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PERLPODSPEC(1)
NAME
perlpodspec - Plain Old Documentation: format specification and notes
DESCRIPTION
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 Definitions
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".
Pod Commands
This section is intended to supplement and clarify the discussion in
"Command Paragraph" in perlpod. These are the currently recognized Pod
commands:
"=head1", "=head2", "=head3", "=head4"
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!
"=pod"
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.
"=cut"
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 black with a "=cut" command. In
that case, the Pod processor must halt parsing of the input file, and
must by default emit a warning.
"=over"
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
"=item"
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.
"=back"
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.
"=begin formatname"
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".
"=end formatname"
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".
"=for formatname text..."
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.
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.
Pod Formatting Codes
(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 perlpod.
"B<text>" -- bold text
See the brief discussion in "Formatting Codes" in perlpod.
"C<code>" -- code text
See the brief discussion in "Formatting Codes" in perlpod.
"F<filename>" -- style for filenames
See the brief discussion in "Formatting Codes" in perlpod.
"X<topic name>" -- an index entry
See the brief discussion in "Formatting Codes" in perlpod.
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 perlpod.
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 perlpod, 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 perlpod, 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 nonbreaking 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, "$z". The difference is that in
the latter, with the S code, those spaces are not "normal" spaces, but
instead are nonbreaking spaces.
If a Pod processor sees any formatting code other than the ones listed
above (as in "N<...>", or "Q<...>", etc.), that processor must by default
treat this as an error. A Pod parser may allow a way for particular
applications to add to the above list of known formatting codes; a Pod
parser might even allow a way to stipulate, for each additional command,
whether it requires some form of special processing, as L<...> does.
Future versions of this specification may add additional formatting codes.
Historical note: A few older Pod processors would not see a ">" as closing
a "C<" code, if the ">" was immediately preceded by a "-". This was so
that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-E<lt>bar>
instead of as equivalent to a "C" formatting code containing only "$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.)
Notes on Implementing Pod Processors
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
"die"ing).
· Pod parsers may emit warnings or error messages ("Unknown E code
E<zslig>!") to STDERR (whether through printing to STDERR, or
"warn"ing/"carp"ing, or "die"ing/"croak"ing), 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 "die"ing/"croak"ing 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 nonbreaking 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 nonbreaking 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 nonbreaking spaces
as an individual character (which I'll call "NBSP"), and others output
to formats that implement nonbreaking 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 nonbreaking 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.
About L<...> Codes
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:
First:
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.)
Second:
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".)
Third:
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.)
Fourth:
The section (AKA "item" in older perlpods), or undef if none.
E.g., in "DESCRIPTION" in Getopt::Std, "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:
Fifth:
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).
Sixth:
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.
About =over...=back Regions
"=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.
About Data Paragraphs and "=begin/=end" Regions
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.
SEE ALSO
perlpod, "PODs: Embedded Documentation" in perlsyn, podchecker
AUTHOR
Sean M. Burke
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