Reference Label Generation Rules for the Khmer Script

Overview

This document specifies a reference set of Label Generation Rules (LGR) for the Khmer script for the second level. The starting point for the development of this LGR can be found in the related Root Zone LGR [RZ-LGR-3-Khmr]. For details and additional background on the script see "Proposal for a Khmer Script Root Zone LGR [Proposal-Khmer]". The format of this file follows [RFC 7940].

This is a DRAFT document released for public comments and not final. Please see the announcement on the ICANN website for public comments on the Second Level Reference LGRs for details on how to submit comments.

Repertoire

The repertoire contains 71 code points for letters used for writing languages in the Khmer script; in addition, 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 a subset of [Unicode 6.3]. For details, see Section 5, "Repertoire" in [Proposal-Khmer]. (The proposal cited has been adopted for the Khmer script portion of the Root Zone LGR.)

For the second level, the repertoire has been augmented with the ASCII digits, U+0030 (0) to U+0039 (9), and U+002D (-) HYPHEN-MINUS for a total of 94 repertoire elements.

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

Variants

This LGR defines two Khmer consonants as 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. See Section 6, "Variants" in [Proposal-Khmer],

Digit Variants: All Khmer digits are treated as semantic variants of the corresponding common (ASCII) digits. By transitivity, they are also semantic variants of any native digits in scripts that also include the common digits. Such transitive relations are deemed to exist implicitly but are not listed explicitly in each reference LGR. (Omitting the listing of these other cross script digit variants does not affect index variant calculation, as the ASCII digit variant being smallest would always be the index variant.) There is a strong resemblance between Thai and Khmer digits, and certain Lao digits. In addition, Khmer digit ZERO is a cross-script homoglyph or near homoglyph of digit ZERO in many other scripts; all of these are already implicit semantic variants by transitivity and therefore not listed here. To keep digit variant sets manageable in zones where multiple scripts are present, no attempt has been made at identifying cross-script variants among digits of different numeric value or between a digit in one script and a letter in another, such as between digit zero and Latin letter 'o'. Other mechanisms may be required to prevent homograph labels.

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 this reference LGR follows the guidelines in [RFC 8228].

Character Classes

Consonants: When more than one consonant occur together in the onset of a grapheme cluster, the additional consonants are written in subscript form, joined with the first consonant in the sequence. In Unicode, this is indicated by preceding each additional consonant with U+17D2 (្) KHMER SIGN COENG. Some consonants have been given the tag "base-only"; these do not take subscript form. Consonants can have light or strong pronunciation indicated by two shifter signs; additionally there are six signs for phonological modifications as discussed below. See Section 5.2, "Consonants" in [Proposal-Khmer].

Independent vowels: These occur independently and cannot combine with any of the marks including dependent vowels and sign. The use of a subscript form, while defined in Unicode, is not supported here. See Section 5.2, "Independent Vowels" in [Proposal-Khmer].

Dependent vowels: The 16 dependent vowels included in the Khmer LGR have been further classified into three subcategories namely dependent vowel 1, dependent vowel 2 and dependent vowel 3, see below. Dependent vowels must follow a consonant, shifter or the Robat sign. They cannot occur independently. See Section 5.3, "Dependent Vowels" in [Proposal-Khmer].

Shifters: 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 two characters are collectively known as consonant shifters and have been given the tag "shifter". See Section 5.5, "Shifters" in [Proposal-Khmer] and reference [210].

Robat: The character U+17CC (៌) KHMER SIGN ROBAT has been given the tag "robat". It follows consonants but unlike other signs may be followed by a dependent vowel. See Sections 5.3, "Dependent Vowels" and 5.4, "Signs" in [Proposal-Khmer].

Signs (Bantoc): The character U+17CB (់) KHMER SIGN BANTOC is used with a subset of consonants, that subset has been given the tag "series-three", see Section 5.4, "Signs" in [Proposal-Khmer] and reference [205].

Signs (Yuukalepintu and Samyoksannya): The character U+17C8 (្) KHMER SIGN YUUKALEAPINTU is written after a consonant to indicate that it is to be followed by a short vowel and a glottal stop. The sign is used with all the consonants. The character U+17D0 (្) KHMER SIGN SAMYOKSANNYA is written above a consonant or a shifter to indicate that the syllable contains a particular short vowel. The sign is used with all the consonants or all the shifters. See Section 5.4, "Signs" in [Proposal-Khmer].

Signs (Nikahit and Reahmuk): 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 Sections 5.3 and 5.4 in [Proposal-Khmer]). 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 Sections 5.3 and 5.4 in [Proposal-Khmer]).

Signs (Toandakhiat): The sign ្ KHMER SIGN TOANDAKHIAT (U+17CD) is written over a final consonant to indicate that it is unpronounced. It is used with all consonants or Khmer vowel SIGN I (U+17B7). The latter has been given a tag "dependent-vowel-3". See Section 5.4, "Signs" in [Proposal-Khmer].

Khmer Digits: U+17E0 (០) to U+17E9 (៩) are a set of Khmer-specific digits. They are used in alternation with the European (common) digits.

Common Digits: U+0030 (0) to U+0039 (9) are the set of digits from the ASCII range.

Whole Label Evaluation (WLE) and Context Rules

Common Rules

• Hyphen Restrictions — restrictions on the allowable placement of hyphens (no leading/ending hyphen and no hyphen in positions 3 and 4). These restrictions are described in section 4.2.3.1 of RFC 5891 [150]. They are implemented here as context rule on U+002D (-) HYPHEN-MINUS.
• Leading Combining Marks — restrictions on the allowable placement of combining marks (no leading combining mark). This rule is described in section 4.2.3.2 of RFC 5891 [150].
• One non-ASCII code point — An IDN label must contain one non-ASCII code point, as described in Section 4.2.4 of RFC 5891 [150].
• Digit-mixing — no mixing between different digit sets (European and Khmer digits) is allowed; implemented here as a WLE rule with associated action.

Default Actions

Actions include the default actions for LGRs as well as that needed to invalidate labels with misplaced combining marks. They are marked with ⍟. For a description see [RFC 7940].

Khmer-specific Rules and Action

Rules provided in the LGR as described in Section 7 of [Proposal-Khmer] 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-Khmer]
• Subscript-consonant-limit — A rule that limits the occurrence of subscript-consonant to two. In case of three, the associate action gives the label the disposition invalid. See Section 7.3 in [Proposal-Khmer]
• Coeng-context — A context rule for 17D2 that requires a consonant before it and a consonant other than base-only after it. See Section 7.4 in [Proposal-Khmer]
• 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-Khmer]
• Follows-series-two — A context rule for those code points that must follow series-two consonants. See Section 7.6 in [Proposal-Khmer]
• Follows-series-one — A context rule for those code points that must follow series-one consonants. See Section 7.7 in [Proposal-Khmer]
• Follows-consonant — A context rule for those code points that must always follow a consonant. See Section 7.8 in [Proposal-Khmer]
• Follows-consonant-shifter A context rule for those code points that must always follow a consonant or shifter. See Section 7.9 in [Proposal-Khmer]
• 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-Khmer]
• 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-Khmer]
• Follows-series-three — A context rule for those code points that must follow series-three consonants. See Section 7.12 in [Proposal-Khmer]
• 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-Khmer]

Methodology and Contributors

This reference LGR for Khmer for the 2nd Level has been developed by Michel Suignard and Asmus Freytag, based on the Root Zone LGR for Khmer and information contained or referenced therein, see [RZ-LGR-3-Khmr]. Suitable extensions for the second level have been applied according to the [Guidelines]. The original proposal for a Root Zone LGR for the Khmer script, that this reference LGR is based on, was developed by the Khmer Generation Panel. For more information on methodology and contributors to the underlying Root Zone LGR, see Sections 4 and 8 in [Proposal-Khmer], as well as [RZ-LGR-Overview].

References

The following general references are cited in this document:

[Guidelines]

ICANN, "Guidelines for Developing Reference LGRs for the Second Level, (Los Angeles, California: ICANN, 27 May 2020),
https://www.icann.org/en/system/files/files/lgr-guidelines-second-level-06jan16-en.pdf

[Proposal-Khmer]

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

[RZ-LGR-Overview]

Integration Panel, "Root Zone Label Generation Rules - LGR-3: Overview and Summary", 01 March 2020 (PDF),
https://www.icann.org/sites/default/files/lgr/lgr-3-overview-10jul19-en.pdf

[RZ-LGR-3-Khmr]

ICANN, Root Zone Label Generation Rules for the Khmer Script (und-Khmr) , 10 July 2019 (XML)
https://www.icann.org/sites/default/files/lgr/lgr-3-khmer-script-10jul19-en.xml

[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 second level. please see details in the Table of References below. Reference [3] refers to the Unicode Standard version in which the 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. Entries in the table 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)
common-digits	Tag=Common-digit	10	{0030-0039}		Digits from the ASCII range; ⍟
khmer-digits	Tag=Khmer-digit	10	{17E0-17E9}		Khmer digits
hyphen	Tag=Hyphen	1	{002D}		The Hyphen-minus character ⍟
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	81	{1780-179C 179F-17A2 17A5-17A7 17AA-17B1 17B3 17B6-17CD 17D0 17D2 17E0-17E9}		Any character tagged as Khmer
implicit	Tag=sc:Zyyy	11	{002D 0030-0039}		Any character tagged as Common

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}]]	✔			[150]	RFC 5891 restrictions on placement of combining marks ⍟
hyphen-minus-disallowed	(((start))← ⚓)|(⚓ →((end)))|(((start)..[:hyphen:])← ⚓)		✔	C	[150]	RFC 5891 restrictions on placement of U+002D (-) ⍟
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)
digit-mixing	([:common-digits:].*[:khmer-digits:])|([:khmer-digits:].*[:common-digits:])	✔				restrictions on mixing digits
ascii-only-label	(start)[\u002D\u0030-\u0039]+(end)	✔			[150]	RFC 5891 restriction requiring at least one non-ASCII code point ⍟

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	[150]	labels with leading combining marks are invalid ⍟
2	if label matches	ascii-only-label	→	invalid	[150]	ascii-only labels invalid (not IDNs) ⍟
3	if label matches	digit-mixing	→	invalid		a label violating the restriction on digit mixing is invalid
4	if at least one variant is in	{out-of-repertoire-var}	→	invalid		any variant label with a code point out of repertoire is invalid ⍟
5	if label matches	subscript-consonant-limit	→	invalid		any label with more than two subscript consonants in a row is invalid
6	if at least one variant is in	{blocked}	→	blocked		any variant label containing blocked variants is blocked ⍟
7	if each variant is in	{allocatable}	→	allocatable		variant labels with all variants allocatable are allocatable ⍟
8	if any label (catch-all)		→	valid		catch all (default action) ⍟