perlpodspec
DESCRIPTION
This document is detailed notes on the Pod markup lan
guage. 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 con
cept, 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 con
taining 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 con
sisting of spaces/tabs and then a newline as a blank line
-- the only lines they considered blank were lines con
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 con
tinues 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 para
graph 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 para
graph -- 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 para
graph must be a literal space or tab, and this para
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!
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 posi
tive numeral. The semantics of this numeral is
explained in the "About =over...=back Regions" sec
tion, 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 para
graph 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>) >>
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 match
ing "=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 para
graphs 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 antic
ipate 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 match
ing "=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 "for
matname" 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.
"=encoding encodingname"
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 encod
ingname, which must be an encoding name that Encoding
recognizes. (Encoding's list of supported encodings,
cessors 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.
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 docu
mentation 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 char
acters, and ending with the first matching ">". Exam
ples:
That's what I<you> think!
What's C<dump()> for?
X<C
B<< $foo->bar(); >>
and so on.
In parsing Pod, a notably tricky part is the correct pars
ing of (potentially nested!) formatting codes. Implemen
tors should consult the code in the "parse_text" routine
in Pod::Parser as an example of a correct implementation.
"I<text>" -- italic text
See the brief discussion in "Formatting Codes" in
perlpod.
"B<text>" -- bold text
See the brief discussion in "Formatting Codes" in
perlpod.
"C<code>" -- code text
See the brief discussion in "Formatting Codes" in
perlpod.
"F<filename>" -- style for filenames
See the brief discussion in "Formatting Codes" in
perlpod.
"X<topic name>" -- an index entry
See the brief discussion in "Formatting Codes" in
perlpod.
This code is unusual in that most formatters com
pletely discard this code and its content. Other for
matters will render it with invisible codes that can
be used in building an index of the current document.
"Z<>" -- a null (zero-effect) formatting code
Discussed briefly in "Formatting Codes" in perlpod.
This code is unusual is that it should have no con
tent. That is, a processor may complain if it sees
"Z<potatoes>". Whether or not it complains, the pota
toes text should ignored.
"L<name>" -- a hyperlink
The complicated syntaxes of this code are discussed at
length in "Formatting Codes" in perlpod, and implemen
tation details are discussed below, in "About L<...>
Codes". Parsing the contents of L<content> is tricky.
Notably, the content has to be checked for whether it
looks like a URL, or whether it has to be split on
literal "|" and/or "/" (in the right order!), and so
on, before E<...> codes are resolved.
"E<escape>" -- a character escape
Both signify the monospace (c[ode] style) text con
sisting of "$x", one space, "?", one space, ":", one
space, "$z". The difference is that in the latter,
with the S code, those spaces are not "normal" spaces,
but instead are nonbreaking spaces.
If a Pod processor sees any formatting code other than the
ones listed above (as in "N<...>", or "Q<...>", etc.),
that processor must by default treat this as an error. A
Pod parser may allow a way for particular applications to
add to the above list of known formatting codes; a Pod
parser might even allow a way to stipulate, for each addi
tional command, whether it requires some form of special
processing, as L<...> does.
Future versions of this specification may add additional
formatting codes.
Historical note: A few older Pod processors would not see
a ">" as closing a "C<" code, if the ">" was immediately
preceded by a "-". This was so that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-E<lt>bar>
instead of as equivalent to a "C" formatting code contain
ing only "$foo-", and then a "bar>" outside the "C" for
matting 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 para
graphs:
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
ments and suggestions to do with Pod processing.
· Pod formatters should tolerate lines in verbatim
blocks that are of any length, even if that means hav
ing 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 lit
tle-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 particu
lar Pod file, content is to be stored in memory as
Unicode characters.
· The well known Unicode Byte Order Marks are as fol
lows: 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
"=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 num
bers 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 pro
gram, 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 other
wise (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<...>,
· 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-ori
ented", 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 dis
abling 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 verba
tim 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 para
graph number ("Nested E<>'s in Paragraph #52 of
Thing/Foo.pm!"). Where this is problematic, the para
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 para
graphs. (Some older parsers recognized only two adja
cent newlines as a "blank line" but would not recog
nize 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 equiva
lent mnemonic, as in E<eacute> which is exactly equiv
alent to E<233>.
Characters in the range 32-126 refer to those well
known US-ASCII characters (also defined there by Uni
code, 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 char
acters (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 under
stand "E<sol>" for "/" (solidus, slash), and "E<ver
ters 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 espe
cially for such codes, or by adding a special node-
type to the in-memory document tree. Such "E<identi
fier>" 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, poten
tially 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
· 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 for
mat (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-sym
bol.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 Uni
code characters directly (as (X)HTML does with
"∞", "∞", or "∞"), reducing the
need for idiosyncratic mappings of Uni
code-to-my_escapes.
· It is up to individual Pod formatter to display good
judgment when confronted with an unrenderable charac
ter (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
rency to '[euro]'", or as "magic is enabled if you set
$Currency to '[x20AC]', etc.
A Pod formatter may also note, in a comment or warn
ing, 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" (char
acter 32) in such codes are taken to represent non
breaking spaces. Pod parsers should consider support
ing 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[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 documenta
tion 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 distin
guish at least four attributes:
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 possi
bly 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
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
<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 syntacti
cally and semantically in the current specification,
and section can refer either to a "=headn Heading Con
tent" 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 sev
eral things all seeming to produce the same section
identifier (e.g., in HTML, several things all produc
ing the same anchorname in <a name="anchor
name">...</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<Bname, 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">).
"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 pro
cessors, 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<sec
tion>" 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 sim
ply 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 for
matter 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 proces
sors 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>...</block
quote>. Similarly, "=item" can map to <li> or <dt>.
· Each "=over" ... "=back" region should be one of the
following:
(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/ver
batim 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>" ele
ment in HTML.
Note that with all the above cases, you can determine
which type of "=over" ... "=back" you have, by examin
ing 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 prac
tice, 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
=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 con
struct:
=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 format
ting 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
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}
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 para
graphs (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 bib
liographic 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 ordi
nary/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 "bib
lio" 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 com
mands. For example:
=begin :biblio
Wirth's classic is available in several editions, including:
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" com
mands, nor "=over", nor "=back", nor "=item". For exam
ple, 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.
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 syn
tactically 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 com
mands, 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
"<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
Pod processors should tolerate empty "=begin some
thing"..."=end something" regions, empty "=begin :some
thing"..."=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
=end outer
Z
=end inner
This latter is improper because when the "=end outer" com
mand 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, pod
checker
AUTHOR
Sean M. Burke
perl v5.8.1 2003-09-02 PERLPODSPEC(1)
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