perlpod - the Plain Old Documentation format


perlpod - the Plain Old Documentation format


Pod is a simple-to-use markup language used for writing documentation for Perl, Perl programs, and Perl modules.

Translators are available for converting Pod to various formats like plain text, HTML, man pages, and more.

Pod markup consists of three basic kinds of paragraphs: ordinary, verbatim, and command.

Ordinary Paragraph

Most paragraphs in your documentation will be ordinary blocks of text, like this one. You can simply type in your text without any markup whatsoever, and with just a blank line before and after. When it gets formatted, it will undergo minimal formatting, like being rewrapped, probably put into a proportionally spaced font, and maybe even justified.

You can use formatting codes in ordinary paragraphs, for bold, italic, code-style, hyperlinks, and more. Such codes are explained in the ``Formatting Codes'' section, below.

Verbatim Paragraph

Verbatim paragraphs are usually used for presenting a codeblock or other text which does not require any special parsing or formatting, and which shouldn't be wrapped.

A verbatim paragraph is distinguished by having its first character be a space or a tab. (And commonly, all its lines begin with spaces and/or tabs.) It should be reproduced exactly, with tabs assumed to be on 8-column boundaries. There are no special formatting codes, so you can't italicize or anything like that. A \ means \, and nothing else.

Command Paragraph

A command paragraph is used for special treatment of whole chunks of text, usually as headings or parts of lists.

All command paragraphs (which are typically only one line long) start with ``='', followed by an identifier, followed by arbitrary text that the command can use however it pleases. Currently recognized commands are

    =head1 Heading Text
    =head2 Heading Text
    =head3 Heading Text
    =head4 Heading Text
    =over indentlevel
    =item stuff
    =begin format
    =end format
    =for format text...

To explain them each in detail:

=head1 Heading Text
=head2 Heading Text
=head3 Heading Text
=head4 Heading Text
Head1 through head4 produce headings, head1 being the highest level. The text in the rest of this paragraph is the content of the heading. For example:
  =head2 Object Attributes

The text ``Object Attributes'' comprises the heading there. (Note that head3 and head4 are recent additions, not supported in older Pod translators.) The text in these heading commands can use formatting codes, as seen here:

  =head2 Possible Values for C<$/>

Such commands are explained in the ``Formatting Codes'' section, below.

=over indentlevel
=item stuff...
Item, over, and back require a little more explanation: ``=over'' starts a region specifically for the generation of a list using ``=item'' commands, or for indenting (groups of) normal paragraphs. At the end of your list, use ``=back'' to end it. The indentlevel option to ``=over'' indicates how far over to indent, generally in ems (where one em is the width of an ``M'' in the document's base font) or roughly comparable units; if there is no indentlevel option, it defaults to four. (And some formatters may just ignore whatever indentlevel you provide.) In the stuff in =item stuff..., you may use formatting codes, as seen here:
  =item Using C<$|> to Control Buffering

Such commands are explained in the ``Formatting Codes'' section, below.

Note also that there are some basic rules to using ``=over'' ... ``=back'' regions:

To end a Pod block, use a blank line, then a line beginning with ``=cut'', and a blank line after it. This lets Perl (and the Pod formatter) know that this is where Perl code is resuming. (The blank line before the ``=cut'' is not technically necessary, but many older Pod processors require it.)

The ``=pod'' command by itself doesn't do much of anything, but it signals to Perl (and Pod formatters) that a Pod block starts here. A Pod block starts with any command paragraph, so a ``=pod'' command is usually used just when you want to start a Pod block with an ordinary paragraph or a verbatim paragraph. For example:
  =item stuff()
  This function does stuff.
  sub stuff {
  Remember to check its return value, as in:
    stuff() || die "Couldn't do stuff!";

=begin formatname
=end formatname
=for formatname text...
For, begin, and end will let you have regions of text/code/data that are not generally interpreted as normal Pod text, but are passed directly to particular formatters, or are otherwise special. A formatter that can use that format will use the region, otherwise it will be completely ignored.

A command ``=begin formatname'', some paragraphs, and a command ``=end formatname'', mean that the text/data inbetween is meant for formatters that understand the special format called formatname. For example,

  =begin html
  <hr> <img src="thang.png">
  <p> This is a raw HTML paragraph </p>
  =end html

The command ``=for formatname text...'' specifies that the remainder of just this paragraph (starting right after formatname) is in that special format.

  =for html <hr> <img src="thang.png">
  <p> This is a raw HTML paragraph </p>

This means the same thing as the above ``=begin html'' ... ``=end html'' region.

That is, with ``=for'', you can have only one paragraph's worth of text (i.e., the text in ``=foo targetname text...''), but with ``=begin targetname'' ... ``=end targetname'', you can have any amount of stuff inbetween. (Note that there still must be a blank line after the ``=begin'' command and a blank line before the ``=end'' command.

Here are some examples of how to use these:

  =begin html
  <br>Figure 1.<br><IMG SRC="figure1.png"><br>
  =end html
  =begin text
    |  foo        |
    |        bar  |
  ^^^^ Figure 1. ^^^^
  =end text

Some format names that formatters currently are known to accept include ``roff'', ``man'', ``latex'', ``tex'', ``text'', and ``html''. (Some formatters will treat some of these as synonyms.)

A format name of ``comment'' is common for just making notes (presumably to yourself) that won't appear in any formatted version of the Pod document:

  =for comment
  Make sure that all the available options are documented!

Some formatnames will require a leading colon (as in "=for :formatname", or "=begin :formatname" ... "=end :formatname"), to signal that the text is not raw data, but instead is Pod text (i.e., possibly containing formatting codes) that's just not for normal formatting (e.g., may not be a normal-use paragraph, but might be for formatting as a footnote).

And don't forget, when using any command, that the command lasts up until the end of its paragraph, not its line. So in the examples below, you can see that every command needs the blank line after it, to end its paragraph.

Some examples of lists include:

  =item *
  First item
  =item *
  Second item
  =item Foo()
  Description of Foo function
  =item Bar()
  Description of Bar function

Formatting Codes

In ordinary paragraphs and in some command paragraphs, various formatting codes (a.k.a. ``interior sequences'') can be used:

I<text> -- italic text
Used for emphasis (``be I<careful!>'') and parameters (``redo I<LABEL>'')

B<text> -- bold text
Used for switches (``perl's B<-n> switch''), programs (``some systems provide a B<chfn> for that''), emphasis (``be B<careful!>''), and so on (``and that feature is known as B<autovivification>'').

C<code> -- code text
Renders code in a typewriter font, or gives some other indication that this represents program text (``C<gmtime($^T)>'') or some other form of computerese (``C<drwxr-xr-x>'').

L<name> -- a hyperlink
There are various syntaxes, listed below. In the syntaxes given, text, name, and section cannot contain the characters '/' and '|'; and any '<' or '>' should be matched.

A section is started by the named heading or item. For example, L<perlvar/$.> or L<perlvar/"$."> both link to the section started by ``=item $.'' in perlvar. And L<perlsyn/For Loops> or L<perlsyn/"For Loops"> both link to the section started by ``=head2 For Loops'' in perlsyn.

To control what text is used for display, you use ``L<text|...>'', as in:

Or you can link to a web page:

E<escape> -- a character escape
Very similar to HTML/XML &foo; ``entity references'':
F<filename> -- used for filenames
Typically displayed in italics. Example: ``F<.cshrc>''

S<text> -- text contains non-breaking spaces
This means that the words in text should not be broken across lines. Example: S<$x ? $y : $z>.

X<topic name> -- an index entry
This is ignored by most formatters, but some may use it for building indexes. It always renders as empty-string. Example: X<absolutizing relative URLs>

Z<> -- a null (zero-effect) formatting code
This is rarely used. It's one way to get around using an E<...> code sometimes. For example, instead of ``NE<lt>3'' (for ``N<3'') you could write ``NZ<><3'' (the ``Z<>'' breaks up the ``N'' and the ``<'' so they can't be considered the part of a (fictitious) ``N<...>'' code.

Most of the time, you will need only a single set of angle brackets to delimit the beginning and end of formatting codes. However, sometimes you will want to put a real right angle bracket (a greater-than sign, '>') inside of a formatting code. This is particularly common when using a formatting code to provide a different font-type for a snippet of code. As with all things in Perl, there is more than one way to do it. One way is to simply escape the closing bracket using an E code:

    C<$a E<lt>=E<gt> $b>

This will produce: ``$a <=> $b''

A more readable, and perhaps more ``plain'' way is to use an alternate set of delimiters that doesn't require a single ``>'' to be escaped. With the Pod formatters that are standard starting with perl5.5.660, doubled angle brackets (``<<'' and ``>>'') may be used if and only if there is whitespace right after the opening delimiter and whitespace right before the closing delimiter! For example, the following will do the trick:

    C<< $a <=> $b >>

In fact, you can use as many repeated angle-brackets as you like so long as you have the same number of them in the opening and closing delimiters, and make sure that whitespace immediately follows the last '<' of the opening delimiter, and immediately precedes the first '>' of the closing delimiter. (The whitespace is ignored.) So the following will also work:

    C<<< $a <=> $b >>>
    C<<<<  $a <=> $b     >>>>

And they all mean exactly the same as this:

    C<$a E<lt>=E<gt> $b>

As a further example, this means that if you wanted to put these bits of code in C (code) style:

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

you could do it like so:

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

which is presumably easier to read than the old way:

    C<open(X, "E<gt>E<gt>thing.dat") || die $!>
    C<$foo-E<gt>bar(); >>

This is currently supported by pod2text (Pod::Text), pod2man (Pod::Man), and any other pod2xxx or Pod::Xxxx translators that use Pod::Parser 1.093 or later, or Pod::Tree 1.02 or later.

The Intent

The intent is simplicity of use, not power of expression. Paragraphs look like paragraphs (block format), so that they stand out visually, and so that I could run them through fmt easily to reformat them (that's F7 in my version of vi, or Esc Q in my version of emacs). I wanted the translator to always leave the ' and ` and " quotes alone, in verbatim mode, so I could slurp in a working program, shift it over four spaces, and have it print out, er, verbatim. And presumably in a monospace font.

The Pod format is not necessarily sufficient for writing a book. Pod is just meant to be an idiot-proof common source for nroff, HTML, TeX, and other markup languages, as used for online documentation. Translators exist for pod2text, pod2html, pod2man (that's for nroff(1) and troff(1)), pod2latex, and pod2fm. Various others are available in CPAN.

Embedding Pods in Perl Modules

You can embed Pod documentation in your Perl modules and scripts. Start your documentation with an empty line, a ``=head1'' command at the beginning, and end it with a ``=cut'' command and an empty line. Perl will ignore the Pod text. See any of the supplied library modules for examples. If you're going to put your Pod at the end of the file, and you're using an __END__ or __DATA__ cut mark, make sure to put an empty line there before the first Pod command.

  =head1 NAME
  Time::Local - efficiently compute time from local and GMT time

Without that empty line before the ``=head1'', many translators wouldn't have recognized the ``=head1'' as starting a Pod block.

Hints for Writing Pod


the perlpodspec manpage, PODs: Embedded Documentation in the perlsyn manpage, the perlnewmod manpage, the perldoc manpage, pod2html, pod2man, podchecker.


Larry Wall, Sean M. Burke

 perlpod - the Plain Old Documentation format