatever 29feature Y is (like expanding tabs in verbatim paragraphs), although 30it implicates that such an option I<may> be provided. 31 32=head1 Pod Definitions 33 34Pod is embedded in files, typically Perl source files, although you 35can write a file that's nothing but Pod. 36 37A B<line> in a file consists of zero or more non-newline characters, 38terminated by either a newline or the end of the file. 39 40A B<newline sequence> is usually a platform-dependent concept, but 41Pod parsers should understand it to mean any of CR (ASCII 13), LF 42(ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in 43addition to any other system-specific meaning. The first CR/CRLF/LF 44sequence in the file may be used as the basis for identifying the 45newline sequence for parsing the rest of the file. 46 47A B<blank line> is a line consisting entirely of zero or more spaces 48(ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file. 49A B<non-blank line> is a line containing one or more characters other 50than space or tab (and terminated by a newline or end-of-file). 51 52(I<Note:> Many older Pod parsers did not accept a line consisting of 53spaces/tabs and then a newline as a blank line. The only lines they 54considered blank were lines consisting of I<no characters at all>, 55terminated by a newline.) 56 57B<Whitespace> is used in this document as a blanket term for spaces, 58tabs, and newline sequences. (By itself, this term usually refers 59to literal whitespace. That is, sequences of whitespace characters 60in Pod source, as opposed to "EE<lt>32>", which is a formatting 61code that I<denotes> a whitespace character.) 62 63A B<Pod parser> is a module meant for parsing Pod (regardless of 64whether this involves calling callbacks or building a parse tree or 65directly formatting it). A B<Pod formatter> (or B<Pod translator>) 66is a module or program that converts Pod to some other format (HTML, 67plaintext, TeX, PostScript, RTF). A B<Pod processor> might be a 68formatter or translator, or might be a program that does something 69else with the Pod (like counting words, scanning for index points, 70etc.). 71 72Pod content is contained in B<Pod blocks>. A Pod block starts with a 73line that matches C<m/\A=[a-zA-Z]/>, and continues up to the next line 74that matches C<m/\A=cut/> or up to the end of the file if there is 75no C<m/\A=cut/> line. 76 77=for comment 78 The current perlsyn says: 79 [beginquote] 80 Note that pod translators should look at only paragraphs beginning 81 with a pod directive (it makes parsing easier), whereas the compiler 82 actually knows to look for pod escapes even in the middle of a 83 paragraph. This means that the following secret stuff will be ignored 84 by both the compiler and the translators. 85 $a=3; 86 =secret stuff 87 warn "Neither POD nor CODE!?" 88 =cut back 89 print "got $a\n"; 90 You probably shouldn't rely upon the warn() being podded out forever. 91 Not all pod translators are well-behaved in this regard, and perhaps 92 the compiler will become pickier. 93 [endquote] 94 I think that those paragraphs should just be removed; paragraph-based 95 parsing seems to have been largely abandoned, because of the hassle 96 with non-empty blank lines messing up what people meant by "paragraph". 97 Even if the "it makes parsing easier" bit were especially true, 98 it wouldn't be worth the confusion of having perl and pod2whatever 99 actually disagree on what can constitute a Pod block. 100 101Note that a parser is not expected to distinguish between something that 102looks like pod, but is in a quoted string, such as a here document. 103 104Within a Pod block, there are B<Pod paragraphs>. A Pod paragraph 105consists of non-blank lines of text, separated by one or more blank 106lines. 107 108For purposes of Pod processing, there are four types of paragraphs in 109a Pod block: 110 111=over 112 113=item * 114 115A command paragraph (also called a "directive"). The first line of 116this paragraph must match C<m/\A=[a-zA-Z]/>. Command paragraphs are 117typically one line, as in: 118 119 =head1 NOTES 120 121 =item * 122 123But they may span several (non-blank) lines: 124 125 =for comment 126 Hm, I wonder what it would look like if 127 you tried to write a BNF for Pod from this. 128 129 =head3 Dr. Strangelove, or: How I Learned to 130 Stop Worrying and Love the Bomb 131 132I<Some> command paragraphs allow formatting codes in their content 133(i.e., after the part that matches C<m/\A=[a-zA-Z]\S*\s*/>), as in: 134 135 =head1 Did You Remember to C<use strict;>? 136 137In other words, the Pod processing handler for "head1" will apply the 138same processing to "Did You Remember to CE<lt>use strict;>?" that it 139would to an ordinary paragraph (i.e., formatting codes like 140"CE<lt>...>") are parsed and presumably formatted appropriately, and 141whitespace in the form of literal spaces and/or tabs is not 142significant. 143 144=item * 145 146A B<verbatim paragraph>. The first line of this paragraph must be a 147literal space or tab, and this paragraph must not be inside a "=begin 148I<identifier>", ... "=end I<identifier>" sequence unless 149"I<identifier>" begins with a colon (":"). That is, if a paragraph 150starts with a literal space or tab, but I<is> inside a 151"=begin I<identifier>", ... "=end I<identifier>" region, then it's 152a data paragraph, unless "I<identifier>" begins with a colon. 153 154Whitespace I<is> significant in verbatim paragraphs (although, in 155processing, tabs are probably expanded). 156 157=item * 158 159An B<ordinary paragraph>. A paragraph is an ordinary paragraph 160if its first line matches neither C<m/\A=[a-zA-Z]/> nor 161C<m/\A[ \t]/>, I<and> if it's not inside a "=begin I<identifier>", 162... "=end I<identifier>" sequence unless "I<identifier>" begins with 163a colon (":"). 164 165=item * 166 167A B<data paragraph>. This is a paragraph that I<is> inside a "=begin 168I<identifier>" ... "=end I<identifier>" sequence where 169"I<identifier>" does I<not> begin with a literal colon (":"). In 170some sense, a data paragraph is not part of Pod at all (i.e., 171effectively it's "out-of-band"), since it's not subject to most kinds 172of Pod parsing; but it is specified here, since Pod 173parsers need to be able to call an event for it, or store it in some 174form in a parse tree, or at least just parse I<around> it. 175 176=back 177 178For example: consider the following paragraphs: 179 180 # <- that's the 0th column 181 182 =head1 Foo 183 184 Stuff 185 186 $foo->bar 187 188 =cut 189 190Here, "=head1 Foo" and "=cut" are command paragraphs because the first 191line of each matches C<m/\A=[a-zA-Z]/>. "I<[space][space]>$foo->bar" 192is a verbatim paragraph, because its first line starts with a literal 193whitespace character (and there's no "=begin"..."=end" region around). 194 195The "=begin I<identifier>" ... "=end I<identifier>" commands stop 196paragraphs that they surround from being parsed as ordinary or verbatim 197paragraphs, if I<identifier> doesn't begin with a colon. This 198is discussed in detail in the section 199L</About Data Paragraphs and "=beginE<sol>=end" Regions>. 200 201=head1 Pod Commands 202 203This section is intended to supplement and clarify the discussion in 204L<perlpod/"Command Paragraph">. These are the currently recognized 205Pod commands: 206 207=over 208 209=item "=head1", "=head2", "=head3", "=head4", "=head5", "=head6" 210 211This command indicates that the text in the remainder of the paragraph 212is a heading. That text may contain formatting codes. Examples: 213 214 =head1 Object Attributes 215 216 =head3 What B<Not> to Do! 217 218Both C<=head5> and C<=head6> were added in 2020 and might not be 219supported on all Pod parsers. L<Pod::Simple> 3.41 was released on October 2202020 and supports both of these providing support for all 221L<Pod::Simple>-based Pod parsers. 222 223=item "=pod" 224 225This command indicates that this paragraph begins a Pod block. (If we 226are already in the middle of a Pod block, this command has no effect at 227all.) If there is any text in this command paragraph after "=pod", 228it must be ignored. Examples: 229 230 =pod 231 232 This is a plain Pod paragraph. 233 234 =pod This text is ignored. 235 236=item "=cut" 237 238This command indicates that this line is the end of this previously 239started Pod block. If there is any text after "=cut" on the line, it must be 240ignored. Examples: 241 242 =cut 243 244 =cut The documentation ends here. 245 246 =cut 247 # This is the first line of program text. 248 sub foo { # This is the second. 249 250It is an error to try to I<start> a Pod block with a "=cut" command. In 251that case, the Pod processor must halt parsing of the input file, and 252must by default emit a warning. 253 254=item "=over" 255 256This command indicates that this is the start of a list/indent 257region. If there is any text following the "=over", it must consist 258of only a nonzero positive numeral. The semantics of this numeral is 259explained in the L</"About =over...=back Regions"> section, further 260below. Formatting codes are not expanded. Examples: 261 262 =over 3 263 264 =over 3.5 265 266 =over 267 268=item "=item" 269 270This command indicates that an item in a list begins here. Formatting 271codes are processed. The semantics of the (optional) text in the 272remainder of this paragraph are 273explained in the L</"About =over...=back Regions"> section, further 274below. Examples: 275 276 =item 277 278 =item * 279 280 =item * 281 282 =item 14 283 284 =item 3. 285 286 =item C<< $thing->stuff(I<dodad>) >> 287 288 =item For transporting us beyond seas to be tried for pretended 289 offenses 290 291 =item He is at this time transporting large armies of foreign 292 mercenaries to complete the works of death, desolation and 293 tyranny, already begun with circumstances of cruelty and perfidy 294 scarcely paralleled in the most barbarous ages, and totally 295 unworthy the head of a civilized nation. 296 297=item "=back" 298 299This command indicates that this is the end of the region begun 300by the most recent "=over" command. It permits no text after the 301"=back" command. 302 303=item "=begin formatname" 304 305=item "=begin formatname parameter" 306 307This marks the following paragraphs (until the matching "=end 308formatname") as being for some special kind of processing. Unless 309"formatname" begins with a colon, the contained non-command 310paragraphs are data paragraphs. But if "formatname" I<does> begin 311with a colon, then non-command paragraphs are ordinary paragraphs 312or data paragraphs. This is discussed in detail in the section 313L</About Data Paragraphs and "=beginE<sol>=end" Regions>. 314 315It is advised that formatnames match the regexp 316C<m/\A:?[-a-zA-Z0-9_]+\z/>. Everything following whitespace after the 317formatname is a parameter that may be used by the formatter when dealing 318with this region. This parameter must not be repeated in the "=end" 319paragraph. Implementors should anticipate future expansion in the 320semantics and syntax of the first parameter to "=begin"/"=end"/"=for". 321 322=item "=end formatname" 323 324This marks the end of the region opened by the matching 325"=begin formatname" region. If "formatname" is not the formatname 326of the most recent open "=begin formatname" region, then this 327is an error, and must generate an error message. This 328is discussed in detail in the section 329L</About Data Paragraphs and "=beginE<sol>=end" Regions>. 330 331=item "=for formatname text..." 332 333This is synonymous with: 334 335 =begin formatname 336 337 text... 338 339 =end formatname 340 341That is, it creates a region consisting of a single paragraph; that 342paragraph is to be treated as a normal paragraph if "formatname" 343begins with a ":"; if "formatname" I<doesn't> begin with a colon, 344then "text..." will constitute a data paragraph. There is no way 345to use "=for formatname text..." to express "text..." as a verbatim 346paragraph. 347 348=item "=encoding encodingname" 349 350This command, which should occur early in the document (at least 351before any non-US-ASCII data!), declares that this document is 352encoded in the encoding I<encodingname>, which must be 353an encoding name that L<Encode> recognizes. (Encode's list 354of supported encodings, in L<Encode::Supported>, is useful here.) 355If the Pod parser cannot decode the declared encoding, it 356should emit a warning and may abort parsing the document 357altogether. 358 359A document having more than one "=encoding" line should be 360considered an error. Pod processors may silently tolerate this if 361the not-first "=encoding" lines are just duplicates of the 362first one (e.g., if there's a "=encoding utf8" line, and later on 363another "=encoding utf8" line). But Pod processors should complain if 364there are contradictory "=encoding" lines in the same document 365(e.g., if there is a "=encoding utf8" early in the document and 366"=encoding big5" later). Pod processors that recognize BOMs 367may also complain if they see an "=encoding" line 368that contradicts the BOM (e.g., if a document with a UTF-16LE 369BOM has an "=encoding shiftjis" line). 370 371=back 372 373If a Pod processor sees any command other than the ones listed 374above (like "=head", or "=haed1", or "=stuff", or "=cuttlefish", 375or "=w123"), that processor must by default treat this as an 376error. It must not process the paragraph beginning with that 377command, must by default warn of this as an error, and may 378abort the parse. A Pod parser may allow a way for particular 379applications to add to the above list of known commands, and to 380stipulate, for each additional command, whether formatting 381codes should be processed. 382 383Future versions of this specification may add additional 384commands. 385 386 387 388=head1 Pod Formatting Codes 389 390(Note that in previous drafts of this document and of perlpod, 391formatting codes were referred to as "interior sequences", and 392this term may still be found in the documentation for Pod parsers, 393and in error messages from Pod processors.) 394 395There are two syntaxes for formatting codes: 396 397=over 398 399=item * 400 401A formatting code starts with a capital letter (just US-ASCII [A-Z]) 402followed by a "<", any number of characters, and ending with the first 403matching ">". Examples: 404 405 That's what I<you> think! 406 407 What's C<CORE::dump()> for? 408 409 X<C<chmod> and C<unlink()> Under Different Operating Systems> 410 411=item * 412 413A formatting code starts with a capital letter (just US-ASCII [A-Z]) 414followed by two or more "<"'s, one or more whitespace characters, 415any number of characters, one or more whitespace characters, 416and ending with the first matching sequence of two or more ">"'s, where 417the number of ">"'s equals the number of "<"'s in the opening of this 418formatting code. Examples: 419 420 That's what I<< you >> think! 421 422 C<<< open(X, ">>thing.dat") || die $! >>> 423 424 B<< $foo->bar(); >> 425 426With this syntax, the whitespace character(s) after the "CE<lt><<" 427and before the ">>>" (or whatever letter) are I<not> renderable. They 428do not signify whitespace, are merely part of the formatting codes 429themselves. That is, these are all synonymous: 430 431 C<thing> 432 C<< thing >> 433 C<< thing >> 434 C<<< thing >>> 435 C<<<< 436 thing 437 >>>> 438 439and so on. 440 441Finally, the multiple-angle-bracket form does I<not> alter the interpretation 442of nested formatting codes, meaning that the following four example lines are 443identical in meaning: 444 445 B<example: C<$a E<lt>=E<gt> $b>> 446 447 B<example: C<< $a <=> $b >>> 448 449 B<example: C<< $a E<lt>=E<gt> $b >>> 450 451 B<<< example: C<< $a E<lt>=E<gt> $b >> >>> 452 453=back 454 455In parsing Pod, a notably tricky part is the correct parsing of 456(potentially nested!) formatting codes. Implementors should 457consult the code in the C<parse_text> routine in Pod::Parser as an 458example of a correct implementation. 459 460=over 461 462=item C<IE<lt>textE<gt>> -- italic text 463 464See the brief discussion in L<perlpod/"Formatting Codes">. 465 466=item C<BE<lt>textE<gt>> -- bold text 467 468See the brief discussion in L<perlpod/"Formatting Codes">. 469 470=item C<CE<lt>codeE<gt>> -- code text 471 472See the brief discussion in L<perlpod/"Formatting Codes">. 473 474=item C<FE<lt>filenameE<gt>> -- style for filenames 475 476See the brief discussion in L<perlpod/"Formatting Codes">. 477 478=item C<XE<lt>topic nameE<gt>> -- an index entry 479 480See the brief discussion in L<perlpod/"Formatting Codes">. 481 482This code is unusual in that most formatters completely discard 483this code and its content. Other formatters will render it with 484invisible codes that can be used in building an index of 485the current document. 486 487=item C<ZE<lt>E<gt>> -- a null (zero-effect) formatting code 488 489Discussed briefly in L<perlpod/"Formatting Codes">. 490 491This code is unusual in that it should have no content. That is, 492a processor may complain if it sees C<ZE<lt>potatoesE<gt>>. Whether 493or not it complains, the I<potatoes> text should ignored. 494 495=item C<LE<lt>nameE<gt>> -- a hyperlink 496 497The complicated syntaxes of this code are discussed at length in 498L<perlpod/"Formatting Codes">, and implementation details are 499discussed below, in L</"About LE<lt>...E<gt> Codes">. Parsing the 500contents of LE<lt>content> is tricky. Notably, the content has to be 501checked for whether it looks like a URL, or whether it has to be split 502on literal "|" and/or "/" (in the right order!), and so on, 503I<before> EE<lt>...> codes are resolved. 504 505=item C<EE<lt>escapeE<gt>> -- a character escape 506 507See L<perlpod/"Formatting Codes">, and several points in 508L</Notes on Implementing Pod Processors>. 509 510=item C<SE<lt>textE<gt>> -- text contains non-breaking spaces 511 512This formatting code is syntactically simple, but semantically 513complex. What it means is that each space in the printable 514content of this code signifies a non-breaking space. 515 516Consider: 517 518 C<$x ? $y : $z> 519 520 S<C<$x ? $y : $z>> 521 522Both signify the monospace (c[ode] style) text consisting of 523"$x", one space, "?", one space, ":", one space, "$z". The 524difference is that in the latter, with the S code, those spaces 525are not "normal" spaces, but instead are non-breaking spaces. 526 527=back 528 529 530If a Pod processor sees any formatting code other than the ones 531listed above (as in "NE<lt>...>", or "QE<lt>...>", etc.), that 532processor must by default treat this as an error. 533A Pod parser may allow a way for particular 534applications to add to the above list of known formatting codes; 535a Pod parser might even allow a way to stipulate, for each additional 536command, whether it requires some form of special processing, as 537LE<lt>...> does. 538 539Future versions of this specification may add additional 540formatting codes. 541 542Historical note: A few older Pod processors would not see a ">" as 543closing a "CE<lt>" code, if the ">" was immediately preceded by 544a "-". This was so that this: 545 546 C<$foo->bar> 547 548would parse as equivalent to this: 549 550 C<$foo-E<gt>bar> 551 552instead of as equivalent to a "C" formatting code containing 553only "$foo-", and then a "bar>" outside the "C" formatting code. This 554problem has since been solved by the addition of syntaxes like this: 555 556 C<< $foo->bar >> 557 558Compliant parsers must not treat "->" as special. 559 560Formatting codes absolutely cannot span paragraphs. If a code is 561opened in one paragraph, and no closing code is found by the end of 562that paragraph, the Pod parser must close that formatting code, 563and should complain (as in "Unterminated I code in the paragraph 564starting at line 123: 'Time objects are not...'"). So these 565two paragraphs: 566 567 I<I told you not to do this! 568 569 Don't make me say it again!> 570 571...must I<not> be parsed as two paragraphs in italics (with the I 572code starting in one paragraph and starting in another.) Instead, 573the first paragraph should generate a warning, but that aside, the 574above code must parse as if it were: 575 576 I<I told you not to do this!> 577 578 Don't make me say it again!E<gt> 579 580(In SGMLish jargon, all Pod commands are like block-level 581elements, whereas all Pod formatting codes are like inline-level 582elements.) 583 584 585 586=head1 Notes on Implementing Pod Processors 587 588The following is a long section of miscellaneous requirements 589and suggestions to do with Pod processing. 590 591=over 592 593=item * 594 595Pod formatters should tolerate lines in verbatim blocks that are of 596any length, even if that means having to break them (possibly several 597times, for very long lines) to avoid text running off the side of the 598page. Pod formatters may warn of such line-breaking. Such warnings 599are particularly appropriate for lines are over 100 characters long, which 600are usually not intentional. 601 602=item * 603 604Pod parsers must recognize I<all> of the three well-known newline 605formats: CR, LF, and CRLF. See L<perlport|perlport>. 606 607=item * 608 609Pod parsers should accept input lines that are of any length. 610 611=item * 612 613Since Perl recognizes a Unicode Byte Order Mark at the start of files 614as signaling that the file is Unicode encoded as in UTF-16 (whether 615big-endian or little-endian) or UTF-8, Pod parsers should do the 616same. Otherwise, the character encoding should be understood as 617being UTF-8 if the first highbit byte sequence in the file seems 618valid as a UTF-8 sequence, or otherwise as CP-1252 (earlier versions of 619this specification used Latin-1 instead of CP-1252). 620 621Future versions of this specification may specify 622how Pod can accept other encodings. Presumably treatment of other 623encodings in Pod parsing would be as in XML parsing: whatever the 624encoding declared by a particular Pod file, content is to be 625stored in memory as Unicode characters. 626 627=item * 628 629The well known Unicode Byte Order Marks are as follows: if the 630file begins with the two literal byte values 0xFE 0xFF, this is 631the BOM for big-endian UTF-16. If the file begins with the two 632literal byte value 0xFF 0xFE, this is the BOM for little-endian 633UTF-16. On an ASCII platform, if the file begins with the three literal 634byte values 6350xEF 0xBB 0xBF, this is the BOM for UTF-8. 636A mechanism portable to EBCDIC platforms is to: 637 638 my $utf8_bom = "\x{FEFF}"; 639 utf8::encode($utf8_bom); 640 641=for comment 642 use bytes; print map sprintf(" 0x%02X", ord $_), split '', "\x{feff}"; 643 0xEF 0xBB 0xBF 644 645=for comment 646 If toke.c is modified to support UTF-32, add mention of those here. 647 648=item * 649 650A naive, but often sufficient heuristic on ASCII platforms, for testing 651the first highbit 652byte-sequence in a BOM-less file (whether in code or in Pod!), to see 653whether that sequence is valid as UTF-8 (RFC 2279) is to check whether 654that the first byte in the sequence is in the range 0xC2 - 0xFD 655I<and> whether the next byte is in the range 6560x80 - 0xBF. If so, the parser may conclude that this file is in 657UTF-8, and all highbit sequences in the file should be assumed to 658be UTF-8. Otherwise the parser should treat the file as being 659in CP-1252. (A better check, and which works on EBCDIC platforms as 660well, is to pass a copy of the sequence to 661L<utf8::decode()|utf8> which performs a full validity check on the 662sequence and returns TRUE if it is valid UTF-8, FALSE otherwise. This 663function is always pre-loaded, is fast because it is written in C, and 664will only get called at most once, so you don't need to avoid it out of 665performance concerns.) 666In the unlikely circumstance that the first highbit 667sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one 668can cater to our heuristic (as well as any more intelligent heuristic) 669by prefacing that line with a comment line containing a highbit 670sequence that is clearly I<not> valid as UTF-8. A line consisting 671of simply "#", an e-acute, and any non-highbit byte, 672is sufficient to establish this file's encoding. 673 674=for comment 675 If/WHEN some brave soul makes these heuristics into a generic 676 text-file class (or PerlIO layer?), we can presumably delete 677 mention of these icky details from this file, and can instead 678 tell people to just use appropriate class/layer. 679 Auto-recognition of newline sequences would be another desirable 680 feature of such a class/layer. 681 HINT HINT HINT. 682 683=for comment 684 "The probability that a string of characters 685 in any other encoding appears as valid UTF-8 is low" - RFC2279 686 687=item * 688 689Pod processors must treat a "=for [label] [content...]" paragraph as 690meaning the same thing as a "=begin [label]" paragraph, content, and 691an "=end [label]" paragraph. (The parser may conflate these two 692constructs, or may leave them distinct, in the expectation that the 693formatter will nevertheless treat them the same.) 694 695=item * 696 697When rendering Pod to a format that allows comments (i.e., to nearly 698any format other than plaintext), a Pod formatter must insert comment 699text identifying its name and version number, and the name and 700version numbers of any modules it might be using to process the Pod. 701Minimal examples: 702 703 %% POD::Pod2PS v3.14159, using POD::Parser v1.92 704 705 <!-- Pod::HTML v3.14159, using POD::Parser v1.92 --> 706 707 {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08} 708 709 .\" Pod::Man version 3.14159, using POD::Parser version 1.92 710 711Formatters may also insert additional comments, including: the 712release date of the Pod formatter program, the contact address for 713the author(s) of the formatter, the current time, the name of input 714file, the formatting options in effect, version of Perl used, etc. 715 716Formatters may also choose to note errors/warnings as comments, 717besides or instead of emitting them otherwise (as in messages to 718STDERR, or C<die>ing). 719 720=item * 721 722Pod parsers I<may> emit warnings or error messages ("Unknown E code 723EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or 724C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> allow 725suppressing all such STDERR output, and instead allow an option for 726reporting errors/warnings 727in some other way, whether by triggering a callback, or noting errors 728in some attribute of the document object, or some similarly unobtrusive 729mechanism -- or even by appending a "Pod Errors" section to the end of 730the parsed form of the document. 731 732=item * 733 734In cases of exceptionally aberrant documents, Pod parsers may abort the 735parse. Even then, using C<die>ing/C<croak>ing is to be avoided; where 736possible, the parser library may simply close the input file 737and add text like "*** Formatting Aborted ***" to the end of the 738(partial) in-memory document. 739 740=item * 741 742In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>) 743are understood (i.e., I<not> verbatim paragraphs, but I<including> 744ordinary paragraphs, and command paragraphs that produce renderable 745text, like "=head1"), literal whitespace should generally be considered 746"insignificant", in that one literal space has the same meaning as any 747(nonzero) number of literal spaces, literal newlines, and literal tabs 748(as long as this produces no blank lines, since those would terminate 749the paragraph). Pod parsers should compact literal whitespace in each 750processed paragraph, but may provide an option for overriding this 751(since some processing tasks do not require it), or may follow 752additional special rules (for example, specially treating 753period-space-space or period-newline sequences). 754 755=item * 756 757Pod parsers should not, by default, try to coerce apostrophe (') and 758quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to 759turn backtick (`) into anything else but a single backtick character 760(distinct from an open quote character!), nor "--" into anything but 761two minus signs. They I<must never> do any of those things to text 762in CE<lt>...> formatting codes, and never I<ever> to text in verbatim 763paragraphs. 764 765=item * 766 767When rendering Pod to a format that has two kinds of hyphens (-), one 768that's a non-breaking hyphen, and another that's a breakable hyphen 769(as in "object-oriented", which can be split across lines as 770"object-", newline, "oriented"), formatters are encouraged to 771generally translate "-" to non-breaking hyphen, but may apply 772heuristics to convert some of these to breaking hyphens. 773 774=item * 775 776Pod formatters should make reasonable efforts to keep words of Perl 777code from being broken across lines. For example, "Foo::Bar" in some 778formatting systems is seen as eligible for being broken across lines 779as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should 780be avoided where possible, either by disabling all line-breaking in 781mid-word, or by wrapping particular words with internal punctuation 782in "don't break this across lines" codes (which in some formats may 783not be a single code, but might be a matter of inserting non-breaking 784zero-width spaces between every pair of characters in a word.) 785 786=item * 787 788Pod parsers should, by default, expand tabs in verbatim paragraphs as 789they are processed, before passing them to the formatter or other 790processor. Parsers may also allow an option for overriding this. 791 792=item * 793 794Pod parsers should, by default, remove newlines from the end of 795ordinary and verbatim paragraphs before passing them to the 796formatter. For example, while the paragraph you're reading now 797could be considered, in Pod source, to end with (and contain) 798the newline(s) that end it, it should be processed as ending with 799(and containing) the period character that ends this sentence. 800 801=item * 802 803Pod parsers, when reporting errors, should make some effort to report 804an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near 805line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph 806number ("Nested EE<lt>>'s in Paragraph #52 of Thing/Foo.pm!"). Where 807this is problematic, the paragraph number should at least be 808accompanied by an excerpt from the paragraph ("Nested EE<lt>>'s in 809Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for 810the CE<lt>interest rate> attribute...'"). 811 812=item * 813 814Pod parsers, when processing a series of verbatim paragraphs one 815after another, should consider them to be one large verbatim 816paragraph that happens to contain blank lines. I.e., these two 817lines, which have a blank line between them: 818 819 use Foo; 820 821 print Foo->VERSION 822 823should be unified into one paragraph ("\tuse Foo;\n\n\tprint 824Foo->VERSION") before being passed to the formatter or other 825processor. Parsers may also allow an option for overriding this. 826 827While this might be too cumbersome to implement in event-based Pod 828parsers, it is straightforward for parsers that return parse trees. 829 830=item * 831 832Pod formatters, where feasible, are advised to avoid splitting short 833verbatim paragraphs (under twelve lines, say) across pages. 834 835=item * 836 837Pod parsers must treat a line with only spaces and/or tabs on it as a 838"blank line" such as separates paragraphs. (Some older parsers 839recognized only two adjacent newlines as a "blank line" but would not 840recognize a newline, a space, and a newline, as a blank line. This 841is noncompliant behavior.) 842 843=item * 844 845Authors of Pod formatters/processors should make every effort to 846avoid writing their own Pod parser. There are already several in 847CPAN, with a wide range of interface styles -- and one of them, 848Pod::Simple, comes with modern versions of Perl. 849 850=item * 851 852Characters in Pod documents may be conveyed either as literals, or by 853number in EE<lt>n> codes, or by an equivalent mnemonic, as in 854EE<lt>eacute> which is exactly equivalent to EE<lt>233>. The numbers 855are the Latin1/Unicode values, even on EBCDIC platforms. 856 857When referring to characters by using a EE<lt>n> numeric code, numbers 858in the range 32-126 refer to those well known US-ASCII characters (also 859defined there by Unicode, with the same meaning), which all Pod 860formatters must render faithfully. Characters whose EE<lt>E<gt> numbers 861are in the ranges 0-31 and 127-159 should not be used (neither as 862literals, 863nor as EE<lt>number> codes), except for the literal byte-sequences for 864newline (ASCII 13, ASCII 13 10, or ASCII 10), and tab (ASCII 9). 865 866Numbers in the range 160-255 refer to Latin-1 characters (also 867defined there by Unicode, with the same meaning). Numbers above 868255 should be understood to refer to Unicode characters. 869 870=item * 871 872Be warned 873that some formatters cannot reliably render characters outside 32-126; 874and many are able to handle 32-126 and 160-255, but nothing above 875255. 876 877=item * 878 879Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for 880less-than and greater-than, Pod parsers must understand "EE<lt>sol>" 881for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar, 882pipe). Pod parsers should also understand "EE<lt>lchevron>" and 883"EE<lt>rchevron>" as legacy codes for characters 171 and 187, i.e., 884"left-pointing double angle quotation mark" = "left pointing 885guillemet" and "right-pointing double angle quotation mark" = "right 886pointing guillemet". (These look like little "<<" and ">>", and they 887are now preferably expressed with the HTML/XHTML codes "EE<lt>laquo>" 888and "EE<lt>raquo>".) 889 890=item * 891 892Pod parsers should understand all "EE<lt>html>" codes as defined 893in the entity declarations in the most recent XHTML specification at 894C<www.W3.org>. Pod parsers must understand at least the entities 895that define characters in the range 160-255 (Latin-1). Pod parsers, 896when faced with some unknown "EE<lt>I<identifier>>" code, 897shouldn't simply replace it with nullstring (by default, at least), 898but may pass it through as a string consisting of the literal characters 899E, less-than, I<identifier>, greater-than. Or Pod parsers may offer the 900alternative option of processing such unknown 901"EE<lt>I<identifier>>" codes by firing an event especially 902for such codes, or by adding a special node-type to the in-memory 903document tree. Such "EE<lt>I<identifier>>" may have special meaning 904to some processors, or some processors may choose to add them to 905a special error report. 906 907=item * 908 909Pod parsers must also support the XHTML codes "EE<lt>quot>" for 910character 34 (doublequote, "), "EE<lt>amp>" for character 38 911(ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, '). 912 913=item * 914 915Note that in all cases of "EE<lt>whateverE<gt>", I<whatever> (whether 916an htmlname, or a number in any base) must consist only of 917alphanumeric characters -- that is, I<whatever> must match 918C<m/\A\w+\z/>. So S<"EE<lt> 0 1 2 3 E<gt>"> is invalid, because 919it contains spaces, which aren't alphanumeric characters. This 920presumably does not I<need> special treatment by a Pod processor; 921S<" 0 1 2 3 "> doesn't look like a number in any base, so it would 922presumably be looked up in the table of HTML-like names. Since 923there isn't (and cannot be) an HTML-like entity called S<" 0 1 2 3 ">, 924this will be treated as an error. However, Pod processors may 925treat S<"EE<lt> 0 1 2 3 E<gt>"> or "EE<lt>e-acute>" as I<syntactically> 926invalid, potentially earning a different error message than the 927error message (or warning, or event) generated by a merely unknown 928(but theoretically valid) htmlname, as in "EE<lt>qacute>" 929[sic]. However, Pod parsers are not required to make this 930distinction. 931 932=item * 933 934Note that EE<lt>number> I<must not> be interpreted as simply 935"codepoint I<number> in the current/native character set". It always 936means only "the character represented by codepoint I<number> in 937Unicode." (This is identical to the semantics of &#I<number>; in XML.) 938 939This will likely require many formatters to have tables mapping from 940treatable Unicode codepoints (such as the "\xE9" for the e-acute 941character) to the escape sequences or codes necessary for conveying 942such sequences in the target output format. A converter to *roff 943would, for example know that "\xE9" (whether conveyed literally, or via 944a EE<lt>...> sequence) is to be conveyed as "e\\*'". 945Similarly, a program rendering Pod in a Mac OS application window, would 946presumably need to know that "\xE9" maps to codepoint 142 in MacRoman 947encoding that (at time of writing) is native for Mac OS. Such 948Unicode2whatever mappings are presumably already widely available for 949common output formats. (Such mappings may be incomplete! Implementers 950are not expected to bend over backwards in an attempt to render 951Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any 952of the other weird things that Unicode can encode.) And 953if a Pod document uses a character not found in such a mapping, the 954formatter should consider it an unrenderable character. 955 956=item * 957 958If, surprisingly, the implementor of a Pod formatter can't find a 959satisfactory pre-existing table mapping from Unicode characters to 960escapes in the target format (e.g., a decent table of Unicode 961characters to *roff escapes), it will be necessary to build such a 962table. If you are in this circumstance, you should begin with the 963characters in the range 0x00A0 - 0x00FF, which is mostly the heavily 964used accented characters. Then proceed (as patience permits and 965fastidiousness compels) through the characters that the (X)HTML 966standards groups judged important enough to merit mnemonics 967for. These are declared in the (X)HTML specifications at the 968www.W3.org site. At time of writing (September 2001), the most recent 969entity declaration files are: 970 971 http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent 972 http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent 973 http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent 974 975Then you can progress through any remaining notable Unicode characters 976in the range 0x2000-0x204D (consult the character tables at 977www.unicode.org), and whatever else strikes your fancy. For example, 978in F<xhtml-symbol.ent>, there is the entry: 979 980 <!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech --> 981 982While the mapping "infin" to the character "\x{221E}" will (hopefully) 983have been already handled by the Pod parser, the presence of the 984character in this file means that it's reasonably important enough to 985include in a formatter's table that maps from notable Unicode characters 986to the codes necessary for rendering them. So for a Unicode-to-*roff 987mapping, for example, this would merit the entry: 988 989 "\x{221E}" => '\(in', 990 991It is eagerly hoped that in the future, increasing numbers of formats 992(and formatters) will support Unicode characters directly (as (X)HTML 993does with C<∞>, C<∞>, or C<∞>), reducing the need 994for idiosyncratic mappings of Unicode-to-I<my_escapes>. 995 996=item * 997 998It is up to individual Pod formatter to display good judgement when 999confronted with an unrenderable character (which is distinct from an 1000unknown EE<lt>thing> sequence that the parser couldn't resolve to 1001anything, renderable or not). It is good practice to map Latin letters 1002with diacritics (like "EE<lt>eacute>"/"EE<lt>233>") to the corresponding 1003unaccented US-ASCII letters (like a simple character 101, "e"), but 1004clearly this is often not feasible, and an unrenderable character may 1005be represented as "?", or the like. In attempting a sane fallback 1006(as from EE<lt>233> to "e"), Pod formatters may use the 1007%Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or 1008L<Text::Unidecode|Text::Unidecode>, if available. 1009 1010For example, this Pod text: 1011 1012 magic is enabled if you set C<$Currency> to 'E<euro>'. 1013 1014may be rendered as: 1015"magic is enabled if you set C<$Currency> to 'I<?>'" or as 1016"magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as 1017"magic is enabled if you set C<$Currency> to '[x20AC]', etc. 1018 1019A Pod formatter may also note, in a comment or warning, a list of what 1020unrenderable characters were encountered. 1021 1022=item * 1023 1024EE<lt>...> may freely appear in any formatting code (other than 1025in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The 1026EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The 1027EE<lt>euro>1,000,000 Solution|Million::Euros>". 1028 1029=item * 1030 1031Some Pod formatters output to formats that implement non-breaking 1032spaces as an individual character (which I'll call "NBSP"), and 1033others output to formats that implement non-breaking spaces just as 1034spaces wrapped in a "don't break this across lines" code. Note that 1035at the level of Pod, both sorts of codes can occur: Pod can contain a 1036NBSP character (whether as a literal, or as a "EE<lt>160>" or 1037"EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo 1038IE<lt>barE<gt> baz>" codes, where "mere spaces" (character 32) in 1039such codes are taken to represent non-breaking spaces. Pod 1040parsers should consider supporting the optional parsing of "SE<lt>foo 1041IE<lt>barE<gt> baz>" as if it were 1042"fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the 1043optional parsing of groups of words joined by NBSP's as if each group 1044were in a SE<lt>...> code, so that formatters may use the 1045representation that maps best to what the output format demands. 1046 1047=item * 1048 1049Some processors may find that the C<SE<lt>...E<gt>> code is easiest to 1050implement by replacing each space in the parse tree under the content 1051of the S, with an NBSP. But note: the replacement should apply I<not> to 1052spaces in I<all> text, but I<only> to spaces in I<printable> text. (This 1053distinction may or may not be evident in the particular tree/event 1054model implemented by the Pod parser.) For example, consider this 1055unusual case: 1056 1057 S<L</Autoloaded Functions>> 1058 1059This means that the space in the middle of the visible link text must 1060not be broken across lines. In other words, it's the same as this: 1061 1062 L<"AutoloadedE<160>Functions"/Autoloaded Functions> 1063 1064However, a misapplied space-to-NBSP replacement could (wrongly) 1065produce something equivalent to this: 1066 1067 L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions> 1068 1069...which is almost definitely not going to work as a hyperlink (assuming 1070this formatter outputs a format supporting hypertext). 1071 1072Formatters may choose to just not support the S format code, 1073especially in cases where the output format simply has no NBSP 1074character/code and no code for "don't break this stuff across lines". 1075 1076=item * 1077 1078Besides the NBSP character discussed above, implementors are reminded 1079of the existence of the other "special" character in Latin-1, the 1080"soft hyphen" character, also known as "discretionary hyphen", 1081i.e. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> = 1082C<EE<lt>shyE<gt>>). This character expresses an optional hyphenation 1083point. That is, it normally renders as nothing, but may render as a 1084"-" if a formatter breaks the word at that point. Pod formatters 1085should, as appropriate, do one of the following: 1) render this with 1086a code with the same meaning (e.g., "\-" in RTF), 2) pass it through 1087in the expectation that the formatter understands this character as 1088such, or 3) delete it. 1089 1090For example: 1091 1092 sigE<shy>action 1093 manuE<shy>script 1094 JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi 1095 1096These signal to a formatter that if it is to hyphenate "sigaction" 1097or "manuscript", then it should be done as 1098"sig-I<[linebreak]>action" or "manu-I<[linebreak]>script" 1099(and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> doesn't 1100show up at all). And if it is 1101to hyphenate "Jarkko" and/or "Hietaniemi", it can do 1102so only at the points where there is a C<EE<lt>shyE<gt>> code. 1103 1104In practice, it is anticipated that this character will not be used 1105often, but formatters should either support it, or delete it. 1106 1107=item * 1108 1109If you think that you want to add a new command to Pod (like, say, a 1110"=biblio" command), consider whether you could get the same 1111effect with a for or begin/end sequence: "=for biblio ..." or "=begin 1112biblio" ... "=end biblio". Pod processors that don't understand 1113"=for biblio", etc, will simply ignore it, whereas they may complain 1114loudly if they see "=biblio". 1115 1116=item * 1117 1118Throughout this document, "Pod" has been the preferred spelling for 1119the name of the documentation format. One may also use "POD" or 1120"pod". For the documentation that is (typically) in the Pod 1121format, you may use "pod", or "Pod", or "POD". Understanding these 1122distinctions is useful; but obsessing over how to spell them, usually 1123is not. 1124 1125=back 1126 1127 1128 1129 1130 1131=head1 About LE<lt>...E<gt> Codes 1132 1133As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...> 1134code is the most complex of the Pod formatting codes. The points below 1135will hopefully clarify what it means and how processors should deal 1136with it. 1137 1138=over 1139 1140=item * 1141 1142In parsing an LE<lt>...> code, Pod parsers must distinguish at least 1143four attributes: 1144 1145=over 1146 1147=item First: 1148 1149The link-text. If there is none, this must be C<undef>. (E.g., in 1150"LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions". 1151In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no 1152link text. Note that link text may contain formatting.) 1153 1154=item Second: 1155 1156The possibly inferred link-text; i.e., if there was no real link 1157text, then this is the text that we'll infer in its place. (E.g., for 1158"LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".) 1159 1160=item Third: 1161 1162The name or URL, or C<undef> if none. (E.g., in "LE<lt>Perl 1163Functions|perlfunc>", the name (also sometimes called the page) 1164is "perlfunc". In "LE<lt>/CAVEATS>", the name is C<undef>.) 1165 1166=item Fourth: 1167 1168The section (AKA "item" in older perlpods), or C<undef> if none. E.g., 1169in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "DESCRIPTION" is the section. (Note 1170that this is not the same as a manpage section like the "5" in "man 5 1171crontab". "Section Foo" in the Pod sense means the part of the text 1172that's introduced by the heading or item whose text is "Foo".) 1173 1174=back 1175 1176Pod parsers may also note additional attributes including: 1177 1178=over 1179 1180=item Fifth: 1181 1182A flag for whether item 3 (if present) is a URL (like 1183"http://lists.perl.org" is), in which case there should be no section 1184attribute; a Pod name (like "perldoc" and "Getopt::Std" are); or 1185possibly a man page name (like "crontab(5)" is). 1186 1187=item Sixth: 1188 1189The raw original LE<lt>...> content, before text is split on 1190"|", "/", etc, and before EE<lt>...> codes are expanded. 1191 1192=back 1193 1194(The above were numbered only for concise reference below. It is not 1195a requirement that these be passed as an actual list or array.) 1196 1197For example: 1198 1199 L<Foo::Bar> 1200 => undef, # link text 1201 "Foo::Bar", # possibly inferred link text 1202 "Foo::Bar", # name 1203 undef, # section 1204 'pod', # what sort of link 1205 "Foo::Bar" # original content 1206 1207 L<Perlport's section on NL's|perlport/Newlines> 1208 => "Perlport's section on NL's", # link text 1209 "Perlport's section on NL's", # possibly inferred link text 1210 "perlport", # name 1211 "Newlines", # section 1212 'pod', # what sort of link 1213 "Perlport's section on NL's|perlport/Newlines" 1214 # original content 1215 1216 L<perlport/Newlines> 1217 => undef, # link text 1218 '"Newlines" in perlport', # possibly inferred link text 1219 "perlport", # name 1220 "Newlines", # section 1221 'pod', # what sort of link 1222 "perlport/Newlines" # original content 1223 1224 L<crontab(5)/"DESCRIPTION"> 1225 => undef, # link text 1226 '"DESCRIPTION" in crontab(5)', # possibly inferred link text 1227 "crontab(5)", # name 1228 "DESCRIPTION", # section 1229 'man', # what sort of link 1230 'crontab(5)/"DESCRIPTION"' # original content 1231 1232 L</Object Attributes> 1233 => undef, # link text 1234 '"Object Attributes"', # possibly inferred link text 1235 undef, # name 1236 "Object Attributes", # section 1237 'pod', # what sort of link 1238 "/Object Attributes" # original content 1239 1240 L<https://www.perl.org/> 1241 => undef, # link text 1242 "https://www.perl.org/", # possibly inferred link text 1243 "https://www.perl.org/", # name 1244 undef, # section 1245 'url', # what sort of link 1246 "https://www.perl.org/" # original content 1247 1248 L<Perl.org|https://www.perl.org/> 1249 => "Perl.org", # link text 1250 "https://www.perl.org/", # possibly inferred link text 1251 "https://www.perl.org/", # name 1252 undef, # section 1253 'url', # what sort of link 1254 "Perl.org|https://www.perl.org/" # original content 1255 1256Note that you can distinguish URL-links from anything else by the 1257fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So 1258C<LE<lt>http://www.perl.comE<gt>> is a URL, but 1259C<LE<lt>HTTP::ResponseE<gt>> isn't. 1260 1261=item * 1262 1263In case of LE<lt>...> codes with no "text|" part in them, 1264older formatters have exhibited great variation in actually displaying 1265the link or cross reference. For example, LE<lt>crontab(5)> would render 1266as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage" 1267or just "C<crontab(5)>". 1268 1269Pod processors must now treat "text|"-less links as follows: 1270 1271 L<name> => L<name|name> 1272 L</section> => L<"section"|/section> 1273 L<name/section> => L<"section" in name|name/section> 1274 1275=item * 1276 1277Note that section names might contain markup. I.e., if a section 1278starts with: 1279 1280 =head2 About the C<-M> Operator 1281 1282or with: 1283 1284 =item About the C<-M> Operator 1285 1286then a link to it would look like this: 1287 1288 L<somedoc/About the C<-M> Operator> 1289 1290Formatters may choose to ignore the markup for purposes of resolving 1291the link and use only the renderable characters in the section name, 1292as in: 1293 1294 <h1><a name="About_the_-M_Operator">About the <code>-M</code> 1295 Operator</h1> 1296 1297 ... 1298 1299 <a href="somedoc#About_the_-M_Operator">About the <code>-M</code> 1300 Operator" in somedoc</a> 1301 1302=item * 1303 1304Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>> 1305links from C<LE<lt>name/itemE<gt>> links (and their targets). These 1306have been merged syntactically and semantically in the current 1307specification, and I<section> can refer either to a "=headI<n> Heading 1308Content" command or to a "=item Item Content" command. This 1309specification does not specify what behavior should be in the case 1310of a given document having several things all seeming to produce the 1311same I<section> identifier (e.g., in HTML, several things all producing 1312the same I<anchorname> in <a name="I<anchorname>">...</a> 1313elements). Where Pod processors can control this behavior, they should 1314use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the 1315I<first> "Bar" section in Foo. 1316 1317But for some processors/formats this cannot be easily controlled; as 1318with the HTML example, the behavior of multiple ambiguous 1319<a name="I<anchorname>">...</a> is most easily just left up to 1320browsers to decide. 1321 1322=item * 1323 1324In a C<LE<lt>text|...E<gt>> code, text may contain formatting codes 1325for formatting or for EE<lt>...> escapes, as in: 1326 1327 L<B<ummE<234>stuff>|...> 1328 1329For C<LE<lt>...E<gt>> codes without a "name|" part, only 1330C<EE<lt>...E<gt>> and C<ZE<lt>E<gt>> codes may occur. That is, 1331authors should not use "C<LE<lt>BE<lt>Foo::BarE<gt>E<gt>>". 1332 1333Note, however, that formatting codes and ZE<lt>>'s can occur in any 1334and all parts of an LE<lt>...> (i.e., in I<name>, I<section>, I<text>, 1335and I<url>). 1336 1337Authors must not nest LE<lt>...> codes. For example, "LE<lt>The 1338LE<lt>Foo::Bar> man page>" should be treated as an error. 1339 1340=item * 1341 1342Note that Pod authors may use formatting codes inside the "text" 1343part of "LE<lt>text|name>" (and so on for LE<lt>text|/"sec">). 1344 1345In other words, this is valid: 1346 1347 Go read L<the docs on C<$.>|perlvar/"$."> 1348 1349Some output formats that do allow rendering "LE<lt>...>" codes as 1350hypertext, might not allow the link-text to be formatted; in 1351that case, formatters will have to just ignore that formatting. 1352 1353=item * 1354 1355At time of writing, C<LE<lt>nameE<gt>> values are of two types: 1356either the name of a Pod page like C<LE<lt>Foo::BarE<gt>> (which 1357might be a real Perl module or program in an @INC / PATH 1358directory, or a .pod file in those places); or the name of a Unix 1359man page, like C<LE<lt>crontab(5)E<gt>>. In theory, C<LE<lt>chmodE<gt>> 1360is ambiguous between a Pod page called "chmod", or the Unix man page 1361"chmod" (in whatever man-section). However, the presence of a string 1362in parens, as in "crontab(5)", is sufficient to signal that what 1363is being discussed is not a Pod page, and so is presumably a 1364Unix man page. The distinction is of no importance to many 1365Pod processors, but some processors that render to hypertext formats 1366may need to distinguish them in order to know how to render a 1367given C<LE<lt>fooE<gt>> code. 1368 1369=item * 1370 1371Previous versions of perlpod allowed for a C<LE<lt>sectionE<gt>> syntax (as in 1372C<LE<lt>Object AttributesE<gt>>), which was not easily distinguishable from 1373C<LE<lt>nameE<gt>> syntax and for C<LE<lt>"section"E<gt>> which was only 1374slightly less ambiguous. This syntax is no longer in the specification, and 1375has been replaced by the C<LE<lt>/sectionE<gt>> syntax (where the slash was 1376formerly optional). Pod parsers should tolerate the C<LE<lt>"section"E<gt>> 1377syntax, for a while at least. The suggested heuristic for distinguishing 1378C<LE<lt>sectionE<gt>> from C<LE<lt>nameE<gt>> is that if it contains any 1379whitespace, it's a I<section>. Pod processors should warn about this being 1380deprecated syntax. 1381 1382=back 1383 1384=head1 About =over...=back Regions 1385 1386"=over"..."=back" regions are used for various kinds of list-like 1387structures. (I use the term "region" here simply as a collective 1388term for everything from the "=over" to the matching "=back".) 1389 1390=over 1391 1392=item * 1393 1394The non-zero numeric I<indentlevel> in "=over I<indentlevel>" ... 1395"=back" is used for giving the formatter a clue as to how many 1396"spaces" (ems, or roughly equivalent units) it should tab over, 1397although many formatters will have to convert this to an absolute 1398measurement that may not exactly match with the size of spaces (or M's) 1399in the document's base font. Other formatters may have to completely 1400ignore the number. The lack of any explicit I<indentlevel> parameter is 1401equivalent to an I<indentlevel> value of 4. Pod processors may 1402complain if I<indentlevel> is present but is not a positive number 1403matching C<m/\A(\d*\.)?\d+\z/>. 1404 1405=item * 1406 1407Authors of Pod formatters are reminded that "=over" ... "=back" may 1408map to several different constructs in your output format. For 1409example, in converting Pod to (X)HTML, it can map to any of 1410<ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or 1411<blockquote>...</blockquote>. Similarly, "=item" can map to <li> or 1412<dt>. 1413 1414=item * 1415 1416Each "=over" ... "=back" region should be one of the following: 1417 1418=over 1419 1420=item * 1421 1422An "=over" ... "=back" region containing only "=item *" commands, 1423each followed by some number of ordinary/verbatim paragraphs, other 1424nested "=over" ... "=back" regions, "=for..." paragraphs, and 1425"=begin"..."=end" regions. 1426 1427(Pod processors must tolerate a bare "=item" as if it were "=item 1428*".) Whether "*" is rendered as a literal asterisk, an "o", or as 1429some kind of real bullet character, is left up to the Pod formatter, 1430and may depend on the level of nesting. 1431 1432=item * 1433 1434An "=over" ... "=back" region containing only 1435C<m/\A=item\s+\d+\.?\s*\z/> paragraphs, each one (or each group of them) 1436followed by some number of ordinary/verbatim paragraphs, other nested 1437"=over" ... "=back" regions, "=for..." paragraphs, and/or 1438"=begin"..."=end" codes. Note that the numbers must start at 1 1439in each section, and must proceed in order and without skipping 1440numbers. 1441 1442(Pod processors must tolerate lines like "=item 1" as if they were 1443"=item 1.", with the period.) 1444 1445=item * 1446 1447An "=over" ... "=back" region containing only "=item [text]" 1448commands, each one (or each group of them) followed by some number of 1449ordinary/verbatim paragraphs, other nested "=over" ... "=back" 1450regions, or "=for..." paragraphs, and "=begin"..."=end" regions. 1451 1452The "=item [text]" paragraph should not match 1453C<m/\A=item\s+\d+\.?\s*\z/> or C<m/\A=item\s+\*\s*\z/>, nor should it 1454match just C<m/\A=item\s*\z/>. 1455 1456=item * 1457 1458An "=over" ... "=back" region containing no "=item" paragraphs at 1459all, and containing only some number of 1460ordinary/verbatim paragraphs, and possibly also some nested "=over" 1461... "=back" regions, "=for..." paragraphs, and "=begin"..."=end" 1462regions. Such an itemless "=over" ... "=back" region in Pod is 1463equivalent in meaning to a "<blockquote>...</blockquote>" element in 1464HTML. 1465 1466=back 1467 1468Note that with all the above cases, you can determine which type of 1469"=over" ... "=back" you have, by examining the first (non-"=cut", 1470non-"=pod") Pod paragraph after the "=over" command. 1471 1472=item * 1473 1474Pod formatters I<must> tolerate arbitrarily large amounts of text 1475in the "=item I<text...>" paragraph. In practice, most such 1476paragraphs are short, as in: 1477 1478 =item For cutting off our trade with all parts of the world 1479 1480But they may be arbitrarily long: 1481 1482 =item For transporting us beyond seas to be tried for pretended 1483 offenses 1484 1485 =item He is at this time transporting large armies of foreign 1486 mercenaries to complete the works of death, desolation and 1487 tyranny, already begun with circumstances of cruelty and perfidy 1488 scarcely paralleled in the most barbarous ages, and totally 1489 unworthy the head of a civilized nation. 1490 1491=item * 1492 1493Pod processors should tolerate "=item *" / "=item I<number>" commands 1494with no accompanying paragraph. The middle item is an example: 1495 1496 =over 1497 1498 =item 1 1499 1500 Pick up dry cleaning. 1501 1502 =item 2 1503 1504 =item 3 1505 1506 Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs. 1507 1508 =back 1509 1510=item * 1511 1512No "=over" ... "=back" region can contain headings. Processors may 1513treat such a heading as an error. 1514 1515=item * 1516 1517Note that an "=over" ... "=back" region should have some 1518content. That is, authors should not have an empty region like this: 1519 1520 =over 1521 1522 =back 1523 1524Pod processors seeing such a contentless "=over" ... "=back" region, 1525may ignore it, or may report it as an error. 1526 1527=item * 1528 1529Processors must tolerate an "=over" list that goes off the end of the 1530document (i.e., which has no matching "=back"), but they may warn 1531about such a list. 1532 1533=item * 1534 1535Authors of Pod formatters should note that this construct: 1536 1537 =item Neque 1538 1539 =item Porro 1540 1541 =item Quisquam Est 1542 1543 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci 1544 velit, sed quia non numquam eius modi tempora incidunt ut 1545 labore et dolore magnam aliquam quaerat voluptatem. 1546 1547 =item Ut Enim 1548 1549is semantically ambiguous, in a way that makes formatting decisions 1550a bit difficult. On the one hand, it could be mention of an item 1551"Neque", mention of another item "Porro", and mention of another 1552item "Quisquam Est", with just the last one requiring the explanatory 1553paragraph "Qui dolorem ipsum quia dolor..."; and then an item 1554"Ut Enim". In that case, you'd want to format it like so: 1555 1556 Neque 1557 1558 Porro 1559 1560 Quisquam Est 1561 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci 1562 velit, sed quia non numquam eius modi tempora incidunt ut 1563 labore et dolore magnam aliquam quaerat voluptatem. 1564 1565 Ut Enim 1566 1567But it could equally well be a discussion of three (related or equivalent) 1568items, "Neque", "Porro", and "Quisquam Est", followed by a paragraph 1569explaining them all, and then a new item "Ut Enim". In that case, you'd 1570probably want to format it like so: 1571 1572 Neque 1573 Porro 1574 Quisquam Est 1575 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci 1576 velit, sed quia non numquam eius modi tempora incidunt ut 1577 labore et dolore magnam aliquam quaerat voluptatem. 1578 1579 Ut Enim 1580 1581But (for the foreseeable future), Pod does not provide any way for Pod 1582authors to distinguish which grouping is meant by the above 1583"=item"-cluster structure. So formatters should format it like so: 1584 1585 Neque 1586 1587 Porro 1588 1589 Quisquam Est 1590 1591 Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci 1592 velit, sed quia non numquam eius modi tempora incidunt ut 1593 labore et dolore magnam aliquam quaerat voluptatem. 1594 1595 Ut Enim 1596 1597That is, there should be (at least roughly) equal spacing between 1598items as between paragraphs (although that spacing may well be less 1599than the full height of a line of text). This leaves it to the reader 1600to use (con)textual cues to figure out whether the "Qui dolorem 1601ipsum..." paragraph applies to the "Quisquam Est" item or to all three 1602items "Neque", "Porro", and "Quisquam Est". While not an ideal 1603situation, this is preferable to providing formatting cues that may 1604be actually contrary to the author's intent. 1605 1606=back 1607 1608 1609 1610=head1 About Data Paragraphs and "=begin/=end" Regions 1611 1612Data paragraphs are typically used for inlining non-Pod data that is 1613to be used (typically passed through) when rendering the document to 1614a specific format: 1615 1616 =begin rtf 1617 1618 \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par} 1619 1620 =end rtf 1621 1622The exact same effect could, incidentally, be achieved with a single 1623"=for" paragraph: 1624 1625 =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par} 1626 1627(Although that is not formally a data paragraph, it has the same 1628meaning as one, and Pod parsers may parse it as one.) 1629 1630Another example of a data paragraph: 1631 1632 =begin html 1633 1634 I like <em>PIE</em>! 1635 1636 <hr>Especially pecan pie! 1637 1638 =end html 1639 1640If these were ordinary paragraphs, the Pod parser would try to 1641expand the "EE<lt>/em>" (in the first paragraph) as a formatting 1642code, just like "EE<lt>lt>" or "EE<lt>eacute>". But since this 1643is in a "=begin I<identifier>"..."=end I<identifier>" region I<and> 1644the identifier "html" doesn't begin have a ":" prefix, the contents 1645of this region are stored as data paragraphs, instead of being 1646processed as ordinary paragraphs (or if they began with a spaces 1647and/or tabs, as verbatim paragraphs). 1648 1649As a further example: At time of writing, no "biblio" identifier is 1650supported, but suppose some processor were written to recognize it as 1651a way of (say) denoting a bibliographic reference (necessarily 1652containing formatting codes in ordinary paragraphs). The fact that 1653"biblio" paragraphs were meant for ordinary processing would be 1654indicated by prefacing each "biblio" identifier with a colon: 1655 1656 =begin :biblio 1657 1658 Wirth, Niklaus. 1976. I<Algorithms + Data Structures = 1659 Programs.> Prentice-Hall, Englewood Cliffs, NJ. 1660 1661 =end :biblio 1662 1663This would signal to the parser that paragraphs in this begin...end 1664region are subject to normal handling as ordinary/verbatim paragraphs 1665(while still tagged as meant only for processors that understand the 1666"biblio" identifier). The same effect could be had with: 1667 1668 =for :biblio 1669 Wirth, Niklaus. 1976. I<Algorithms + Data Structures = 1670 Programs.> Prentice-Hall, Englewood Cliffs, NJ. 1671 1672The ":" on these identifiers means simply "process this stuff 1673normally, even though the result will be for some special target". 1674I suggest that parser APIs report "biblio" as the target identifier, 1675but also report that it had a ":" prefix. (And similarly, with the 1676above "html", report "html" as the target identifier, and note the 1677I<lack> of a ":" prefix.) 1678 1679Note that a "=begin I<identifier>"..."=end I<identifier>" region where 1680I<identifier> begins with a colon, I<can> contain commands. For example: 1681 1682 =begin :biblio 1683 1684 Wirth's classic is available in several editions, including: 1685 1686 =for comment 1687 hm, check abebooks.com for how much used copies cost. 1688 1689 =over 1690 1691 =item 1692 1693 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.> 1694 Teubner, Stuttgart. [Yes, it's in German.] 1695 1696 =item 1697 1698 Wirth, Niklaus. 1976. I<Algorithms + Data Structures = 1699 Programs.> Prentice-Hall, Englewood Cliffs, NJ. 1700 1701 =back 1702 1703 =end :biblio 1704 1705Note, however, a "=begin I<identifier>"..."=end I<identifier>" 1706region where I<identifier> does I<not> begin with a colon, should not 1707directly contain "=head1" ... "=head4" commands, nor "=over", nor "=back", 1708nor "=item". For example, this may be considered invalid: 1709 1710 =begin somedata 1711 1712 This is a data paragraph. 1713 1714 =head1 Don't do this! 1715 1716 This is a data paragraph too. 1717 1718 =end somedata 1719 1720A Pod processor may signal that the above (specifically the "=head1" 1721paragraph) is an error. Note, however, that the following should 1722I<not> be treated as an error: 1723 1724 =begin somedata 1725 1726 This is a data paragraph. 1727 1728 =cut 1729 1730 # Yup, this isn't Pod anymore. 1731 sub excl { (rand() > .5) ? "hoo!" : "hah!" } 1732 1733 =pod 1734 1735 This is a data paragraph too. 1736 1737 =end somedata 1738 1739And this too is valid: 1740 1741 =begin someformat 1742 1743 This is a data paragraph. 1744 1745 And this is a data paragraph. 1746 1747 =begin someotherformat 1748 1749 This is a data paragraph too. 1750 1751 And this is a data paragraph too. 1752 1753 =begin :yetanotherformat 1754 1755 =head2 This is a command paragraph! 1756 1757 This is an ordinary paragraph! 1758 1759 And this is a verbatim paragraph! 1760 1761 =end :yetanotherformat 1762 1763 =end someotherformat 1764 1765 Another data paragraph! 1766 1767 =end someformat 1768 1769The contents of the above "=begin :yetanotherformat" ... 1770"=end :yetanotherformat" region I<aren't> data paragraphs, because 1771the immediately containing region's identifier (":yetanotherformat") 1772begins with a colon. In practice, most regions that contain 1773data paragraphs will contain I<only> data paragraphs; however, 1774the above nesting is syntactically valid as Pod, even if it is 1775rare. However, the handlers for some formats, like "html", 1776will accept only data paragraphs, not nested regions; and they may 1777complain if they see (targeted for them) nested regions, or commands, 1778other than "=end", "=pod", and "=cut". 1779 1780Also consider this valid structure: 1781 1782 =begin :biblio 1783 1784 Wirth's classic is available in several editions, including: 1785 1786 =over 1787 1788 =item 1789 1790 Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.> 1791 Teubner, Stuttgart. [Yes, it's in German.] 1792 1793 =item 1794 1795 Wirth, Niklaus. 1976. I<Algorithms + Data Structures = 1796 Programs.> Prentice-Hall, Englewood Cliffs, NJ. 1797 1798 =back 1799 1800 Buy buy buy! 1801 1802 =begin html 1803 1804 <img src='wirth_spokesmodeling_book.png'> 1805 1806 <hr> 1807 1808 =end html 1809 1810 Now now now! 1811 1812 =end :biblio 1813 1814There, the "=begin html"..."=end html" region is nested inside 1815the larger "=begin :biblio"..."=end :biblio" region. Note that the 1816content of the "=begin html"..."=end html" region is data 1817paragraph(s), because the immediately containing region's identifier 1818("html") I<doesn't> begin with a colon. 1819 1820Pod parsers, when processing a series of data paragraphs one 1821after another (within a single region), should consider them to 1822be one large data paragraph that happens to contain blank lines. So 1823the content of the above "=begin html"..."=end html" I<may> be stored 1824as two data paragraphs (one consisting of 1825"<img src='wirth_spokesmodeling_book.png'>\n" 1826and another consisting of "<hr>\n"), but I<should> be stored as 1827a single data paragraph (consisting of 1828"<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n"). 1829 1830Pod processors should tolerate empty 1831"=begin I<something>"..."=end I<something>" regions, 1832empty "=begin :I<something>"..."=end :I<something>" regions, and 1833contentless "=for I<something>" and "=for :I<something>" 1834paragraphs. I.e., these should be tolerated: 1835 1836 =for html 1837 1838 =begin html 1839 1840 =end html 1841 1842 =begin :biblio 1843 1844 =end :biblio 1845 1846Incidentally, note that there's no easy way to express a data 1847paragraph starting with something that looks like a command. Consider: 1848 1849 =begin stuff 1850 1851 =shazbot 1852 1853 =end stuff 1854 1855There, "=shazbot" will be parsed as a Pod command "shazbot", not as a data 1856paragraph "=shazbot\n". However, you can express a data paragraph consisting 1857of "=shazbot\n" using this code: 1858 1859 =for stuff =shazbot 1860 1861The situation where this is necessary, is presumably quite rare. 1862 1863Note that =end commands must match the currently open =begin command. That 1864is, they must properly nest. For example, this is valid: 1865 1866 =begin outer 1867 1868 X 1869 1870 =begin inner 1871 1872 Y 1873 1874 =end inner 1875 1876 Z 1877 1878 =end outer 1879 1880while this is invalid: 1881 1882 =begin outer 1883 1884 X 1885 1886 =begin inner 1887 1888 Y 1889 1890 =end outer 1891 1892 Z 1893 1894 =end inner 1895 1896This latter is improper because when the "=end outer" command is seen, the 1897currently open region has the formatname "inner", not "outer". (It just 1898happens that "outer" is the format name of a higher-up region.) This is 1899an error. Processors must by default report this as an error, and may halt 1900processing the document containing that error. A corollary of this is that 1901regions cannot "overlap". That is, the latter block above does not represent 1902a region called "outer" which contains X and Y, overlapping a region called 1903"inner" which contains Y and Z. But because it is invalid (as all 1904apparently overlapping regions would be), it doesn't represent that, or 1905anything at all. 1906 1907Similarly, this is invalid: 1908 1909 =begin thing 1910 1911 =end hting 1912 1913This is an error because the region is opened by "thing", and the "=end" 1914tries to close "hting" [sic]. 1915 1916This is also invalid: 1917 1918 =begin thing 1919 1920 =end 1921 1922This is invalid because every "=end" command must have a formatname 1923parameter. 1924 1925=head1 SEE ALSO 1926 1927L<perlpod>, L<perlsyn/"PODs: Embedded Documentation">, 1928L<podchecker> 1929 1930=head1 AUTHOR 1931 1932Sean M. Burke 1933 1934=cut 1935 1936 1937