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Label Generation Rules for the Thai Language

Overview

This document specifies a set of Label Generation Rules (LGR) for the Thai language for the second level domain or domains identified above. The ultimate starting point for the development of this LGR can be found in the related Root Zone LGR [RZ-LGR-Thai]. Note that while it is the Thai Script Root Zone LGR that forms the starting point, the LGR defined here covers the Thai Language. The format of this file follows [RFC 7940]. This LGR is adapted from the “Reference LGR for the Second Level for the Thai Language” [Ref-LGR-th-Thai], for details, see Change History below.

For details and additional background on the Thai script, see “Proposal for a Thai Script Root Zone LGR” [Proposal-Thai].

Standalone LGR: This LGR is designed to be used in a zone that does not cater to IDNs other than those valid under this LGR. This LGR lacks features that would allow its use in the context of another LGR in the same zone, and it may contain other features incompatible with such use.

Repertoire

The repertoire contains 68 code points for letters used in the Thai language. In addition, three sequences have been defined. The sequence U+0E4D U+0E32  ํา  was defined to replace the disallowed U+0E33 ำ SARA AM and to facilitate implementation of the WLE rule follows-consonant-tone as a context rule. The other two sequences were defined to restrict U+0E45 ๅ LAKKHANGYAO from appearing in any context other than these sequences. Accordingly, while U+0E45 ๅ is not listed by itself this brings the total of code points avaliable for letters to 69.

The repertoire is a subset of [Unicode 11.0.0]. For details, see Section 5, “Repertoire” in [Proposal-Thai]. (The proposal cited has been adopted for the Thai 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; Thai digits, U+0E50 ๐ to U+0E59 ๙; U+002D - HYPHEN-MINUS; the abbreviation mark U+0E2F ฯ THAI CHARACTER PAIYANNOI; and the repetition mark U+0E46 ๆ THAI CHARACTER MAIYAMOK. Also, a special sequence using the abbreviation mark has been added for a total of 95 repertoire elements.

Repertoire Listing: Each code point or range is tagged with the script or scripts with which the code point is used, and one or more references documenting sufficient justification for inclusion in the repertoire, see “References” below. Comments provide additional information for some code points.

Variants

This LGR defines no variants for letters, see Section 6, “Variants” in [Proposal-Thai].

Digit Variants: All Thai 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 variant relations are deemed to exist implicitly by transitivity but are not listed explicitly in each reference LGR. Instead, if needed, they are applied by using the Common LGR in label processing.

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.

Consonants: There are 44 characters that are classified as consonants; code points from this subset have been given the tag “cons”. See Section 5.1, “Consonants” in [Proposal-Thai].

Vowels: 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 tags “fv1”,"fv2", “fv3”, “av”, “bv”, or “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). See Section 5.2, “Vowels” in [Proposal-Thai].

Tones: 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”. See section 5.3, “Tone Marks” in [Proposal-Thai].

Diacritical Marks: There are 3 above 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 “Diacritics” in [Proposal-Thai].

• 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 ำ THAI CHARACTER SARA AM, which is in common use. However, NIKHAHIT may also be used by itself.

A fourth above diacritic, U+0E4E  ๎  YAMAKKAN, has been excluded from the Root Zone 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, a below diacritic, which has been given the tag “bd”. 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.

Abbreviation Mark: The abbreviation mark (or ellipsis) U+0E2F ฯ PAIYANNOI should not be at the beginning position and must end the label. An exception to this rule has been provided for by a special sequence.

Repetition Mark: The repetition mark U+0E46 ๆ MAIYAMOK repeats anything before it. Therefore, it should not be at the beginning position. The repetition mark must not be followed by any combining mark, or follow a leading vowel to prevent unstable rendering. A repetition mark may be repeated. A context rule has been defined to limit the repetition mark to acceptable context, however, some of the restrictions are taken care of by other rules, for example, the rule that leading vowel must be followed by a consonant.

Thai Digits: U+0E50 ๐ to U+0E59 ๙ are a set of Thai-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

The LGR includes the rules and actions to implement the following restrictions, some of which are mandated by the IDNA protocol. They are marked with ⍟ and included here by default.

• 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].
• Digit-mixing — no mixing between different digit sets (European and Thai digits) is allowed; implemented here as a WLE rule with associated action.

Default Actions

This LGR includes the complete set default actions for LGRs as well as the action needed to invalidate labels with misplaced combining marks. They are marked with ⍟. For a description see [RFC 7940] and [150]. An additional action is triggered by mixed digit labels.

Thai-specific Rules

The rules provided in this LGR as described in Section 7 of [Proposal-Thai] 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-Thai]
• A below-vowel must follow a consonant — See Section 7.3 in [Proposal-Thai]
• An above-vowel must follow a consonant — See Section 7.3 in [Proposal-Thai]
• A below diacritic must follow a consonant — See Section 7.3 in [Proposal-Thai]
• An above-diacritic MAITAIKHU must follow a consonant — See Section 7.3 in [Proposal-Thai]
• A vowel MAIHAN AKAT must be in between a consonant and either tone or consonant — See Section 7.4 in [Proposal-Thai]
• A vowel SARA A can follow a consonant, a tone or a vowel SARA AA — See Section 7.5 in [Proposal-Thai]
• 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-Thai]
• A tone mark, THANTHAKHAT, NIKHAHIT can only follow a consonant, above-vowel or below-vowel — See section 7.7 and 7.8 in [Proposal-Thai]
• An abbreviation mark, PAIYANNOI cannot be at the beginning of the label and must end it — rule SL1 for second level
• A repetition mark, MAIYAMOK can precede a repetition mark, an independent vowel, or end of label; it cannot start a label — rule SL2 for second level

Methodology and Contributors

The LGR in this document has been adapted from the corresponding Reference LGR for the Second Level. The Second Level Reference LGR for the Thai Language was developed by Michel Suignard and Asmus Freytag, based on the Root Zone LGR for the Thai script and information contained or referenced therein; see [RZ-LGR-Thai]. Suitable extensions for the second level have been applied according to the [Guidelines] and with community input. As part of that process, features have been removed or modified as appropriate for the target language. The original proposal for a Root Zone LGR for the Thai script, that this LGR is based on, was developed by the Thai Generation Panel. For more information on methodology and contributors to the underlying Root Zone LGR, see Sections 4 and 8 in [Proposal-Thai], as well as [RZ-LGR-Overview].

Changes from Version Dated 15 December 2020

Language tag has been updated.

Changes from Version Dated 18 May 2021

Unicode Version has been updated.

Changes from Version Dated 24 January 2024

Adopted from the Second Level Reference LGR for the Thai Language [Ref-LGR-th-Thai] without normative changes.

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-27may20-en.pdf

[Proposal-Thai]

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

[Ref-LGR-th-Thai]

ICANN, Second Level Reference Label Generation Rules for the Thai Language (th-Thai), 24 January 2024 (XML)
https://www.icann.org/sites/default/files/packages/lgr/lgr-second-level-thai-language-24jan24-en.xml
non-normative HTML presentation:
https://www.icann.org/sites/default/files/packages/lgr/lgr-second-level-thai-language-24jan24-en.html

[RFC 7940]

Davies, K. and A. Freytag, “Representing Label Generation Rulesets Using XML”, RFC 7940, August 2016,
https://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 (RZ LGR-5): Overview and Summary”, 26 May 2022 (PDF),
https://www.icann.org/sites/default/files/lgr/rz-lgr-5-overview-26may22-en.pdf

[RZ-LGR-Thai]

ICANN, Root Zone Label Generation Rules for the Thai Script (und-Thai), 26 May 2022 (XML)
https://www.icann.org/sites/default/files/lgr/rz-lgr-5-thai-script-26may22-en.xml

[Unicode 11.0.0]

The Unicode Consortium. The Unicode Standard, Version 11.0.0, (Mountain View, CA: The Unicode Consortium, 2018. ISBN 978-1-936213-19-1)
https://www.unicode.org/versions/Unicode11.0.0/

For references consulted particularly in designing the repertoire for the Thai Language for the second level 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-Thai] for justifying the inclusion of for the corresponding code points. Entries in the table may have multiple source reference values. In the listing of whole label evaluation and context rules, reference [150] indicates the source for common rules.

Character Classes

Number of named classes	12
Implicit (except script)	5
Implict defined by script tag	2

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
leading-vowel	Tag=lv	5	{0E40-0E44}		Any Thai leading vowel
repetition	Tag=rep	1	{0E46}		The Thai repetition mark, MAIYAMOK
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
common-digits	Tag=Common-digit	10	{0030-0039}		Digits from the ASCII range; ⍟
thai-digits	Tag=Thai-digit	10	{0E50-0E59}		Thai digits
implicit	Tag=abbr	1	{0E2F}		The character tagged as abbr
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=sc:Thai	80	{0E01-0E32 0E34-0E3A 0E40-0E44 0E46-0E4D 0E50-0E59}		Any character tagged as Thai
implicit	Tag=sc:Zyyy	11	{002D 0030-0039}		Any character tagged as Common

Legend

Members or Ranges

Lists the members of the class as code points (xxx) or as ranges of code points (xxx-yyy). Any class too numerous to list in full is elided with "...".

Tag=ttt

A named or implicit class defined by all code points that share the given tag value (ttt).

Implicit

An anonymous class implicitly defined based on tag value and for which there is no named equivalent.

Combined

A named class defined by set operations on other classes using the following syntax:

[: :] - named or implicit character set

Reference to a named character set [:name:] or an implicit character set [:tag:]. A leading “^” before name or tag indicates the set complement.

∪, ∩, ∖, ∆ - set operators

Sets may be combined by set operators (∪ = union, ∩ = intersection, ∖ = difference, ∆ = symmetric difference).

Whole label evaluation and context rules

Number of rules	11
Used to trigger actions	2
Used as context rule (C)	9
Anchored context rules	9

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)..\u002D)← ⚓︎)		✔	C	[150]	RFC 5891 restrictions on placement of U+002D - ⍟
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 ะ THAI CHARACTER SARA A can follow a consonant, a tone or U+0E32 า THAI CHARACTER SARA AA
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
follows-any-precedes-end	(.)← ⚓︎ →((end))		✔	C	[102]	WLE SL1: An abbreviation mark, PAIYANNOI cannot be at the beginning of the label and must end it
follows-any-precedes-rep-cons-lv-end	(.)← ⚓︎ →([:consonant:]|[:repetition:]|[:leading-vowel:]|(end))		✔	C	[102]	WLE SL2: A repetition mark, MAIYAMOK cannot be at the beginning of the label and must be followed by itself, a consonat or a leading vowel
digit-mixing	([:common-digits:].*[:thai-digits:])|([:thai-digits:].*[:common-digits:])	✔				restrictions on mixing digits

Legend

Used as Trigger

This rule triggers one of the actions listed below.

Used as Context

This rule defines a required or prohibited context for a code point C or variant V.

Anchor

This rule has a placeholder for the code point for which it is evaluated.

Regular Expression

A regular expression equivalent to the rule, shown in a modified notation as noted:

⚓︎ - context anchor

Placeholder for the actual code point when a context is evaluated. The code point must occur at the position corresponding to the anchor. Rules containing an anchor cannot be used as triggers.

(...)← - look-behind

If present encloses required context preceding the anchor.

→(..) - look-ahead

If present encloses required context following the anchor.

( ) - group

An anonymous nested rule is used to group match operators.

(... | ...) - choice

When there is more than one alternative in a rule, the choices are separated by the alternation operator (...|...).

start or end

(start) matches the start of the label; (end) matches the end of the label.

. - any code point

. matches any code point.

*, +, ?, {n,m} - count operators

* indicates 0 or more, + indicates one or more, and ? indicates up to one instance. {n,m} indicates at least n and at most m instances.

[: :] - named or implicit character set

Reference to a named character set [:name:] or an implicit character set [:tag:]. A leading “^” before name or tag indicates the set complement.

[\p{ }] - property

Set of all characters matching a given value for a Unicode property [\p{prop=val}]. Note: uppercase “\P” defines the complement of a property set.

∪, ∩, ∖, ∆ - set operators

Sets may be combined by set operators (∪ = union, ∩ = intersection, ∖ = difference, ∆ = symmetric difference).

∅= - empty set

Indicates that the following set is empty because of the result of set operations, or because none of its elements is part of the repertoire defined here. A rule with a non-optional empty set never matches.

⍟ - default rule

Rules marked with ⍟ are included by default and may or may not be triggered by any possible label under this LGR.

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	digit-mixing	→	invalid		a label violating the restriction on digit mixing is invalid
3	if at least one variant is in	{out-of-repertoire-var}	→	invalid		any variant label with a code point out of repertoire 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) ⍟

Legend

{...} - variant type set

In the “Rule/Variant Set” column, the notation {...} means a set of variant types.

⍟ - default action

Actions marked with ⍟ are included by default and may or may not be triggered by any possible label under this LGR.

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