Root Zone Label Generation Rules for the Khmer Script

Overview

This file contains Label Generation Rules (LGR) for the Khmer script for the Root zone. For more details on this LGR and its development see "Proposal for a Khmer Script Root Zone LGR [Proposal]". The format of this file follows [RFC 7940].

Repertoire

In addition to the 71 code points according to Section 5 "Repertoire", in [Proposal], two Khmer subscript consonant sequences have been defined. The repertoire only includes code points used by languages that are actively written in the Khmer script. The repertoire is based on [MSR-4], which is a subset of [Unicode 6.3].

Each code point or range is tagged with the script or scripts that the code point is used with, and one or more references documenting sufficient justification for inclusion in the repertoire, see "References" below.

Variants

According to Section 6, "Variants" in [Proposal], two Khmer consonants are variants of each other in their subscript form only. This is captured by a variant relation between the two subscript sequences, which are listed explicitly in the repertoire.

Variant Disposition: All variants are of type "blocked", making labels that differ only by these variants mutually exclusive: whichever label containing either of these variants is chosen earlier would be delegated, while any other equivalent label should be blocked.

This LGR does not define allocatable variants.

The specification of variants in the Root Zone LGR follows the guidelines in [RFC 8228].

Character Classes

Some consonants have been given the tag "base-only"; these do not take subscript form.

The character U+17C9 (៉) KHMER SIGN MUUSIKATOAN is used with a subset of first series consonants, code points from this subset have been given the tag "series-one".

The character U+17CA (៊) KHMER SIGN TRIISAP is used with subset of second series consonants and with U+1794 (ប) KHMER LETTER BA. Code points from this subset have been given the tag "series-two". The series, and the sets of code points used with these two signs are defined in [210]. The character U+17CB (់) KHMER SIGN BANTOC is used with a subset of consonants, that subset has been given the tag "series-three" [205]. The characters U+17C9 (៉) KHMER SIGN MUUSIKATOAN and U+17CA (៊) KHMER SIGN TRIISAP are collectively known as consonant shifters and have been given the tag "shifter".

The character U+17CC (៌) KHMER SIGN ROBAT has been given the tag "robat".

The character U+17C6 (ំ) KHMER SIGN NIKAHIT is used with consonants and some dependent vowels. These dependent vowels have been given a tag "dependent-vowel-1" (see Section 5.3 in [Proposal]). The character U+17C7 (ះ) KHMER SIGN REAHMUK is used with consonants and some dependent vowels. These dependent vowels have been given a tag "dependent-vowel-2" (see Section 5.3 in [Proposal]). The code point U+17B7 (ិ) KHMER VOWEL SIGN I has been given a tag "dependent-vowel-3".

Whole Label Evaluation (WLE) and Context Rules

Default Whole Label Evaluation Rules and Actions

The LGR includes the set of required default WLE rules and actions applicable to the Root Zone and defined in [MSR-4]. They are marked with ⍟. The default prohibition on leading combining marks is equivalent to ensuring that a label only starts with a consonant or independent-vowel.

Khmer-specific Rules and Action

Rules provided in the LGR as described in Section 7 of [Proposal] constrain possible labels so that they reasonably conform to Khmer syllable structure. These constraints are presented as context rules on the affected code points, except for the whole-label rule and associated action that enforce the limit on the number of consecutive subscripted consonants.

The rules are:

• Subscript-consonant — A rule that specifies allowable consonant sequence. See Section 7.2 in [Proposal]
• Subscript-consonant-limit — It limits the occurrence of subscript-consonant to two. In case of three, label is stated as invalid. See Section 7.3 in [Proposal]
• Coeng-context — A context rule for 17D2 that must have a consonant or base-only before it and a consonant after it. See Section 7.4 in [Proposal]
• Follows-consonant-robat-shifter — A context rule for those code points that must follow a consonant, robat or shifter. See Section 7.5 in [Proposal]
• Follows-series-two — A context rule for those code points that must follow series-two consonants. See Section 7.6 in [Proposal]
• Follows-series-one — A context rule for those code points that must follow series-one consonants. See Section 7.7 in [Proposal]
• Follows-consonant — A context rule for those code points that must always follow a consonant. See Section 7.8 in [Proposal]
• Follows-consonant-shifter A context rule for those code points that must always follow a consonant or shifter. See Section 7.9 in [Proposal]
• Follows-consonant-depvowel-1-shifter — A context rule for those code points that must follow consonant, shifter or dependent-vowel-1. See Section 7.10 in [Proposal]
• Follows-consonant-depvowel-2-shifter — A context rule for those code points that must follow consonant, shifter or dependent-vowel-2. See Section 7.11 in [Proposal]
• Follows-series-three — A context rule for those code points that must follow series-three consonants. See Section 7.12 in [Proposal]
• Follows-consonant-or-vowel-i — A context rule for those code points that must follow a consonant or 17B7 KHMER VOWEL SIGN I. See Section 7.13 in [Proposal]

Methodology and Contributors

For methodology and contributors, see Sections 4 and 8 of [Proposal].

References

The following general references are cited in this document:

[MSR-4]

Integration Panel, "Maximal Starting Repertoire — MSR-4 Overview and Rationale", 7 February 2019,
https://www.icann.org/en/system/files/files/msr-4-overview-25jan19-en.pdf

[Proposal]

Proposal for Khmer Script Root Zone LGR, 15 August 2016,
https://www.icann.org/en/system/files/files/proposal-khmer-lgr-15aug16-en.pdf

[RFC 7940]

Davies, K. and A. Freytag, "Representing Label Generation Rulesets Using XML", RFC 7940, August 2016,
http://www.rfc-editor.org/info/rfc7940

[RFC 8228]

A. Freytag, "Guidance on Designing Label Generation Rulesets (LGRs) Supporting Variant Labels", RFC 8228, August 2017,
https://www.rfc-editor.org/info/rfc8228

[Unicode 6.3]

The Unicode Consortium. The Unicode Standard, Version 6.3.0, (Mountain View, CA: The Unicode Consortium, 2013. ISBN 978-1-936213-08-5)
http://www.unicode.org/versions/Unicode6.3.0/

For references consulted particularly in designing the repertoire for the Khmer script for the Root Zone please see details in the Table of References below. Reference [3] refers to Unicode Standard version in which corresponding code points were initially encoded. References [100], [203], [204], [205], [206], [207], [208], [209] & [210] correspond to sources justifying the inclusion of or classification for the corresponding code points. Single code point or ranges may have multiple source reference values.

4.1 Character Classes

The following table lists all named and implicit classes with their definition and a list of their members intersected with the current repertoire (for larger classes, this list is elided).

Name	Definition	Count	Members or Ranges	Ref	Comment
consonant	Tag=consonant	33	{1780-179C 179F-17A2}		Any Khmer consonant
consonant-but-not-base-only	combined = [[:consonant:]∖[:base-only:]]	32	{1780-179C 179F-17A0 17A2}		Any Khmer consonant that is not base-only
dependent-vowel-1	Tag=dependent-vowel-1	2	{17B6 17BB}		Dependent Khmer vowels used with NIKAHIT
dependent-vowel-2	Tag=dependent-vowel-2	5	{17B7 17B9 17BB 17C1 17C4}		Dependent Khmer vowels used with REAHMUK
dependent-vowel-3	Tag=dependent-vowel-3	1	{17B7}		Dependent Khmer vowel used with TOANDAKHIAT
robat	Tag=robat	1	{17CC}		Khmer ROBAT
series-one	Tag=series-one	4	{1794 179F-17A0 17A2}		Khmer consonants used with MUUSIKATOAN
series-three	Tag=series-three	9	{1780 1784-1785 1789 178F 1793-1794 179B 179F}		Khmer consonants used with BANTOC
series-two	Tag=series-two	7	{1784 1789 1794 1798-179A 179C}		Khmer consonants used with TRIISAP
shifter	Tag=shifter	2	{17C9-17CA}		Any Khmer shifter (MUUSIKATOAN or TRIISAP)
implicit	Tag=base-only	1	{17A1}		The character tagged as base-only
implicit	Tag=coeng	1	{17D2}		The character tagged as coeng
implicit	Tag=dependent-vowel	16	{17B6-17C5}		Any character tagged as dependent-vowel
implicit	Tag=independent-vowel	12	{17A5-17A7 17AA-17B1 17B3}		Any character tagged as independent-vowel
implicit	Tag=sign	6	{17C6-17C8 17CB 17CD 17D0}		Any character tagged as sign
implicit	Tag=sc:Khmr	71	{1780-179C 179F-17A2 17A5-17A7 17AA-17B1 17B3 17B6-17CD 17D0 17D2}		Any character tagged as Khmer

4.2 Whole label evaluation and context rules

The following table lists all named rules defined in the LGR and indicates whether they are used as trigger in an action or as context (when or not-when) for a code point or variant.

Name	Regular Expression	Used as Trigger	Anchor	Used as Context	Ref	Comment
leading-combining-mark	(start)[[\p{gc=Mn}]∪[\p{gc=Mc}]]	✔				Default WLE rule matching labels with leading combining marks ⍟
subscript-consonant	\u17D2[:consonant-but-not-base-only:]					WLE Rule 2: allowable subscript consonant sequence
subscript-consonant-limit	(:subscript-consonant:){3,3}	✔				WLE Rule 3: more than two subscript consonants
coeng-context	([:consonant:])← ⚓ →([:consonant-but-not-base-only:])		✔	C		WLE Rule 4: checks for 17D2 and its surrounding code points
follows-consonant-robat-shifter	([:shifter:]|[:consonant:]|[:robat:])← ⚓		✔	C		WLE Rule 5: makes sure that dependent vowel follows a consonant or a shifter or a robat
follows-series-two	([:series-two:])← ⚓		✔	C		WLE Rule 6: checks sequence for shifter 17C9 MUUSIKATOAN
follows-series-one	([:series-one:])← ⚓		✔	C		WLE Rule 7: checks sequence for shifter 17CA TRIISAP
follows-consonant	([:consonant:])← ⚓		✔	C		WLE Rule 8: checks if sign code point or subscript consonant follows a consonant
follows-consonant-shifter	([:consonant:]|[:shifter:])← ⚓		✔	C		WLE Rule 9: checks if 17D0 KHMER SIGN SAMYOKSANNYA follows a consonant or a shifter
follows-consonant-depvowel-1-shifter	([:consonant:]|[:dependent-vowel-1:]|[:shifter:])← ⚓		✔	C		WLE Rule 10: checks if 17C6 KHMER SIGN NIKAHIT follows a consonant or a dependent vowel-1 or a shifter
follows-consonant-depvowel-2-shifter	([:consonant:]|[:dependent-vowel-2:]|[:shifter:])← ⚓		✔	C		WLE Rule 11: checks if 17C7 KHMER SIGN REAHMUK follows a consonant or a dependent vowel-2 or a shifter
follows-series-three	([:series-three:])← ⚓		✔	C		WLE Rule 12: checks if 17CB KHMER SIGN BANTOC code point follows a series-three consonant
follows-consonant-or-vowel-i	([:consonant:][:dependent-vowel-3:]?)← ⚓		✔	C		WLE Rule 13: checks if 17CD KHMER SIGN TOANDAKHIAT follows a consonant or 17B7 KHMER VOWEL SIGN I (Dependent-vowel-3)

4.3 Actions

The following table lists the actions that are used to assign dispositions to labels and variant labels based on the specified conditions. The order of actions defines their precedence: the first action triggered by a label is the one defining its disposition.

#	Condition	Rule / Variant Set		Disposition	Ref	Comment
1	if label matches	leading-combining-mark	→	invalid		labels with leading combining marks are invalid ⍟
2	if at least one variant is in	{out-of-repertoire-var}	→	invalid		any variant label with a code point out of repertoire is invalid ⍟
3	if label matches	subscript-consonant-limit	→	invalid		any label with more than two subscript consonants in a row is invalid
4	if at least one variant is in	{blocked}	→	blocked		any variant label containing blocked variants is blocked ⍟
5	if each variant is in	{allocatable}	→	allocatable		variant labels with all variants allocatable are allocatable ⍟
6	if any label (catch-all)		→	valid		catch all (default action) ⍟