CLDR-18318 kbd: escaping: update to spec

- also added modification section

Author: srl295

docs/ldml/tr35-keyboards.md

@@ -2195,30 +2195,48 @@ _Attribute:_ `from` (required)
     The hex escaping is case insensitive. The value may not match a surrogate or illegal character, nor a marker character.
     The form `\u{…}` is preferred as it is the same regardless of codepoint length.
 
-- **Fixed character classes and escapes**
+- **Fixed character classes**
 
-    `\s \S \t \r \n \f \v \\ \$ \d \w \D \W \0`
+    `\s \S \t \r \n \f \v \d \w \D \W`
 
     The value of these classes do not change with Unicode versions.
 
     `\s` for example is exactly `[\f\n\r\t\v\u{00a0}\u{1680}\u{2000}-\u{200a}\u{2028}\u{2029}\u{202f}\u{205f}\u{3000}\u{feff}]`
 
-    `\\` and `\$` evaluate to `\` and `$`, respectively.
+- **Escapes**
+
+    `\$ \( \) \* \+ \. \/ \? \[ \\ \] \^ \{ \| \}`
+
+    For example, `\\`, `\*`, and `\$` match `\`, `*`, and `$`, respectively.
+
+    Some of these characters (such as `*`) aren't actually used as syntax in the keyboard transform syntax.
+    However, they are required to be escaped in keyboard transforms, to avoid confusion or problems with characters which are syntax in regular expressions.
+
+    Sequences not listed here as **Fixed Character Classes** nor as **Escapes** are disallowed.
+    For example, `\0` (octal escape) and `\1` (backreference) are not allowed.
+    `\a` is not defined as a character class and is also disallowed.
 
 - **Character classes**
 
     `[abc]` `[^def]` `[a-z]` `[ॲऄ-आइ-ऋ]` `[\u{093F}-\u{0944}\u{0962}\u{0963}]`
 
-    - supported
-    - no Unicode properties such as `\p{…}`
-    - Warning: Character classes look superficially similar to [`uset`](#element-uset) elements, but they are distinct and referenced with the `$[...usetId]` notation in transforms. The `uset` notation cannot be embedded directly in a transform.
+    If the character class begins with a caret (`^`) then it is a negation, matching all characters except for those listed.
+
+    Unicode properties such as `\p{…}` are not allowed.
+
+    One additional escape is allowed within character classes besides those listed above: `\-`, for escaping the hyphen character.
+
+    **Note**: Character classes look superficially similar to [`uset`](#element-uset) elements, but they are distinct and referenced with the `$[...usetId]` notation in transforms. The `uset` notation cannot be embedded directly in a transform.
 
 - **Bounded quantifier**
 
     `{x,y}`
 
-    `x` and `y` are required single digits representing the minimum and maximum number of occurrences.
-    `x` must be ≥ 0, `y` must be ≥ x and ≥ 1
+    `x` and `y` are required single digits (`1` to `9`) representing the minimum and maximum number of occurrences.
+    
+    `x` must be ≥ 0, `y` must be ≥ x and ≥ 1. 
+
+    Unbounded quantifiers such as `{3,}` are not allowed.
 
 - **Optional Specifier**
 

docs/ldml/tr35.md

@@ -223,11 +223,12 @@ The LDML specification is divided into the following parts:
   * [Number symbols and formats without numberSystem](#number-symbols-and-formats-without-numbersystem)
   * [Clarified `currencyData` element ordering](#clarified-currencydata-element-ordering)
   * [Semantic Datetime Skeletons](#semantic-datetime-skeletons)
+  * [Improvements to Keyboard Transforms](#improvements-to-keyboard-transforms)
   * [Well-formed identifiers](#well-formed-identifiers)
 
 ## <a name="Introduction" href="#Introduction">Introduction</a>
 
-Not long ago, computer systems were like separate worlds, isolated from one another. The internet and related events have changed all that. A single system can be built of many different components, hardware and software, all needing to work together. Many different technologies have been important in bridging the gaps; in the internationalization arena, Unicode has provided a lingua franca for communicating textual data. However, there remain differences in the locale data used by different systems.
+Not long ago, computer systems were like separate worlds, isolated from one another. The internet and related events have changed all that. A single system can be built of many different compFonents, hardware and software, all needing to work together. Many different technologies have been important in bridging the gaps; in the internationalization arena, Unicode has provided a lingua franca for communicating textual data. However, there remain differences in the locale data used by different systems.
 
 The best practice for internationalization is to store and communicate language-neutral data, and format that data for the client. This formatting can take place on any of a number of the components in a system; a server might format data based on the user's locale, or it could be that a client machine does the formatting. The same goes for parsing data, and locale-sensitive analysis of data.
 
@@ -4356,6 +4357,10 @@ Other contributors to CLDR are listed on the [CLDR Project Page](https://www.uni
 - Added a [Time Precision](tr35-dates#Time_Precision) option, replacing the discrete time field sets.
 - Updated the algorithm for mapping to standard skeletons.
 
+### Improvements to Keyboard Transforms
+- Added rigorous EBNF definition of transform syntax to [Transform Grammar](tr35-keyboards.md#transform-grammar).
+- Corrected and clarified escaping behavior under [Regex-like Syntax](tr35-keyboards.md#regex-like-syntax)
+
 **Changes in LDML Version 46.1 (Differences from Version 46)**
 
 ### Well-formed identifiers