PERLPODSPEC(1) Perl Programmers Reference Guide PERLPODSPEC(1)
NAME
perlpodspec - Plain Old Documentation: format specification and notes
DESCRIPTION
This document is detailed notes on the Pod markup language. Most peo
ple 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 doesnt do Y, its against this specification, and
should really be fixed. "X should do Y" means that its recommended,
but X may fail to do Y, if theres 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 wouldnt 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 doesnt 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 thats nothing but Pod.
A line in a file consists of zero or more non-newline characters, ter
minated by either a newline or the end of the file.
A newline sequence is usually a platform-dependent concept, but Pod
parsers should understand it to mean any of CR (ASCII 13), LF (ASCII
10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in addition
to any other system-specific meaning. The first CR/CRLF/LF sequence in
the file may be used as the basis for identifying the newline sequence
for parsing the rest of the file.
A blank line is a line consisting entirely of zero or more spaces
(ASCII 32) or tabs (ASCII 9), and terminated by a newline or
end-of-file. A non-blank line is a line containing one or more charac
ters 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, termi
nated 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 , and continues up to the next line that
matches "m/\A=cut/" -- or up to the end of the file, if there is no
"m/\A=cut/" line.
Within a Pod block, there are Pod paragraphs. A Pod paragraph consists
of non-blank lines of text, separated by one or more blank lines.
For purposes of Pod processing, there are four types of paragraphs in a
Pod block:
A command paragraph (also called a "directive"). The first line of
this paragraph must match "m/\A=[a-zA-Z]/". Command paragraphs are
typically one line, as in:
=head1 NOTES
=item *
But they may span several (non-blank) lines:
=for comment
Hm, I wonder what it would look like if
you tried to write a BNF for Pod from this.
=head3 Dr. Strangelove, or: How I Learned to
Stop Worrying and Love the Bomb
Some command paragraphs allow formatting codes in their content
(i.e., after the part that matches "m/\A=[a-zA-Z]\S*\s*/"), as in:
=head1 Did You Remember to C?
In other words, the Pod processing handler for "head1" will apply
the same processing to "Did You Remember to C?" 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 signif
icant.
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 "identi
fier" 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 its a data paragraph, unless
"identifier" begins with a colon.
Whitespace is significant in verbatim paragraphs (although, in pro
cessing, tabs are probably expanded).
An ordinary paragraph. A paragraph is an ordinary paragraph if its
first line matches neither "m/\A=[a-zA-Z]/" nor "m/\A[ \t]/", and
if its not inside a "=begin identifier", ... "=end identifier"
sequence unless "identifier" begins with a colon (":").
A data paragraph. This is a paragraph that is inside a "=begin
identifier" ... "=end identifier" sequence where "identifier" does
not begin with a literal colon (":"). In some sense, a data para
graph is not part of Pod at all (i.e., effectively its
"out-of-band"), since its not subject to most kinds of Pod pars
ing; 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:
# <- thats 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 theres 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 doesnt 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 para
graph is a heading. That text may contain formatting codes. Exam
ples:
=head1 Object Attributes
=head3 What B 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 con
sist 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. Format
ting 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) >>
=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 for
matname") as being for some special kind of processing. Unless
"formatname" begins with a colon, the contained non-command para
graphs are data paragraphs. But if "formatname" does begin with a
colon, then non-command paragraphs are ordinary paragraphs or data
paragraphs. This is discussed in detail in the section "About Data
Paragraphs and "=begin/=end" Regions".
It is advised that formatnames match the regexp
"m/\A:?[-a-zA-Z0-9_]+\z/". Implementors should anticipate future
expansion in the semantics and syntax of the first parameter to
"=begin"/"=end"/"=for".
"=end formatname"
This marks the end of the region opened by the matching "=begin
formatname" region. If "formatname" is not the formatname of the
most recent open "=begin formatname" region, then this is an error,
and must generate an error message. This is discussed in detail in
the section "About Data Paragraphs and "=begin/=end" Regions".
"=for formatname text..."
This is synonymous with:
=begin formatname
text...
=end formatname
That is, it creates a region consisting of a single paragraph; that
paragraph is to be treated as a normal paragraph if "formatname"
begins with a ":"; if "formatname" doesnt 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 para
graph.
"=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 encodingname, which must be an encoding
name that Encoding recognizes. (Encodings list of supported
encodings, in Encoding::Supported, is useful here.) If the Pod
parser cannot decode the declared encoding, it should emit a warn
ing and may abort parsing the document altogether.
A document having more than one "=encoding" line should be consid
ered an error. Pod processors may silently tolerate this if the
not-first "=encoding" lines are just duplicates of the first one
(e.g., if theres a "=use utf8" line, and later on another "=use
utf8" line). But Pod processors should complain if there are con
tradictory "=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 com
mands, and to stipulate, for each additional command, whether format
ting 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, format
ting codes were referred to as "interior sequences", and this term may
still be found in the documentation for Pod parsers, and in error mes
sages from Pod processors.)
There are two syntaxes for formatting codes:
A formatting code starts with a capital letter (just US-ASCII
[A-Z]) followed by a "<", any number of characters, and ending with
the first matching ">". Examples:
Thats what I think!
Whats C for?
X and C Under Different Operating Systems>
A formatting code starts with a capital letter (just US-ASCII
[A-Z]) followed by two or more "<"s, one or more whitespace
characters, any number of characters, one or more whitespace char
acters, 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:
Thats 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
C<< thing >>
C<< thing >>
C<<< thing >>>
C<<<<
thing
>>>>
and so on.
In parsing Pod, a notably tricky part is the correct parsing of (poten
tially nested!) formatting codes. Implementors should consult the code
in the "parse_text" routine in Pod::Parser as an example of a correct
implementation.
"I" -- italic text
See the brief discussion in "Formatting Codes" in perlpod.
"B" -- bold text
See the brief discussion in "Formatting Codes" in perlpod.
"C" -- code text
See the brief discussion in "Formatting Codes" in perlpod.
"F" -- style for filenames
See the brief discussion in "Formatting Codes" in perlpod.
"X" -- 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 cur
rent 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". Whether or not it
complains, the potatoes text should ignored.
"L" -- a hyperlink
The complicated syntaxes of this code are discussed at length in
"Formatting Codes" in perlpod, and implementation details are dis
cussed below, in "About L<...> Codes". Parsing the contents of
L is tricky. Notably, the content has to be checked for
whether it looks like a URL, or whether it has to be split on lit
eral "|" and/or "/" (in the right order!), and so on, before E<...>
codes are resolved.
"E" -- a character escape
See "Formatting Codes" in perlpod, and several points in "Notes on
Implementing Pod Processors".
"S" -- text contains non-breaking spaces
This formatting code is syntactically simple, but semantically com
plex. What it means is that each space in the printable content of
this code signifies a non-breaking space.
Consider:
C<$x ? $y : $z>
S>
Both signify the monospace (c[ode] style) text consisting of "$x",
one space, "?", one space, ":", one space, "$z". The difference is
that in the latter, with the S code, those spaces are not "normal"
spaces, but instead are non-breaking spaces.
If a Pod processor sees any formatting code other than the ones listed
above (as in "N<...>", or "Q<...>", etc.), that processor must by
default treat this as an error. A Pod parser may allow a way for par
ticular applications to add to the above list of known formatting
codes; a Pod parser might even allow a way to stipulate, for each addi
tional command, whether it requires some form of special processing, as
L<...> does.
Future versions of this specification may add additional formatting
codes.
Historical note: A few older Pod processors would not see a ">" as
closing a "C<" code, if the ">" was immediately preceded by a "-".
This was so that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-Ebar>
instead of as equivalent to a "C" formatting code containing only
"$foo-", and then a "bar>" outside the "C" formatting code. This prob
lem 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
...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
Dont make me say it again!E
(In SGMLish jargon, all Pod commands are like block-level elements,
whereas all Pod formatting codes are like inline-level elements.)
Notes on Implementing Pod Processors
The following is a long section of miscellaneous requirements and sug
gestions to do with Pod processing.
Pod formatters should tolerate lines in verbatim blocks that are of
any length, even if that means having to break them (possibly sev
eral 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 char
acters long, which are usually not intentional.
Pod parsers must recognize all of the three well-known newline for
mats: CR, LF, and CRLF. See perlport.
Pod parsers should accept input lines that are of any length.
Since Perl recognizes a Unicode Byte Order Mark at the start of
files as signaling that the file is Unicode encoded as in UTF-16
(whether big-endian or little-endian) or UTF-8, Pod parsers should
do the same. Otherwise, the character encoding should be under
stood 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 mem
ory as Unicode characters.
The well known Unicode Byte Order Marks are as follows: if the
file begins with the two literal byte values 0xFE 0xFF, this is the
BOM for big-endian UTF-16. If the file begins with the two literal
byte value 0xFF 0xFE, this is the BOM for little-endian UTF-16. If
the file begins with the three literal byte values 0xEF 0xBB 0xBF,
this is the BOM for UTF-8.
A naive but sufficient heuristic for testing the first highbit
byte-sequence in a BOM-less file (whether in code or in Pod!), to
see whether that sequence is valid as UTF-8 (RFC 2279) is to check
whether that the first byte in the sequence is in the range 0xC0 -
0xFD and whether the next byte is in the range 0x80 - 0xBF. If so,
the parser may conclude that this file is in UTF-8, and all highbit
sequences in the file should be assumed to be UTF-8. Otherwise the
parser should treat the file as being in Latin-1. In the unlikely
circumstance that the first highbit sequence in a truly non-UTF-8
file happens to appear to be UTF-8, one can cater to our heuristic
(as well as any more intelligent heuristic) by prefacing that line
with a comment line containing a highbit sequence that is clearly
not valid as UTF-8. A line consisting of simply "#", an e-acute,
and any non-highbit byte, is sufficient to establish this files
encoding.
This documents requirements and suggestions about encodings do not
apply to Pod processors running on non-ASCII platforms, notably
EBCDIC platforms.
Pod processors must treat a "=for [label] [content...]" paragraph
as meaning the same thing as a "=begin [label]" paragraph, content,
and an "=end [label]" paragraph. (The parser may conflate these
two constructs, or may leave them distinct, in the expectation that
the formatter will nevertheless treat them the same.)
When rendering Pod to a format that allows comments (i.e., to
nearly any format other than plaintext), a Pod formatter must
insert comment text identifying its name and version number, and
the name and version numbers of any modules it might be using to
process the Pod. Minimal examples:
%% POD::Pod2PS v3.14159, using POD::Parser v1.92
{\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
.\" Pod::Man version 3.14159, using POD::Parser version 1.92
Formatters may also insert additional comments, including: the
release date of the Pod formatter program, the contact address for
the author(s) of the formatter, the current time, the name of input
file, the formatting options in effect, version of Perl used, etc.
Formatters may also choose to note errors/warnings as comments,
besides or instead of emitting them otherwise (as in messages to
STDERR, or "die"ing).
Pod parsers may emit warnings or error messages ("Unknown E code
E!") to STDERR (whether through printing to STDERR, or
"warn"ing/"carp"ing, or "die"ing/"croak"ing), but must allow sup
pressing all such STDERR output, and instead allow an option for
reporting errors/warnings in some other way, whether by triggering
a callback, or noting errors in some attribute of the document
object, or some similarly unobtrusive mechanism -- or even by
appending a "Pod Errors" section to the end of the parsed form of
the document.
In cases of exceptionally aberrant documents, Pod parsers may abort
the parse. Even then, using "die"ing/"croak"ing is to be avoided;
where possible, the parser library may simply close the input file
and add text like "*** Formatting Aborted ***" to the end of the
(partial) in-memory document.
In paragraphs where formatting codes (like E<...>, B<...>) are
understood (i.e., not verbatim paragraphs, but including ordinary
paragraphs, and command paragraphs that produce renderable text,
like "=head1"), literal whitespace should generally be considered
"insignificant", in that one literal space has the same meaning as
any (nonzero) number of literal spaces, literal newlines, and lit
eral tabs (as long as this produces no blank lines, since those
would terminate the paragraph). Pod parsers should compact literal
whitespace in each processed paragraph, but may provide an option
for overriding this (since some processing tasks do not require
it), or may follow additional special rules (for example, specially
treating period-space-space or period-newline sequences).
Pod parsers should not, by default, try to coerce apostrophe ()
and quote (") into smart quotes (little 9s, 66s, 99s, 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 thats a non-breaking hyphen, and another thats a breakable
hyphen (as in "object-oriented", which can be split across lines as
"object-", newline, "oriented"), formatters are encouraged to gen
erally translate "-" to non-breaking hyphen, but may apply heuris
tics to convert some of these to breaking hyphens.
Pod formatters should make reasonable efforts to keep words of Perl
code from being broken across lines. For example, "Foo::Bar" in
some formatting systems is seen as eligible for being broken across
lines as "Foo::" newline "Bar" or even "Foo::-" newline "Bar".
This should be avoided where possible, either by disabling all
line-breaking in mid-word, or by wrapping particular words with
internal punctuation in "dont break this across lines" codes
(which in some formats may not be a single code, but might be a
matter of inserting non-breaking zero-width spaces between every
pair of characters in a word.)
Pod parsers should, by default, expand tabs in verbatim paragraphs
as they are processed, before passing them to the formatter or
other processor. Parsers may also allow an option for overriding
this.
Pod parsers should, by default, remove newlines from the end of
ordinary and verbatim paragraphs before passing them to the format
ter. For example, while the paragraph youre reading now could be
considered, in Pod source, to end with (and contain) the newline(s)
that end it, it should be processed as ending with (and containing)
the period character that ends this sentence.
Pod parsers, when reporting errors, should make some effort to
report an approximate line number ("Nested E<>s in Paragraph #52,
near line 633 of Thing/Foo.pm!"), instead of merely noting the
paragraph number ("Nested E<>s in Paragraph #52 of
Thing/Foo.pm!"). Where this is problematic, the paragraph number
should at least be accompanied by an excerpt from the paragraph
("Nested E<>s in Paragraph #52 of Thing/Foo.pm, which begins
Read/write accessor for the C attribute...").
Pod parsers, when processing a series of verbatim paragraphs one
after another, should consider them to be one large verbatim para
graph that happens to contain blank lines. I.e., these two lines,
which have a blank line between them:
use Foo;
print Foo->VERSION
should be unified into one paragraph ("\tuse Foo;\n\n\tprint
Foo->VERSION") before being passed to the formatter or other pro
cessor. Parsers may also allow an option for overriding this.
While this might be too cumbersome to implement in event-based Pod
parsers, it is straightforward for parsers that return parse trees.
Pod formatters, where feasible, are advised to avoid splitting
short verbatim paragraphs (under twelve lines, say) across pages.
Pod parsers must treat a line with only spaces and/or tabs on it as
a "blank line" such as separates paragraphs. (Some older parsers
recognized only two adjacent newlines as a "blank line" but would
not recognize a newline, a space, and a newline, as a blank line.
This is noncompliant behavior.)
Authors of Pod formatters/processors should make every effort to
avoid writing their own Pod parser. There are already several in
CPAN, with a wide range of interface styles -- and one of them,
Pod::Parser, comes with modern versions of Perl.
Characters in Pod documents may be conveyed either as literals, or
by number in E codes, or by an equivalent mnemonic, as in
E 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 codes), except for the literal byte-sequences for
newline (13, 13 10, or 10), and tab (9).
Characters in the range 160-255 refer to Latin-1 characters (also
defined there by Unicode, with the same meaning). Characters above
255 should be understood to refer to Unicode characters.
Be warned that some formatters cannot reliably render characters
outside 32-126; and many are able to handle 32-126 and 160-255, but
nothing above 255.
Besides the well-known "E" and "E" codes for less-than and
greater-than, Pod parsers must understand "E" for "/"
(solidus, slash), and "E" for "|" (vertical bar, pipe).
Pod parsers should also understand "E" and "E"
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" and
"E".)
Pod parsers should understand all "E" 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" code,
shouldnt simply replace it with nullstring (by default, at least),
but may pass it through as a string consisting of the literal char
acters E, less-than, identifier, greater-than. Or Pod parsers may
offer the alternative option of processing such unknown "E" codes by firing an event especially for such codes, or by
adding a special node-type to the in-memory document tree. Such
"E" 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" for charac
ter 34 (doublequote, "), "E" for character 38 (ampersand, &),
and "E" for character 39 (apostrophe, ).
Note that in all cases of "E", whatever (whether an html
name, 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 arent
alphanumeric characters. This presumably does not need special
treatment by a Pod processor; " 0 1 2 3 " doesnt look like a num
ber in any base, so it would presumably be looked up in the table
of HTML-like names. Since there isnt (and cannot be) an HTML-like
entity called " 0 1 2 3 ", this will be treated as an error. How
ever, Pod processors may treat "E< 0 1 2 3 >" or "E" as
syntactically invalid, potentially earning a different error mes
sage than the error message (or warning, or event) generated by a
merely unknown (but theoretically valid) htmlname, as in
"E" [sic]. However, Pod parsers are not required to make
this distinction.
Note that E 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 lit
erally, or via a E<...> sequence) is to be conveyed as "e\\*".
Similarly, a program rendering Pod in a Mac OS application window,
would presumably need to know that "\xE9" maps to codepoint 142 in
MacRoman encoding that (at time of writing) is native for Mac OS.
Such Unicode2whatever mappings are presumably already widely avail
able for common output formats. (Such mappings may be incomplete!
Implementers are not expected to bend over backwards in an attempt
to render Cherokee syllabics, Etruscan runes, Byzantine musical
symbols, or any of the other weird things that Unicode can encode.)
And if a Pod document uses a character not found in such a mapping,
the formatter should consider it an unrenderable character.
If, surprisingly, the implementor of a Pod formatter cant find a
satisfactory pre-existing table mapping from Unicode characters to
escapes in the target format (e.g., a decent table of Unicode char
acters to *roff escapes), it will be necessary to build such a ta
ble. If you are in this circumstance, you should begin with the
characters in the range 0x00A0 - 0x00FF, which is mostly the heav
ily 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 charac
ters in the range 0x2000-0x204D (consult the character tables at
www.unicode.org), and whatever else strikes your fancy. For exam
ple, in xhtml-symbol.ent, there is the entry:
While the mapping "infin" to the character "\x{221E}" will (hope
fully) have been already handled by the Pod parser, the presence of
the character in this file means that its reasonably important
enough to include in a formatters 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 for
mats (and formatters) will support Unicode characters directly (as
(X)HTML does with "∞", "∞", or "∞"), reducing
the need for idiosyncratic mappings of Unicode-to-my_escapes.
It is up to individual Pod formatter to display good judgment when
confronted with an unrenderable character (which is distinct from
an unknown E sequence that the parser couldnt resolve to
anything, renderable or not). It is good practice to map Latin
letters with diacritics (like "E"/"E<233>") to the corre
sponding 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.
may be rendered as: "magic is enabled if you set $Currency to ?"
or as "magic is enabled if you set $Currency to [euro]", or as
"magic is enabled if you set $Currency to [x20AC], etc.
A Pod formatter may also note, in a comment or warning, a list of
what unrenderable characters were encountered.
E<...> may freely appear in any formatting code (other than in
another E<...> or in an Z<>). That is, "X1,000,000
Solution>" is valid, as is "L1,000,000 Solution|Mil
lion::Euros>".
Some Pod formatters output to formats that implement non-breaking
spaces as an individual character (which Ill call "NBSP"), and
others output to formats that implement non-breaking spaces just as
spaces wrapped in a "dont 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" code); and Pod can contain "S baz>" codes,
where "mere spaces" (character 32) in such codes are taken to rep
resent non-breaking spaces. Pod parsers should consider supporting
the optional parsing of "S baz>" as if it were "fooNB
SPINBSPbaz", and, going the other way, the optional parsing of
groups of words joined by NBSPs 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 con
tent 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 particu
lar tree/event model implemented by the Pod parser.) For example,
consider this unusual case:
S>
This means that the space in the middle of the visible link text
must not be broken across lines. In other words, its 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, espe
cially in cases where the output format simply has no NBSP charac
ter/code and no code for "dont break this stuff across lines".
Besides the NBSP character discussed above, implementors are
reminded of the existence of the other "special" character in
Latin-1, the "soft hyphen" character, also known as "discretionary
hyphen", i.e. "E<173>" = "E<0xAD>" = "E"). This character
expresses an optional hyphenation point. That is, it normally ren
ders 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:
sigEaction
manuEscript
JarkEko HieEtaEnieEmi
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 doesnt hyphenate it, then
the "E" doesnt 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" 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 dont
understand "=for biblio", etc, will simply ignore it, whereas they
may complain loudly if they see "=biblio".
Throughout this document, "Pod" has been the preferred spelling for
the name of the documentation format. One may also use "POD" or
"pod". For the documentation that is (typically) in the Pod for
mat, you may use "pod", or "Pod", or "POD". Understanding these
distinctions is useful; but obsessing over how to spell them, usu
ally is not.
About L<...> Codes
As you can tell from a glance at perlpod, the L<...> code is the most
complex of the Pod formatting codes. The points below will hopefully
clarify what it means and how processors should deal with it.
In parsing an L<...> code, Pod parsers must distinguish at least
four attributes:
First:
The link-text. If there is none, this must be undef. (E.g.,
in "L", the link-text is "Perl Func
tions". In "L" 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 well infer in its place.
(E.g., for "L", the inferred link text is
"Getopt::Std".)
Third:
The name or URL, or undef if none. (E.g., in "L", the name -- also sometimes called the page --
is "perlfunc". In "L", 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 thats introduced by the head
ing 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
=> undef, # link text
"Foo::Bar", # possibly inferred link text
"Foo::Bar", # name
undef, # section
pod, # what sort of link
"Foo::Bar" # original content
L
=> "Perlports section on NLs", # link text
"Perlports section on NLs", # possibly inferred link text
"perlport", # name
"Newlines", # section
pod, # what sort of link
"Perlports section on NLs|perlport/Newlines" # orig. content
L
=> undef, # link text
"Newlines" in perlport, # possibly inferred link text
"perlport", # name
"Newlines", # section
pod, # what sort of link
"perlport/Newlines" # original content
L
=> 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
=> undef, # link text
"Object Attributes", # possibly inferred link text
undef, # name
"Object Attributes", # section
pod, # what sort of link
"/Object Attributes" # original content
L" is a URL, but "L" isnt.
In case of L<...> codes with no "text|" part in them, older format
ters have exhibited great variation in actually displaying the link
or cross reference. For example, L 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 => L
L => L<"section"|/section>
L => L<"section" in name|name/section>
Note that section names might contain markup. I.e., if a section
starts with:
=head2 About the C<-M> Operator
or with:
=item About the C<-M> Operator
then a link to it would look like this:
L Operator>
Formatters may choose to ignore the markup for purposes of resolv
ing the link and use only the renderable characters in the section
name, as in:
...
About the -M
Operator" in somedoc
Previous versions of perlpod distinguished "L"
links from "L" links (and their targets). These have
been merged syntactically and semantically in the current specifi
cation, 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 sec
tion identifier (e.g., in HTML, several things all producing the
same anchorname in ... elements). Where
Pod processors can control this behavior, they should use the first
such anchor. That is, "L" refers to the first "Bar" sec
tion in Foo.
But for some processors/formats this cannot be easily controlled;
as with the HTML example, the behavior of multiple ambiguous ... 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" codes (like L), and
must not attempt "L" codes. This
restriction avoids many problems in parsing and rendering L<...>
codes.
In a "L" code, text may contain formatting codes for for
matting or for E<...> escapes, as in:
Lstuff>|...>
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>"".
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 man page>" should be treated as an error.
Note that Pod authors may use formatting codes inside the "text"
part of "L" (and so on for L).
In other words, this is valid:
Go read L|perlvar/"$.">
Some output formats that do allow rendering "L<...>" codes as
hypertext, might not allow the link-text to be formatted; in that
case, formatters will have to just ignore that formatting.
At time of writing, "L" values are of two types: either the
name of a Pod page like "L" (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". In theory, "L" 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 dis
tinction is of no importance to many Pod processors, but some pro
cessors that render to hypertext formats may need to distinguish
them in order to know how to render a given "L" code.
Previous versions of perlpod allowed for a "L" syntax (as
in ""L""), which was not easily distinguishable
from "L" syntax. This syntax is no longer in the specifica
tion, and has been replaced by the "L<"section">" syntax (where the
quotes were formerly optional). Pod parsers should tolerate the
"L" syntax, for a while at least. The suggested heuristic
for distinguishing "L" from "L" is that if it con
tains any whitespace, its a section. Pod processors may warn
about this being deprecated syntax.
About =over...=back Regions
"=over"..."=back" regions are used for various kinds of list-like
structures. (I use the term "region" here simply as a collective term
for everything from the "=over" to the matching "=back".)
The non-zero numeric indentlevel in "=over indentlevel" ...
"=back" is used for giving the formatter a clue as to how many
"spaces" (ems, or roughly equivalent units) it should tab over,
although many formatters will have to convert this to an absolute
measurement that may not exactly match with the size of spaces (or
Ms) in the documents 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
, ... , ... , or .... Similarly, "=item" can map to or
.
Each "=over" ... "=back" region should be one of the following:
An "=over" ... "=back" region containing only "=item *" com
mands, each followed by some number of ordinary/verbatim para
graphs, other nested "=over" ... "=back" regions, "=for..."
paragraphs, and "=begin"..."=end" regions.
(Pod processors must tolerate a bare "=item" as if it were
"=item *".) Whether "*" is rendered as a literal asterisk, an
"o", or as some kind of real bullet character, is left up to
the Pod formatter, and may depend on the level of nesting.
An "=over" ... "=back" region containing only
"m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each group
of them) followed by some number of ordinary/verbatim para
graphs, other nested "=over" ... "=back" regions, "=for..."
paragraphs, and/or "=begin"..."=end" codes. Note that the num
bers must start at 1 in each section, and must proceed in order
and without skipping numbers.
(Pod processors must tolerate lines like "=item 1" as if they
were "=item 1.", with the period.)
An "=over" ... "=back" region containing only "=item [text]"
commands, each one (or each group of them) followed by some
number of ordinary/verbatim paragraphs, other nested "=over"
... "=back" regions, or "=for..." paragraphs, and
"=begin"..."=end" regions.
The "=item [text]" paragraph should not match
"m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor
should it match just "m/\A=item\s*\z/".
An "=over" ... "=back" region containing no "=item" paragraphs
at all, and containing only some number of ordinary/verbatim
paragraphs, and possibly also some nested "=over" ... "=back"
regions, "=for..." paragraphs, and "=begin"..."=end" regions.
Such an itemless "=over" ... "=back" region in Pod is equiva
lent in meaning to a "... " element in
HTML.
Note that with all the above cases, you can determine which type of
"=over" ... "=back" you have, by examining the first (non-"=cut",
non-"=pod") Pod paragraph after the "=over" command.
Pod formatters must tolerate arbitrarily large amounts of text in
the "=item text..." paragraph. In practice, most such paragraphs
are short, as in:
=item For cutting off our trade with all parts of the world
But they may be arbitrarily long:
=item For transporting us beyond seas to be tried for pretended
offenses
=item He is at this time transporting large armies of foreign
mercenaries to complete the works of death, desolation and
tyranny, already begun with circumstances of cruelty and perfidy
scarcely paralleled in the most barbarous ages, and totally
unworthy the head of a civilized nation.
Pod processors should tolerate "=item *" / "=item number" commands
with no accompanying paragraph. The middle item is an example:
=over
=item 1
Pick up dry cleaning.
=item 2
=item 3
Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
=back
No "=over" ... "=back" region can contain headings. Processors may
treat such a heading as an error.
Note that an "=over" ... "=back" region should have some content.
That is, authors should not have an empty region like this:
=over
=back
Pod processors seeing such a contentless "=over" ... "=back"
region, may ignore it, or may report it as an error.
Processors must tolerate an "=over" list that goes off the end of
the document (i.e., which has no matching "=back"), but they may
warn about such a list.
Authors of Pod formatters should note that this construct:
=item Neque
=item Porro
=item Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
=item Ut Enim
is semantically ambiguous, in a way that makes formatting decisions
a bit difficult. On the one hand, it could be mention of an item
"Neque", mention of another item "Porro", and mention of another
item "Quisquam Est", with just the last one requiring the explana
tory paragraph "Qui dolorem ipsum quia dolor..."; and then an item
"Ut Enim". In that case, youd want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But it could equally well be a discussion of three (related or
equivalent) items, "Neque", "Porro", and "Quisquam Est", followed
by a paragraph explaining them all, and then a new item "Ut Enim".
In that case, youd probably want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But (for the forseeable future), Pod does not provide any way for
Pod authors to distinguish which grouping is meant by the above
"=item"-cluster structure. So formatters should format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
That is, there should be (at least roughly) equal spacing between
items as between paragraphs (although that spacing may well be less
than the full height of a line of text). This leaves it to the
reader to use (con)textual cues to figure out whether the "Qui
dolorem ipsum..." paragraph applies to the "Quisquam Est" item or
to all three items "Neque", "Porro", and "Quisquam Est". While not
an ideal situation, this is preferable to providing formatting cues
that may be actually contrary to the authors intent.
About Data Paragraphs and "=begin/=end" Regions
Data paragraphs are typically used for inlining non-Pod data that is to
be used (typically passed through) when rendering the document to a
specific format:
=begin rtf
\par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
=end rtf
The exact same effect could, incidentally, be achieved with a single
"=for" paragraph:
=for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
(Although that is not formally a data paragraph, it has the same mean
ing as one, and Pod parsers may parse it as one.)
Another example of a data paragraph:
=begin html
I like PIE !
" or "E". But since this is in a "=begin identi
fier"..."=end identifier" region and the identifier "html" doesnt
begin have a ":" prefix, the contents of this region are stored as data
paragraphs, instead of being processed as ordinary paragraphs (or if
they began with a spaces and/or tabs, as verbatim paragraphs).
As a further example: At time of writing, no "biblio" identifier is
supported, but suppose some processor were written to recognize it as a
way of (say) denoting a bibliographic reference (necessarily containing
formatting codes in ordinary paragraphs). The fact that "biblio" para
graphs were meant for ordinary processing would be indicated by prefac
ing each "biblio" identifier with a colon:
=begin :biblio
Wirth, Niklaus. 1976. I Prentice-Hall, Englewood Cliffs, NJ.
=end :biblio
This would signal to the parser that paragraphs in this begin...end
region are subject to normal handling as ordinary/verbatim paragraphs
(while still tagged as meant only for processors that understand the
"biblio" identifier). The same effect could be had with:
=for :biblio
Wirth, Niklaus. 1976. I Prentice-Hall, Englewood Cliffs, NJ.
The ":" on these identifiers means simply "process this stuff normally,
even though the result will be for some special target". I suggest
that parser APIs report "biblio" as the target identifier, but also
report that it had a ":" prefix. (And similarly, with the above
"html", report "html" as the target identifier, and note the lack of a
":" prefix.)
Note that a "=begin identifier"..."=end identifier" region where iden
tifier begins with a colon, can contain commands. For example:
=begin :biblio
Wirths classic is available in several editions, including:
=for comment
hm, check abebooks.com for how much used copies cost.
=over
=item
Wirth, Niklaus. 1975. I
Teubner, Stuttgart. [Yes, its in German.]
=item
Wirth, Niklaus. 1976. I Prentice-Hall, Englewood Cliffs, NJ.
=back
=end :biblio
Note, however, a "=begin identifier"..."=end identifier" region where
identifier does not begin with a colon, should not directly contain
"=head1" ... "=head4" commands, nor "=over", nor "=back", nor "=item".
For example, this may be considered invalid:
=begin somedata
This is a data paragraph.
=head1 Dont 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 isnt Pod anymore.
sub excl { (rand() > .5) ? "hoo!" : "hah!" }
=pod
This is a data paragraph too.
=end somedata
And this too is valid:
=begin someformat
This is a data paragraph.
And this is a data paragraph.
=begin someotherformat
This is a data paragraph too.
And this is a data paragraph too.
=begin :yetanotherformat
=head2 This is a command paragraph!
This is an ordinary paragraph!
And this is a verbatim paragraph!
=end :yetanotherformat
=end someotherformat
Another data paragraph!
=end someformat
The contents of the above "=begin :yetanotherformat" ... "=end :yetan
otherformat" region arent data paragraphs, because the immediately
containing regions identifier (":yetanotherformat") begins with a
colon. In practice, most regions that contain data paragraphs will
contain only data paragraphs; however, the above nesting is syntacti
cally valid as Pod, even if it is rare. However, the handlers for some
formats, like "html", will accept only data paragraphs, not nested
regions; and they may complain if they see (targeted for them) nested
regions, or commands, other than "=end", "=pod", and "=cut".
Also consider this valid structure:
=begin :biblio
Wirths classic is available in several editions, including:
=over
=item
Wirth, Niklaus. 1975. I
Teubner, Stuttgart. [Yes, its in German.]
=item
Wirth, Niklaus. 1976. I Prentice-Hall, Englewood Cliffs, NJ.
=back
Buy buy buy!
=begin html
