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Message Styling This specification defines a formatted text syntax for use in instant messages with simple text styling. &LEGALNOTICE; 0393 Experimental Standards Track Standards Council XMPP Core XEP-0001 styling &sam; 0.1.2 2018-01-13 ssw

Clarify block quote and plain text parsing and formatting behavior.

0.1.1 2018-01-12 ssw

Minor clarifications and updates, add security considerations, and expand the glossary.

0.1.0 2017-11-22 XEP Editor (ssw)

First draft approved by the XMPP Council.

 0.0.1 2017-10-28 ssw

First draft.

Historically, XMPP has had no system for simple text styling. Instead, specifications like &xep0071; that require full layout engines have been used, leading to numerous security issues with implementations. Some entities have also performed their own styling based on identifiers in the body. While this has worked well in the past, it is not interoperable and leads to entities each supporting their own informal styling languages.

This specification aims to provide a single, interoperable formatted text syntax that can be used by entities that do not require full layout engines.

Many important terms used in this document are defined in &unicode;. The terms "left-to-right" (LTR) and "right-to-left" (RTL) are defined in &uax9;. The term "formatted text" is defined in &rfc7764;.

Block
Any chunk of text that can be parsed unambiguously in one pass. Blocks may contain one or more children which may be other blocks or spans. For example:
  • A single line of text comprising one or more spans
  • A block quotation
  • A preformatted code block
Formal markup language
A structured markup language such as LaTeX, SGML, HTML, or XML that is formally defined and may include metadata unrelated to formatting or text style.
Plain text
Text that does not convey any particular formatting or interpretation of the text by computer programs.
Span
A group of text that is rendered inline and where the entire group is rendered in the same manner. Spans may be either plain text with no formatting applied, or may be formatted text that is enclosed by two styling directives. Spans are always children of blocks and may not escape from their containing block. The following all contain spans marked by parenthesis:
  • (plain span)
  • (*strong span*)
  • (_emphasized span_)
  • (_emphasized span containing (*strong span*)_)
  • (span one )(*span two*)
Styling directive
A character or set of characters that indicates the beginning of a span or block. For example, in certain contexts the characters '*' (U+002A ASTERISK), and '_' (U+005F LOW LINE) may be styling directives that indicate the beginning of a strong or emphasis span and the string '```' (U+0060 GRAVE ACCENT) may be a styling directive that indicate the beginning of a preformatted code block.
Whitespace character
Any Unicode scalar value which has the property "White_Space" or is in category Z in the Unicode Character Database.

Matches of spans between two styling directives MUST contain some text between the two styling directives and the opening styling directive MUST be located at the beginning of the line, or after a whitespace character. The opening styling directive MUST NOT be followed by a whitespace character and the closing styling directive MUST NOT be preceeded by a whitespace character. Spans are always parsed from the beginning of the byte stream to the end and are lazily matched. Characters that would be styling directives but do not follow these rules are not considered when matching and thus may be present between two other styling directives.

For example, each of the following would be styled as indicated:

  • *strong*
  • plain *strong* plain
  • *strong* plain *strong*
  • *strong*plain*
  • * plain *strong*

Nothing would be styled in the following messages (where "\n" represents a new line):

  • not strong*
  • *not strong
  • *not \n strong*
  • *not *strong
  • **
  • ****

Any text inside of a block that is not part of another span is implicitly considered to be inside of a "plain text" span.

(Two spans, both )(*alike in dignity*) ]]>

Text enclosed by '*' (U+002A ASTERISK) is strong and SHOULD be displayed with a heavier font weight than the surrounding text (bold).

The full title is "Twelfth Night, or What You Will" but *most* people shorten it. ]]>

Text enclosed by '_' (U+005F LOW LINE) is emphasized and SHOULD be displayed in italics.

The full title is _Twelfth Night, or What You Will_ but _most_ people shorten it. ]]>

Text enclosed by '~' (U+007E TILDE) SHOULD be displayed with a horizontal line through the middle.

Everyone ~dis~likes cake. ]]>

Text enclosed by a '`' (U+0060 GRAVE ACCENT) is a preformatted span SHOULD be displayed inline in a monospace font. A preformatted span may only contain a single plain span. Inline formatting directives inside the preformatted span are not rendered. For example, the following all contain valid preformatted spans:

  • This is `monospace`
  • This is `*monospace*`
  • This is *`monospace and bold`*
Wow, I can write in `monospace`! ]]>

Parsers implementing message styling will first parse blocks and then parse child blocks or spans if allowed by the specific block type.

Groups of lines that are not part of any other block are considered a "plain" block, as are individual lines of text that are not inside of a preformatted text block. Plain blocks are not bound by styling directives and do not imply formatting themselves, but they may contain spans which imply formatting.

((There are four blocks in this body marked by parens,) (but there is no *formatting) (as spans* may not escape blocks.)) ]]>

A preformatted text block is started by a line beginning with "```" (U+0060 GRAVE ACCENT), and ended by a line containing only three grave accents or the end of the parent block (whichever comes first). Preformatted text blocks cannot contain child blocks and other styling MUST NOT be rendered inside the block. Text inside a preformatted block SHOULD be displayed in a monospace font.

```ignored (println "Hello, world!") ``` This should show up as monospace, preformatted text ⤴ ]]> > ``` > (println "Hello, world!") The entire blockquote is a preformatted text block, but this line is plaintext! ]]>

A quotation is indicated by one or more lines with a byte stream beginning with a '>' (U+003E GREATER-THAN SIGN). Block quotes may contain any child block, including other quotations. Lines inside the block quote MUST have leading spaces trimmed before parsing the child block. It is RECOMMENDED that text inside of a block quote be indented or distinguished from the surrounding text in some other way.

> That that is, is. Said the old hermit of Prague. ]]> >> That that is, is. > Said the old hermit of Prague. Who? ]]>

This document does not define a regular grammar and thus styling cannot be matched by a regular expression. Instead, a simple parser can be constructed by first parsing all text into blocks and then recursively parsing the child-blocks inside block quotations, the spans inside individual lines, and by returning the text inside preformatted blocks without modification.

It is RECOMMENDED that formatting characters be displayed and formatted in the same manner as the text they apply to. For example, the string "*emphasis*" would be rendered as "*emphasis*".

When displaying text with formatting, developers should take care to ensure sufficient contrast exists between styled and unstyled text so that users with vision deficiencies are able to distinguish between the two.

Formatted text may also be rendered poorly by screen readers. When applying formatting it may be desirable to include directives to exclude formatting characters from being read.

Authors of message styling parsers should take care that improperly formatted messages cannot lead to buffer overruns or code execution.

This document requires no interaction with &IANA;.

This specification requires no interaction with the ®ISTRAR;

This document does not define any new XML structure requiring a schema.