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Label Generation Rules for the Tamil Script

Overview

This document specifies a set of Label Generation Rules (LGR) for the Tamil script for the second level domain or domains identified above. The starting point for the development of this LGR can be found in the related Root Zone LGR [RZ-LGR-Taml]. The format of this file follows [RFC 7940]. This LGR is adapted from the “Reference LGR for the Second Level for the Tamil Script” [Ref-LGR-und-Taml], for details, see Change History below.

For details and additional background on the Tamil script, see “Proposal for a Tamil Script Root Zone Label Generation Rule-Set (LGR)” [Proposal-Tamil].

Repertoire

The repertoire contains 48 code points for letters and 4 sequences used to write the Tamil language. The repertoire is a subset of [Unicode 11.0.0]. For details, see Section 5, “Repertoire” in [Proposal-Tamil]. (The proposal cited has been adopted for the Tamil 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, plus U+002D - HYPHEN-MINUS, for a total of 63 repertoire elements.

Any code points outside the Tamil Script repertoire that are targets for out-of-repertoire variants would be included here only if the variant is listed in this file. In this case they are identified as a reflexive (identity) variant of type “out-of-repertoire-var”. Whether or not they are listed, they do not form part of the repertoire.

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

Variants

The variants defined in this LGR are limited to those required for use in zones not shared with any other script. As such, this LGR does not define cross-script variants. However, using this LGR concurrently with any LGR for Malayalam in the same zone will create potential cross-script issues. For details, see Section 6, “Variants” in [Proposal-Tamil]. Mitigation of these cross-script variants can be addressed by using the Common LGR. For details, see Section 3, “Use of Multiple Reference LGRs in the Same Zone” in [Level-2-Overview]. In addition to variants defined by this LGR, the full variant information related to this script and required for concurrent use with the Malayalam LGR(s) can be found in the following LGR: [Ref-LGR-Tamil-Full-Variant-Script]

This LGR defines four sequences as in-script variants. Two of them are variants to single code points; the other two are variants of each other; all defined variants look exactly alike and can cause confusion even to a careful observer. For detais, see Section 6, “Variants” in [Proposal-Tamil].

Digit Variants: The Tamil reference LGR does not include native digits, so there are no semantic variants defined.

However, some Tamil letters are homoglyphs of native digits. For example, U+0BE7 ௧ TAMIL DIGIT ONE and U+0BED ௭ TAMIL DIGIT SEVEN are homoglyphs of U+0B95 க TAMIL LETTER KA and U+0B8E எ TAMIL LETTER E respectively.

Should an extension be contemplated that adds native digits, these might require variant relations. At the same time, however, any native digits would normally be semantic variants of the ASCII digits. In any zone containing multiple scripts, these two types of variant relation for digits may lead to complication once transitivity is applied.

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 labels should be blocked. There is no preference among these labels.

This LGR does not define allocatable variants.

The specification of variants in this LGR follows the guidelines in [RFC 8228].

For historical reasons, there are two sequences that both represent the same akshar (Shri) with exactly the same rendered form. These two sequences are thus semantically identical and users may choose either one, except that a single label must use one of these forms consistently. This variant pair is thus being defined as “allocatable” variant. (For more details see Section 6.1 in [Proposal-Tamil].)

Character Classes

Tamil is an alphasyllabary and the heart of the writing system is the akshar. This is the unit which is instinctively recognized by users of the script. As encoded, the writing system of Tamil is composed of Consonants, the Implicit Vowel Killer: Halant, Vowels and Visarga/Aytham.

Consonants: More details in Section 3.3.1, “The Consonants” in [Proposal-Tamil].

Halant / Virama: All consonants contain an implicit vowel /a/. A special sign is needed to denote that this implicit vowel is stripped off. This is known as the Pulli and encoded as U+0BCD  ்  TAMIL SIGN VIRAMA. The virama thus joins two adjacent consonants. In Tamil, there are only two cases where this forms conjuncts. More details in Section 3.3.2, “Virama/Pulli” of the [Proposal-Tamil].

Vowels and Matras: Separate symbols exist for all Vowels that are pronounced independently either at the beginning or after another vowel sound. To indicate a Vowel sound other than the implicit one following a consonant, a Vowel sign (matra) is attached to the consonant. Since the consonant has a built-in /a/, there are equivalent Matras for all vowels excepting the அ. More details in Section 3.3.3, “Vowels” in [Proposal-Tamil].

Visarga: The Visarga (or Aytham) is used in Tamil to represent a sound very close to /ḵ/. More details in Section 3.3.4, “Visarga/Aytham” in [Proposal-Tamil].

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

By default, the LGR includes the rules and actions to implement the following restrictions mandated by the IDNA protocol. They are marked with ⍟.

• 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].

Default Actions

This LGR includes the 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].

Tamil-specific Rules and Action

These rules have been drafted to ensure that the prospective Tamil label conforms to akshar formation norms as desired in Tamil script. These norms are exclusively presented as context rules.

The following symbols are used in the WLE rules:
C → Consonant
M → Matra
H → Haleant / Virama =Pulli
X → Visarga = Aytham

The rules are:

1. H: must be preceded by C
2. M: must be preceded by C
3. X: cannot be preceded by X

The following whole-label rule and associated action prevent the mixing of two allocatable variants of the same sequence within the same label. This reduces overproduction of variant labels.

4. Two representations of ‘Shri’ cannot be mixed in the same label

More details in Section 7, “Whole Label Evaluation Rules (WLE)” in [Proposal-Tamil].

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 Tamil Script was developed by Michel Suignard and Asmus Freytag, based on the Root Zone LGR for the Tamil script and information contained or referenced therein; see [RZ-LGR-Taml]. Suitable extensions for the second level have been applied according to the [Guidelines] and with community input. The original proposal for a Root Zone LGR for the Tamil script, that this reference LGR is based on, was developed by the Neo-Brahmi Generation Panel (NBGP). For more information on methodology and contributors to the underlying Root Zone LGR, see Sections 4 and 8 in [Proposal-Tamil], as well as [RZ-LGR-Overview].

Changes from Version Dated 15 December 2020

Unicode Version has been updated.

Changes from Version Dated 24 January 2024

Adopted from the Second Level Reference LGR for the Tamil Script [Ref-LGR-und-Taml] 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

[Level-2-Overview]

Internet Corporation for Assigned Names and Numbers, (ICANN),“Reference Label Generation Rules (LGR) for the Second Level: Overview and Summary” (PDF), (Los Angeles, California: ICANN, 24 January 2024),
https://www.icann.org/en/system/files/files/level2-lgr-overview-summary-24jan24-en.pdf

[Proposal-Tamil]

Neo-Brahmi Generation Panel, “Proposal for a Tamil Script Root Zone Label Generation Rule-Set (LGR)”, 6 March 2019,
https://www.icann.org/en/system/files/files/proposal-tamil-lgr-06mar19-en.pdf

[Ref-LGR-und-Taml]

ICANN, Second Level Reference Label Generation Rules for the Tamil Script (und-Taml), 24 January 2024 (XML)
https://www.icann.org/sites/default/files/packages/lgr/lgr-second-level-tamil-script-24jan24-en.xml
non-normative HTML presentation:
https://www.icann.org/sites/default/files/packages/lgr/lgr-second-level-tamil-script-24jan24-en.html

[Ref-LGR-Tamil-Full-Variant-Script]

ICANN, Second Level Reference Label Generation Rules for the Tamil Script (und-Taml), 24 January 2024 (XML)
https://www.icann.org/sites/default/files/packages/lgr/lgr-second-level-tamil-full-variant-script-24jan24-en.xml
non-normative HTML presentation:
https://www.icann.org/sites/default/files/packages/lgr/lgr-second-level-tamil-full-variant-script-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-Taml]

ICANN, Root Zone Label Generation Rules for the Tamil Script (und-Taml), 26 May 2022 (XML)
https://www.icann.org/sites/default/files/lgr/rz-lgr-5-tamil-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 Tamil Script for the second level please see details in the Table of References below.

References [0] and [7] refer to the Unicode Standard versions in which the corresponding code points were initially encoded. References [101] and above correspond to sources given in [Proposal-Tamil] justifying the inclusion of 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	2
Implicit (except script)	4
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
C	Tag=Consonant	23	{0B95 0B99-0B9A 0B9C 0B9E-0B9F 0BA3-0BA4 0BA8-0BAA 0BAE-0BB9}		Any Tamil consonant
X	Tag=Visarga	1	{0B83}		The Tamil Visarga (aytham)
implicit	Tag=Common-digit	10	{0030-0039}		Any character tagged as Common-digit
implicit	Tag=Halant	1	{0BCD}		The character tagged as Halant
implicit	Tag=Matra	11	{0BBE-0BC2 0BC6-0BC8 0BCA-0BCC}		Any character tagged as Matra
implicit	Tag=Vowel	12	{0B85-0B8A 0B8E-0B90 0B92-0B94}		Any character tagged as Vowel
implicit	Tag=sc:Taml	48	{0B83 0B85-0B8A 0B8E-0B90 0B92-0B95 0B99-0B9A 0B9C 0B9E-0B9F 0BA3-0BA4 0BA8-0BAA 0BAE-0BB9 0BBE-0BC2 0BC6-0BC8 0BCA-0BCD}		Any character tagged as Tamil
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.

Whole label evaluation and context rules

Number of rules	5
Used to trigger actions	2
Used as context rule (C)	3
Anchored context rules	3

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 - ⍟
follows-C	([:C:])← ⚓︎		✔	C		Section 7, WLE 1, 2: H and M must be preceded by C
preceded-by-X	([:X:])← ⚓︎		✔	C		Section 7, WLE 3 , X cannot be preceded by X
no-mix-sri-shri	(\u0BB6\u0BCD\u0BB0\u0BC0.*\u0BB8\u0BCD\u0BB0\u0BC0)|(\u0BB8\u0BCD\u0BB0\u0BC0.*\u0BB6\u0BCD\u0BB0\u0BC0)	✔				Section 7: WLE 4: Two representations of ‘Shri’ cannot be mixed in the same label

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).

⍟ - 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 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 label matches	no-mix-sri-shri	→	invalid		do not mix two representations of ‘Shri’ in the same label
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: out-of-repertoire-var. This is not necessarily an error.