Label Generation Rules for Lao

Overview

This file contains Label Generation Rules (LGR) for the Lao script as would be appropriate for the Root zone. For more details on this LGR, see "Proposal for a Lao Script Root Zone LGR [Proposal]". The format of this file follows [RFC 7940].

Repertoire

In addition to the 51 code points according to Section 5 “Repertoire” in [Proposal], the sequence 0EB2 0EB0 has been defined to facilitate implementation of WLE rule follows-vafter-context as a context rule. The repertoire only includes code points used by languages that are actively written in the Khmer script. The repertoire is based on [MSR-2], which is a subset of Unicode 6.3 [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

This LGR defines no variants.

Character Classes

Some consonants have been given the tag of Cf, which indicates final consonants. Other character classes that have been used are semi-consonant, tone-mark, vowel-above, vowel-before, vowel-below and vowel-after. See Section 5 of the [Proposal].

Whole Label Evaluation (WLE) Rules

Default Whole Label Evaluation Rules

The LGR includes the set of required default WLE rules and actions applicable to the Root Zone and defined in [MSR-2]. They are marked with ⍟. The default prohibition on leading combining marks is equivalent to ensuring that a label only starts with a consonant or vowel-before.

Lao-specific Rules

Rules provided in the LGR as described in Section 7 of [Proposal] reasonably restrict labels so that they conform to Lao syllable structure. These constraints are presented exclusively as context rules.

The rules are:

• follows-consonant — A context rule for semi-consonant. See Section 7 in [Proposal]. (WLE Rule 1)
• precedes-consonant — A context rule for vowel-before. See Section 7 in [Proposal]. (WLE Rule 2)
• follows-main-consonant — A context rule for vowel-below, and vowel-above. See Section 7 in [Proposal]. (WLE Rule 3)
• follows-C-tonemark-vabove — A context rule for vowel-after. See Section 7 in [Proposal]. (WLE Rule 4)
• follows-vbefore-consonant-cluster — A context rule for a vowel-after sequence. It incorporates consonant-cluster. See Section 7 in [Proposal]. (WLE Rule 5)
• follows-C-vabove-vbelow — A context rule for tone mark. See Section 7 in [Proposal]. (WLE Rule 6)
• follows-Cf — A context rule for LAO CANCELLATION MARK ໌ (U+0ECC (໌) ) . See Section 7 in [Proposal]. (WLE Rule 7)
• repetition-mark-limit.—. A rule that limits the occurrence of LAO KO LA ໆ (U+0EC6 (ໆ) ) at the label end. See Section 7 in [Proposal]. (WLE Rule 8)

No context rules apply to “consonant” code points. For discussion, see Section 5.1 “Consonants” in [Proposal].

Methodology and Contributors

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

References

Reference value ("ref" attribute) 0 refers to Unicode Standard versions in which corresponding code points were initially encoded. Reference values 201, 202, 203, 204, 205, 206, & 207 correspond to sources justifying the inclusion of or classification for the corresponding code points. Single code points or ranges may have multiple source reference values.

Reference values ("ref" attribute") from 201 and up refer to specific sources cited for the corresponding code points in the "[Proposal]".

In addition, the following references are cited in this document:

[MSR-2]

Integration Panel, "Maximal Starting Repertoire — MSR-2 Overview and Rationale", 14 April 2015
https://www.icann.org/en/system/files/files/msr-2-overview-14apr15-en.pdf

[Proposal]

Lao Generation Panel, "Proposal for Lao Script Root Zone LGR",
https://www.icann.org/en/system/files/files/proposal-lao-lgr-31jan17-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.

[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 more details for references, refer to the Table of References below. Several of these references refer to a figure in an appendix of the [Proposal] document.

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
Cf	Tag=Cf	14 Elements:	{0E81 0E87 0E8A 0E8D 0E94 0E97 0E99-0E9A 0E9F 0EA1 0EA3 0EA5 0EA7 0EAA}
consonant	Tag=consonant	27 Elements:	{0E81-0E82 0E84 0E87-0E88 0E8A 0E8D 0E94-0E97 0E99-0E9F 0EA1-0EA3 0EA5 0EA7 0EAA-0EAB 0EAD-0EAE}
semi-consonant	Tag=semi-consonant	1 Elements:	{0EBC}
tone-mark	Tag=tone-mark	4 Elements:	{0EC8-0ECB}
vowel-above	Tag=vowel-above	7 Elements:	{0EB1 0EB4-0EB7 0EBB 0ECD}
vowel-below	Tag=vowel-below	2 Elements:	{0EB8-0EB9}
implicit	Tag=vowel-after	3 Elements:	{0EB0 0EB2 0EBD}
implicit	Tag=vowel-before	5 Elements:	{0EC0-0EC4}
implicit	Tag=sign	2 Elements:	{0EC6 0ECC}
implicit	Tag=sc:Laoo	51 Elements:	{0E81-0E82 0E84 0E87-0E88 0E8A 0E8D 0E94-0E97 0E99-0E9F 0EA1-0EA3 0EA5 0EA7 0EAA-0EAB 0EAD-0EAE ...}

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. (Any use of context rules for variants is not indicated).

Name	Used as Trigger	Used as Context	Anchor	Regular Expression	Ref	Comment
leading-combining-mark	✔			(^[[\p{gc=Mn}][∅=\p{gc=Mc}]])		Default WLE rule from MSR-2 matching labels with leading combining marks ⍟
follows-consonant		✔	✔	((?<=[:consonant:])⚓)		WLE Rule 1: A semi-consonant must follow a consonant
precedes-consonant		✔	✔	(⚓(?=[:consonant:]))		WLE Rule 2: A vowel-before precedes a main consonant cluster
follows-main-consonant		✔	✔	((?<=([:consonant:]|[:semi-consonant:]))⚓)		WLE Rule 3: A vowel-above, and vowel-below follow a main consonant C
follows-C-tonemark-vabove		✔	✔	((?<=([:consonant:]|[:semi-consonant:]|[:tone-mark:]|[:vowel-above:]))⚓)		WLE Rule 4: A vowel-after follows a main consonant, tone-mark or vowel-above
consonant-cluster				([:consonant:]{1,2}[:semi-consonant:]?)		Defining consonant cluster for WLE Rule 5
follows-vbefore-consonant-cluster		✔	✔	((?<=\u0EC0(:consonant-cluster:))⚓)		WLE Rule 5: The sequence (0EB2 0EB0) follows a vowel before, and a consonant cluster
follows-C-vabove-vbelow		✔	✔	((?<=([:consonant:]|[:semi-consonant:]|[:vowel-above:]|[:vowel-below:]))⚓)		WLE Rule 6: A tone-mark follows a main consonant, vowel-above or vowel-below
follows-Cf		✔	✔	((?<=[:Cf:])⚓)		WLE Rule 7: The sign 0ECC can only occur after final consonants
repetition-mark-limit		✔	✔	(⚓(?=\u0EC6{0,2}$))		WLE Rule 8: The sign 0EC6 can only occur 0 to 3 times at the end of the label

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 at least one variant is in	{blocked}	→	blocked		any variant label containing blocked variants is blocked ⍟
4	if each variant is in	{allocatable}	→	allocatable		variant labels with all variants allocatable are allocatable ⍟
5	if any label (catch-all)		→	valid		catch all (default action) ⍟

Note: The following variant types are used in one or more actions, but are not defined in this LGR: out-of-repertoire-var, blocked, allocatable. This is not necessarily an error.