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PERLPODSPEC

Section: Perl Programmers Reference Guide (1)
Updated: 2017-06-30
Index Return to Main Contents

NAME

perlpodspec - Plain Old Documentation: format specification and notes

DESCRIPTION

This document is detailed notes on the Pod markup language. Most people will only have to read perlpod to know how to write in Pod, but this document may answer some incidental questions to do with parsing and rendering Pod.

In this document, ``must'' / ``must not'', ``should'' / ``should not'', and ``may'' have their conventional (cf. RFC 2119) meanings: ``X must do Y'' means that if X doesn't do Y, it's against this specification, and should really be fixed. ``X should do Y'' means that it's recommended, but X may fail to do Y, if there's a good reason. ``X may do Y'' is merely a note that X can do Y at will (although it is up to the reader to detect any connotation of "and I think it would be nice if X did Y`` versus ''it wouldn't really bother me if X did Y").

Notably, when I say ``the parser should do Y'', the parser may fail to do Y, if the calling application explicitly requests that the parser not do Y. I often phrase this as ``the parser should, by default, do Y.'' This doesn't require the parser to provide an option for turning off whatever feature Y is (like expanding tabs in verbatim paragraphs), although it implicates that such an option may be provided.

Pod Definitions

Pod is embedded in files, typically Perl source files, although you can write a file that's nothing but Pod.

A line in a file consists of zero or more non-newline characters, terminated by either a newline or the end of the file.

A newline sequence is usually a platform-dependent concept, but Pod parsers should understand it to mean any of CR (ASCII 13), LF (ASCII 10), or a CRLF (ASCII 13 followed immediately by ASCII 10), in addition to any other system-specific meaning. The first CR/CRLF/LF sequence in the file may be used as the basis for identifying the newline sequence for parsing the rest of the file.

A blank line is a line consisting entirely of zero or more spaces (ASCII 32) or tabs (ASCII 9), and terminated by a newline or end-of-file. A non-blank line is a line containing one or more characters other than space or tab (and terminated by a newline or end-of-file).

(Note: Many older Pod parsers did not accept a line consisting of spaces/tabs and then a newline as a blank line. The only lines they considered blank were lines consisting of no characters at all, terminated by a newline.)

Whitespace is used in this document as a blanket term for spaces, tabs, and newline sequences. (By itself, this term usually refers to literal whitespace. That is, sequences of whitespace characters in Pod source, as opposed to ``E<32>'', which is a formatting code that denotes a whitespace character.)

A Pod parser is a module meant for parsing Pod (regardless of whether this involves calling callbacks or building a parse tree or directly formatting it). A Pod formatter (or Pod translator) is a module or program that converts Pod to some other format (HTML, plaintext, TeX, PostScript, RTF). A Pod processor might be a formatter or translator, or might be a program that does something else with the Pod (like counting words, scanning for index points, etc.).

Pod content is contained in Pod blocks. A Pod block starts with a line that matches "m/\A=[a-zA-Z]/", and continues up to the next line that matches "m/\A=cut/" or up to the end of the file if there is no "m/\A=cut/" line.

Within a Pod block, there are Pod paragraphs. A Pod paragraph consists of non-blank lines of text, separated by one or more blank lines.

For purposes of Pod processing, there are four types of paragraphs in a Pod block:

*

A command paragraph (also called a ``directive''). The first line of this paragraph must match "m/\A=[a-zA-Z]/". Command paragraphs are typically one line, as in:

  =head1 NOTES

  =item *

But they may span several (non-blank) lines:

  =for comment
  Hm, I wonder what it would look like if
  you tried to write a BNF for Pod from this.

  =head3 Dr. Strangelove, or: How I Learned to
  Stop Worrying and Love the Bomb

Some command paragraphs allow formatting codes in their content (i.e., after the part that matches "m/\A=[a-zA-Z]\S*\s*/"), as in:

  =head1 Did You Remember to C<use strict;>?

In other words, the Pod processing handler for ``head1'' will apply the same processing to ``Did You Remember to C<use strict;>?'' that it would to an ordinary paragraph (i.e., formatting codes like ``C<...>'') are parsed and presumably formatted appropriately, and whitespace in the form of literal spaces and/or tabs is not significant.

*

A verbatim paragraph. The first line of this paragraph must be a literal space or tab, and this paragraph must not be inside a "=begin identifier``, ... ''=end identifier`` sequence unless ''identifier`` begins with a colon ('':"). That is, if a paragraph starts with a literal space or tab, but is inside a "=begin identifier``, ... ''=end identifier`` region, then it's a data paragraph, unless ''identifier" begins with a colon.

Whitespace is significant in verbatim paragraphs (although, in processing, tabs are probably expanded).

*

An ordinary paragraph. A paragraph is an ordinary paragraph if its first line matches neither "m/\A=[a-zA-Z]/" nor "m/\A[ \t]/", and if it's not inside a "=begin identifier``, ... ''=end identifier`` sequence unless ''identifier`` begins with a colon ('':").

*

A data paragraph. This is a paragraph that is inside a "=begin identifier`` ... ''=end identifier`` sequence where ''identifier" does not begin with a literal colon (``:''). In some sense, a data paragraph is not part of Pod at all (i.e., effectively it's ``out-of-band''), since it's not subject to most kinds of Pod parsing; but it is specified here, since Pod parsers need to be able to call an event for it, or store it in some form in a parse tree, or at least just parse around it.

For example: consider the following paragraphs:

  # <- that's the 0th column

  =head1 Foo

  Stuff

    $foo->bar

  =cut

Here, ``=head1 Foo'' and ``=cut'' are command paragraphs because the first line of each matches "m/\A=[a-zA-Z]/". "[space][space]$foo->bar`` is a verbatim paragraph, because its first line starts with a literal whitespace character (and there's no ''=begin``...''=end" region around).

The "=begin identifier`` ... ''=end identifier" commands stop paragraphs that they surround from being parsed as ordinary or verbatim paragraphs, if identifier doesn't begin with a colon. This is discussed in detail in the section ``About Data Paragraphs and ''=begin/=end`` Regions''.

Pod Commands

This section is intended to supplement and clarify the discussion in ``Command Paragraph'' in perlpod. These are the currently recognized Pod commands:

=head1, =head2, =head3, =head4

This command indicates that the text in the remainder of the paragraph is a heading. That text may contain formatting codes. Examples:

  =head1 Object Attributes

  =head3 What B<Not> to Do!

=pod

This command indicates that this paragraph begins a Pod block. (If we are already in the middle of a Pod block, this command has no effect at all.) If there is any text in this command paragraph after ``=pod'', it must be ignored. Examples:

  =pod

  This is a plain Pod paragraph.

  =pod This text is ignored.

=cut

This command indicates that this line is the end of this previously started Pod block. If there is any text after ``=cut'' on the line, it must be ignored. Examples:

  =cut

  =cut The documentation ends here.

  =cut
  # This is the first line of program text.
  sub foo { # This is the second.

It is an error to try to start a Pod 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

=begin formatname parameter

This marks the following paragraphs (until the matching ``=end formatname'') as being for some special kind of processing. Unless ``formatname'' begins with a colon, the contained non-command paragraphs are data paragraphs. But if ``formatname'' does begin with a colon, then non-command paragraphs are ordinary paragraphs or data paragraphs. This is discussed in detail in the section ``About Data Paragraphs and ''=begin/=end`` Regions''.

It is advised that formatnames match the regexp "m/\A:?[-a-zA-Z0-9_]+\z/". Everything following whitespace after the formatname is a parameter that may be used by the formatter when dealing with this region. This parameter must not be repeated in the ``=end'' paragraph. Implementors should anticipate future expansion in the semantics and syntax of the first parameter to ``=begin''/``=end''/``=for''.

=end formatname

This marks the end of the region opened by the matching ``=begin formatname'' region. If ``formatname'' is not the formatname of the most recent open ``=begin formatname'' region, then this is an error, and must generate an error message. This is discussed in detail in the section ``About Data Paragraphs and ''=begin/=end`` Regions''.

=for formatname text...

This is synonymous with:

     =begin formatname

     text...

     =end formatname

That is, it creates a region consisting of a single paragraph; that paragraph is to be treated as a normal paragraph if ``formatname'' begins with a ``:''; if ``formatname'' doesn't begin with a colon, then ``text...'' will constitute a data paragraph. There is no way to use ``=for formatname text...'' to express ``text...'' as a verbatim paragraph.

=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 Encode recognizes. (Encode's list of supported encodings, in Encode::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 ``=encoding utf8'' line, and later on another ``=encoding 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.

Pod Formatting Codes

(Note that in previous drafts of this document and of perlpod, formatting codes were referred to as ``interior sequences'', and this term may still be found in the documentation for Pod parsers, and in error messages from Pod processors.)

There are two syntaxes for formatting codes:

*

A formatting code starts with a capital letter (just US-ASCII [A-Z]) followed by a ``<'', any number of characters, and ending with the first matching ``>''. Examples:

    That's what I<you> think!

    What's C<dump()> for?

    X<C<chmod> and C<unlink()> Under Different Operating Systems>

*

A formatting code starts with a capital letter (just US-ASCII [A-Z]) followed by two or more ``<'''s, one or more whitespace characters, any number of characters, one or more whitespace characters, and ending with the first matching sequence of two or more ``>'''s, where the number of ``>'''s equals the number of ``<'''s in the opening of this formatting code. Examples:

    That's what I<< you >> think!

    C<<< open(X, ">>thing.dat") || die $! >>>

    B<< $foo->bar(); >>

With this syntax, the whitespace character(s) after the ``C<<<'' and before the ``>>>'' (or whatever letter) are not renderable. They do not signify whitespace, are merely part of the formatting codes themselves. That is, these are all synonymous:

    C<thing>
    C<< thing >>
    C<<           thing     >>
    C<<<   thing >>>
    C<<<<
    thing
               >>>>

and so on.

Finally, the multiple-angle-bracket form does not alter the interpretation of nested formatting codes, meaning that the following four example lines are identical in meaning:

  B<example: C<$a E<lt>=E<gt> $b>>

  B<example: C<< $a <=> $b >>>

  B<example: C<< $a E<lt>=E<gt> $b >>>

  B<<< example: C<< $a E<lt>=E<gt> $b >> >>>

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