perlpodspec - Plain Old Documentation: format specification
and notes
This document is detailed notes on the Pod markup language.
Most people will only have to read perlpod to know
how to write in Pod, but this document may answer some
incidental questions to do with parsing and rendering Pod.
In this document, "must" / "must not", "should" / "should
not", and "may" have their conventional (cf. RFC 2119)
meanings: "X must do Y" means that if X doesn't do Y, it's
against this specification, and should really be fixed.
"X should do Y" means that it's recommended, but X may
fail to do Y, if there's a good reason. "X may do Y" is
merely a note that X can do Y at will (although it is up
to the reader to detect any connotation of "and I think it
would be nice if X did Y" versus "it wouldn't really
bother me if X did Y").
Notably, when I say "the parser should do Y", the parser
may fail to do Y, if the calling application explicitly
requests that the parser not do Y. I often phrase this as
"the parser should, by default, do Y." This doesn't
require the parser to provide an option for turning off
whatever feature Y is (like expanding tabs in verbatim
paragraphs), although it implicates that such an option
may be provided.
Pod is embedded in files, typically Perl source files --
although you can write a file that's nothing but Pod.
A line in a file consists of zero or more non-newline
characters, terminated by either a newline or the end of
the file.
A newline sequence is usually a platform-dependent concept,
but Pod parsers should understand it to mean any of
CR (ASCII 13), LF (ASCII 10), or a CRLF (ASCII 13 followed
immediately by ASCII 10), in addition to any other systemspecific
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/0 tinues up to
the next line that matches "m/0 up to the end of the file,
if there is no "m/1 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:
o A command paragraph (also called a "directive"). The
first line of this paragraph must match
"m/0 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/0
=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.
o 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).
o An ordinary paragraph. A paragraph is an ordinary
paragraph if its first line matches neither
"m/0begin identifier", ... "=end identifier"
sequence unless "identifier" begins with a colon
(":").
o 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
$foo->bar
=cut
Here, "=head1 Foo" and "=cut" are command paragraphs
because the first line of each matches "m/1
"[space][space]$foo->bar" is a verbatim paragraph, because
its first line starts with a literal whitespace character
(and there's no "=begin"..."=end" region around).
The "=begin identifier" ... "=end identifier" commands
stop paragraphs that they surround from being parsed as
data or verbatim paragraphs, if identifier doesn't begin
with a colon. This is discussed in detail in the section
"About Data Paragraphs and "=begin/=end" Regions".
This section is intended to supplement and clarify the
discussion in "Command Paragraph" in 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 block with a
"=cut" command. In that case, the Pod processor must
halt parsing of the input file, and must by default
emit a warning.
"=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/0 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 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.
"=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,
in Encoding::Supported, is useful here.) If the Pod
parser cannot decode the declared encoding, it should
emit a warning and may abort parsing the document
altogether.
A document having more than one "=encoding" line
should be considered an error. Pod processors may
silently tolerate this if the not-first "=encoding"
lines are just duplicates of the first one (e.g., if
there's a "=use utf8" line, and later on another "=use
utf8" line). But Pod processors should complain if
there are contradictory "=encoding" lines in the same
document (e.g., if there is a "=encoding utf8" early
in the document and "=encoding big5" later). Pod processors
that recognize BOMs may also complain if they
see an "=encoding" line that contradicts the BOM
(e.g., if a document with a UTF-16LE BOM has an
"=encoding shiftjis" line).
If a Pod processor sees any command other than the ones
listed above (like "=head", or "=haed1", or "=stuff", or
"=cuttlefish", or "=w123"), that processor must by default
treat this as an error. It must not process the paragraph
beginning with that command, must by default warn of this
as an error, and may abort the parse. A Pod parser may
allow a way for particular applications to add to the
above list of known commands, and to stipulate, for each
additional command, whether formatting codes should be
processed.
Future versions of this specification may add additional
commands.
(Note that in previous drafts of this document and of
perlpod, formatting codes were referred to as "interior
sequences", and this term may still be found in the documentation
for Pod parsers, and in error messages from Pod
processors.)
There are two syntaxes for formatting codes:
o 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>
o 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 non-breaking space.
Consider:
C<$x ? $y : $z>
S<C<$x ? $y : $z>>
Both signify the monospace (c[ode] style) text consisting
of "$x", one space, "?", one space, ":", one
space, "$z". The difference is that in the latter,
with the S code, those spaces are not "normal" spaces,
but instead are non-breaking spaces.
If a Pod processor sees any formatting code other than the
ones listed above (as in "N<...>", or "Q<...>", etc.),
that processor must by default treat this as an error. A
Pod parser may allow a way for particular applications to
add to the above list of known formatting codes; a Pod
parser might even allow a way to stipulate, for each additional
command, whether it requires some form of special
processing, as L<...> does.
Future versions of this specification may add additional
formatting codes.
Historical note: A few older Pod processors would not see
a ">" as closing a "C<" code, if the ">" was immediately
preceded by a "-". This was so that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-E<lt>bar>
instead of as equivalent to a "C" formatting code containing
only "$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 [Toc] [Back] The following is a long section of miscellaneous requirements
and suggestions to do with Pod processing.
o 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.
o Pod parsers must recognize all of the three well-known
newline formats: CR, LF, and CRLF. See perlport.
o Pod parsers should accept input lines that are of any
length.
o 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.
o 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.
o 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.
o This document's requirements and suggestions about
encodings do not apply to Pod processors running on
non-ASCII platforms, notably EBCDIC platforms.
o 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.)
o 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 -->
{occomm generated by Pod::Tree::RTF 3.14159 using
Pod::Tree 1.08}
.
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).
o 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.
o 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.
o 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).
o 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.
o When rendering Pod to a format that has two kinds of
hyphens (-), one that's a non-breaking hyphen, and
another that's a breakable hyphen (as in "object-oriented",
which can be split across lines as "object-",
newline, "oriented"), formatters are encouraged to
generally translate "-" to non-breaking hyphen, but
may apply heuristics to convert some of these to
breaking hyphens.
o 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.)
o 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.
o 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.
o 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...'").
o 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 ("use
Foo;0rint 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.
o Pod formatters, where feasible, are advised to avoid
splitting short verbatim paragraphs (under twelve
lines, say) across pages.
o 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.)
o 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.
o 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.
o 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.
o 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>".)
o 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 nodetype
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.
o 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, ').
o 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/1 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.
o 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 " 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 " (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 " 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.
o 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-sym-
bol.ent, there is the entry:
<!ENTITY infin "∞"> <!-- infinity, U+221E
ISOtech -->
While the mapping "infin" to the character "}"
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:
"}" => '',
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.
o 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.
o 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>".
o Some Pod formatters output to formats that implement
non-breaking spaces as an individual character (which
I'll call "NBSP"), and others output to formats that
implement non-breaking spaces just as spaces wrapped
in a "don't break this across lines" code. Note that
at the level of Pod, both sorts of codes can occur:
Pod can contain a NBSP character (whether as a literal,
or as a "E<160>" or "E<nbsp>" code); and Pod can
contain "S<foo I<bar> baz>" codes, where "mere spaces"
(character 32) in such codes are taken to represent
non-breaking spaces. Pod parsers should consider supporting
the optional parsing of "S<foo I<bar> baz>" as
if it were "fooNBSPI<bar>NBSPbaz", and, going the
other way, the optional parsing of groups of words
joined by NBSP's as if each group were in a S<...>
code, so that formatters may use the representation
that maps best to what the output format demands.
o 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".
o 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.
o 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".
o 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.
o 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/1 is a URL, but "L<HTTP::Response>"
isn't.
o 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>
o 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>
o 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="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.
o 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.
o 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.
o 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.
o 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.
o 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
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