This file contains Label Generation Rules (LGR) for the Khmer script for the Root zone. For more details on this LGR and its development, as well as background on the script see "Proposal for a Khmer Script Root Zone LGR [Proposal-Khmer]". This file is one of a set of LGR files that together form an integrated LGR for the DNS Root Zone [RZ-LGR-4]. The format of this file follows [RFC 7940].
In addition to the 71 code points according to Section 5, "Repertoire", in [Proposal-Khmer], 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].
As part of the Root Zone, this LGR includes neither digits nor the HYPHEN-MINUS.
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.
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],
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].
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].
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.
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:
The Root Zone LGR for the Khmer script was developed by the Khmer Generation Panel. For methodology and contributors, see Sections 4 and 8 in [Proposal-Khmer], as well as [RZ-LGR-4-Overview].
The following general references are cited in this document:
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 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.
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