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perlpodspec - Plain Old Documentation: format specification and notes
This document is detailed notes on the Pod markup language. Most people will only have to read perlpod to know how to write in Pod, but this document may answer some incidental questions to do with parsing and rendering Pod.
In this document, ``must'' / ``must not'', ``should'' / ``should not'', and ``may'' have their conventional (cf. RFC 2119) meanings: ``X must do Y'' means that if X doesn't do Y, it's against this specification, and should really be fixed. ``X should do Y'' means that it's recommended, but X may fail to do Y, if there's a good reason. ``X may do Y'' is merely a note that X can do Y at will (although it is up to the reader to detect any connotation of ``and I think it would be nice if X did Y'' versus ``it wouldn't really bother me if X did Y'').
Notably, when I say ``the parser should do Y'', the parser may fail to do Y, if the calling application explicitly requests that the parser not do Y. I often phrase this as ``the parser should, by default, do Y.'' This doesn't require the parser to provide an option for turning off whatever feature Y is (like expanding tabs in verbatim paragraphs), although it implicates that such an option may be provided.
Pod is embedded in files, typically Perl source files -- although you can write a file that's nothing but Pod.
A line in a file consists of zero or more non-newline characters, terminated by either a newline or the end of the file.
A newline sequence is usually a platform-dependent concept, but Pod parsers should understand it to mean any of CR (ASCII 13), LF (ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in addition to any other system-specific meaning. The first CR/CRLF/LF sequence in the file may be used as the basis for identifying the newline sequence for parsing the rest of the file.
A blank line is a line consisting entirely of zero or more spaces (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file. A non-blank line is a line containing one or more characters other than space or tab (and terminated by a newline or end-of-file).
(Note: Many older Pod parsers did not accept a line consisting of spaces/tabs and then a newline as a blank line -- the only lines they considered blank were lines consisting of no characters at all, terminated by a newline.)
Whitespace is used in this document as a blanket term for spaces, tabs, and newline sequences. (By itself, this term usually refers to literal whitespace. That is, sequences of whitespace characters in Pod source, as opposed to ``E<32>'', which is a formatting code that denotes a whitespace character.)
A Pod parser is a module meant for parsing Pod (regardless of whether this involves calling callbacks or building a parse tree or directly formatting it). A Pod formatter (or Pod translator) is a module or program that converts Pod to some other format (HTML, plaintext, TeX, PostScript, RTF). A Pod processor might be a formatter or translator, or might be a program that does something else with the Pod (like wordcounting it, scanning for index points, etc.).
Pod content is contained in Pod blocks. A Pod block starts with a
line that matches <m/\A=[a-zA-Z]/>, and continues up to the next line
that matches m/\A=cut/ -- or up to the end of the file, if there is
no m/\A=cut/ line.
Within a Pod block, there are Pod paragraphs. A Pod paragraph consists of non-blank lines of text, separated by one or more blank lines.
For purposes of Pod processing, there are four types of paragraphs in a Pod block:
A command paragraph (also called a ``directive''). The first line of
this paragraph must match m/\A=[a-zA-Z]/. Command paragraphs are
typically one line, as in:
=head1 NOTES
=item *
But they may span several (non-blank) lines:
=for comment Hm, I wonder what it would look like if you tried to write a BNF for Pod from this.
=head3 Dr. Strangelove, or: How I Learned to Stop Worrying and Love the Bomb
Some command paragraphs allow formatting codes in their content
(i.e., after the part that matches m/\A=[a-zA-Z]\S*\s*/), as in:
=head1 Did You Remember to C<use strict;>?
In other words, the Pod processing handler for ``head1'' will apply the same processing to ``Did You Remember to C<use strict;>?'' that it would to an ordinary paragraph -- i.e., formatting codes (like ``C<...>'') are parsed and presumably formatted appropriately, and whitespace in the form of literal spaces and/or tabs is not significant.
A verbatim paragraph. The first line of this paragraph must be a literal space or tab, and this paragraph must not be inside a ``=begin identifier'', ... ``=end identifier'' sequence unless ``identifier'' begins with a colon (``:''). That is, if a paragraph starts with a literal space or tab, but is inside a ``=begin identifier'', ... ``=end identifier'' region, then it's a data paragraph, unless ``identifier'' begins with a colon.
Whitespace is significant in verbatim paragraphs (although, in processing, tabs are probably expanded).
An ordinary paragraph. A paragraph is an ordinary paragraph
if its first line matches neither m/\A=[a-zA-Z]/ nor
m/\A[ \t]/, and if it's not inside a ``=begin identifier'',
... ``=end identifier'' sequence unless ``identifier'' begins with
a colon (``:'').
A data paragraph. This is a paragraph that is inside a ``=begin identifier'' ... ``=end identifier'' sequence where ``identifier'' does not begin with a literal colon (``:''). In some sense, a data paragraph is not part of Pod at all (i.e., effectively it's ``out-of-band''), since it's not subject to most kinds of Pod parsing; but it is specified here, since Pod parsers need to be able to call an event for it, or store it in some form in a parse tree, or at least just parse around it.
For example: consider the following paragraphs:
# <- that's the 0th column
=head1 Foo
Stuff
$foo->bar
=cut
Here, ``=head1 Foo'' and ``=cut'' are command paragraphs because the first
line of each matches m/\A=[a-zA-Z]/. ``[space][space]$foo->bar''
is a verbatim paragraph, because its first line starts with a literal
whitespace character (and there's no ``=begin''...``=end'' region around).
The ``=begin identifier'' ... ``=end identifier'' commands stop paragraphs that they surround from being parsed as data or verbatim paragraphs, if identifier doesn't begin with a colon. This is discussed in detail in the section About Data Paragraphs and ``=begin/=end'' Regions.
This section is intended to supplement and clarify the discussion in Command Paragraph in the perlpod manpage. These are the currently recognized Pod commands:
This command indicates that the text in the remainder of the paragraph is a heading. That text may contain formatting codes. Examples:
=head1 Object Attributes
=head3 What B<Not> to Do!
This command indicates that this paragraph begins a Pod block. (If we are already in the middle of a Pod block, this command has no effect at all.) If there is any text in this command paragraph after ``=pod'', it must be ignored. Examples:
=pod
This is a plain Pod paragraph.
=pod This text is ignored.
This command indicates that this line is the end of this previously started Pod block. If there is any text after ``=cut'' on the line, it must be ignored. Examples:
=cut
=cut The documentation ends here.
=cut
# This is the first line of program text.
sub foo { # This is the second.
It is an error to try to start a Pod block with a ``=cut'' command. In that case, the Pod processor must halt parsing of the input file, and must by default emit a warning.
This command indicates that this is the start of a list/indent region. If there is any text following the ``=over'', it must consist of only a nonzero positive numeral. The semantics of this numeral is explained in the About =over...=back Regions section, further below. Formatting codes are not expanded. Examples:
=over 3
=over 3.5
=over
This command indicates that an item in a list begins here. Formatting codes are processed. The semantics of the (optional) text in the remainder of this paragraph are explained in the About =over...=back Regions section, further below. Examples:
=item
=item *
=item *
=item 14
=item 3.
=item C<< $thing->stuff(I<dodad>) >>
=item For transporting us beyond seas to be tried for pretended offenses
=item He is at this time transporting large armies of foreign mercenaries to complete the works of death, desolation and tyranny, already begun with circumstances of cruelty and perfidy scarcely paralleled in the most barbarous ages, and totally unworthy the head of a civilized nation.
This command indicates that this is the end of the region begun by the most recent ``=over'' command. It permits no text after the ``=back'' command.
This marks the following paragraphs (until the matching ``=end formatname'') as being for some special kind of processing. Unless ``formatname'' begins with a colon, the contained non-command paragraphs are data paragraphs. But if ``formatname'' does begin with a colon, then non-command paragraphs are ordinary paragraphs or data paragraphs. This is discussed in detail in the section About Data Paragraphs and ``=begin/=end'' Regions.
It is advised that formatnames match the regexp
m/\A:?[-a-zA-Z0-9_]+\z/. Implementors should anticipate future
expansion in the semantics and syntax of the first parameter
to ``=begin''/``=end''/``=for''.
This marks the end of the region opened by the matching ``=begin formatname'' region. If ``formatname'' is not the formatname of the most recent open ``=begin formatname'' region, then this is an error, and must generate an error message. This is discussed in detail in the section About Data Paragraphs and ``=begin/=end'' Regions.
This is synonymous with:
=begin formatname
text...
=end formatname
That is, it creates a region consisting of a single paragraph; that paragraph is to be treated as a normal paragraph if ``formatname'' begins with a ``:''; if ``formatname'' doesn't begin with a colon, then ``text...'' will constitute a data paragraph. There is no way to use ``=for formatname text...'' to express ``text...'' as a verbatim paragraph.
This command, which should occur early in the document (at least before any non-US-ASCII data!), declares that this document is encoded in the encoding encodingname, which must be an encoding name that the Encoding manpage recognizes. (Encoding's list of supported encodings, in the Encoding::Supported manpage, is useful here.) If the Pod parser cannot decode the declared encoding, it should emit a warning and may abort parsing the document altogether.
A document having more than one ``=encoding'' line should be considered an error. Pod processors may silently tolerate this if the not-first ``=encoding'' lines are just duplicates of the first one (e.g., if there's a ``=use utf8'' line, and later on another ``=use utf8'' line). But Pod processors should complain if there are contradictory ``=encoding'' lines in the same document (e.g., if there is a ``=encoding utf8'' early in the document and ``=encoding big5'' later). Pod processors that recognize BOMs may also complain if they see an ``=encoding'' line that contradicts the BOM (e.g., if a document with a UTF-16LE BOM has an ``=encoding shiftjis'' line).
If a Pod processor sees any command other than the ones listed above (like ``=head'', or ``=haed1'', or ``=stuff'', or ``=cuttlefish'', or ``=w123''), that processor must by default treat this as an error. It must not process the paragraph beginning with that command, must by default warn of this as an error, and may abort the parse. A Pod parser may allow a way for particular applications to add to the above list of known commands, and to stipulate, for each additional command, whether formatting codes should be processed.
Future versions of this specification may add additional commands.
(Note that in previous drafts of this document and of perlpod, formatting codes were referred to as ``interior sequences'', and this term may still be found in the documentation for Pod parsers, and in error messages from Pod processors.)
There are two syntaxes for formatting codes:
A formatting code starts with a capital letter (just US-ASCII [A-Z]) followed by a ``<'', any number of characters, and ending with the first matching ``>''. Examples:
That's what I<you> think!
What's C<dump()> for?
X<C<chmod> and C<unlink()> Under Different Operating Systems>
A formatting code starts with a capital letter (just US-ASCII [A-Z]) followed by two or more ``<'''s, one or more whitespace characters, any number of characters, one or more whitespace characters, and ending with the first matching sequence of two or more ``>'''s, where the number of ``>'''s equals the number of ``<'''s in the opening of this formatting code. Examples:
That's what I<< you >> think!
C<<< open(X, ">>thing.dat") || die $! >>>
B<< $foo->bar(); >>
With this syntax, the whitespace character(s) after the ``C<<<''
and before the ``>>'' (or whatever letter) are not renderable -- they
do not signify whitespace, are merely part of the formatting codes
themselves. That is, these are all synonymous:
C<thing>
C<< thing >>
C<< thing >>
C<<< thing >>>
C<<<<
thing
>>>>
and so on.
In parsing Pod, a notably tricky part is the correct parsing of
(potentially nested!) formatting codes. Implementors should
consult the code in the parse_text routine in Pod::Parser as an
example of a correct implementation.
I<text> -- italic text
See the brief discussion in Formatting Codes in the perlpod manpage.
B<text> -- bold text
See the brief discussion in Formatting Codes in the perlpod manpage.
C<code> -- code text
See the brief discussion in Formatting Codes in the perlpod manpage.
F<filename> -- style for filenames
See the brief discussion in Formatting Codes in the perlpod manpage.
X<topic name> -- an index entry
See the brief discussion in Formatting Codes in the perlpod manpage.
This code is unusual in that most formatters completely discard this code and its content. Other formatters will render it with invisible codes that can be used in building an index of the current document.
Z<> -- a null (zero-effect) formatting code
Discussed briefly in Formatting Codes in the perlpod manpage.
This code is unusual is that it should have no content. That is,
a processor may complain if it sees Z<potatoes>. Whether
or not it complains, the potatoes text should ignored.
L<name> -- a hyperlink
The complicated syntaxes of this code are discussed at length in Formatting Codes in the perlpod manpage, and implementation details are discussed below, in About L<...> Codes. Parsing the contents of L<content> is tricky. Notably, the content has to be checked for whether it looks like a URL, or whether it has to be split on literal ``|'' and/or ``/'' (in the right order!), and so on, before E<...> codes are resolved.
E<escape> -- a character escape
See Formatting Codes in the perlpod manpage, and several points in Notes on Implementing Pod Processors.
S<text> -- text contains non-breaking spaces
This formatting code is syntactically simple, but semantically complex. What it means is that each space in the printable content of this code signifies a non-breaking space.
Consider:
C<$x ? $y : $z>
S<C<$x ? $y : $z>>
Both signify the monospace (c[ode] style) text consisting of ``$x'', one space, ``?'', one space, ``:'', one space, ``$z''. The difference is that in the latter, with the S code, those spaces are not ``normal'' spaces, but instead are non-breaking spaces.
If a Pod processor sees any formatting code other than the ones listed above (as in ``N<...>'', or ``Q<...>'', etc.), that processor must by default treat this as an error. A Pod parser may allow a way for particular applications to add to the above list of known formatting codes; a Pod parser might even allow a way to stipulate, for each additional command, whether it requires some form of special processing, as L<...> does.
Future versions of this specification may add additional formatting codes.
Historical note: A few older Pod processors would not see a ``>'' as closing a ``C<'' code, if the ``>'' was immediately preceded by a ``-''. This was so that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-E<gt>bar>
instead of as equivalent to a ``C'' formatting code containing only ``$foo-'', and then a ``bar>'' outside the ``C'' formatting code. This problem has since been solved by the addition of syntaxes like this:
C<< $foo->bar >>
Compliant parsers must not treat ``->'' as special.
Formatting codes absolutely cannot span paragraphs. If a code is opened in one paragraph, and no closing code is found by the end of that paragraph, the Pod parser must close that formatting code, and should complain (as in ``Unterminated I code in the paragraph starting at line 123: 'Time objects are not...'''). So these two paragraphs:
I<I told you not to do this!
Don't make me say it again!>
...must not be parsed as two paragraphs in italics (with the I code starting in one paragraph and starting in another.) Instead, the first paragraph should generate a warning, but that aside, the above code must parse as if it were:
I<I told you not to do this!>
Don't make me say it again!E<gt>
(In SGMLish jargon, all Pod commands are like block-level elements, whereas all Pod formatting codes are like inline-level elements.)
The following is a long section of miscellaneous requirements and suggestions to do with Pod processing.
Pod formatters should tolerate lines in verbatim blocks that are of any length, even if that means having to break them (possibly several times, for very long lines) to avoid text running off the side of the page. Pod formatters may warn of such line-breaking. Such warnings are particularly appropriate for lines are over 100 characters long, which are usually not intentional.
Pod parsers must recognize all of the three well-known newline formats: CR, LF, and CRLF. See perlport.
Pod parsers should accept input lines that are of any length.
Since Perl recognizes a Unicode Byte Order Mark at the start of files as signaling that the file is Unicode encoded as in UTF-16 (whether big-endian or little-endian) or UTF-8, Pod parsers should do the same. Otherwise, the character encoding should be understood as being UTF-8 if the first highbit byte sequence in the file seems valid as a UTF-8 sequence, or otherwise as Latin-1.
Future versions of this specification may specify how Pod can accept other encodings. Presumably treatment of other encodings in Pod parsing would be as in XML parsing: whatever the encoding declared by a particular Pod file, content is to be stored in memory as Unicode characters.
The well known Unicode Byte Order Marks are as follows: if the file begins with the two literal byte values 0xFE 0xFF, this is the BOM for big-endian UTF-16. If the file begins with the two literal byte value 0xFF 0xFE, this is the BOM for little-endian UTF-16. If the file begins with the three literal byte values 0xEF 0xBB 0xBF, this is the BOM for UTF-8.
A naive but sufficient heuristic for testing the first highbit byte-sequence in a BOM-less file (whether in code or in Pod!), to see whether that sequence is valid as UTF-8 (RFC 2279) is to check whether that the first byte in the sequence is in the range 0xC0 - 0xFD and whether the next byte is in the range 0x80 - 0xBF. If so, the parser may conclude that this file is in UTF-8, and all highbit sequences in the file should be assumed to be UTF-8. Otherwise the parser should treat the file as being in Latin-1. In the unlikely circumstance that the first highbit sequence in a truly non-UTF-8 file happens to appear to be UTF-8, one can cater to our heuristic (as well as any more intelligent heuristic) by prefacing that line with a comment line containing a highbit sequence that is clearly not valid as UTF-8. A line consisting of simply ``#'', an e-acute, and any non-highbit byte, is sufficient to establish this file's encoding.
This document's requirements and suggestions about encodings do not apply to Pod processors running on non-ASCII platforms, notably EBCDIC platforms.
Pod processors must treat a ``=for [label] [content...]'' paragraph as meaning the same thing as a ``=begin [label]'' paragraph, content, and an ``=end [label]'' paragraph. (The parser may conflate these two constructs, or may leave them distinct, in the expectation that the formatter will nevertheless treat them the same.)
When rendering Pod to a format that allows comments (i.e., to nearly any format other than plaintext), a Pod formatter must insert comment text identifying its name and version number, and the name and version numbers of any modules it might be using to process the Pod. Minimal examples:
%% POD::Pod2PS v3.14159, using POD::Parser v1.92
<!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
{\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
.\" Pod::Man version 3.14159, using POD::Parser version 1.92
Formatters may also insert additional comments, including: the
release date of the Pod formatter program, the contact address for
the author(s) of the formatter, the current time, the name of input
file, the formatting options in effect, version of Perl used, etc.
Formatters may also choose to note errors/warnings as comments,
besides or instead of emitting them otherwise (as in messages to
STDERR, or dieing).
Pod parsers may emit warnings or error messages (``Unknown E code
E<zslig>!'') to STDERR (whether through printing to STDERR, or
warning/carping, or dieing/croaking), but must allow
suppressing all such STDERR output, and instead allow an option for
reporting errors/warnings
in some other way, whether by triggering a callback, or noting errors
in some attribute of the document object, or some similarly unobtrusive
mechanism -- or even by appending a ``Pod Errors'' section to the end of
the parsed form of the document.
In cases of exceptionally aberrant documents, Pod parsers may abort the
parse. Even then, using dieing/croaking is to be avoided; where
possible, the parser library may simply close the input file
and add text like ``*** Formatting Aborted ***'' to the end of the
(partial) in-memory document.
In paragraphs where formatting codes (like E<...>, B<...>) are understood (i.e., not verbatim paragraphs, but including ordinary paragraphs, and command paragraphs that produce renderable text, like ``=head1''), literal whitespace should generally be considered ``insignificant'', in that one literal space has the same meaning as any (nonzero) number of literal spaces, literal newlines, and literal tabs (as long as this produces no blank lines, since those would terminate the paragraph). Pod parsers should compact literal whitespace in each processed paragraph, but may provide an option for overriding this (since some processing tasks do not require it), or may follow additional special rules (for example, specially treating period-space-space or period-newline sequences).
Pod parsers should not, by default, try to coerce apostrophe (') and quote (``) into smart quotes (little 9's, 66's, 99's, etc), nor try to turn backtick (`) into anything else but a single backtick character (distinct from an openquote character!), nor ''--`` into anything but two minus signs. They must never do any of those things to text in C<...> formatting codes, and never ever to text in verbatim paragraphs.
When rendering Pod to a format that has two kinds of hyphens (-), one that's a non-breaking hyphen, and another that's a breakable hyphen (as in ``object-oriented'', which can be split across lines as ``object-'', newline, ``oriented''), formatters are encouraged to generally translate ``-'' to non-breaking hyphen, but may apply heuristics to convert some of these to breaking hyphens.
Pod formatters should make reasonable efforts to keep words of Perl code from being broken across lines. For example, ``Foo::Bar'' in some formatting systems is seen as eligible for being broken across lines as ``Foo::'' newline ``Bar'' or even ``Foo::-'' newline ``Bar''. This should be avoided where possible, either by disabling all line-breaking in mid-word, or by wrapping particular words with internal punctuation in ``don't break this across lines'' codes (which in some formats may not be a single code, but might be a matter of inserting non-breaking zero-width spaces between every pair of characters in a word.)
Pod parsers should, by default, expand tabs in verbatim paragraphs as they are processed, before passing them to the formatter or other processor. Parsers may also allow an option for overriding this.
Pod parsers should, by default, remove newlines from the end of
ordinary and verbatim paragraphs before passing them to the
formatter. For example, while the paragraph you're reading now
could be considered, in Pod source, to end with (and contain)
the newline(s) that end it, it should be processed as ending with
(and containing) the period character that ends this sentence.
Pod parsers, when reporting errors, should make some effort to report an approximate line number (``Nested E<>'s in Paragraph #52, near line 633 of Thing/Foo.pm!''), instead of merely noting the paragraph number (``Nested E<>'s in Paragraph #52 of Thing/Foo.pm!''). Where this is problematic, the paragraph number should at least be accompanied by an excerpt from the paragraph (``Nested E<>'s in Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for the C<interest rate> attribute...''').
Pod parsers, when processing a series of verbatim paragraphs one after another, should consider them to be one large verbatim paragraph that happens to contain blank lines. I.e., these two lines, which have a blank line between them:
use Foo;
print Foo->VERSION
should be unified into one paragraph (``\tuse Foo;\n\n\tprint Foo->VERSION'') before being passed to the formatter or other processor. Parsers may also allow an option for overriding this.
While this might be too cumbersome to implement in event-based Pod parsers, it is straightforward for parsers that return parse trees.
Pod formatters, where feasible, are advised to avoid splitting short verbatim paragraphs (under twelve lines, say) across pages.
Pod parsers must treat a line with only spaces and/or tabs on it as a ``blank line'' such as separates paragraphs. (Some older parsers recognized only two adjacent newlines as a ``blank line'' but would not recognize a newline, a space, and a newline, as a blank line. This is noncompliant behavior.)
Authors of Pod formatters/processors should make every effort to avoid writing their own Pod parser. There are already several in CPAN, with a wide range of interface styles -- and one of them, Pod::Parser, comes with modern versions of Perl.
Characters in Pod documents may be conveyed either as literals, or by number in E<n> codes, or by an equivalent mnemonic, as in E<eacute> which is exactly equivalent to E<233>.
Characters in the range 32-126 refer to those well known US-ASCII characters (also defined there by Unicode, with the same meaning), which all Pod formatters must render faithfully. Characters in the ranges 0-31 and 127-159 should not be used (neither as literals, nor as E<number> codes), except for the literal byte-sequences for newline (13, 13 10, or 10), and tab (9).
Characters in the range 160-255 refer to Latin-1 characters (also defined there by Unicode, with the same meaning). Characters above 255 should be understood to refer to Unicode characters.
Be warned that some formatters cannot reliably render characters outside 32-126; and many are able to handle 32-126 and 160-255, but nothing above 255.
Besides the well-known ``E<lt>'' and ``E<gt>'' codes for less-than and greater-than, Pod parsers must understand ``E<sol>'' for ``/'' (solidus, slash), and ``E<verbar>'' for ``|'' (vertical bar, pipe). Pod parsers should also understand ``E<lchevron>'' and ``E<rchevron>'' as legacy codes for characters 171 and 187, i.e., ``left-pointing double angle quotation mark'' = ``left pointing guillemet'' and ``right-pointing double angle quotation mark'' = ``right pointing guillemet''. (These look like little ``<<'' and ``>>'', and they are now preferably expressed with the HTML/XHTML codes ``E<laquo>'' and ``E<raquo>''.)
Pod parsers should understand all ``E<html>'' codes as defined
in the entity declarations in the most recent XHTML specification at
www.W3.org. Pod parsers must understand at least the entities
that define characters in the range 160-255 (Latin-1). Pod parsers,
when faced with some unknown ``E<identifier>'' code,
shouldn't simply replace it with nullstring (by default, at least),
but may pass it through as a string consisting of the literal characters
E, less-than, identifier, greater-than. Or Pod parsers may offer the
alternative option of processing such unknown
``E<identifier>'' codes by firing an event especially
for such codes, or by adding a special node-type to the in-memory
document tree. Such ``E<identifier>'' may have special meaning
to some processors, or some processors may choose to add them to
a special error report.
Pod parsers must also support the XHTML codes ``E<quot>'' for character 34 (doublequote, ``), ''E<amp>`` for character 38 (ampersand, &), and ''E<apos>`` for character 39 (apostrophe, ').
Note that in all cases of ``E<whatever>'', whatever (whether
an htmlname, or a number in any base) must consist only of
alphanumeric characters -- that is, whatever must watch
m/\A\w+\z/. So ``E< 0 1 2 3 >'' is invalid, because
it contains spaces, which aren't alphanumeric characters. This
presumably does not need special treatment by a Pod processor;
`` 0 1 2 3 '' doesn't look like a number in any base, so it would
presumably be looked up in the table of HTML-like names. Since
there isn't (and cannot be) an HTML-like entity called `` 0 1 2 3 '',
this will be treated as an error. However, Pod processors may
treat ``E< 0 1 2 3 >'' or ``E<e-acute>'' as syntactically
invalid, potentially earning a different error message than the
error message (or warning, or event) generated by a merely unknown
(but theoretically valid) htmlname, as in ``E<qacute>''
[sic]. However, Pod parsers are not required to make this
distinction.
Note that E<number> must not be interpreted as simply ``codepoint number in the current/native character set''. It always means only ``the character represented by codepoint number in Unicode.'' (This is identical to the semantics of &#number; in XML.)
This will likely require many formatters to have tables mapping from treatable Unicode codepoints (such as the ``\xE9'' for the e-acute character) to the escape sequences or codes necessary for conveying such sequences in the target output format. A converter to *roff would, for example know that ``\xE9'' (whether conveyed literally, or via a E<...> sequence) is to be conveyed as ``e\\*'''. Similarly, a program rendering Pod in a Mac OS application window, would presumably need to know that ``\xE9'' maps to codepoint 142 in MacRoman encoding that (at time of writing) is native for Mac OS. Such Unicode2whatever mappings are presumably already widely available for common output formats. (Such mappings may be incomplete! Implementers are not expected to bend over backwards in an attempt to render Cherokee syllabics, Etruscan runes, Byzantine musical symbols, or any of the other weird things that Unicode can encode.) And if a Pod document uses a character not found in such a mapping, the formatter should consider it an unrenderable character.
If, surprisingly, the implementor of a Pod formatter can't find a satisfactory pre-existing table mapping from Unicode characters to escapes in the target format (e.g., a decent table of Unicode characters to *roff escapes), it will be necessary to build such a table. If you are in this circumstance, you should begin with the characters in the range 0x00A0 - 0x00FF, which is mostly the heavily used accented characters. Then proceed (as patience permits and fastidiousness compels) through the characters that the (X)HTML standards groups judged important enough to merit mnemonics for. These are declared in the (X)HTML specifications at the www.W3.org site. At time of writing (September 2001), the most recent entity declaration files are:
http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
Then you can progress through any remaining notable Unicode characters in the range 0x2000-0x204D (consult the character tables at www.unicode.org), and whatever else strikes your fancy. For example, in xhtml-symbol.ent, there is the entry:
<!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
While the mapping ``infin'' to the character ``\x{221E}'' will (hopefully) have been already handled by the Pod parser, the presence of the character in this file means that it's reasonably important enough to include in a formatter's table that maps from notable Unicode characters to the codes necessary for rendering them. So for a Unicode-to-*roff mapping, for example, this would merit the entry:
"\x{221E}" => '\(in',
It is eagerly hoped that in the future, increasing numbers of formats
(and formatters) will support Unicode characters directly (as (X)HTML
does with ∞, ∞, or ∞), reducing the need
for idiosyncratic mappings of Unicode-to-my_escapes.
It is up to individual Pod formatter to display good judgment when confronted with an unrenderable character (which is distinct from an unknown E<thing> sequence that the parser couldn't resolve to anything, renderable or not). It is good practice to map Latin letters with diacritics (like ``E<eacute>''/``E<233>'') to the corresponding unaccented US-ASCII letters (like a simple character 101, ``e''), but clearly this is often not feasible, and an unrenderable character may be represented as ``?'', or the like. In attempting a sane fallback (as from E<233> to ``e''), Pod formatters may use the %Latin1Code_to_fallback table in Pod::Escapes, or Text::Unidecode, if available.
For example, this Pod text:
magic is enabled if you set C<$Currency> to 'E<euro>'.
may be rendered as:
``magic is enabled if you set $Currency to '?''' or as
``magic is enabled if you set $Currency to '[euro]''', or as
``magic is enabled if you set $Currency to '[x20AC]', etc.
A Pod formatter may also note, in a comment or warning, a list of what unrenderable characters were encountered.
E<...> may freely appear in any formatting code (other than in another E<...> or in an Z<>). That is, ``X<The E<euro>1,000,000 Solution>'' is valid, as is ``L<The E<euro>1,000,000 Solution|Million::Euros>''.
Some Pod formatters output to formats that implement non-breaking spaces as an individual character (which I'll call ``NBSP''), and others output to formats that implement non-breaking spaces just as spaces wrapped in a ``don't break this across lines'' code. Note that at the level of Pod, both sorts of codes can occur: Pod can contain a NBSP character (whether as a literal, or as a ``E<160>'' or ``E<nbsp>'' code); and Pod can contain ``S<foo I<bar> baz>'' codes, where ``mere spaces'' (character 32) in such codes are taken to represent non-breaking spaces. Pod parsers should consider supporting the optional parsing of ``S<foo I<bar> baz>'' as if it were ``fooNBSPI<bar>NBSPbaz'', and, going the other way, the optional parsing of groups of words joined by NBSP's as if each group were in a S<...> code, so that formatters may use the representation that maps best to what the output format demands.
Some processors may find that the S<...> code is easiest to
implement by replacing each space in the parse tree under the content
of the S, with an NBSP. But note: the replacement should apply not to
spaces in all text, but only to spaces in printable text. (This
distinction may or may not be evident in the particular tree/event
model implemented by the Pod parser.) For example, consider this
unusual case:
S<L</Autoloaded Functions>>
This means that the space in the middle of the visible link text must not be broken across lines. In other words, it's the same as this:
L<"AutoloadedE<160>Functions"/Autoloaded Functions>
However, a misapplied space-to-NBSP replacement could (wrongly) produce something equivalent to this:
L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
...which is almost definitely not going to work as a hyperlink (assuming this formatter outputs a format supporting hypertext).
Formatters may choose to just not support the S format code, especially in cases where the output format simply has no NBSP character/code and no code for ``don't break this stuff across lines''.
Besides the NBSP character discussed above, implementors are reminded
of the existence of the other ``special'' character in Latin-1, the
``soft hyphen'' character, also known as ``discretionary hyphen'',
i.e. E<173> = E<0xAD> =
E<shy>). This character expresses an optional hyphenation
point. That is, it normally renders as nothing, but may render as a
``-'' if a formatter breaks the word at that point. Pod formatters
should, as appropriate, do one of the following: 1) render this with
a code with the same meaning (e.g., ``\-'' in RTF), 2) pass it through
in the expectation that the formatter understands this character as
such, or 3) delete it.
For example:
sigE<shy>action manuE<shy>script JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
These signal to a formatter that if it is to hyphenate ``sigaction''
or ``manuscript'', then it should be done as
``sig-[linebreak]action'' or ``manu-[linebreak]script''
(and if it doesn't hyphenate it, then the E<shy> doesn't
show up at all). And if it is
to hyphenate ``Jarkko'' and/or ``Hietaniemi'', it can do
so only at the points where there is a E<shy> code.
In practice, it is anticipated that this character will not be used often, but formatters should either support it, or delete it.
If you think that you want to add a new command to Pod (like, say, a ``=biblio'' command), consider whether you could get the same effect with a for or begin/end sequence: ``=for biblio ...'' or ``=begin biblio'' ... ``=end biblio''. Pod processors that don't understand ``=for biblio'', etc, will simply ignore it, whereas they may complain loudly if they see ``=biblio''.
Throughout this document, ``Pod'' has been the preferred spelling for the name of the documentation format. One may also use ``POD'' or ``pod''. For the documentation that is (typically) in the Pod format, you may use ``pod'', or ``Pod'', or ``POD''. Understanding these distinctions is useful; but obsessing over how to spell them, usually is not.
As you can tell from a glance at perlpod, the L<...> code is the most complex of the Pod formatting codes. The points below will hopefully clarify what it means and how processors should deal with it.
In parsing an L<...> code, Pod parsers must distinguish at least four attributes:
The link-text. If there is none, this must be undef. (E.g., in ``L<Perl Functions|perlfunc>'', the link-text is ``Perl Functions''. In ``L<Time::HiRes>'' and even ``L<|Time::HiRes>'', there is no link text. Note that link text may contain formatting.)
The possibly inferred link-text -- i.e., if there was no real link text, then this is the text that we'll infer in its place. (E.g., for ``L<Getopt::Std>'', the inferred link text is ``Getopt::Std''.)
The name or URL, or undef if none. (E.g., in ``L<Perl Functions|perlfunc>'', the name -- also sometimes called the page -- is ``perlfunc''. In ``L</CAVEATS>'', the name is undef.)
The section (AKA ``item'' in older perlpods), or undef if none. E.g., in DESCRIPTION in the Getopt::Std manpage, ``DESCRIPTION'' is the section. (Note that this is not the same as a manpage section like the ``5'' in ``man 5 crontab''. ``Section Foo'' in the Pod sense means the part of the text that's introduced by the heading or item whose text is ``Foo''.)
Pod parsers may also note additional attributes including:
A flag for whether item 3 (if present) is a URL (like ``http://lists.perl.org'' is), in which case there should be no section attribute; a Pod name (like ``perldoc'' and ``Getopt::Std'' are); or possibly a man page name (like ``crontab(5)'' is).
The raw original L<...> content, before text is split on ``|'', ``/'', etc, and before E<...> codes are expanded.
(The above were numbered only for concise reference below. It is not a requirement that these be passed as an actual list or array.)
For example:
L<Foo::Bar>
=> undef, # link text
"Foo::Bar", # possibly inferred link text
"Foo::Bar", # name
undef, # section
'pod', # what sort of link
"Foo::Bar" # original content
L<Perlport's section on NL's|perlport/Newlines>
=> "Perlport's section on NL's", # link text
"Perlport's section on NL's", # possibly inferred link text
"perlport", # name
"Newlines", # section
'pod', # what sort of link
"Perlport's section on NL's|perlport/Newlines" # orig. content
L<perlport/Newlines>
=> undef, # link text
'"Newlines" in perlport', # possibly inferred link text
"perlport", # name
"Newlines", # section
'pod', # what sort of link
"perlport/Newlines" # original content
L<crontab(5)/"DESCRIPTION">
=> undef, # link text
'"DESCRIPTION" in crontab(5)', # possibly inferred link text
"crontab(5)", # name
"DESCRIPTION", # section
'man', # what sort of link
'crontab(5)/"DESCRIPTION"' # original content
L</Object Attributes>
=> undef, # link text
'"Object Attributes"', # possibly inferred link text
undef, # name
"Object Attributes", # section
'pod', # what sort of link
"/Object Attributes" # original content
L<http://www.perl.org/>;
=> undef, # link text
"http://www.perl.org/";, # possibly inferred link text
"http://www.perl.org/";, # name
undef, # section
'url', # what sort of link
"http://www.perl.org/"; # original content
Note that you can distinguish URL-links from anything else by the
fact that they match m/\A\w+:[^:\s]\S*\z/. So
L<http://www.perl.com> is a URL, but
L<HTTP::Response> isn't.
In case of L<...> codes with no ``text|'' part in them,
older formatters have exhibited great variation in actually displaying
the link or cross reference. For example, L<crontab(5)> would render
as ``the crontab(5) manpage'', or ``in the crontab(5) manpage''
or just ``crontab(5)''.
Pod processors must now treat ``text|''-less links as follows:
L<name> => L<name|name> L</section> => L<"section"|/section> L<name/section> => L<"section" in name|name/section>
Note that section names might contain markup. I.e., if a section starts with:
=head2 About the C<-M> Operator
or with:
=item About the C<-M> Operator
then a link to it would look like this:
L<somedoc/About the C<-M> Operator>
Formatters may choose to ignore the markup for purposes of resolving the link and use only the renderable characters in the section name, as in:
<h1><a name="About_the_-M_Operator">About the <code>-M</code> Operator</h1>
...
<a href="somedoc#About_the_-M_Operator">About the <code>-M</code> Operator" in somedoc</a>
Previous versions of perlpod distinguished L<name/"section">
links from L<name/item> links (and their targets). These
have been merged syntactically and semantically in the current
specification, and section can refer either to a ``=headn Heading
Content'' command or to a ``=item Item Content'' command. This
specification does not specify what behavior should be in the case
of a given document having several things all seeming to produce the
same section identifier (e.g., in HTML, several things all producing
the same anchorname in <a name=``anchorname''>...</a>
elements). Where Pod processors can control this behavior, they should
use the first such anchor. That is, L<Foo/Bar> refers to the
first ``Bar'' section in Foo.
But for some processors/formats this cannot be easily controlled; as with the HTML example, the behavior of multiple ambiguous <a name=``anchorname''>...</a> is most easily just left up to browsers to decide.
Authors wanting to link to a particular (absolute) URL, must do so only with ``L<scheme:...>'' codes (like L<http://www.perl.org>), and must not attempt ``L<Some Site Name|scheme:...>'' codes. This restriction avoids many problems in parsing and rendering L<...> codes.
In a L<text|...> code, text may contain formatting codes
for formatting or for E<...> escapes, as in:
L<B<ummE<234>stuff>|...>
For L<...> codes without a ``name|'' part, only
E<...> and Z<> codes may occur -- no
other formatting codes. That is, authors should not use
``L<B<Foo::Bar>>''.
Note, however, that formatting codes and Z<>'s can occur in any and all parts of an L<...> (i.e., in name, section, text, and url).
Authors must not nest L<...> codes. For example, ``L<The L<Foo::Bar> man page>'' should be treated as an error.
Note that Pod authors may use formatting codes inside the ``text'' part of ``L<text|name>'' (and so on for L<text|/``sec''>).
In other words, this is valid:
Go read L<the docs on C<$.>|perlvar/"$.">
Some output formats that do allow rendering ``L<...>'' codes as hypertext, might not allow the link-text to be formatted; in that case, formatters will have to just ignore that formatting.
At time of writing, L<name> values are of two types:
either the name of a Pod page like L<Foo::Bar> (which
might be a real Perl module or program in an @INC / PATH
directory, or a .pod file in those places); or the name of a UNIX
man page, like L<crontab(5)>. In theory, L<chmod>
in ambiguous between a Pod page called ``chmod'', or the Unix man page
``chmod'' (in whatever man-section). However, the presence of a string
in parens, as in ``crontab(5)'', is sufficient to signal that what
is being discussed is not a Pod page, and so is presumably a
UNIX man page. The distinction is of no importance to many
Pod processors, but some processors that render to hypertext formats
may need to distinguish them in order to know how to render a
given L<foo> code.
Previous versions of perlpod allowed for a L<section> syntax
(as in ``L<Object Attributes>''), which was not easily distinguishable
from L<name> syntax. This syntax is no longer in the
specification, and has been replaced by the L<"section"> syntax
(where the quotes were formerly optional). Pod parsers should tolerate
the L<section> syntax, for a while at least. The suggested
heuristic for distinguishing L<section> from L<name>
is that if it contains any whitespace, it's a section. Pod processors
may warn about this being deprecated syntax.
``=over''...``=back'' regions are used for various kinds of list-like structures. (I use the term ``region'' here simply as a collective term for everything from the ``=over'' to the matching ``=back''.)
The non-zero numeric indentlevel in ``=over indentlevel'' ...
``=back'' is used for giving the formatter a clue as to how many
``spaces'' (ems, or roughly equivalent units) it should tab over,
although many formatters will have to convert this to an absolute
measurement that may not exactly match with the size of spaces (or M's)
in the document's base font. Other formatters may have to completely
ignore the number. The lack of any explicit indentlevel parameter is
equivalent to an indentlevel value of 4. Pod processors may
complain if indentlevel is present but is not a positive number
matching m/\A(\d*\.)?\d+\z/.
Authors of Pod formatters are reminded that ``=over'' ... ``=back'' may map to several different constructs in your output format. For example, in converting Pod to (X)HTML, it can map to any of <ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or <blockquote>...</blockquote>. Similarly, ``=item'' can map to <li> or <dt>.
Each ``=over'' ... ``=back'' region should be one of the following:
An ``=over'' ... ``=back'' region containing only ``=item *'' commands, each followed by some number of ordinary/verbatim paragraphs, other nested ``=over'' ... ``=back'' regions, ``=for...'' paragraphs, and ``=begin''...``=end'' regions.
(Pod processors must tolerate a bare ``=item'' as if it were ``=item *''.) Whether ``*'' is rendered as a literal asterisk, an ``o'', or as some kind of real bullet character, is left up to the Pod formatter, and may depend on the level of nesting.
An ``=over'' ... ``=back'' region containing only
m/\A=item\s+\d+\.?\s*\z/ paragraphs, each one (or each group of them)
followed by some number of ordinary/verbatim paragraphs, other nested
``=over'' ... ``=back'' regions, ``=for...'' paragraphs, and/or
``=begin''...``=end'' codes. Note that the numbers must start at 1
in each section, and must proceed in order and without skipping
numbers.
(Pod processors must tolerate lines like ``=item 1'' as if they were ``=item 1.'', with the period.)
An ``=over'' ... ``=back'' region containing only ``=item [text]'' commands, each one (or each group of them) followed by some number of ordinary/verbatim paragraphs, other nested ``=over'' ... ``=back'' regions, or ``=for...'' paragraphs, and ``=begin''...``=end'' regions.
The ``=item [text]'' paragraph should not match
m/\A=item\s+\d+\.?\s*\z/ or m/\A=item\s+\*\s*\z/, nor should it
match just m/\A=item\s*\z/.
An ``=over'' ... ``=back'' region containing no ``=item'' paragraphs at all, and containing only some number of ordinary/verbatim paragraphs, and possibly also some nested ``=over'' ... ``=back'' regions, ``=for...'' paragraphs, and ``=begin''...``=end'' regions. Such an itemless ``=over'' ... ``=back'' region in Pod is equivalent in meaning to a ``<blockquote>...</blockquote>'' element in HTML.
Note that with all the above cases, you can determine which type of ``=over'' ... ``=back'' you have, by examining the first (non-``=cut'', non-``=pod'') Pod paragraph after the ``=over'' command.
Pod formatters must tolerate arbitrarily large amounts of text in the ``=item text...'' paragraph. In practice, most such paragraphs are short, as in:
=item For cutting off our trade with all parts of the world
But they may be arbitrarily long:
=item For transporting us beyond seas to be tried for pretended offenses
=item He is at this time transporting large armies of foreign mercenaries to complete the works of death, desolation and tyranny, already begun with circumstances of cruelty and perfidy scarcely paralleled in the most barbarous ages, and totally unworthy the head of a civilized nation.
Pod processors should tolerate ``=item *'' / ``=item number'' commands with no accompanying paragraph. The middle item is an example:
=over
=item 1
Pick up dry cleaning.
=item 2
=item 3
Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
=back
No ``=over'' ... ``=back'' region can contain headings. Processors may treat such a heading as an error.
Note that an ``=over'' ... ``=back'' region should have some content. That is, authors should not have an empty region like this:
=over
=back
Pod processors seeing such a contentless ``=over'' ... ``=back'' region, may ignore it, or may report it as an error.
Processors must tolerate an ``=over'' list that goes off the end of the document (i.e., which has no matching ``=back''), but they may warn about such a list.
Authors of Pod formatters should note that this construct:
=item Neque
=item Porro
=item Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem.
=item Ut Enim
is semantically ambiguous, in a way that makes formatting decisions a bit difficult. On the one hand, it could be mention of an item ``Neque'', mention of another item ``Porro'', and mention of another item ``Quisquam Est'', with just the last one requiring the explanatory paragraph ``Qui dolorem ipsum quia dolor...''; and then an item ``Ut Enim''. In that case, you'd want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But it could equally well be a discussion of three (related or equivalent) items, ``Neque'', ``Porro'', and ``Quisquam Est'', followed by a paragraph explaining them all, and then a new item ``Ut Enim''. In that case, you'd probably want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But (for the forseeable future), Pod does not provide any way for Pod authors to distinguish which grouping is meant by the above ``=item''-cluster structure. So formatters should format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
That is, there should be (at least roughly) equal spacing between items as between paragraphs (although that spacing may well be less than the full height of a line of text). This leaves it to the reader to use (con)textual cues to figure out whether the ``Qui dolorem ipsum...'' paragraph applies to the ``Quisquam Est'' item or to all three items ``Neque'', ``Porro'', and ``Quisquam Est''. While not an ideal situation, this is preferable to providing formatting cues that may be actually contrary to the author's intent.
Data paragraphs are typically used for inlining non-Pod data that is to be used (typically passed through) when rendering the document to a specific format:
=begin rtf
\par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
=end rtf
The exact same effect could, incidentally, be achieved with a single ``=for'' paragraph:
=for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
(Although that is not formally a data paragraph, it has the same meaning as one, and Pod parsers may parse it as one.)
Another example of a data paragraph:
=begin html
I like <em>PIE</em>!
<hr>Especially pecan pie!
=end html
If these were ordinary paragraphs, the Pod parser would try to expand the ``E</em>'' (in the first paragraph) as a formatting code, just like ``E<lt>'' or ``E<eacute>''. But since this is in a ``=begin identifier''...``=end identifier'' region and the identifier ``html'' doesn't begin have a ``:'' prefix, the contents of this region are stored as data paragraphs, instead of being processed as ordinary paragraphs (or if they began with a spaces and/or tabs, as verbatim paragraphs).
As a further example: At time of writing, no ``biblio'' identifier is supported, but suppose some processor were written to recognize it as a way of (say) denoting a bibliographic reference (necessarily containing formatting codes in ordinary paragraphs). The fact that ``biblio'' paragraphs were meant for ordinary processing would be indicated by prefacing each ``biblio'' identifier with a colon:
=begin :biblio
Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ.
=end :biblio
This would signal to the parser that paragraphs in this begin...end region are subject to normal handling as ordinary/verbatim paragraphs (while still tagged as meant only for processors that understand the ``biblio'' identifier). The same effect could be had with:
=for :biblio Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ.
The ``:'' on these identifiers means simply ``process this stuff normally, even though the result will be for some special target''. I suggest that parser APIs report ``biblio'' as the target identifier, but also report that it had a ``:'' prefix. (And similarly, with the above ``html'', report ``html'' as the target identifier, and note the lack of a ``:'' prefix.)
Note that a ``=begin identifier''...``=end identifier'' region where identifier begins with a colon, can contain commands. For example:
=begin :biblio
Wirth's classic is available in several editions, including:
=for comment hm, check abebooks.com for how much used copies cost.
=over
=item
Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.> Teubner, Stuttgart. [Yes, it's in German.]
=item
Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ.
=back
=end :biblio
Note, however, a ``=begin identifier''...``=end identifier'' region where identifier does not begin with a colon, should not directly contain ``=head1'' ... ``=head4'' commands, nor ``=over'', nor ``=back'', nor ``=item''. For example, this may be considered invalid:
=begin somedata
This is a data paragraph.
=head1 Don't do this!
This is a data paragraph too.
=end somedata
A Pod processor may signal that the above (specifically the ``=head1'' paragraph) is an error. Note, however, that the following should not be treated as an error:
=begin somedata
This is a data paragraph.
=cut
# Yup, this isn't Pod anymore.
sub excl { (rand() > .5) ? "hoo!" : "hah!" }
=pod
This is a data paragraph too.
=end somedata
And this too is valid:
=begin someformat
This is a data paragraph.
And this is a data paragraph.
=begin someotherformat
This is a data paragraph too.
And this is a data paragraph too.
=begin :yetanotherformat
=head2 This is a command paragraph!
This is an ordinary paragraph!
And this is a verbatim paragraph!
=end :yetanotherformat
=end someotherformat
Another data paragraph!
=end someformat
The contents of the above ``=begin :yetanotherformat'' ... ``=end :yetanotherformat'' region aren't data paragraphs, because the immediately containing region's identifier (``:yetanotherformat'') begins with a colon. In practice, most regions that contain data paragraphs will contain only data paragraphs; however, the above nesting is syntactically valid as Pod, even if it is rare. However, the handlers for some formats, like ``html'', will accept only data paragraphs, not nested regions; and they may complain if they see (targeted for them) nested regions, or commands, other than ``=end'', ``=pod'', and ``=cut''.
Also consider this valid structure:
=begin :biblio
Wirth's classic is available in several editions, including:
=over
=item
Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.> Teubner, Stuttgart. [Yes, it's in German.]
=item
Wirth, Niklaus. 1976. I<Algorithms + Data Structures = Programs.> Prentice-Hall, Englewood Cliffs, NJ.
=back
Buy buy buy!
=begin html
<img src='wirth_spokesmodeling_book.png'>
<hr>
=end html
Now now now!
=end :biblio
There, the ``=begin html''...``=end html'' region is nested inside the larger ``=begin :biblio''...``=end :biblio'' region. Note that the content of the ``=begin html''...``=end html'' region is data paragraph(s), because the immediately containing region's identifier (``html'') doesn't begin with a colon.
Pod parsers, when processing a series of data paragraphs one after another (within a single region), should consider them to be one large data paragraph that happens to contain blank lines. So the content of the above ``=begin html''...``=end html'' may be stored as two data paragraphs (one consisting of ``<img src='wirth_spokesmodeling_book.png'>\n'' and another consisting of ``<hr>\n''), but should be stored as a single data paragraph (consisting of ``<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n'').
Pod processors should tolerate empty ``=begin something''...``=end something'' regions, empty ``=begin :something''...``=end :something'' regions, and contentless ``=for something'' and ``=for :something'' paragraphs. I.e., these should be tolerated:
=for html
=begin html
=end html
=begin :biblio
=end :biblio
Incidentally, note that there's no easy way to express a data paragraph starting with something that looks like a command. Consider:
=begin stuff
=shazbot
=end stuff
There, ``=shazbot'' will be parsed as a Pod command ``shazbot'', not as a data paragraph ``=shazbot\n''. However, you can express a data paragraph consisting of ``=shazbot\n'' using this code:
=for stuff =shazbot
The situation where this is necessary, is presumably quite rare.
Note that =end commands must match the currently open =begin command. That is, they must properly nest. For example, this is valid:
=begin outer
X
=begin inner
Y
=end inner
Z
=end outer
while this is invalid:
=begin outer
X
=begin inner
Y
=end outer
Z
=end inner
This latter is improper because when the ``=end outer'' command is seen, the currently open region has the formatname ``inner'', not ``outer''. (It just happens that ``outer'' is the format name of a higher-up region.) This is an error. Processors must by default report this as an error, and may halt processing the document containing that error. A corollary of this is that regions cannot ``overlap'' -- i.e., the latter block above does not represent a region called ``outer'' which contains X and Y, overlapping a region called ``inner'' which contains Y and Z. But because it is invalid (as all apparently overlapping regions would be), it doesn't represent that, or anything at all.
Similarly, this is invalid:
=begin thing
=end hting
This is an error because the region is opened by ``thing'', and the ``=end'' tries to close ``hting'' [sic].
This is also invalid:
=begin thing
=end
This is invalid because every ``=end'' command must have a formatname parameter.
the perlpod manpage, PODs: Embedded Documentation in the perlsyn manpage, podchecker
Sean M. Burke