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perlpodspec




DESCRIPTION

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

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

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


Pod Definitions

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

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

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

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

       (Note: Many older Pod parsers did not accept a line con­
       sisting of spaces/tabs and then a newline as a blank line
       -- the only lines they considered blank were lines con­
       PostScript, RTF).  A Pod processor might be a formatter or
       translator, or might be a program that does something else
       with the Pod (like wordcounting it, scanning for index
       points, etc.).

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

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

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

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

             =head1 NOTES

             =item *

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

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

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

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

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

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

       ·   A verbatim paragraph.  The first line of this para­
           graph must be a literal space or tab, and this para­
           inside a "=begin identifier", ... "=end identifier"
           sequence unless "identifier" begins with a colon
           (":").

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

       For example: consider the following paragraphs:

         # <- that's the 0th column

         =head1 Foo

         Stuff

           $foo->bar

         =cut

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

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


Pod Commands

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

       "=head1", "=head2", "=head3", "=head4"
           This command indicates that the text in the remainder
           of the paragraph is a heading.  That text may contain
           formatting codes.  Examples:

             =head1 Object Attributes

             =head3 What B<Not> to Do!

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

             =cut

             =cut The documentation ends here.

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

           It is an error to try to start a Pod black with a
           "=cut" command.  In that case, the Pod processor must
           halt parsing of the input file, and must by default
           emit a warning.

       "=over"
           This command indicates that this is the start of a
           list/indent region.  If there is any text following
           the "=over", it must consist of only a nonzero posi­
           tive numeral.  The semantics of this numeral is
           explained in the "About =over...=back Regions" sec­
           tion, further below.  Formatting codes are not
           expanded.  Examples:

             =over 3

             =over 3.5

             =over

       "=item"
           This command indicates that an item in a list begins
           here.  Formatting codes are processed.  The semantics
           of the (optional) text in the remainder of this para­
           graph are explained in the "About =over...=back
           Regions" section, further below.  Examples:

             =item

             =item *

             =item      *

             =item 14

             =item   3.

             =item C<< $thing->stuff(I<dodad>) >>

           This command indicates that this is the end of the
           region begun by the most recent "=over" command.  It
           permits no text after the "=back" command.

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

           It is advised that formatnames match the regexp
           "m/\A:?[-a-zA-Z0-9_]+\z/".  Implementors should antic­
           ipate future expansion in the semantics and syntax of
           the first parameter to "=begin"/"=end"/"=for".

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

       "=for formatname text..."
           This is synonymous with:

                =begin formatname

                text...

                =end formatname

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

       "=encoding encodingname"
           This command, which should occur early in the document
           (at least before any non-US-ASCII data!), declares
           that this document is encoded in the encoding encod­
           ingname, which must be an encoding name that Encoding
           recognizes.  (Encoding's list of supported encodings,
           cessors that recognize BOMs may also complain if they
           see an "=encoding" line that contradicts the BOM
           (e.g., if a document with a UTF-16LE BOM has an
           "=encoding shiftjis" line).

       If a Pod processor sees any command other than the ones
       listed above (like "=head", or "=haed1", or "=stuff", or
       "=cuttlefish", or "=w123"), that processor must by default
       treat this as an error.  It must not process the paragraph
       beginning with that command, must by default warn of this
       as an error, and may abort the parse.  A Pod parser may
       allow a way for particular applications to add to the
       above list of known commands, and to stipulate, for each
       additional command, whether formatting codes should be
       processed.

       Future versions of this specification may add additional
       commands.


Pod Formatting Codes

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

       There are two syntaxes for formatting codes:

       ·   A formatting code starts with a capital letter (just
           US-ASCII [A-Z]) followed by a "<", any number of char­
           acters, and ending with the first matching ">".  Exam­
           ples:

               That's what I<you> think!

               What's C<dump()> for?

               X<C

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

           and so on.

       In parsing Pod, a notably tricky part is the correct pars­
       ing of (potentially nested!) formatting codes.  Implemen­
       tors should consult the code in the "parse_text" routine
       in Pod::Parser as an example of a correct implementation.

       "I<text>" -- italic text
           See the brief discussion in "Formatting Codes" in
           perlpod.

       "B<text>" -- bold text
           See the brief discussion in "Formatting Codes" in
           perlpod.

       "C<code>" -- code text
           See the brief discussion in "Formatting Codes" in
           perlpod.

       "F<filename>" -- style for filenames
           See the brief discussion in "Formatting Codes" in
           perlpod.

       "X<topic name>" -- an index entry
           See the brief discussion in "Formatting Codes" in
           perlpod.

           This code is unusual in that most formatters com­
           pletely discard this code and its content.  Other for­
           matters will render it with invisible codes that can
           be used in building an index of the current document.

       "Z<>" -- a null (zero-effect) formatting code
           Discussed briefly in "Formatting Codes" in perlpod.

           This code is unusual is that it should have no con­
           tent.  That is, a processor may complain if it sees
           "Z<potatoes>".  Whether or not it complains, the pota­
           toes text should ignored.

       "L<name>" -- a hyperlink
           The complicated syntaxes of this code are discussed at
           length in "Formatting Codes" in perlpod, and implemen­
           tation details are discussed below, in "About L<...>
           Codes".  Parsing the contents of L<content> is tricky.
           Notably, the content has to be checked for whether it
           looks like a URL, or whether it has to be split on
           literal "|" and/or "/" (in the right order!), and so
           on, before E<...> codes are resolved.

       "E<escape>" -- a character escape

           Both signify the monospace (c[ode] style) text con­
           sisting of "$x", one space, "?", one space, ":", one
           space, "$z".  The difference is that in the latter,
           with the S code, those spaces are not "normal" spaces,
           but instead are nonbreaking spaces.

       If a Pod processor sees any formatting code other than the
       ones listed above (as in "N<...>", or "Q<...>", etc.),
       that processor must by default treat this as an error.  A
       Pod parser may allow a way for particular applications to
       add to the above list of known formatting codes; a Pod
       parser might even allow a way to stipulate, for each addi­
       tional command, whether it requires some form of special
       processing, as L<...> does.

       Future versions of this specification may add additional
       formatting codes.

       Historical note:  A few older Pod processors would not see
       a ">" as closing a "C<" code, if the ">" was immediately
       preceded by a "-".  This was so that this:

           C<$foo->bar>

       would parse as equivalent to this:

           C<$foo-E<lt>bar>

       instead of as equivalent to a "C" formatting code contain­
       ing only "$foo-", and then a "bar>" outside the "C" for­
       matting code.  This problem has since been solved by the
       addition of syntaxes like this:

           C<< $foo->bar >>

       Compliant parsers must not treat "->" as special.

       Formatting codes absolutely cannot span paragraphs.  If a
       code is opened in one paragraph, and no closing code is
       found by the end of that paragraph, the Pod parser must
       close that formatting code, and should complain (as in
       "Unterminated I code in the paragraph starting at line
       123: 'Time objects are not...'").  So these two para­
       graphs:

         I<I told you not to do this!

         Don't make me say it again!>

       ...must not be parsed as two paragraphs in italics (with
       the I code starting in one paragraph and starting in
       ments and suggestions to do with Pod processing.

       ·   Pod formatters should tolerate lines in verbatim
           blocks that are of any length, even if that means hav­
           ing to break them (possibly several times, for very
           long lines) to avoid text running off the side of the
           page.  Pod formatters may warn of such line-breaking.
           Such warnings are particularly appropriate for lines
           are over 100 characters long, which are usually not
           intentional.

       ·   Pod parsers must recognize all of the three well-known
           newline formats: CR, LF, and CRLF.  See perlport.

       ·   Pod parsers should accept input lines that are of any
           length.

       ·   Since Perl recognizes a Unicode Byte Order Mark at the
           start of files as signaling that the file is Unicode
           encoded as in UTF-16 (whether big-endian or lit­
           tle-endian) or UTF-8, Pod parsers should do the same.
           Otherwise, the character encoding should be understood
           as being UTF-8 if the first highbit byte sequence in
           the file seems valid as a UTF-8 sequence, or otherwise
           as Latin-1.

           Future versions of this specification may specify how
           Pod can accept other encodings.  Presumably treatment
           of other encodings in Pod parsing would be as in XML
           parsing: whatever the encoding declared by a particu­
           lar Pod file, content is to be stored in memory as
           Unicode characters.

       ·   The well known Unicode Byte Order Marks are as fol­
           lows:  if the file begins with the two literal byte
           values 0xFE 0xFF, this is the BOM for big-endian
           UTF-16.  If the file begins with the two literal byte
           value 0xFF 0xFE, this is the BOM for little-endian
           UTF-16.  If the file begins with the three literal
           byte values 0xEF 0xBB 0xBF, this is the BOM for UTF-8.

       ·   A naive but sufficient heuristic for testing the first
           highbit byte-sequence in a BOM-less file (whether in
           code or in Pod!), to see whether that sequence is
           valid as UTF-8 (RFC 2279) is to check whether that the
           first byte in the sequence is in the range 0xC0 - 0xFD
           and whether the next byte is in the range 0x80 - 0xBF.
           If so, the parser may conclude that this file is in
           UTF-8, and all highbit sequences in the file should be
           assumed to be UTF-8.  Otherwise the parser should
           treat the file as being in Latin-1.  In the unlikely
           circumstance that the first highbit sequence in a
           "=begin [label]" paragraph, content, and an "=end
           [label]" paragraph.  (The parser may conflate these
           two constructs, or may leave them distinct, in the
           expectation that the formatter will nevertheless treat
           them the same.)

       ·   When rendering Pod to a format that allows comments
           (i.e., to nearly any format other than plaintext), a
           Pod formatter must insert comment text identifying its
           name and version number, and the name and version num­
           bers of any modules it might be using to process the
           Pod.  Minimal examples:

             %% POD::Pod2PS v3.14159, using POD::Parser v1.92

             <!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->

             {\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}

             .\" Pod::Man version 3.14159, using POD::Parser version 1.92

           Formatters may also insert additional comments,
           including: the release date of the Pod formatter pro­
           gram, the contact address for the author(s) of the
           formatter, the current time, the name of input file,
           the formatting options in effect, version of Perl
           used, etc.

           Formatters may also choose to note errors/warnings as
           comments, besides or instead of emitting them other­
           wise (as in messages to STDERR, or "die"ing).

       ·   Pod parsers may emit warnings or error messages
           ("Unknown E code E<zslig>!") to STDERR (whether
           through printing to STDERR, or "warn"ing/"carp"ing, or
           "die"ing/"croak"ing), but must allow suppressing all
           such STDERR output, and instead allow an option for
           reporting errors/warnings in some other way, whether
           by triggering a callback, or noting errors in some
           attribute of the document object, or some similarly
           unobtrusive mechanism -- or even by appending a "Pod
           Errors" section to the end of the parsed form of the
           document.

       ·   In cases of exceptionally aberrant documents, Pod
           parsers may abort the parse.  Even then, using
           "die"ing/"croak"ing is to be avoided; where possible,
           the parser library may simply close the input file and
           add text like "*** Formatting Aborted ***" to the end
           of the (partial) in-memory document.

       ·   In paragraphs where formatting codes (like E<...>,

       ·   Pod parsers should not, by default, try to coerce
           apostrophe (') and quote (") into smart quotes (little
           9's, 66's, 99's, etc), nor try to turn backtick (`)
           into anything else but a single backtick character
           (distinct from an openquote character!), nor "--" into
           anything but two minus signs.  They must never do any
           of those things to text in C<...> formatting codes,
           and never ever to text in verbatim paragraphs.

       ·   When rendering Pod to a format that has two kinds of
           hyphens (-), one that's a nonbreaking hyphen, and
           another that's a breakable hyphen (as in "object-ori­
           ented", which can be split across lines as "object-",
           newline, "oriented"), formatters are encouraged to
           generally translate "-" to nonbreaking hyphen, but may
           apply heuristics to convert some of these to breaking
           hyphens.

       ·   Pod formatters should make reasonable efforts to keep
           words of Perl code from being broken across lines.
           For example, "Foo::Bar" in some formatting systems is
           seen as eligible for being broken across lines as
           "Foo::" newline "Bar" or even "Foo::-" newline "Bar".
           This should be avoided where possible, either by dis­
           abling all line-breaking in mid-word, or by wrapping
           particular words with internal punctuation in "don't
           break this across lines" codes (which in some formats
           may not be a single code, but might be a matter of
           inserting non-breaking zero-width spaces between every
           pair of characters in a word.)

       ·   Pod parsers should, by default, expand tabs in verba­
           tim paragraphs as they are processed, before passing
           them to the formatter or other processor.  Parsers may
           also allow an option for overriding this.

       ·   Pod parsers should, by default, remove newlines from
           the end of ordinary and verbatim paragraphs before
           passing them to the formatter.  For example, while the
           paragraph you're reading now could be considered, in
           Pod source, to end with (and contain) the newline(s)
           that end it, it should be processed as ending with
           (and containing) the period character that ends this
           sentence.

       ·   Pod parsers, when reporting errors, should make some
           effort to report an approximate line number ("Nested
           E<>'s in Paragraph #52, near line 633 of
           Thing/Foo.pm!"), instead of merely noting the para­
           graph number ("Nested E<>'s in Paragraph #52 of
           Thing/Foo.pm!").  Where this is problematic, the para­

           should be unified into one paragraph ("\tuse
           Foo;\n\n\tprint Foo->VERSION") before being passed to
           the formatter or other processor.  Parsers may also
           allow an option for overriding this.

           While this might be too cumbersome to implement in
           event-based Pod parsers, it is straightforward for
           parsers that return parse trees.

       ·   Pod formatters, where feasible, are advised to avoid
           splitting short verbatim paragraphs (under twelve
           lines, say) across pages.

       ·   Pod parsers must treat a line with only spaces and/or
           tabs on it as a "blank line" such as separates para­
           graphs.  (Some older parsers recognized only two adja­
           cent newlines as a "blank line" but would not recog­
           nize a newline, a space, and a newline, as a blank
           line.  This is noncompliant behavior.)

       ·   Authors of Pod formatters/processors should make every
           effort to avoid writing their own Pod parser.  There
           are already several in CPAN, with a wide range of
           interface styles -- and one of them, Pod::Parser,
           comes with modern versions of Perl.

       ·   Characters in Pod documents may be conveyed either as
           literals, or by number in E<n> codes, or by an equiva­
           lent mnemonic, as in E<eacute> which is exactly equiv­
           alent to E<233>.

           Characters in the range 32-126 refer to those well
           known US-ASCII characters (also defined there by Uni­
           code, with the same meaning), which all Pod formatters
           must render faithfully.  Characters in the ranges 0-31
           and 127-159 should not be used (neither as literals,
           nor as E<number> codes), except for the literal byte-
           sequences for newline (13, 13 10, or 10), and tab (9).

           Characters in the range 160-255 refer to Latin-1 char­
           acters (also defined there by Unicode, with the same
           meaning).  Characters above 255 should be understood
           to refer to Unicode characters.

       ·   Be warned that some formatters cannot reliably render
           characters outside 32-126; and many are able to handle
           32-126 and 160-255, but nothing above 255.

       ·   Besides the well-known "E<lt>" and "E<gt>" codes for
           less-than and greater-than, Pod parsers must under­
           stand "E<sol>" for "/" (solidus, slash), and "E<ver­
           ters in the range 160-255 (Latin-1).  Pod parsers,
           when faced with some unknown "E<identifier>" code,
           shouldn't simply replace it with nullstring (by
           default, at least), but may pass it through as a
           string consisting of the literal characters E,
           less-than, identifier, greater-than.  Or Pod parsers
           may offer the alternative option of processing such
           unknown "E<identifier>" codes by firing an event espe­
           cially for such codes, or by adding a special node-
           type to the in-memory document tree.  Such "E<identi­
           fier>" may have special meaning to some processors, or
           some processors may choose to add them to a special
           error report.

       ·   Pod parsers must also support the XHTML codes
           "E<quot>" for character 34 (doublequote, "), "E<amp>"
           for character 38 (ampersand, &), and "E<apos>" for
           character 39 (apostrophe, ').

       ·   Note that in all cases of "E<whatever>", whatever
           (whether an htmlname, or a number in any base) must
           consist only of alphanumeric characters -- that is,
           whatever must watch "m/\A\w+\z/".  So "E< 0 1 2 3 >"
           is invalid, because it contains spaces, which aren't
           alphanumeric characters.  This presumably does not
           need special treatment by a Pod processor; " 0 1 2 3 "
           doesn't look like a number in any base, so it would
           presumably be looked up in the table of HTML-like
           names.  Since there isn't (and cannot be) an HTML-like
           entity called " 0 1 2 3 ", this will be treated as an
           error.  However, Pod processors may treat "E< 0 1 2 3
           >" or "E<e-acute>" as syntactically invalid, poten­
           tially earning a different error message than the
           error message (or warning, or event) generated by a
           merely unknown (but theoretically valid) htmlname, as
           in "E<qacute>" [sic].  However, Pod parsers are not
           required to make this distinction.

       ·   Note that E<number> must not be interpreted as simply
           "codepoint number in the current/native character
           set".  It always means only "the character represented
           by codepoint number in Unicode."  (This is identical
           to the semantics of &#number; in XML.)

           This will likely require many formatters to have
           tables mapping from treatable Unicode codepoints (such
           as the "\xE9" for the e-acute character) to the escape
           sequences or codes necessary for conveying such
           sequences in the target output format.  A converter to
           *roff would, for example know that "\xE9" (whether
           conveyed literally, or via a E<...> sequence) is to be
           conveyed as "e\\*'".  Similarly, a program rendering

       ·   If, surprisingly, the implementor of a Pod formatter
           can't find a satisfactory pre-existing table mapping
           from Unicode characters to escapes in the target for­
           mat (e.g., a decent table of Unicode characters to
           *roff escapes), it will be necessary to build such a
           table.  If you are in this circumstance, you should
           begin with the characters in the range 0x00A0 -
           0x00FF, which is mostly the heavily used accented
           characters.  Then proceed (as patience permits and
           fastidiousness compels) through the characters that
           the (X)HTML standards groups judged important enough
           to merit mnemonics for.  These are declared in the
           (X)HTML specifications at the www.W3.org site.  At
           time of writing (September 2001), the most recent
           entity declaration files are:

             http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
             http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
             http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent

           Then you can progress through any remaining notable
           Unicode characters in the range 0x2000-0x204D (consult
           the character tables at www.unicode.org), and whatever
           else strikes your fancy.  For example, in xhtml-sym­
           bol.ent, there is the entry:

             <!ENTITY infin    "&#8734;"> <!-- infinity, U+221E ISOtech -->

           While the mapping "infin" to the character "\x{221E}"
           will (hopefully) have been already handled by the Pod
           parser, the presence of the character in this file
           means that it's reasonably important enough to include
           in a formatter's table that maps from notable Unicode
           characters to the codes necessary for rendering them.
           So for a Unicode-to-*roff mapping, for example, this
           would merit the entry:

             "\x{221E}" => '\(in',

           It is eagerly hoped that in the future, increasing
           numbers of formats (and formatters) will support Uni­
           code characters directly (as (X)HTML does with
           "&infin;", "&#8734;", or "&#x221E;"), reducing the
           need for idiosyncratic mappings of Uni­
           code-to-my_escapes.

       ·   It is up to individual Pod formatter to display good
           judgment when confronted with an unrenderable charac­
           ter (which is distinct from an unknown E<thing>
           sequence that the parser couldn't resolve to anything,
           renderable or not).  It is good practice to map Latin
           letters with diacritics (like "E<eacute>"/"E<233>") to
           rency to '[euro]'", or as "magic is enabled if you set
           $Currency to '[x20AC]', etc.

           A Pod formatter may also note, in a comment or warn­
           ing, a list of what unrenderable characters were
           encountered.

       ·   E<...> may freely appear in any formatting code (other
           than in another E<...> or in an Z<>).  That is, "X<The
           E<euro>1,000,000 Solution>" is valid, as is "L<The
           E<euro>1,000,000 Solution|Million::Euros>".

       ·   Some Pod formatters output to formats that implement
           nonbreaking spaces as an individual character (which
           I'll call "NBSP"), and others output to formats that
           implement nonbreaking spaces just as spaces wrapped in
           a "don't break this across lines" code.  Note that at
           the level of Pod, both sorts of codes can occur: Pod
           can contain a NBSP character (whether as a literal, or
           as a "E<160>" or "E<nbsp>" code); and Pod can contain
           "S<foo I<bar> baz>" codes, where "mere spaces" (char­
           acter 32) in such codes are taken to represent non­
           breaking spaces.  Pod parsers should consider support­
           ing the optional parsing of "S<foo I<bar> baz>" as if
           it were "fooNBSPI<bar>NBSPbaz", and, going the other
           way, the optional parsing of groups of words joined by
           NBSP's as if each group were in a S<...> code, so that
           formatters may use the representation that maps best
           to what the output format demands.

       ·   Some processors may find that the "S<...>" code is
           easiest to implement by replacing each space in the
           parse tree under the content of the S, with an NBSP.
           But note: the replacement should apply not to spaces
           in all text, but only to spaces in printable text.
           (This distinction may or may not be evident in the
           particular tree/event model implemented by the Pod
           parser.)  For example, consider this unusual case:

              S<L[linebreak]action" or "manu-[linebreak]script"
           (and if it doesn't hyphenate it, then the "E<shy>"
           doesn't show up at all).  And if it is to hyphenate
           "Jarkko" and/or "Hietaniemi", it can do so only at the
           points where there is a "E<shy>" code.

           In practice, it is anticipated that this character
           will not be used often, but formatters should either
           support it, or delete it.

       ·   If you think that you want to add a new command to Pod
           (like, say, a "=biblio" command), consider whether you
           could get the same effect with a for or begin/end
           sequence: "=for biblio ..." or "=begin biblio" ...
           "=end biblio".  Pod processors that don't understand
           "=for biblio", etc, will simply ignore it, whereas
           they may complain loudly if they see "=biblio".

       ·   Throughout this document, "Pod" has been the preferred
           spelling for the name of the documentation format.
           One may also use "POD" or "pod".  For the documenta­
           tion that is (typically) in the Pod format, you may
           use "pod", or "Pod", or "POD".  Understanding these
           distinctions is useful; but obsessing over how to
           spell them, usually is not.


About L<...> Codes

       As you can tell from a glance at perlpod, the L<...> code
       is the most complex of the Pod formatting codes.  The
       points below will hopefully clarify what it means and how
       processors should deal with it.

       ·   In parsing an L<...> code, Pod parsers must distin­
           guish at least four attributes:


           Third:
               The name or URL, or undef if none.  (E.g., in
               "L<Perl Functions|perlfunc>", the name -- also
               sometimes called the page -- is "perlfunc".  In
               "L</CAVEATS>", the name is undef.)

           Fourth:
               The section (AKA "item" in older perlpods), or
               undef if none.  E.g., in "DESCRIPTION" in
               Getopt::Std, "DESCRIPTION" is the section.  (Note
               that this is not the same as a manpage section
               like the "5" in "man 5 crontab".  "Section Foo" in
               the Pod sense means the part of the text that's
               introduced by the heading or item whose text is
               "Foo".)

           Pod parsers may also note additional attributes
           including:

           Fifth:
               A flag for whether item 3 (if present) is a URL
               (like "http://lists.perl.org" is), in which case
               there should be no section attribute; a Pod name
               (like "perldoc" and "Getopt::Std" are); or possi­
               bly a man page name (like "crontab(5)" is).

           Sixth:
               The raw original L<...> content, before text is
               split on "|", "/", etc, and before E<...> codes
               are expanded.

           (The above were numbered only for concise reference
           below.  It is not a requirement that these be passed
           as an actual list or array.)

           For example:

             L<Foo::Bar>
               =>  undef,                          # link text
                   "Foo::Bar",                     # possibly inferred link text
                   "Foo::Bar",                     # name
                   undef,                          # section
                   'pod',                          # what sort of link
                   "Foo::Bar"                      # original content

             L<Perlport's section on NL's|perlport/Newlines>
               =>  "Perlport's section on NL's",   # link text
                   "Perlport's section on NL's",   # possibly inferred link text
                   "perlport",                     # name
                   "Newlines",                     # section
                   'pod',                          # what sort of link

             L<crontab(5)/"DESCRIPTION">
               =>  undef,                          # link text
                   '"DESCRIPTION" in crontab(5)',  # possibly inferred link text
                   "crontab(5)",                   # name
                   "DESCRIPTION",                  # section
                   'man',                          # what sort of link
                   'crontab(5)/"DESCRIPTION"'      # original content

             L</Object Attributes>
               =>  undef,                          # link text
                   '"Object Attributes"',          # possibly inferred link text
                   undef,                          # name
                   "Object Attributes",            # section
                   'pod',                          # what sort of link
                   "/Object Attributes"            # original content

             L<http://www.perl.org/>
               =>  undef,                          # link text
                   "http://www.perl.org/",         # possibly inferred link text
                   "http://www.perl.org/",         # name
                   undef,                          # section
                   'url',                          # what sort of link
                   "http://www.perl.org/"          # original content

           Note that you can distinguish URL-links from anything
           else by the fact that they match
           "m/\A\w+:[^:\s]\S*\z/".  So "L<http://www.perl.com>"
           is a URL, but "L<HTTP::Response>" isn't.

       ·   In case of L<...> codes with no "text|" part in them,
           older formatters have exhibited great variation in
           actually displaying the link or cross reference.  For
           example, L<crontab(5)> would render as "the crontab(5)
           manpage", or "in the crontab(5) manpage" or just
           "crontab(5)".

           Pod processors must now treat "text|"-less links as
           follows:

             L<name>         =>  L<name|name>
             L</section>     =>  L<"section"|/section>
             L<name/section> =>  L<"section" in name|name/section>

       ·   Note that section names might contain markup.  I.e.,
           if a section starts with:

             =head2 About the C<-M> Operator

           or with:

             =item About the C<-M> Operator
             <a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
             Operator" in somedoc</a>

       ·   Previous versions of perlpod distinguished
           "L<name/"section">" links from "L<name/item>" links
           (and their targets).  These have been merged syntacti­
           cally and semantically in the current specification,
           and section can refer either to a "=headn Heading Con­
           tent" command or to a "=item Item Content" command.
           This specification does not specify what behavior
           should be in the case of a given document having sev­
           eral things all seeming to produce the same section
           identifier (e.g., in HTML, several things all produc­
           ing the same anchorname in <a name="anchor­
           name">...</a> elements).  Where Pod processors can
           control this behavior, they should use the first such
           anchor.  That is, "L<Foo/Bar>" refers to the first
           "Bar" section in Foo.

           But for some processors/formats this cannot be easily
           controlled; as with the HTML example, the behavior of
           multiple ambiguous <a name="anchorname">...</a> is
           most easily just left up to browsers to decide.

       ·   Authors wanting to link to a particular (absolute)
           URL, must do so only with "L<scheme:...>" codes (like
           L<http://www.perl.org>), and must not attempt "L<Some
           Site Name|scheme:...>" codes.  This restriction avoids
           many problems in parsing and rendering L<...> codes.

       ·   In a "L<text|...>" code, text may contain formatting
           codes for formatting or for E<...> escapes, as in:

             L<Bname, section, text, and url).

           Authors must not nest L<...> codes.  For example,
           "L<The L<Foo::Bar> man page>" should be treated as an
           error.

       ·   Note that Pod authors may use formatting codes inside
           the "text" part of "L<text|name>" (and so on for
           L<text|/"sec">).

           "L<crontab(5)>".  In theory, "L<chmod>" in ambiguous
           between a Pod page called "chmod", or the Unix man
           page "chmod" (in whatever man-section).  However, the
           presence of a string in parens, as in "crontab(5)", is
           sufficient to signal that what is being discussed is
           not a Pod page, and so is presumably a UNIX man page.
           The distinction is of no importance to many Pod pro­
           cessors, but some processors that render to hypertext
           formats may need to distinguish them in order to know
           how to render a given "L<foo>" code.

       ·   Previous versions of perlpod allowed for a "L<sec­
           tion>" syntax (as in ""L<Object Attributes>""), which
           was not easily distinguishable from "L<name>" syntax.
           This syntax is no longer in the specification, and has
           been replaced by the "L<"section">" syntax (where the
           quotes were formerly optional).  Pod parsers should
           tolerate the "L<section>" syntax, for a while at
           least.  The suggested heuristic for distinguishing
           "L<section>" from "L<name>" is that if it contains any
           whitespace, it's a section.  Pod processors may warn
           about this being deprecated syntax.


About =over...=back Regions

       "=over"..."=back" regions are used for various kinds of
       list-like structures.  (I use the term "region" here sim­
       ply as a collective term for everything from the "=over"
       to the matching "=back".)

       ·   The non-zero numeric indentlevel in "=over
           indentlevel" ...  "=back" is used for giving the for­
           matter a clue as to how many "spaces" (ems, or roughly
           equivalent units) it should tab over, although many
           formatters will have to convert this to an absolute
           measurement that may not exactly match with the size
           of spaces (or M's) in the document's base font.  Other
           formatters may have to completely ignore the number.
           The lack of any explicit indentlevel parameter is
           equivalent to an indentlevel value of 4.  Pod proces­
           sors may complain if indentlevel is present but is not
           a positive number matching "m/\A(\d*\.)?\d+\z/".

       ·   Authors of Pod formatters are reminded that "=over"
           ... "=back" may map to several different constructs in
           your output format.  For example, in converting Pod to
           (X)HTML, it can map to any of <ul>...</ul>,
           <ol>...</ol>, <dl>...</dl>, or <blockquote>...</block­
           quote>.  Similarly, "=item" can map to <li> or <dt>.

       ·   Each "=over" ... "=back" region should be one of the
           following:

               (or each group of them) followed by some number of
               ordinary/verbatim paragraphs, other nested "=over"
               ... "=back" regions, "=for..." paragraphs, and/or
               "=begin"..."=end" codes.  Note that the numbers
               must start at 1 in each section, and must proceed
               in order and without skipping numbers.

               (Pod processors must tolerate lines like "=item 1"
               as if they were "=item 1.", with the period.)

           ·   An "=over" ... "=back" region containing only
               "=item [text]" commands, each one (or each group
               of them) followed by some number of ordinary/ver­
               batim paragraphs, other nested "=over" ... "=back"
               regions, or "=for..." paragraphs, and
               "=begin"..."=end" regions.

               The "=item [text]" paragraph should not match
               "m/\A=item\s+\d+\.?\s*\z/" or
               "m/\A=item\s+\*\s*\z/", nor should it match just
               "m/\A=item\s*\z/".

           ·   An "=over" ... "=back" region containing no
               "=item" paragraphs at all, and containing only
               some number of ordinary/verbatim paragraphs, and
               possibly also some nested "=over" ... "=back"
               regions, "=for..." paragraphs, and
               "=begin"..."=end" regions.  Such an itemless
               "=over" ... "=back" region in Pod is equivalent in
               meaning to a "<blockquote>...</blockquote>" ele­
               ment in HTML.

           Note that with all the above cases, you can determine
           which type of "=over" ... "=back" you have, by examin­
           ing the first (non-"=cut", non-"=pod") Pod paragraph
           after the "=over" command.

       ·   Pod formatters must tolerate arbitrarily large amounts
           of text in the "=item text..." paragraph.  In prac­
           tice, most such paragraphs are short, as in:

             =item For cutting off our trade with all parts of the world

           But they may be arbitrarily long:

             =item For transporting us beyond seas to be tried for pretended
             offenses

             =item He is at this time transporting large armies of foreign
             mercenaries to complete the works of death, desolation and
             tyranny, already begun with circumstances of cruelty and perfidy
             scarcely paralleled in the most barbarous ages, and totally
             =item 3

             Stop by the store.  Get Abba Zabas, Stoli, and cheap lawn chairs.

             =back

       ·   No "=over" ... "=back" region can contain headings.
           Processors may treat such a heading as an error.

       ·   Note that an "=over" ... "=back" region should have
           some content.  That is, authors should not have an
           empty region like this:

             =over

             =back

           Pod processors seeing such a contentless "=over" ...
           "=back" region, may ignore it, or may report it as an
           error.

       ·   Processors must tolerate an "=over" list that goes off
           the end of the document (i.e., which has no matching
           "=back"), but they may warn about such a list.

       ·   Authors of Pod formatters should note that this con­
           struct:

             =item Neque

             =item Porro

             =item Quisquam Est

             Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
             velit, sed quia non numquam eius modi tempora incidunt ut
             labore et dolore magnam aliquam quaerat voluptatem.

             =item Ut Enim

           is semantically ambiguous, in a way that makes format­
           ting decisions a bit difficult.  On the one hand, it
           could be mention of an item "Neque", mention of
           another item "Porro", and mention of another item
           "Quisquam Est", with just the last one requiring the
           explanatory paragraph "Qui dolorem ipsum quia
           dolor..."; and then an item "Ut Enim".  In that case,
           you'd want to format it like so:

             Neque

             Porro
             Neque
             Porro
             Quisquam Est
               Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
               velit, sed quia non numquam eius modi tempora incidunt ut
               labore et dolore magnam aliquam quaerat voluptatem.

             Ut Enim

           But (for the forseeable future), Pod does not provide
           any way for Pod authors to distinguish which grouping
           is meant by the above "=item"-cluster structure.  So
           formatters should format it like so:

             Neque

             Porro

             Quisquam Est

               Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
               velit, sed quia non numquam eius modi tempora incidunt ut
               labore et dolore magnam aliquam quaerat voluptatem.

             Ut Enim

           That is, there should be (at least roughly) equal
           spacing between items as between paragraphs (although
           that spacing may well be less than the full height of
           a line of text).  This leaves it to the reader to use
           (con)textual cues to figure out whether the "Qui
           dolorem ipsum..." paragraph applies to the "Quisquam
           Est" item or to all three items "Neque", "Porro", and
           "Quisquam Est".  While not an ideal situation, this is
           preferable to providing formatting cues that may be
           actually contrary to the author's intent.


About Data Paragraphs and "=begin/=end" Regions

       Data paragraphs are typically used for inlining non-Pod
       data that is to be used (typically passed through) when
       rendering the document to a specific format:

         =begin rtf

         \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}

         =end rtf

       The exact same effect could, incidentally, be achieved
       with a single "=for" paragraph:

         =for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}

       If these were ordinary paragraphs, the Pod parser would
       try to expand the "E</em>" (in the first paragraph) as a
       formatting code, just like "E<lt>" or "E<eacute>".  But
       since this is in a "=begin identifier"..."=end identifier"
       region and the identifier "html" doesn't begin have a ":"
       prefix, the contents of this region are stored as data
       paragraphs, instead of being processed as ordinary para­
       graphs (or if they began with a spaces and/or tabs, as
       verbatim paragraphs).

       As a further example: At time of writing, no "biblio"
       identifier is supported, but suppose some processor were
       written to recognize it as a way of (say) denoting a bib­
       liographic reference (necessarily containing formatting
       codes in ordinary paragraphs).  The fact that "biblio"
       paragraphs were meant for ordinary processing would be
       indicated by prefacing each "biblio" identifier with a
       colon:

         =begin :biblio

         Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
         Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

         =end :biblio

       This would signal to the parser that paragraphs in this
       begin...end region are subject to normal handling as ordi­
       nary/verbatim paragraphs (while still tagged as meant only
       for processors that understand the "biblio" identifier).
       The same effect could be had with:

         =for :biblio
         Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
         Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

       The ":" on these identifiers means simply "process this
       stuff normally, even though the result will be for some
       special target".  I suggest that parser APIs report "bib­
       lio" as the target identifier, but also report that it had
       a ":" prefix.  (And similarly, with the above "html",
       report "html" as the target identifier, and note the lack
       of a ":" prefix.)

       Note that a "=begin identifier"..."=end identifier" region
       where identifier begins with a colon, can contain com­
       mands.  For example:

         =begin :biblio

         Wirth's classic is available in several editions, including:
         Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

         =back

         =end :biblio

       Note, however, a "=begin identifier"..."=end identifier"
       region where identifier does not begin with a colon,
       should not directly contain "=head1" ... "=head4" com­
       mands, nor "=over", nor "=back", nor "=item".  For exam­
       ple, this may be considered invalid:

         =begin somedata

         This is a data paragraph.

         =head1 Don't do this!

         This is a data paragraph too.

         =end somedata

       A Pod processor may signal that the above (specifically
       the "=head1" paragraph) is an error.  Note, however, that
       the following should not be treated as an error:

         =begin somedata

         This is a data paragraph.

         =cut

         # Yup, this isn't Pod anymore.
         sub excl { (rand() > .5) ? "hoo!" : "hah!" }

         =pod

         This is a data paragraph too.

         =end somedata

       And this too is valid:

         =begin someformat

         This is a data paragraph.

           And this is a data paragraph.

         =begin someotherformat

         This is a data paragraph too.

         Another data paragraph!

         =end someformat

       The contents of the above "=begin :yetanotherformat" ...
       "=end :yetanotherformat" region aren't data paragraphs,
       because the immediately containing region's identifier
       (":yetanotherformat") begins with a colon.  In practice,
       most regions that contain data paragraphs will contain
       only data paragraphs; however, the above nesting is syn­
       tactically valid as Pod, even if it is rare.  However, the
       handlers for some formats, like "html", will accept only
       data paragraphs, not nested regions; and they may complain
       if they see (targeted for them) nested regions, or com­
       mands, other than "=end", "=pod", and "=cut".

       Also consider this valid structure:

         =begin :biblio

         Wirth's classic is available in several editions, including:

         =over

         =item

         Wirth, Niklaus.  1975.  I<Algorithmen und Datenstrukturen.>
         Teubner, Stuttgart.  [Yes, it's in German.]

         =item

         Wirth, Niklaus.  1976.  I<Algorithms + Data Structures =
         Programs.>  Prentice-Hall, Englewood Cliffs, NJ.

         =back

         Buy buy buy!

         =begin html

         <img src='wirth_spokesmodeling_book.png'>

         <hr>

         =end html

         Now now now!

         =end :biblio

       There, the "=begin html"..."=end html" region is nested
       "<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").

       Pod processors should tolerate empty "=begin some­
       thing"..."=end something" regions, empty "=begin :some­
       thing"..."=end :something" regions, and contentless "=for
       something" and "=for :something" paragraphs.  I.e., these
       should be tolerated:

         =for html

         =begin html

         =end html

         =begin :biblio

         =end :biblio

       Incidentally, note that there's no easy way to express a
       data paragraph starting with something that looks like a
       command.  Consider:

         =begin stuff

         =shazbot

         =end stuff

       There, "=shazbot" will be parsed as a Pod command
       "shazbot", not as a data paragraph "=shazbot\n".  However,
       you can express a data paragraph consisting of
       "=shazbot\n" using this code:

         =for stuff =shazbot

       The situation where this is necessary, is presumably quite
       rare.

       Note that =end commands must match the currently open
       =begin command.  That is, they must properly nest.  For
       example, this is valid:

         =begin outer

         X

         =begin inner

         Y

         =end inner

         =end outer

         Z

         =end inner

       This latter is improper because when the "=end outer" com­
       mand is seen, the currently open region has the formatname
       "inner", not "outer".  (It just happens that "outer" is
       the format name of a higher-up region.)  This is an error.
       Processors must by default report this as an error, and
       may halt processing the document containing that error.  A
       corollary of this is that regions cannot "overlap" --
       i.e., the latter block above does not represent a region
       called "outer" which contains X and Y, overlapping a
       region called "inner" which contains Y and Z.  But because
       it is invalid (as all apparently overlapping regions would
       be), it doesn't represent that, or anything at all.

       Similarly, this is invalid:

         =begin thing

         =end hting

       This is an error because the region is opened by "thing",
       and the "=end" tries to close "hting" [sic].

       This is also invalid:

         =begin thing

         =end

       This is invalid because every "=end" command must have a
       formatname parameter.


SEE ALSO

       perlpod, "PODs: Embedded Documentation" in perlsyn, pod­
       checker


AUTHOR

       Sean M. Burke

perl v5.8.1                 2003-09-02             PERLPODSPEC(1)
  

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