Root Zone Label Generation Rules for the Thai Script

Overview

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

Repertoire

In addition to the 68 code points according to Section 5 “Repertoire” in [Proposal], three sequences have been defined. The sequence U+0E4D U+0E32 (ํา) was defined to replace the disallowed U+0E33 (SARA AM) and to facilitate implementation of WLE rule follows-consonant-tone as a context rule. The other two sequences were defined to restrict U+0E45 (LAKKHANGYAO) from appearing in any context other than these sequences. Accordingly, while U+0E45 (ๅ) is not listed by itself it brings the total of distinct code points to 69.

The repertoire only includes code points used by languages that are actively written in the Thai 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], this LGR defines no variants.

Character Classes

The Thai Script is an abugida in which consonant–vowel sequences are written as a unit: each unit is based on a consonant letter, and vowel, tone mark or diacritic notation are secondary. It is written with the combining marks stacked above or below the base consonant, like diacritics in European languages. However, although the concepts are quite similar, the implementations are significantly different.

There are 44 characters that are classified as consonants, code points from this subset have been given the tag "cons".

The 18 vowel symbols pronounced after a consonant are non-sequential: they can be located before (lv) , after (fv), above (av) or below (bv) the consonant, or in a combination of these positions, code points from this subset have been given the tag "fv1","fv2","fv3","av","bv","lv". There are three code point sequences defined that include vowels. (Code point sequences do not carry tag values; instead, for code point sequences the subset values are identified in comments).

There are 5 phonemic tones: mid, low, falling, high, and rising. These 5 tones are represented by 4 tone marks plus the absence of a mark. Code points from this subset have been given the tag "tone".

There are 3 diacritic symbols that have been included here and given the tag "ad". They differ in their frequency and purpose of usage. See also the discussion in section 5.4 in [Proposal].

• U+0E47 (MAITAIKHU) and U+0E4C (THANTHAKHAT) are commonly used for words in everyday communication
• U+0E4D (NIKHAHIT) is included because of its use to decompose U+0E33 ( ําา ) THAI CHARACTER SARA AM, which is in common use. However, NIKHAHIT may also be used by itself.

The Thai GP decided to exclude a fourth above diacritic, U+0E4E (YAMAKKAN), from the LGR repertoire because it is rarely used in Modern Thai or even in older Pali manuscripts; it is more common to replace it with U+0E3A (PHINTHU). Moreover, excluding U+0E4E (YAMAKKAN) also eliminates the chance of confusion between U+0E4E (YAMAKKAN) and U+0E4C (THANTHAKHAT). Both look similar, are always placed at the same position in the word cell, and they are normally displayed in a small size.

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 ⍟.

Thai-specific Rules

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

The rules are:

• A leading-vowel must precede a consonant - See Section 7.2 in [Proposal]
• A below-vowel must follow a consonant - See Section 7.3 in [Proposal]
• An above-vowel must follow a consonant - See Section 7.3 in [Proposal]
• A below diacritic must follow a consonant - See Section 7.3 in [Proposal]
• An above-diacritic MAITAIKHU must follow a consonant - See Section 7.3 in [Proposal]
• A vowel MAIHAN AKAT must be in between a consonant and either tone or consonant - See Section 7.4 in [Proposal]
• A vowel SARA A can follow a consonant, a tone or a vowel SARA AA - See Section 7.5 in [Proposal]
• A vowel SARA AA, or an above diacritic NIKHAHIT followed by a vowel SARA AA can follow a consonant or a tone - See Sections 7.6 and 7.9 in [Proposal]
• A tone-mark, THANTHAKHAT, NIKHAHIT can only follow a consonant, above-vowel or below-vowel - See section 7.7 and 7.8 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 a Thai Script Root Zone LGR, 25 May 2017,
https://www.icann.org/en/system/files/files/proposal-thai-lgr-25may17-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 references consulted particularly in designing the repertoire for the Thai script for the Root Zone please see details in the Table of References below. Reference [0] refers to the Unicode Standard version in which corresponding code points were initially encoded. References [100] and [101] correspond to sources given in [Proposal] for justifying the inclusion of 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
above-vowel	Tag=av	5	{0E31 0E34-0E37}		Any Thai above vowel
below-vowel	Tag=bv	2	{0E38-0E39}		Any Thai below vowel
consonant	Tag=cons	44	{0E01-0E23 0E25 0E27-0E2E}		Any Thai consonant
sara-aa	Tag=sara-aa	1	{0E32}		Thai SARA AA
tone	Tag=tone	4	{0E48-0E4B}		Any Thai tone mark
c-av-bv	combined = [[:consonant:]∪[:above-vowel:]∪[:below-vowel:]]	51	{0E01-0E23 0E25 0E27-0E2E 0E31 0E34-0E39}		Any Thai consonant, vowel-above or vowel-below
ct	combined = [[:consonant:]∪[:tone:]]	48	{0E01-0E23 0E25 0E27-0E2E 0E48-0E4B}		Any Thai consonant or tone mark
ctaa	combined = [[:consonant:]∪[:tone:]∪[:sara-aa:]]	49	{0E01-0E23 0E25 0E27-0E2E 0E32 0E48-0E4B}		Any Thai consonant, tone or sara-aa
implicit	Tag=ad	3	{0E47 0E4C-0E4D}		Any character tagged as ad
implicit	Tag=bd	1	{0E3A}		The character tagged as bd
implicit	Tag=fv1	2	{0E30 0E32}		Any character tagged as fv1
implicit	Tag=fv3	2	{0E24 0E26}		Any character tagged as fv3
implicit	Tag=lv	5	{0E40-0E44}		Any character tagged as lv
implicit	Tag=sc:Thai	68	{0E01-0E2E 0E30-0E32 0E34-0E3A 0E40-0E44 0E47-0E4D}		Any character tagged as Thai

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 ⍟
precedes-consonant	⚓ →([:consonant:])		✔	C		WLE 7.2: check if current cp is preceding a consonant
follows-consonant	([:consonant:])← ⚓		✔	C		WLE 7.3: check if current cp is following a consonant
between-consonant-and-ct	([:consonant:])← ⚓ →([:ct:])		✔	C		WLE 7.4: check if current cp is in between a consonant and either tone or consonant
follow-consonant-tone-sara-aa	([:ctaa:])← ⚓		✔	C		WLE 7.5: U+0E30 (THAI CHARACTER SARA A, ะ) can follow a consonant, a tone or U+0E32 (THAI CHARACTER SARA AA, า)
follows-consonant-tone	([:ct:])← ⚓		✔	C		WLE 7.6, 7.9: check if current cp is following a consonant or a tone
follows-consonant-av-bv	([:c-av-bv:])← ⚓		✔	C		WLE 7.7, 7.8: A tone-mark, THANTHAKHAT, NIKAHIT can only follow a consonant, above-vowel or below-vowel

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 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) ⍟