Message Styling

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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; 0.1.0 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.

Clients that do not support this specification MUST still be able to receive messages sent by clients using this specification and display them in a human-readable form.
Clients that support this specification MUST NOT be required to use a layout engine such as HTML or LaTeX.
Messages formatted using this specification MUST NOT hinder readability on receiving clients regardless of client background color, contrast, or window size.
Messages formatted using this specification MUST NOT hinder readability by users with color vision deficiency or impaired vision.
Messages formatted with this specification MUST render correctly in locales with right-to-left (RTL) layouts without causing confusion.
Clients that support this specification MUST NOT be required to extract metadata unrelated to formatting or text style from the message.
Servers MUST NOT need to implement any new functionality for this specification to be supported.

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;.

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.
Whitespace character: Any Unicode scalar value which has the property "White_Space" or is in category Z in the Unicode Character Database.

As a user sending an instant message to a friend, I want to be able to emphasize an important part of my message.
As a software developer, I want to be able to send pre-formatted, monospace, block or inline text to another developer.
As a multi-user chat user I want to quote something someone said earlier in the chat and make it evident that the text is a quotation.

A block is any chunk of text that can be parsed unambiguously in one pass.

A single line of text comprising one or more spans
A block quotation
A preformatted code block

A span is 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 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*)

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 also not be followed by a whitespace character. 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
**
****

Text enclosed by '*' (U+002A ASTERISK) is strong SHOULD be displayed as 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 plaintext. 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`! ]]>

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.

> 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 predictive recursive descent or LALR parser may be constructed. For instance, 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 plain 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.

OPTIONAL.

REQUIRED.

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.