Unicode Standard Annex #14

Line Breaking Properties

Summary

This annex presents the specification of line breaking properties for Unicode characters as well as a default algorithm for determining line break opportunities. A model implementation using pair tables is also provided.

Contents

• 1  Overview and Scope
• 2  Definitions
• 3  Introduction
  • 3.1  Determining Line Break Opportunities
• 4  Conformance
  • 4.1  Line Breaking Algorithm
  • 4.2  Line Breaking Properties
  • 4.3  Higher-Level Protocols
• 5  Line Breaking Properties
  • 5.1  Description of Line Breaking Properties
  • 5.2  Dictionary Usage
  • 5.3  Use of Soft Hyphen
  • 5.4  Use of Double Hyphen
  • 5.5  Tibetan Line Breaking
• 6  Line Breaking Algorithm
  • 6.1  Non-tailorable Line Breaking Rules
  • 6.2  Tailorable Line Breaking Rules
• 7  Pair Table-Based Implementation
  • 7.1  Minimal Table
  • 7.2  Extended Context
  • 7.3  Example Pair Table
  • 7.4  Sample Code
  • 7.5  Combining Marks
  • 7.6  Conjoining Jamos
• 8  Customization
  • 8.1  Types of Tailoring
  • 8.2  Examples of Customization
• 9  Implementation Notes
  • 9.1  Combining Marks in Regular Expression-Based Implementations
  • 9.2  Legacy Support for Space Character as Base for Combining Marks

• References
• Acknowledgments
• Modifications

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1 Overview and Scope

The text of The Unicode Standard [Unicode] presents a limited description of some of the characters with specific functions in line breaking, but does not give a complete specification of line breaking behavior. This annex provides more detailed information about default line breaking behavior reflecting best practices for the support of multilingual texts.

For most Unicode characters, considerable variation in line breaking behavior can be expected, including variation based on local or stylistic preferences. For that reason, the line breaking properties provided for these characters are informative. Some characters are intended to explicitly influence line breaking. Their line breaking behavior is therefore expected to be identical across all implementations. The Unicode Standard assigns normative line breaking properties to those characters. The Unicode Line Breaking Algorithm is a tailorable set of rules that uses these line breaking properties in context to determine line break opportunities.

This annex opens with formal definitions, a summary of the line breaking task and a brief section on conformance requirements. Three main sections follow:

• Section 5, Line Breaking Properties, contains a narrative description of the line breaking behavior of the characters of the Unicode Standard, grouping them in alphabetical order by line breaking class.
• Section 6, Line Breaking Algorithm, provides a set of rules listed in order of precedence that constitute a line breaking algorithm.
• Section 7, Pair Table-Based Implementation, describes an efficient pair table-based implementation of the algorithm.

The final two sections discuss issues of customization and implementation.

• Section 8, Customization, provides a discussion of how to tailor the algorithm.
• Section 9, Implementation Notes, provides additional information to implementers using regular expression-based techniques or requiring legacy support for combining marks.

2 Definitions

All terms not defined here shall be as defined in the Unicode Standard [Unicode5.0]. The notation defined in this annex differs somewhat from the notation defined elsewhere in the Unicode Standard. All other notation used here without an explicit definition shall be as defined elsewhere in the Unicode Standard.

LD1   Line Fitting:  The process of determining how much text will fit on a line of text, given the available space between the margins and the actual display width of the text.

LD2  Line Break:  The position in the text where one line ends and the next one starts.

LD3  Line Break Opportunity:  A place where a line is allowed to end.

• Whether a given position in the text is a valid line break opportunity depends on the context as well as the line breaking rules in force.

LD4  Line Breaking:  The process of selecting one among several line break opportunities such that the resulting line is optimal or ends at a user-requested explicit line break.

LD5  Line Breaking Property: A character property with enumerated values, as listed in Table 1, and separated into normative and informative.

• Line breaking property values are used to classify characters and, taken in context, determine the type of break.

LD6  Line Breaking Class: A class of characters with the same line breaking property value.

The Line Breaking Classes are described in Section 5.1, Description of Line Breaking Properties.

LD7  Mandatory Break: A line must break following a character that has the mandatory break property.

Such a break is also known as a forced break and is indicated in the rules as B !, where B is the character with the mandatory break property.

LD8  Direct Break:  A line break opportunity exists between two adjacent characters of the given line breaking classes.

A direct break is indicated in the rules below as B ÷ A, where B is the character class of the character before and A is the character class of the character after the break. If they are separated by one or more space characters, a break opportunity also exists after the last space. In the pair table, the optional space characters are not shown.

LD9  Indirect Break:  A line break opportunity exists between two characters of the given line breaking classes only if they are separated by one or more spaces.

• For an indirect break, a break opportunity exists after the last space. No break opportunity exists if the characters are immediately adjacent.

An indirect break is indicated in the pair table in Table 2 as B % A, where B is the character class of the character before and A is the character class of the character after the break. Even though space characters are not shown in the pair table, an indirect break can occur only if one or more spaces follow B. In the notation of the rules in Section 6, Line Breaking Algorithm, this would be represented as two rules: B × A and B SP+ ÷ A.

LD10  Prohibited Break: No line break opportunity exists between two characters of the given line breaking classes, even if they are separated by one or more space characters.

A direct break is indicated in the pair table in Table 2 as B ^ A, where B is the character class of the character before and A is the character class of the character after the break, and the optional space characters are not shown. In the notation of the rules in Section 6, Line Breaking Algorithm, this would be expressed as a rule of the form: B SP* × A.

LD11  Hyphenation: Hyphenation uses language-specific rules to provide additional line break opportunities within a word.

• Hyphenation improves the layout of narrow columns, especially for languages with many longer words, such as German or Finnish. For the purpose of this annex, it is assumed that hyphenation is equivalent to inserting soft hyphen characters. All other aspects of hyphenation are outside the scope of this annex.

Table 1 provides a summary listing of all line breaking classes while Section 5.1, Description of Line Breaking Properties, provides a detailed description of each line breaking class, including detailed overview of the line breaking behavior for characters of that class.

Table 1. Line Breaking Classes (* = non-tailorable)

Class	Descriptive Name	Examples	Characters with This Property...
Non-tailorable Line Breaking Classes
BK *	Mandatory Break	NL, PS	Cause a line break (after)
CR *	Carriage Return	CR	Cause a line break (after), except between CR and LF
LF *	Line Feed	LF	Cause a line break (after)
CM *	Attached Characters and Combining Marks	Combining marks, control codes	Prohibit a line break between the character and the preceding character
NL *	Next Line	NEL	Cause a line break (after)
SG *	Surrogates	Surrogates	Should not occur in well-formed text
WJ *	Word Joiner	WJ	Prohibit line breaks before or after
ZW *	Zero Width Space	ZWSP	Provide a break opportunity
GL *	Non-breaking (“Glue”)	CGJ, NBSP, ZWNBSP	Prohibit line breaks before or after
SP *	Space	SPACE	Generally provide a line break opportunity after the character, enable indirect breaks
Break Opportunities
B2	Break Opportunity Before and After	Em dash	Provide a line break opportunity before and after the character
BA	Break Opportunity After	Spaces, hyphens	Generally provide a line break opportunity after the character
BB	Break Opportunity Before	Punctuation used in dictionaries	Generally provide a line break opportunity before the character
HY	Hyphen	HYPHEN-MINUS	Provide a line break opportunity after the character, except in numeric context
CB	Contingent Break Opportunity	Inline objects	Provide a line break opportunity contingent on additional information
Characters Prohibiting Certain Breaks
CL	Closing Punctuation	“)”, “]”, “}”, etc.	Prohibit line breaks before
EX	Exclamation/ Interrogation	“!”, “?”, etc.	Prohibit line breaks before
IN	Inseparable	Leaders	Allow only indirect line breaks between pairs
NS	Nonstarter	small kana	Allow only indirect line breaks before
OP	Opening Punctuation	“(“, “[“, “{“, etc.	Prohibit line breaks after
QU	Ambiguous Quotation	Quotation marks	Act like they are both opening and closing
Numeric Context
IS	Infix Separator (Numeric)	. ,	Prevent breaks after any and before numeric
NU	Numeric	Digits	Form numeric expressions for line breaking purposes
PO	Postfix (Numeric)	%, ¢	Do not break following a numeric expression
PR	Prefix (Numeric)	$, £, ¥, etc.	Do not break in front of a numeric expression
SY	Symbols Allowing Break After	/	Prevent a break before, and allow a break after
Other Characters
AI	Ambiguous (Alphabetic or Ideographic)	Characters with Ambiguous East Asian Width	Act like AL when the resolved EAW is N; otherwise, act as ID
AL	Ordinary Alphabetic and Symbol Characters	Alphabets and regular symbols	Are alphabetic characters or symbols that are used with alphabetic characters
H2	Hangul LV Syllable	Hangul	Form Korean syllable blocks
H3	Hangul LVT Syllable	Hangul	Form Korean syllable blocks
ID	Ideographic	Ideographs	Break before or after, except in some numeric context
JL	Hangul L Jamo	Conjoining jamo	Form Korean syllable blocks
JV	Hangul V Jamo	Conjoining jamo	Form Korean syllable blocks
JT	Hangul T Jamo	Conjoining jamo	Form Korean syllable blocks
SA	Complex Context Dependent (South East Asian)	South East Asian: Thai, Lao, Khmer	Provide a line break opportunity contingent on additional, language-specific context analysis
XX	Unknown	Unassigned, private-use	Have as yet unknown line breaking behavior or unassigned code positions

 

3 Introduction

Lines are broken as result of one of two conditions. The first condition is the presence of an explicit line breaking character. The second condition results from a formatting algorithm having selected among available line break opportunities; ideally the chosen line break results in the optimal layout of the text.

Different formatting algorithms may use different methods to determine an optimal line break. For example, simple implementations consider a single line at a time, trying to find a locally optimal line break. A basic, yet widely used approach is to allow no compression or expansion of the intercharacter and interword spaces and consider the longest line that fits. When compression or expansion is allowed, a locally optimal line break seeks to balance the relative merits of the resulting amounts of compression and expansion for different line break candidates.

When expanding or compressing interword space according to common typographical practice, only the spaces marked by U+0020 SPACE, U+00A0 NO-BREAK SPACE, and U+3000 IDEOGRAPHIC SPACE are subject to compression, and only spaces marked by U+0020 SPACE, U+00A0 NO-BREAK SPACE, and occasionally spaces marked by U+2009 THIN SPACE are subject to expansion. All other space characters normally have fixed width. When expanding or compressing intercharacter space, the presence of U+200B ZERO WIDTH SPACE or U+2060 WORD JOINER is always ignored.

Local custom or document style determines whether and to what degree expansion of intercharacter space is allowed in justifying a line. In languages, such as German, where intercharacter space is commonly used to mark  e m p h a s i s (like this), allowing variable intercharacter spacing would have the unintended effect of adding random emphasis, and should therefore be avoided.

In table headings that use Han ideographs, even extreme amounts of intercharacter space commonly occur as short texts are spread out across the entire available space to distribute the characters evenly from end to end.

More complex formatting algorithms may take into account the interaction of line breaking decisions for the whole paragraph. The well-known text layout system [T_EX] implements an example of such a globally optimal strategy that may make complex tradeoffs across an entire paragraph to avoid unnecessary hyphenation and other legal, but inferior breaks. For a description of this strategy, see [Knuth78].

The definition of optimal line breaks is outside the scope of this annex, as are methods for their selection. For the purpose of this annex, what is important is not so much what defines the optimal amount of text on the line, but how to determine all legal line break opportunities. Whether and how any given line break opportunity is actually used is up to the full layout system. Some layout systems will further evaluate the raw line break opportunities returned from the line breaking algorithm and apply additional rules. T_EX, for example, uses line break opportunities based on hyphens only as a last resort.

Finally, most text layout systems will support an emergency mode that handles the case of an unusual line that contains no ordinary line break opportunities. In such line layout emergencies, line breaks are placed with no regard to the ordinary line breaking behavior of the characters involved.

3.1 Determining Line Break Opportunities

Three principal styles of context analysis determine line break opportunities.

1. Western: spaces and hyphens are used to determine breaks
2. East Asian: lines can break anywhere, unless prohibited
3. South East Asian: line breaks require morphological analysis

The Western style is commonly used for scripts employing the space character. Hyphenation is often used with space-based line breaking to provide additional line break opportunities—however, it requires knowledge of the language and it may need user interaction or overrides.

The second style of context analysis is used with East Asian ideographic and syllabic scripts. In these scripts, lines can break anywhere, except before or after certain characters. The precise set of prohibited line breaks may depend on user preference or local custom and is commonly tailorable.

Korean makes use of both styles of line break. When Korean text is justified, the second style is commonly used, even for interspersed Latin letters. But when ragged margins are used, the Western style (relying on spaces) is commonly used instead, even for ideographs.

The third style is used for scripts such as Thai, which do not use spaces, but which restrict word breaks to syllable boundaries, the determination of which requires knowledge of the language comparable to that required by a hyphenation algorithm. Such an algorithm is beyond the scope of the Unicode Standard.

For multilingual text, the Western and East Asian styles can be unified into a single set of specifications, based on the information in this annex. Unicode characters have explicit line breaking properties assigned to them. These can be utilized with these two styles of context analysis for line break opportunities. Customization for user preferences or document style can then be achieved by tailoring that specification.

In bidirectional text, line breaks takes are determined before applying rule L1 of the Unicode Bidirectional Algorithm [Bidi]. However, line breaking is strictly independent of directional properties of the characters or of any auxiliary information determined by the application of rules of that algorithm.

4 Conformance

There is no single method for determining line breaks; the rules may differ based on user preference and document layout. Therefore the information in this annex, including the specification of the line breaking algorithm, must allow for the necessary flexibility in determining line breaks according to different conventions. However, some characters have been encoded explicitly for their effect on line breaking. Users adding such characters to a text expect that they will have the desired effect. For that reason, these characters have been given non-tailorable line breaking behavior. The conformance requirements are spelled out in the following subsections.

At times, this specification recommends best practice. These recommendations are not normative and conformance with this specification does not depend on their realization. These recommendations contain the expression “This specification recommends ...”, or some similar wording.

4.1 Line Breaking Algorithm

UAX14-C1. In the absence of a permissible higher-level protocol, a process that determines line breaks in Unicode text, and that purports to implement the Unicode Line Breaking Algorithm, shall do so in accordance with the specifications in this annex.

• As is the case for all other Unicode algorithms, this specification is a logical description—particular implementations can have more efficient mechanisms as long as they produce the same results. See C18 in Chapter 3, Conformance, of [Unicode], and the notes following.

• The line breaking algorithm specifies part of the intrinsic semantics of characters specifically encoded for their line breaking behavior and, therefore, is required for conformance to the Unicode Standard where text containing such characters is broken into lines.

UAX14-C2. The permissible higher-level protocols are described in Section 4.3, Higher-Level Protocols.

4.2 Line Breaking Properties

All line breaking classes are normative, but overridable, except for those line breaking classes marked with * in Table 1, which are not overridable.

4.3 Higher-Level Protocols

There are many different ways to break lines of text, and the Unicode Standard does not intend to unnecessarily restrict the ways in which implementations can do this. However, for characters that are encoded solely or primarily for their line breaking behavior, interpretation of these characters must be consistent with their semantics as defined by their normative line breaking behavior. This leads to the following permissible higher-level protocols:

UAX14-HL1.  Override rule 2 and report a break at the start of text.

• A higher-level protocol may report a break at the start of text (sot). As written, the rule is intended to ensure that the line breaking algorithm always produces lines that have at least one character in them. However, an analysis in terms of text boundaries would more naturally report a boundary at the sot, leaving it to any client to skip past that boundary in breaking lines.

UAX14-HL2.  Tailor any tailorable line break class.

• A higher-level protocol may change the algorithm to produce results as if the membership of any tailorable line break class had been changed.

UAX14-HL3. Override any rule in Section 6.2,Tailorable Line Breaking Rules, or add new rules to that section.

• A higher-level protocol my change the algorithm to produce results as if any of the rules in Section 6.2, Tailorable Line Breaking Rules, had been deleted or amended, or as if new rules had been added.

Because of the way the specification is set up, HL2 and HL3 have no effect on the results for text containing only characters of the non-tailorable line breaking classes. However, they allow for unrestricted tailoring of the results for texts containing only characters from the tailorable line breaking classes as well as wide latitude in defining the behavior of mixed texts.

5 Line Breaking Properties

This section provides detailed narrative descriptions of the line breaking behavior of many Unicode characters. In a few instances, the descriptions in this section provide additional detail about handling a given character at the end of a line, which goes beyond the simple determination of line breaks.

This section also summarizes the membership of character classes for each value of the line breaking property. Note that the mnemonic names for the line break classes are intended neither as exhaustive descriptions of their membership nor as indicators of their entire range of behaviors in the line breaking process. Instead, their main purpose is to serve as unique, yet broadly mnemonic labels. In other words, as long as their line break behavior is identical, otherwise unrelated characters will be found grouped together in the same line break class.

The classification by property values defined in this section and in the data file is used as input into two algorithms defined in Section 6, Line Breaking Algorithm, and Section 7, Pair Table-Based Implementation. These sections describe workable default line breaking methods. Section 8, Customization, discusses how the default line breaking behavior can be tailored to the needs of particular languages for particular document styles and user preferences.

Data File

The full classification of all Unicode characters by their line breaking properties is available in the file LineBreak.txt [Data14] in the Unicode Character Database [UCD]. This is a tab-delimited, two-column, plain text file, with code position and line breaking class. A comment at the end of each line indicates the character name. Ideographic, Hangul, Surrogate, and Private Use ranges are collapsed by giving a range in the first column.

Future Updates

As more scripts are added to the Unicode Standard and become more widely implemented and used on computers, more line breaking classes may be added or the assignment of line breaking class may be changed for some characters. Implementations should not make any assumptions to the contrary. Any future updates will be reflected in the latest version of the data file. (See the Unicode Character Database [UCD] for any specific version of the data file.)

5.1 Description of Line Breaking Properties

Line breaking classes are listed alphabetically. Each line breaking class is marked with an annotation in parentheses with the following meanings:

(A)—the class allows a break opportunity after in specified contexts

(XA)—the class prevents a break opportunity after in specified contexts

(B)—the class allows a break opportunity before in specified contexts

(XB)—the class prevents a break opportunity before in specified contexts

(P)—the class allows a break opportunity for a pair of same characters

(XP)—the class prevents a break opportunity for a pair of same characters

Note: The use of the letters B and A in these annotations marks the position of the break opportunity relative to the character. It is not to be confused with the use of the same letters in the other parts of this annex, where they indicate the positions of the characters relative to the break opportunity.

AI: Ambiguous (Alphabetic or Ideograph)

Some characters that ordinarily act like alphabetic or symbol characters (which have the AL line breaking class) are treated like ideographs (line breaking class ID) in certain East Asian legacy contexts. Their line breaking behavior therefore depends on the context. In the absence of appropriate context information, they are treated as class AL, but see the note at the end of this description.

As originally defined, the line break class AI contained all characters with East Asian Width property A (ambiguous width) that would otherwise be AL in this classification. They take the AL line breaking class only when their resolved width is N (narrow) and take the line breaking class ID when their resolved width is W (wide). For more information on East Asian Width and how to resolve it, see Unicode Standard Annex #11, East Asian Width [EAW].

The original definition included many Latin, Greek, and Cyrillic characters. These characters are now classified by default as AL because use of the AL line breaking class better corresponds to modern practice. Where strict compatibility with older legacy implementations is desired, some of these characters need to be treated as ID in certain contexts. This can be done by always tailoring them to ID or by continuing to classify them as AI and resolving them to ID where required.

As part of the same revision, the set of ambiguous characters has been extended to completely encompass the enclosed alphanumeric characters used for numbering of bullets.

As updated, the AI line breaking class includes all characters with East Asian Width A that are outside the range U+0000..U+1FFF, plus the following characters:

24EA	CIRCLED DIGIT ZERO
2780..2793	DINGBAT CIRCLED SANS-SERIF DIGIT ONE..DINGBAT NEGATIVE CIRCLED SANS-SERIF NUMBER TEN

The line breaking rules in Section 6, Line Breaking Algorithm, and the pair table in Section 7, Pair Table-Based Implementation, assume that all ambiguous characters have been resolved appropriately as part of assigning line breaking classes to the input characters.

Note: Normally characters with class AI are resolved to either ID or AL. However, the following two characters are used as punctuation marks in Spanish, where they would behave more like a character of class OP. Implementations might therefore wish to tailor these characters to class OP for use in Spanish.

00A1	INVERTED EXCLAMATION MARK
00BF	INVERTED QUESTION MARK

AL: Ordinary Alphabetic and Symbol Characters (XP)

Ordinary characters require other characters to provide break opportunities; otherwise, no line breaks are allowed between pairs of them. However, this behavior is tailorable. In some Far Eastern documents, it may be desirable to allow breaking between pairs of ordinary characters—particularly Latin characters and symbols.

Note: Use ZWSP as a manual override to provide break opportunities around alphabetic or symbol characters.

Except as listed explicitly below as part of another line breaking class, and except as assigned class AI or ID based on East Asian Width, this class contains the following characters:

ALPHABETIC—all remaining characters of General Categories Lu, Ll, Lt, Lm, and Lo
SYMBOLS—all remaining characters of General Categories Sm, Sk, and So
NON-DECIMAL NUMBERS—all remaining characters of General Categories Nl, and No
PUNCTUATION—all remaining characters of General Categories Pc, Pd, and Po

Plus these characters:

0600..0603	ARABIC NUMBER SIGN..ARABIC SIGN SAFHA
06DD	ARABIC END OF AYAH

070F	SYRIAC ABBREVIATION MARK
2061..2063	FUNCTION APPLICATION..INVISIBLE SEPARATOR

These characters occur in the middle or at the beginning of words or alphanumeric or symbol sequences. However, when alphabetic characters are tailored to allow breaks, these characters should not allow breaks after.

BA: Break Opportunity After (A)

Like SPACE, the characters in this class provide a break opportunity; unlike SPACE, they do not take part in determining indirect breaks. They can be subdivided into several categories.

Breaking Spaces

Breaking spaces are the following subset of characters with General_Category Zs:

1680	OGHAM SPACE MARK

2000	EN QUAD
2001	EM QUAD
2002	EN SPACE
2003	EM SPACE
2004	THREE-PER-EM SPACE
2005	FOUR-PER-EM SPACE
2006	SIX-PER-EM SPACE
2008	PUNCTUATION SPACE
2009	THIN SPACE
200A	HAIR SPACE
205F	MEDIUM MATHEMATICAL SPACE

All of these space characters have a specific width, but otherwise behave as breaking spaces. In setting a justified line, none of these spaces normally changes in width, except for THIN SPACE when used in mathematical notation. See also the SP property. 

The Ogham space mark is rendered visibly between words but should be elided at the end of a line.

See the ID property for U+3000 IDEOGRAPHIC SPACE. For a list of all space characters in the Unicode Standard, see Section 6.2, General Punctuation, in [Unicode5.0].

Tabs

0009

TAB

Except for the effect of the location of the tab stops, the tab character acts similarly to a space for the purpose of line breaking.

Conditional Hyphens

00AD	SOFT HYPHEN (SHY)

SHY marks an optional place where a line break may occur inside a word. It can be used with all scripts. SHY is rendered invisibly and has no width: it merely indicates an optional line break. The rendering of the optional line break depends on the script. For the Latin script, rendering the line break typically means displaying a hyphen at the end of the line; however, some languages require a change in spelling surrounding a line break. For examples, see Section 5.3, Use of Soft Hyphen.

Breaking Hyphens

Breaking hyphens establish explicit break opportunities immediately after each occurrence.

058A	ARMENIAN HYPHEN
2010	HYPHEN
2012	FIGURE DASH
2013	EN DASH

Hyphens are graphic characters with width. Because, unlike spaces, they print, they are included in the measured part of the preceding line, except where the layout style allows hyphens to hang into the margins.

Visible Word Dividers

The following are other forms of visible word dividers that provide break opportunities:

05BE	HEBREW PUNCTUATION MAQAF
0F0B	TIBETAN MARK INTERSYLLABIC TSHEG
1361	ETHIOPIC WORDSPACE
17D5	KHMER SIGN BARIYOOSAN
10100	AEGEAN WORD SEPARATOR LINE
10101	AEGEAN WORD SEPARATOR DOT
10102	AEGEAN CHECK MARK
1039F	UGARITIC WORD DIVIDER
103D0	OLD PERSIAN WORD DIVIDER
12470	CUNEIFORM PUNCTUATION SIGN OLD ASSYRIAN WORD DIVIDER

The Tibetan tsheg is a visible mark, but it functions effectively like a space to separate words (or other units) in Tibetan. It provides a break opportunity after itself. For additional information, see Section 5.5, Tibetan Line Breaking.

The ethiopic word space is a visible word delimiter and is kept on the previous line. In contrast, U+1360 ETHIOPIC SECTION MARK is typically used in a sequence of several such marks on a separate line, and separated by spaces. As such lines are typically marked with separate hard line breaks (BK), the section mark is treated like an ordinary symbol and given line break class AL.

2027	HYPHENATION POINT

A hyphenation point is a raised dot, which is mainly used in dictionaries and similar works to visibly indicate syllabification of words. Syllable breaks are potential line break opportunities in the middle of words. When an actual line break falls inside a word containing hyphenation point characters, the hyphenation point is rendered as a regular hyphen at the end of the line.

007C	VERTICAL LINE

In some dictionaries, a vertical bar is used instead of a hyphenation point. In this usage, U+0323 COMBINING DOT BELOW is used to mark stressed syllables, so all breaks are marked by the vertical bar. For an actual break opportunity, the vertical bar is rendered as a hyphen.

Word Separators

Historic texts, especially ancient ones, often do not use spaces, even for scripts where modern use of spaces is standard. Special punctuation was used to mark word boundaries in such texts. For modern text processing these should be treated as line break opportunities by default. WJ can be used to override this default, where necessary.

16EB	RUNIC SINGLE DOT PUNCTUATION
16EC	RUNIC MULTIPLE DOT PUNCTUATION
16ED	RUNIC CROSS PUNCTUATION
2056	THREE DOT PUNCTUATION
2058	FOUR DOT PUNCTUATION
2059	FIVE DOT PUNCTUATION
205A	TWO DOT PUNCTUATION
205B	FOUR DOT MARK
205D	TRICOLON
205E	VERTICAL FOUR DOTS

Dandas

DEVANAGARI DANDA is similar to a full stop. The danda or historically related symbols are used with several other Indic scripts. Unlike a full stop, the danda is not used in number formatting. DEVANAGARI DOUBLE DANDA marks the end of a verse. It also has analogues in other scripts.

0964	DEVANAGARI DANDA
0965	DEVANAGARI DOUBLE DANDA
0E5A	THAI CHARACTER ANGKHANKHU
104A	MYANMAR SIGN LITTLE SECTION
104B	MYANMAR SIGN SECTION
1735	PHILIPPINE SINGLE PUNCTUATION
1736	PHILIPPINE DOUBLE PUNCTUATION
17D4	KHMER SIGN KHAN
17D5	KHMER SIGN BARIYOOSAN
17D8	KHMER SIGN BEYYAL
17DA	KHMER SIGN KOOMUUT
10A56	KHAROSHTHI PUNCTUATION DANDA
10A57	KHAROSHTHI PUNCTUATION DOUBLE DANDA

Tibetan

0F85	TIBETAN MARK PALUTA
0F34	TIBETAN MARK BSDUS RTAGS
0F7F	TIBETAN SIGN RNAM BCAD
0FBE	TIBETAN KU RU KHA
0FBF	TIBETAN KU RU KHA BZHI MIG CAN

For additional information, see Section 5.5, Tibetan Line Breaking.

Other Terminating Punctuation

Termination punctuation stays with the line, but otherwise allows a break after it. This is similar to EX, except that the latter may be separated by a space from the preceding word without allowing a break, whereas these marks are used without spaces.

1802	MONGOLIAN COMMA
1803	MONGOLIAN FULL STOP
1804	MONGOLIAN COLON
1805	MONGOLIAN FOUR DOTS
1808	MONGOLIAN MANCHU COMMA
1809	MONGOLIAN MANCHU FULL STOP
1A1E	BUGINESE PALLAWA
2CF9	COPTIC OLD NUBIAN FULL STOP
2CFA	COPTIC OLD NUBIAN DIRECT QUESTION MARK
2CFB	COPTIC OLD NUBIAN INDIRECT QUESTION MARK
2CFC	COPTIC OLD NUBIAN VERSE DIVIDER
2CFE	COPTIC FULL STOP
2CFF	COPTIC MORPHOLOGICAL DIVIDER
10A50	KHAROSHTHI PUNCTUATION DOT
10A51	KHAROSHTHI PUNCTUATION SMALL CIRCLE
10A52	KHAROSHTHI PUNCTUATION CIRCLE
10A53	KHAROSHTHI PUNCTUATION CRESCENT BAR
10A54	KHAROSHTHI PUNCTUATION MANGALAM
10A55	KHAROSHTHI PUNCTUATION LOTUS

BB: Break Opportunities Before (B)

Characters of this line break class move to the next line at a line break and thus provide a line break opportunity before.

Dictionary Use

00B4	ACUTE ACCENT

In some dictionaries, stressed syllables are indicated with a spacing acute accent instead of the hyphenation point. In this case the accent moves to the next line, and the preceding line ends with a hyphen.

02C8	MODIFIER LETTER VERTICAL LINE
02CC	MODIFIER LETTER LOW VERTICAL LINE

These characters are used in dictionaries to indicate stress and secondary stress when IPA is used. Both are prefixes to the stressed syllable in IPA. Breaking before them keeps them with the syllable.

Note: It is hard to find actual examples in most dictionaries because the pronunciation fields usually occur right after the headword, and the columns are wide enough to prevent line breaks in most pronunciations.

Tibetan Head Letters

0F01	TIBETAN MARK GTER YIG MGO TRUNCATED A
0F02	TIBETAN MARK GTER YIG MGO -UM RNAM BCAD MA
0F03	TIBETAN MARK GTER YIG MGO -UM GTER TSHEG MA
0F04	TIBETAN MARK INITIAL YIG MGO MDUN MA
0F06	TIBETAN MARK CARET YIG MGO PHUR SHAD MA
0F07	TIBETAN MARK YIG MGO TSHEG SHAD MA
0F09	TIBETAN MARK BSKUR YIG MGO
0F0A	TIBETAN MARK BKA- SHOG YIG MGO
0FD0	TIBETAN MARK BSKA- SHOG GI MGO RGYAN
0FD1	TIBETAN MARK MNYAM YIG GI MGO RGYAN

Tibetan head letters allow a break before. For more information, see Section 5.5, Tibetan Line Breaking.

Mongolian

1806	MONGOLIAN TODO SOFT HYPHEN

Despite its name, this Mongolian character is not an invisible control like SOFT HYPHEN, but rather a visible character like a regular hyphen. Unlike the hyphen, MONGOLIAN TODO SOFT HYPHEN stays with the following line.  Whenever optional line breaks are to be marked, SOFT HYPHEN should be used instead.

B2: Break Opportunity Before and After (B/A/XP)

2014

EM DASH

The EM DASH is used to set off parenthetical text. Normally, it is used without spaces. However, this is language dependent. For example, in Swedish, spaces are used around the EM DASH. Line breaks can occur before and after an EM DASH, but not between a pair of them. Such pairs are sometimes used instead of a single quotation dash. For that reason, the line should not be broken between EM DASHes even though not all fonts use connecting glyphs for the EM DASH.

BK: Mandatory Break (A) (Non-tailorable)

Explicit breaks act independently of the surrounding characters. No characters can be added to the BK class as part of tailoring, but implementations are not required to support the VT character.

000C	FORM FEED (FF)
000B	LINE TABULATION (VT)

FORM FEED separates pages. The text on the new page starts at the beginning of the line. No paragraph formatting is applied.

2028	LINE SEPARATOR (LS)

The text after the Line Separator starts at the beginning of the line. No paragraph formatting is applied. This is similar to HTML <BR>.

2029	PARAGRAPH SEPARATOR (PS)

The text of the new paragraph starts at the beginning of the line. Paragraph formatting is applied.

Newline Function (NLF)

Newline Functions are defined in the Unicode Standard as providing additional explicit breaks. They are not individual characters, but are encoded as sequences of the control characters NEL, LF, and CR. If a character sequence for a Newline Function contains more than one character, it is kept together. The particular sequences that form an NLF depend on the implementation and other circumstances as described in Section 5.8, Newline Guidelines, of [Unicode5.0].

This specification defines the NLF implicitly. It defines the three character classes CR, LF, and NL. Their line break behavior, defined in rule LB5 in Section 6.1, Non-tailorable Line Breaking Rules, is to break after NL, LF, or CR, but not between CR and LF.

CB: Contingent Break Opportunity (B/A)

By default, there is a break opportunity both before and after any inline object. Object-specific line breaking behavior is implemented in the associated object itself, and where available can override the default to prevent either or both of the default break opportunities. Using U+FFFC OBJECT REPLACEMENT CHARACTER allows the object anchor to take a character position in the string.

FFFC	OBJECT REPLACEMENT CHARACTER

Object-specific line break behavior is best implemented by querying the object itself, not by replacing the CB line breaking class by another class.

CL: Closing Punctuation (XB)

The closing character of any set of paired punctuation must be kept with the preceding character, and the same applies to all forms of wide comma and full stop. This line break class contains the following characters plus any characters of General_Category Pe in the Unicode Character Database.

3001..3002	IDEOGRAPHIC COMMA..IDEOGRAPHIC FULL STOP
FE11	PRESENTATION FORM FOR VERTICAL IDEOGRAPHIC COMMA
FE12	PRESENTATION FORM FOR VERTICAL IDEOGRAPHIC FULL STOP
FE50	SMALL COMMA
FE52	SMALL FULL STOP
FF0C	FULLWIDTH COMMA
FF0E	FULLWIDTH FULL STOP
FF61	HALFWIDTH IDEOGRAPHIC FULL STOP
FF64	HALFWIDTH IDEOGRAPHIC COMMA

CM: Attached Characters and Combining Marks (XB)

Combining Characters

Combining character sequences are treated as units for the purpose of line breaking. The line breaking behavior of the sequence is that of the base character.

The preferred base character for showing combining marks in isolation is U+00A0 No-Break SPACE.  If a line break before or after the combining sequence is desired, U+200B ZERO WIDTH SPACE can be used. The use of U+0020 SPACE as a base character is deprecated.

For most purposes, combining characters take on the properties of their base characters, and that is how the CM class is treated in rule LB9 of this specification. As a result, if the sequence <0021, 20E4> is used to represent a triangle enclosing an exclamation point, it is effectively treated as EX, the line break class of the exclamation mark. If U+2061 CAUTION SIGN had been used, which also looks like an exclamation point inside a triangle, it would have the line break class of AL. Only the latter corresponds to the line breaking behavior expected by users for this symbol. To avoid surprising behavior, always use a base character that is a symbol when using enclosing combining marks (General_Category Me).

The CM line break class includes all combining characters with General_Category Mc, Me, and Mn, unless listed explicitly elsewhere. This includes viramas.

Control and Formatting Characters

Most control and formatting characters are ignored in line breaking and do not contribute to the line width. By giving them class CM, the line breaking behavior of the last preceding character that is not of class CM affects the line breaking behavior. 

Note: When control codes and format characters are rendered visibly during editing, more graceful layout might be achieved by assigning them the AL or ID class instead. 

The CM line break class includes all characters of General_Category Cc and Cf, unless listed explicitly elsewhere.

CR: Carriage Return (A) (Non-tailorable)

000D	CARRIAGE RETURN (CR)

A CR indicates a mandatory break after, unless followed by a LF. See also the discussion under BK.

Note: On some platforms the character sequence <CR, CR, LF> is used to indicate the location of actual line breaks, whereas <CR, LF> is treated like a hard line break. As soon as a user edits the text, the location of all the <CR, CR, LF> sequences may change as the new text breaks differently, while the relative position of any <CR, LF> to the surrounding text stays the same. This convention allows an editor to return a buffer and the client to tell which text is displayed on which line by counting the number of <CR, CR, LF> and <CR, LF> sequences.

EX: Exclamation/Interrogation (XB)

Characters in this line break class behave like closing characters, except in relation to postfix (PO) and non-starter characters (NS).

0021	EXCLAMATION MARK
003F	QUESTION MARK
05C6	HEBREW PUNCTUATION NUN HAFUKHA
060C	ARABIC COMMA
061B	ARABIC SEMICOLON
061E	ARABIC TRIPLE DOT PUNCTUATION MARK
061F	ARABIC QUESTION MARK
066A	ARABIC PERCENT SIGN
06D4	ARABIC FULL STOP
07F9	NKO EXCLAMATION MARK
0F0D	TIBETAN MARK SHAD
0F0E	TIBETAN MARK NYIS SHAD
0F0F	TIBETAN MARK TSHEG SHAD
0F10	TIBETAN MARK NYIS TSHEG SHAD
0F11	TIBETAN MARK RIN CHEN SPUNGS SHAD
0F14	TIBETAN MARK GTER TSHEG
1944	LIMBU EXCLAMATION MARK
1945	LIMBU QUESTION MARK
2762	HEAVY EXCLAMATION MARK ORNAMENT
2763	HEAVY HEART EXCLAMATION MARK ORNAMENT
A876	PHAGS-PA MARK SHAD
A877	PHAGS-PA MARK DOUBLE SHAD
FE15	PRESENTATION FORM FOR VERTICAL EXCLAMATION MARK
FE16	PRESENTATION FORM FOR VERTICAL QUESTION MARK
FE56..FE57	SMALL QUESTION MARK..SMALL EXCLAMATION MARK
FF01	FULLWIDTH EXCLAMATION MARK
FF1F	FULLWIDTH QUESTION MARK

GL: Non-breaking (“Glue”) (XB/XA) (Non-tailorable)

Non-breaking characters prohibit breaks on either side, but that prohibition can be overridden by SP or ZW. In particular, when NBSP follows SPACE, there is a break opportunity after the SPACE and NBSP will go as visible space onto the next line. See also WJ. The following lists the characters of line break class GL with additional description.

00A0	NO-BREAK SPACE (NBSP)
202F	NARROW NO-BREAK SPACE (NNBSP)
180E	MONGOLIAN VOWEL SEPARATOR (MVS)

NO-BREAK SPACE is the preferred character to use where two words should be visually separated but kept on the same line, as in the case of a title and a name “Dr.<NBSP>Joseph Becker”. When SPACE follows NBSP, there is no break, because there never is a break in front of SPACE.  NARROW NO-BREAK SPACE is used in Mongolian. The mongolian vowel separator acts like a NNBSP in its line breaking behavior. It additionally affects the shaping of certain vowel characters as described in Section 13.2, Mongolian, of [Unicode5.0].

034F	COMBINING GRAPHEME JOINER

This character has no visible glyph and its presence indicates that adjoining characters are to be treated as a graphemic unit, therefore preventing line breaks between them.

2007	FIGURE SPACE

This is the preferred space to use in numbers. It has the same width as a digit and keeps the number together for the purpose of line breaking.

2011	NON-BREAKING HYPHEN (NBHY)

This is the preferred character to use where words must be hyphenated but may not be broken at the hyphen.

0F08	TIBETAN MARK SBRUL SHAD
0F0C	TIBETAN MARK DELIMITER TSHEG BSTAR
0F12	TIBETAN MARK RGYA GRAM SHAD

The TSHEG BstAR looks exactly like a Tibetan tsheg, but can be used to prevent a break like no-break space. It inhibits breaking on either side. For more information, see Section 5.5, Tibetan Line Breaking.

035C..0362

COMBINING DOUBLE BREVE BELOW..COMBINING DOUBLE RIGHTWARDS ARROW BELOW

These diacritics span two characters, so no word or line breaks are possible on either side.

H2: Hangul LV Syllable (B/A)

This class includes all characters of Hangul Syllable Type LV.

Together with conjoining jamos, Hangul syllables form Korean Syllable Blocks, which are kept together; see [Boundaries]. Korean uses space-based line breaking in many styles of documents. To support these, Hangul syllables and conjoining jamo need to be tailored to use class AL, while the default in this specification is class ID, which supports the case of Korean documents not using space-based line breaking. See Section 8.1, Types of Tailoring. See also JL, JT, JV, and H3.

H3: Hangul LVT Syllable (B/A)

This class includes all characters of Hangul Syllable Type LVT. See also JL, JT, JV, and H2.

HY: Hyphen (XA)

002D	HYPHEN-MINUS

Some additional context analysis is required to distinguish usage of this character as a hyphen from its usage as a minus sign (or indicator of numerical range). If used as hyphen, it acts like hyphen.

Note: Some typescript conventions use runs of HYPHEN-MINUS to stand in for longer dashes or horizontal rules. If actual character code conversion is not performed and it is desired to treat them like the characters or layout elements they stand for, line breaking needs to support these runs explicitly.

ID: Ideographic (B/A)

Note: This class includes characters other than Han ideographs.

Characters with this property do not require other characters to provide break opportunities; lines can ordinarily break before and after and between pairs of ideographic characters. The ID line break class consists of the following characters:

2E80..2FFF	CJK, KANGXI RADICALS, DESCRIPTION SYMBOLS
3000	IDEOGRAPHIC SPACE
3040..309F	Hiragana (except small characters)
30A0..30FF	Katakana (except small characters)
3400..4DBF	CJK UNIFIED IDEOGRAPHS EXTENSION A
4E00..9FAF	CJK UNIFIED IDEOGRAPHS
F900..FAFF	CJK COMPATIBILITY IDEOGRAPHS
A000..A48F	YI SYLLABLES
A490..A4CF	YI RADICALS
FE62..FE66	SMALL PLUS SIGN to SMALL EQUALS SIGN
FF10..FF19	WIDE DIGITS
20000..2A6D6	CJK UNIFIED IDEOGRAPHS EXTENSION B
2F800..2FA1D	CJK COMPATIBILITY IDEOGRAPHS SUPPLEMENT

It also includes all of the FULLWIDTH LATIN letters and all of the blocks in the range 3000..33FF not covered elsewhere.

Note: Use U+2060 WORD JOINER as a manual override to prevent break opportunities around characters of class ID.

U+3000 IDEOGRAPHIC SPACE may be subject to expansion or compression during line justification.

Korean

Korean is encoded with conjoining jamo, Hangul syllables, or both. See also JL, JT, JV, H2, and H3. The following set of compatibility jamo is treated as ID by default.

3130..318F

HANGUL COMPATIBILITY JAMO

IN: Inseparable Characters (XP)

Leaders

These characters are intended to be used in consecutive sequence. There is never a line break between two character of this class.

2024	ONE DOT LEADER
2025	TWO DOT LEADER
2026	HORIZONTAL ELLIPSIS
FE19	PRESENTATION FORM FOR VERTICAL HORIZONTAL ELLIPSIS

Horizontal ellipsis can be used as a three-dot leader.

IS: Numeric Separator (Infix) (XB)

Characters that usually occur inside a numerical expression may not be separated from the numeric characters that follow, unless a space character intervenes. For example, there is no break in “100.00” or “10,000”, nor in “12:59”.

002C	COMMA
002E	FULL STOP
003A	COLON
003B	SEMICOLON
037E	GREEK QUESTION MARK (canonically equivalent to 003B)
0589	ARMENIAN FULL STOP
060D	ARABIC DATE SEPARATOR
07F8	NKO COMMA
2044	FRACTION SLASH
FE10	PRESENTATION FORM FOR VERTICAL COMMA
FE13	PRESENTATION FORM FOR VERTICAL COLON
FE14	PRESENTATION FORM FOR VERTICAL SEMICOLON

When not used in a numeric context, infix separators are sentence-ending punctuation. Therefore they always prevent breaks before.

JL: Hangul L Jamo (B)

The JL line break class consists of all characters of Hangul Syllable Type L.

Conjoining jamos form Korean Syllable Blocks, which are kept together; see [Boundaries]. Korean uses space-based line breaking in many styles of documents. To support these, Hangul syllables and conjoining jamo need to be tailored to use class AL, while the default in this specification is class ID, which supports the case of Korean documents not using space-based line breaking. See Section 8.1, Types of Tailoring. See also JT, JV, H2, and H3.

JT: Hangul T Jamo (A)

The JT line break class consists of all characters of Hangul Syllable Type T. See also JL, JV, H2, and H3.

JV: Hangul V Jamo (XA/XB)

The JV line break class consists of all characters of Hangul Syllable Type V. See also JL, JT, H2, and H3.

LF: Line Feed (A) (Non-tailorable)

000A	LINE FEED (LF)

There is a mandatory break after any LF character, but see the discussion under BK.

NL: Next Line (A) (Non-tailorable) 

0085	NEXT LINE (NEL)

The NL class acts like BK in all respects (there is a mandatory break after any NEL character). It cannot be tailored, but implementations are not required to support the NEL character; see the discussion under BK.

NS: Nonstarters (XB)

Nonstarter characters cannot start a line, but unlike CL they may allow a break in some context when they follow one or more space characters. Nonstarters include

0E5A..0E5B	THAI CHARACTER ANGKHANKHU..THAI CHARACTER KHOMUT
17D4	KHMER SIGN KHAN
17D6	KHMER SIGN CAMNUC PII KUUH
203C	DOUBLE EXCLAMATION MARK
203D	INTERROBANG
2047	DOUBLE QUESTION MARK
2048	QUESTION EXCLAMATION MARK
2049	EXCLAMATION QUESTION MARK
3005	IDEOGRAPHIC ITERATION MARK
301C	WAVE DASH
303C	MASU MARK
303B	VERTICAL IDEOGRAPHIC ITERATION MARK
309B.. 309E	KATAKANA-HIRAGANA VOICED SOUND MARK..HIRAGANA VOICED ITERATION MARK
30A0	KATAKANA-HIRAGANA DOUBLE HYPHEN
30FB..30FE	KATAKANA MIDDLE DOT..KATAKANA VOICED ITERATION MARK
A015	YI SYLLABLE WU
FE54..FE55	SMALL SEMICOLON..SMALL COLON
FF1A..FF1B	FULLWIDTH COLON.. FULLWIDTH SEMICOLON
FF65	HALFWIDTH KATAKANA MIDDLE DOT
FF70	HALFWIDTH KATAKANA-HIRAGANA PROLONGED SOUND MARK
FF9E..FF9F	HALFWIDTH KATAKANA VOICED SOUND MARK..HALFWIDTH KATAKANA SEMI-VOICED SOUND MARK

plus all Hiragana, Katakana, and Halfwidth Katakana “small” characters.

Note: Optionally, the NS restriction may be relaxed and characters treated like ID to achieve a more permissive style of line breaking, especially in some East Asian document styles.

NU: Numeric (XP)

These characters behave like ordinary characters in the context of ordinary characters but activate the prefix and postfix behavior of prefix and postfix characters.

Numeric characters consist of decimal digits (all characters of General_Category Nd), except those with East_Asian_Width F (Fullwidth), plus these characters:

066B	ARABIC DECIMAL SEPARATOR
066C	ARABIC THOUSANDS SEPARATOR

Unlike with IS, the Arabic numeric punctuation does not occur as sentence terminal punctuation outside numbers.

OP: Opening Punctuation (XA)

The opening character of any set of paired punctuation must be kept with the following character. The OP line break class consists of all characters of General_Category Ps in the Unicode Character Database.

PO: Postfix (Numeric) (XB)

Characters that usually follow a numerical expression may not be separated from preceding numeric characters or preceding closing characters, even if one or more space characters intervene. For example, there is no break in “(12.00) %”.

Some of these characters—in particular, degree sign and percent sign—can appear on both sides of a numeric expression. Therefore the line breaking algorithm by default does not break between PO and numbers or letters on either side.

The list of postfix characters is

0025	PERCENT SIGN
00A2	CENT SIGN
00B0	DEGREE SIGN
060B	AFGHANI SIGN
20300	PER MILLE SIGN
2031	PER TEN THOUSAND SIGN
2032..2037	PRIME..REVERSED TRIPLE PRIME
20A7	PESETA SIGN
2103	DEGREE CELSIUS
2109	DEGREE FAHRENHEIT
FDFC	RIAL SIGN
FE6A	SMALL PERCENT SIGN
FF05	FULLWIDTH PERCENT SIGN
FFE0	FULLWIDTH CENT SIGN

Alphabetic characters are also widely used as unit designators in a postfix position. For purposes of line breaking, their classification as alphabetic is sufficient to keep them together with the preceding number.

PR: Prefix (Numeric) (XA)

Characters that usually precede a numerical expression may not be separated from following numeric characters or following opening characters, even if a space character intervenes. For example, there is no break in “$ (100.00)”.

Many currency signs can appear on both sides, or even the middle, of a numeric expression. Therefore the line breaking algorithm, by default, does not break between PR and numbers or letters on either side.

The PR line break class consists of all currency symbols (General_Category Sc) except as listed explicitly in PO, as well as the following:

002B	PLUS SIGN
005C	REVERSE SOLIDUS
00B1	PLUS-MINUS
2116	NUMERO SIGN
2212	MINUS SIGN
2213	MINUS-OR-PLUS-SIGN

Note: Many currency symbols may be used either as prefix or as postfix, depending on local convention. For details on the conventions used, see [CLDR].

QU: Ambiguous Quotation (XB/XA)

Some paired characters can be either opening or closing depending on usage. The default is to treat them as both opening and closing.

Note: If language information is available, it can be used to determine which character is used as the opening quote and which as the closing quote. See the information in Section 6.2, General Punctuation, in [Unicode5.0].

The QU line break class consists of characters of General_Category Pf or Pi in the Unicode Character Database as well as

0022	QUOTATION MARK
0027	APOSTROPHE
275B	HEAVY SINGLE TURNED COMMA QUOTATION MARK ORNAMENT
275C	HEAVY SINGLE COMMA QUOTATION MARK ORNAMENT
275D	HEAVY DOUBLE TURNED COMMA QUOTATION MARK ORNAMENT
275E	HEAVY DOUBLE COMMA QUOTATION MARK ORNAMENT

U+23B6 BOTTOM SQUARE BRACKET OVER TOP SQUARE BRACKET is subtly different from the others in this class, in that it is both an opening and a closing punctuation character at the same time. However, its use is limited to certain vertical text modes in terminal emulation. Instead of creating a one-of-a-kind class for this rarely used character, assigning it to the QU class approximates the intended behavior.

SA: Complex-Context Dependent (South East Asian) (P)

Runs of these characters require morphological analysis to determine break opportunities. This is similar to, for example, a hyphenation algorithm. For the characters that have this property, no line breaks will be found otherwise. Therefore complex context analysis, often involving dictionary lookup of some form, is required to determine non-emergency line breaks.

If such analysis is not available, they should be treated as AL.

Note: These characters can be mapped into their equivalent line breaking classes as the result of dictionary lookup, thus permitting a logical separation of this algorithm from the morphological analysis.

The class SA consists of all characters of General_Category Cf, Lo, Lm, Mn, or Mc in the following ranges, except as noted elsewhere:

0E00..0E7F	Thai
0E80..0EFF	Lao
1000..109F	Myanmar
1780..17FF	Khmer
1950..197F	Tai Le
1980..19DF	New Tai Lue

SG: Surrogates (XP) (Non-tailorable)

Line break class SG comprises all code points with General_Category Cs. The line breaking behavior of isolated surrogates is undefined. 

Note: The use of this line breaking class is deprecated. It was of limited usefulness for UTF-16 implementations that do not support characters beyond the BMP. The correct implementation is to resolve a pair of surrogates into a supplementary character before line breaking. 

SP: Space (A) (Non-tailorable)

The space characters are explicit break opportunities; however, spaces at the end of a line are not measured for fit. If there is a sequence of space characters, and breaking after any of the space characters would result in the same visible line, then the line breaking position after the last space character in the sequence is the locally most optimal one. In other words, because the last character measured for fit is before the space character, any number of space characters are kept together invisibly on the previous line and the first non-space character starts the next line.

0020	SPACE (SP)

Note: By default, SPACE, but none of the other breaking spaces, is used in determining an indirect break. For other breaking space characters, see BA.

SY: Symbols Allowing Break After (A)

URLs are now so common in regular plain text that they must be taken into account when assigning general-purpose line breaking properties. The SY line breaking property is intended to provide a break after, but not in front of, digits so as to not break “1/2” or “06/07/99”.

002F

SOLIDUS

Slash (solidus) is allowed as an additional, limited break opportunity to improve layout of Web addresses. As a side effect, some common abbreviations such as “w/o” or “A/S”, which normally would not be broken, acquire a line break opportunity. The recommendation in this case is for the layout system not to utilize a line break opportunity allowed by SY unless the distance between it and the next line break opportunity exceeds an implementation-defined minimal distance.

Note: Normally, symbols are treated as AL. However, symbols can be added to this line breaking class or classes BA, BB, and B2 by tailoring. This can be used to allow additional line breaks—for example, after “=”. Mathematics requires additional specifications for line breaking, which are outside the scope of this annex.

WJ: Word Joiner (XB/XA) (Non-tailorable)

These characters glue together left and right neighbor characters such that they are kept on the same line.

2060	WORD JOINER (WJ)
FEFF	ZERO WIDTH NO-BREAK SPACE (ZWNBSP)

The word joiner character is the preferred choice for an invisible character to keep other characters together that would otherwise be split across the line at a direct break. The character FEFF has the same effect, but because it is also used in an unrelated way as a byte order mark, the use of the WJ as the preferred interword glue simplifies the handling of FEFF.

By definition, WJ and ZWNBSP take precedence over the action of SP, but not ZW.

XX: Unknown (XP)

The XX line break class consists of all characters with General_Category Co and all code points with General_Category Cn.

Unassigned code positions, private-use characters, and characters for which reliable line breaking information is not available are assigned this default line breaking property. The default behavior for this class is identical to class AL. Users can manually insert ZWSP or word joiner around characters of class XX to allow or prevent breaks as needed. 

In addition, implementations can override or tailor this default behavior—for example, by assigning characters the property ID or another class. Doing so may give better default behavior for their users. There are other possible means of determining the desired behavior of private-use characters. For example, one implementation might treat any private-use character in ideographic context as ID, while another implementation might support a method for assigning specific properties to specific definitions of private-use characters. The details of such use of private-use characters are outside the scope of this standard.

For supplementary characters, a useful default is to treat characters in the range 10000..1FFFD as AL and characters in the ranges 20000..2FFFD and 30000..3FFFD as ID, until the implementation can be revised to take into account the actual line breaking properties for these characters.

For more information on handling default property values for unassigned characters, see the discussion on default property values in Section 5.3, Unknown and Missing Characters, of [Unicode5.0].

The line breaking rules in Section 6, Line Breaking Algorithm, and the pair table in Section 7, Pair Table-Based Implementation, assume that all unknown characters have been assigned one of the other line breaking classes, such as AL, as part of assigning line breaking classes to the input characters.

ZW: Zero Width Space (A) (Non-tailorable)

200B	ZERO WIDTH SPACE (ZWSP)

This character is used to enable additional (invisible) break opportunities wherever SPACE cannot be used. As its name implies, it normally has no width. However, its presence between two characters does not prevent increased letter spacing in justification.

5.2 Dictionary Usage

Dictionaries follow specific conventions that guide their use of special characters to indicate features of the terms they list. Marks used for some of these conventions may occur near line break opportunities and therefore interact with line breaking. For example, in one dictionary a natural hyphen in a word becomes a tilde dash when the word is split.

Examples of conventions used in several dictionaries are briefly described in this subsection. Where possible, the default line breaking properties for characters commonly used in dictionaries have been assigned to accommodate these and similar conventions. However, implementing the full conventions in dictionaries requires special support.

Looking up the noun “syllable” in eight dictionaries yields eight different conventions:

Dictionary of the English Language (Samuel Johnson, 1843) SY´LLABLE where ´ is an oversized U+02B9 and follows the vowel of the main syllable (not the syllable itself).

Oxford English Dictionary (1st Edition) si·lă'bl where · is a slightly raised middle dot indicating the vowel of the stressed syllable (similar to Johnson’s acute). The letter ă is U+0103. The ' is an apostrophe.

Oxford English Dictionary (2nd Edition) has gone to IPA 'sIləb(ə)l where ' is U+02C8, I is U+026A, and ə is U+0259 (both times). The ' comes before the stressed syllable. The () indicate the schwa may be omitted.

Chambers English Dictionary (7th Edition) sil´ə-bl where the stressed syllable is followed by ´ U+02B9, ə is U+0259, and - is a hyphen. When splitting a word like abate´- ment, the stress mark ´ goes after stressed syllable followed by the hyphen. No special convention is used when splitting at hyphen.

BBC English Dictionary sIləbl where I is <U+026A, U+0332> and ə is U+0259. The vowel of the stressed syllable is underlined.

Collins Cobuild English Language Dictionary sIləbə°l where I is <U+026A, U+0332> and has the same meaning as in the BBC English Dictionary. The ə is U+0259 (both times). The ° is a U+2070 and indicates the schwa may be omitted.

Readers Digest Great Illustrated Dictionary syl·la·ble (sílləb'l) The spelling of the word has hyphenation points (· is a U+2027) followed by phonetic spelling. The vowel of the stressed syllable is given an accent, rather than being followed by an accent. The ' is an apostrophe.

Webster’s 3rd New International Dictionary syl·la·ble /'siləbəl/ The spelling of the word has hyphenation points (· is a U+2027) and is followed by phonetic spelling. The stressed syllable is preceded by ' U+02C8. The ə’s are schwas as usual. Webster’s splits words at the end of a line with a normal hyphen. A U+2E17 DOUBLE OBLIQUE HYPHEN indicates that a hyphenated word is split at the hyphen.

Some dictionaries use a character that looks like a vertical series of four dots to indicate places where there is a syllable, but no allowable break. This can be represented by a sequence of U+205E VERTICAL FOUR DOTS followed by U+2060 WORD JOINER.

5.3 Use of Soft Hyphen

Unlike U+2010 HYPHEN, which always has a visible rendition, the character U+00AD SOFT HYPHEN (SHY) is an invisible format character that merely indicates a preferred intraword line break position. If the line is broken at that point, then whatever mechanism is appropriate for intraword line breaks should be invoked, just as if the line break had been triggered by another mechanism, such as a dictionary lookup. Depending on the language and the word, that may produce different visible results—for example

• Simply inserting a hyphen glyph 
• Inserting a hyphen glyph and changing spelling in the divided word parts 
• Not showing any visible change and simply breaking at that point
• Inserting a hyphen glyph at the beginning of the new line

The following are a few examples of spelling changes. Each example shows the line break as “ / ” and any inserted hyphens. There are many other cases.

• In pre-reform German orthography, a “c” before the hyphenation point can change into a “k”: “Drucker” hyphenates into “Druk- / ker”.
• In modern Dutch, an e-diaeresis after the hyphenation point can change into a simple “e”: “geërfde” hyphenates into “ge- / erfde”, and “geëerd” into “ge-/ eerd”.
• In German and Swedish, a consonant is sometimes doubled: Swedish “tuggummi”; hyphenates into “tugg- / gummi”.
• In Dutch, a letter can disappear: “opaatje” hyphenates into “opa- / tje”.

The inserted hyphen glyph can take a wide variety of shapes, as appropriate for the situation. Examples include shapes like U+2010 HYPHEN, U+058A ARMENIAN HYPHEN, U+180A MONGOLIAN NIRUGU, or U+1806 MONGOLIAN TODO SOFT HYPHEN.

When a SHY is used to represent a possible hyphenation location, the spelling is that of the word without hyphenation: “tug<SHY>gummi”. It is up to the line breaking implementation to make any necessary spelling changes when such a possible hyphenation is actually used.

Sometimes it is desirable to encode text that includes line breaking decisions and will not be further broken into lines. If such text includes hyphenations, the spelling must reflect the changes due to hyphenation: “tugg<U+2010>/ gummi”, including the appropriate character for any inserted hyphen. For a list of dash-like characters in Unicode, see Section 6.2, General Punctuation, in [Unicode5.0].

Hyphenation, and therefore the SHY, can be used with the Arabic script. If the rendering system breaks at that point, the display—including shaping—should be what is appropriate for the given language. For example, sometimes a hyphen-like mark is placed on the end of the line. This mark looks like a kashida, but is not connected to the letter preceding it, looking as if the mark is placed and the line divided after the contextual shapes for the line have been determined. For more information on shaping, see [Bidi] and Section 8.2, Arabic, of [Unicode5.0].

There are three types of hyphens: explicit hyphens, conditional hyphens, and dictionary-inserted hyphens resulting from a hyphenation process. There is no character code for the third kind of hyphen. If a distinction is desired, the fact that a hyphen is dictionary-inserted must be represented out of band or by using another control code instead of SHY.

The action of a hyphenation algorithm is equivalent to the insertion of a SHY. However, when a word contains an explicit SHY, it is customarily treated as overriding the action of the hyphenator for that word.

5.4 Use of Double Hyphen

In some fonts, noticeably Fraktur fonts, it is customary to use a double-stroke form of the hyphen, usually oblique. Such use is merely a font-based glyph variation and does not affect line breaking in any way. In texts using such a font, automatic hyphenation or SHY would also result in the display of a double-stroke, oblique hyphen.

In some dictionaries, such as Webster’s 3rd New International Dictionary, double-stroke, oblique hyphens are used to indicate a hyphen at the end of the line that should be retained when the term shown is not line wrapped. It is not necessary to store a special character in the data; one merely needs to substitute the glyph of any ordinary hyphen that winds up at the end of a line. In such convention, automatic hyphenation or SHY would result in the display of an ordinary hyphen without further substitution.

Certain linguistic notations make use of a double-stroke, oblique hyphen to indicate specific features. The U+2E17 DOUBLE OBLIQUE HYPHEN character used in this case is not a hyphen and does not represent a line break opportunity. Automatic hyphenation or SHY would result in the display of an ordinary hyphen.

5.5 Tibetan Line Breaking

The Tibetan script uses spaces sparingly, relying instead on the tsheg. There is no punctuation equivalent to a period in Tibetan; Tibetan shad characters indicate the end of a “phrase,” not a sentence. “Phrases” are often metrical—that is, written after every N syllables—and a new sentence can often start within the middle of a phrase. Sentence boundaries need to be determined grammatically rather than by punctuation.

Traditionally there is nothing akin to a paragraph in Tibetan text. It is typical to have many pages of text without a paragraph break—that is, without an explicit line break. The closest thing to a paragraph in Tibetan is a new section or topic starting with U+0F12 or U+0F08. However, these occur inline: one section ends and a new one starts on the same line, and the new section is marked only by the presence of one of these characters.

Some modern books, newspapers, and magazines format text more like English with a break before each section or topic—and (often) the title of the section on a separate line. Where this is done, authors insert an explicit line break. Western punctuation (full stop, question mark, exclamation mark, comma, colon, semicolon, quotes) is starting to appear in Tibetan documents, particularly those published in India, Bhutan, and Nepal. Because there are no formal rules for their use in Tibetan, they get treated generically by default. In Tibetan documents published in China, CJK bracket and punctuation characters occur frequently; these should be treated as in Chinese written horizontally.

Note: The detailed rules for formatting Tibetan texts are complex, and the original assignment of line break classes was found to be wholly insufficient for the purpose. In [Unicode4.1], the assignment of line break classes for Tibetan was revised significantly in an attempt to better model Tibetan line breaking behavior. No new rules or line break classes were added.

The set of line break classes for Tibetan should provide a good starting point, even though there is limited practical experience in their implementation. As more experience is gained, some modifications, possibly including new rules or additional line break classes, can be expected.

It is the stated intention of the Unicode Consortium to review these assignments in a future version and to furnish a more detailed and complete description of Tibetan line breaking and line formatting behavior.

6 Line Breaking Algorithm

Unicode Standard Annex #29, “Text Boundaries” [Boundaries], describes a particular method for boundary detection. It is based on a set of hierarchical rules and character classifications. That method is well suited for implementation of some of the advanced heuristics for line breaking.

A slightly simplified implementation of such an algorithm can be devised that uses a two-dimensional table to resolve break opportunities between pairs or characters. It is described in Section 7, Pair Table-Based Implementation.

The line breaking algorithm presented in this section can be expressed in a series of rules that take line breaking classes defined in Section 5.2, Description of Line Breaking Properties, as input. The title of each rule contains a mnemonic summary of the main effect of the rule. The formal statement of each line breaking rules consists either of a remap rule or of one or more regular expressions containing one or more line breaking classes and one of three special symbols indicating the type of line break opportunity:

! Mandatory break at the indicated position

× No break allowed at the indicated position

÷ Break allowed at the indicated position

The rules are applied in order. That is, there is an implicit “otherwise” at the front of each rule following the first. It is possible to construct alternate sets of such rules that are fully equivalent. To be equivalent, an alternate set of rules must have the same effect.

The distinction between a direct break and an indirect break as defined in Section 2, Definitions, is handled in rule LB18, which explicitly considers the effect of SP. Because rules are applied in order, allowing breaks following SP in rule LB18 implies that any prohibited break in rules LB19–LB30 is equivalent to an indirect break.

The examples for each rule use representative characters, where ‘H’ stands for an ideographs, ‘h’ for small kana, and ‘9’ for digits. Except where a rule contains no expressions, the italicized text of the rule is intended merely as a handy summary.

The algorithm consists of a part for which tailoring is prohibited and a freely tailorable part.

6.1 Non-tailorable Line Breaking Rules

The rules in this subsection and the membership in the classes BK, CM, CR, GL, LF, NL, WJ, and ZW are not tailorable; see Section 4, Conformance.

Resolve line breaking classes:

LB1  Assign a line breaking class to each code point of the input. Resolve AI, CB, SA, SG, and XX into other line breaking classes depending on criteria outside the scope of this algorithm.

In the absence of such criteria, it is recommended that classes AI, SA, SG, and XX be resolved to AL, except that characters of class SA that have General_Category Mn or Mc be resolved to CM (see SA). Unresolved class CB is handled in rule LB20.

Start and end of text:

There are two special logical positions: sot, which occurs before the first character in the text, and eot, which occurs after the last character in the text. Thus an empty string would consist of sot followed immediately by eot. With these two definitions, the line break rules for start and end of text can be specified as follows:

LB2  Never break at the start of text.

sot ×

LB3  Always break at the end of text.

! eot

These two rules are designed to deal with degenerate cases, so that there is at least one character on each line, and at least one line break for the whole text. Emergency line breaking behavior usually also allows line breaks anywhere on the line if a legal line break cannot be found. This has the effect of preventing text from running into the margins.

Mandatory breaks:

A hard line break can consist of BK or a Newline Function (NLF) as described in Section 5.8, Newline Guidelines, of [Unicode5.0]. These three rules are designed to handle the line ending and line separating characters as described there.

LB4  Always break after hard line breaks (but never between CR and LF).

BK !

LB5  Treat CR followed by LF, as well as CR, LF, and NL as hard line breaks.

CR × LF

CR !

LF !

NL !

LB6  Do not break before hard line breaks.

× ( BK | CR | LF | NL )

 

Explicit breaks and non-breaks:

LB7  Do not break before spaces or zero width space.

× SP

× ZW

LB8  Break after zero width space.

ZW ÷

Combining marks:

See also Section 9.2, Legacy Support for Space Character as Base for Combining Marks.

LB9  Do not break a combining character sequence; treat it as if it has the line breaking class of the base character in all of the following rules.

Treat X CM* as if it were X.

where X is any line break class except  BK, CR, LF, NL, SP, or ZW.

At any possible break opportunity between CM and a following character, CM behaves as if it had the type of its base character. Note that despite the summary title of this rule it is not limited to standard combining character sequences. For the purposes of line breaking, sequences containing most of the control codes or layout control characters are treated like combining sequences.

LB10  Treat any remaining combining mark as AL.

Treat any remaining CM as it if were AL.

This catches the case where a CM is the first character on the line or follows SP, BK, CR, LF, NL, or ZW.

Word joiner:

LB11  Do not break before or after Word joiner and related characters.

× WJ

WJ ×

Non-breaking characters:

LB12  Do not break before or after NBSP and related characters.

[^SP] × GL

GL ×

Unlike the case for WJ, inserting a SP overrides the non-breaking nature of a GL. The expression [^SP] designates any line break class other than SP. The symbol ^ is used, instead of !, to avoid confusion with the use of ! to indicate an explicit break.

6.2 Tailorable Line Breaking Rules

The following rules and classes referenced in them can be tailored by a conformant implementations; see Section 4, Conformance.

Opening and closing:

These have special behavior with respect to spaces, and therefore come before rule 19.

LB13  Do not break before ‘]’ or ‘!’ or ‘;’ or ‘/’, even after spaces.

× CL

× EX

× IS

× SY

LB14  Do not break after ‘[’, even after spaces.

OP SP* ×

LB15  Do not break within ‘”[’, even with intervening spaces.

QU SP* × OP

LB16  Do not break within ‘]h’, even with intervening spaces.

CL SP* × NS

LB17  Do not break within ‘——’, even with intervening spaces.

B2 SP* × B2

Spaces:

LB18  Break after spaces.

SP ÷

Special case rules:

LB19  Do not break before or after  quotation marks, such as ‘ ” ’.

× QU

QU ×

LB20  Break before and after unresolved CB.

÷ CB

CB ÷

Conditional breaks should be resolved external to the line breaking rules. However, the default action is to treat unresolved CB as breaking before and after.

LB21  Do not break before hyphen-minus, other hyphens, fixed-width spaces, small kana, and other non-starters, or after acute accents.

× BA

× HY

× NS

BB ×

LB22  Do not break between two ellipses, or between letters or numbers and ellipsis.

AL × IN

ID × IN

IN × IN

NU × IN

Examples: ‘9...’, ‘a...’, ‘H...’

Numbers:

Do not break alphanumerics.

LB23  Do not break within ‘a9’, ‘3a’, or ‘H%’.

ID × PO

AL × NU

NU × AL

LB24  Do not break between prefix and letters or ideographs.

PR × ID

PR × AL

PO × AL

In general, lines should not be broken inside numbers of the form described by the following regular expression:

 ( PR | PO ) ? ( OP | HY ) ? NU (NU | SY | IS) * CL ?  ( PR | PO ) ?

Examples:  $(12.35)    2,1234    (12)¢    12.54¢

The default line breaking algorithm approximates this with the following rule.  Note that some cases have already been handled, such as ‘9,’, ‘[9’. For a tailoring that supports the regular expression directly, see Section 8.2, Examples of Customization.

LB25  Do not break between the following pairs of classes relevant to numbers:

CL × PO

CL × PR

NU × PO

NU × PR

PO × OP

PO × NU

PR × OP

PR × NU

HY × NU

IS × NU

NU × NU

SY × NU

Example pairs: ‘$9’, ‘$[’, ‘$-’, ‘-9’, ‘/9’, ‘99’, ‘,9’, ‘9%’ ‘]%’

Korean syllable blocks

Conjoining jamo, Hangul syllables, or combinations of both form Korean Syllable Blocks. Such blocks are effectively treated as if they were Hangul syllables; no breaks can occur in the middle of a syllable block. See Unicode Standard Annex #29, “Text Boundaries” [Boundaries], for more information on Korean Syllable Blocks.

LB26 Do not break a Korean syllable.

JL × (JL | JV | H2 | H3)

(JV | H2) × (JV | JT)

(JT | H3) × JT

The effective line breaking class for the syllable block matches the line breaking class for Hangul syllables, which is ID by default. This is achieved by the following rule:

LB27 Treat a Korean Syllable Block the same as ID.

(JL | JV | JT | H2 | H3) × IN

(JL | JV | JT | H2 | H3) × PO

PR × (JL | JV | JT | H2 | H3)

When Korean uses SPACE for line breaking, these classes, as well as characters of class ID, are often tailored to AL; see Section 8, Customization.

Finally, join alphabetic letters into words and break everything else.

LB28  Do not break between alphabetics (“at”).

AL × AL

LB29  Do not break between numeric punctuation and alphabetics (“e.g.”).

IS × AL

LB30 Do not break between letters, numbers, or ordinary symbols and opening or closing punctuation.

(AL | NU) × OP

CL × (AL | NU)

The purpose of this rule is to prevent breaks in common cases where a part of a word appears between delimiters—for example, in “person(s)”.

LB31  Break everywhere else.

ALL ÷

÷ ALL

7 Pair Table-Based Implementation

A two-dimensional table can be used to resolve break opportunities between pairs of characters. The rows of the table are labeled with the possible values of the line breaking property of the leading character in the pair. The columns are labeled with the line breaking class for the following character of the pair. Each intersection is labeled with the resulting line break opportunity.

The Japanese standard JIS X 4051-1995 [JIS] provides an example of such a table-based definition. However, it uses line breaking classes whose membership is not solely determined by the line breaking property (as in this annex), but in some cases by heuristic analysis or markup of the text.

The implementation provided here directly uses the line breaking classes defined previously. 

7.1 Minimal Table

If two rows of the table have identical values and the corresponding columns also have identical values, then the two line breaking classes can be coalesced. For example, the JIS standard uses 20 classes, of which only 14 appear to be unique. Any minimal table representation is unique, except for trivial reordering of rows and columns.

7.2 Extended Context

Most of the rules in Section 6, Line Breaking Algorithm, involve only pairs of characters, or they apply to a single line break class preceded or followed by any character. These rules can be represented directly in a pair table. However, rules LB14–LB17 require extended context to handle spaces.

By broadening the definition of a pair from B A, where B is the line breaking class before a break and A the one after, to B SP* A, where SP* is an optional run of space characters, the same table can be used to distinguish between cases where SP can or cannot provide a line break opportunity (that is, direct and indirect breaks). Rules equivalent to the ones given in Section 6, Line Breaking Algorithm, can be formulated without explicit use of SP by using % to express indirect breaks instead. These rules can then be simplified to involve only pairs of classes—that is, only constructions of the form:

B ÷ A

B % A

B × A

where either A or B may be empty. These simplified rules can be automatically translated into a pair table, as in Table 2. Line breaking analysis then proceeds by pair table lookup as explained below.  (For readability in table layout, the symbol ^ is used in the table instead of × and _ is used instead of ÷.)

Rule LB9 requires extended context for handling combining marks. This extended context must also be built into the code that interprets the pair table.  For convenience in detecting the condition where A = CM, the symbols # and @ are used in the pair table, instead of % and ^, respectively. See Section 7.5, Combining Marks.

7.3 Example Pair Table

Table 2 implements the line breaking behavior described in this annex, with the limitation that only context of the form B  SP*  A is considered. BK, CR, LF, NL, and SP classes are handled explicitly in the outer loop, as given in the code sample below. Pair context of the form B CM* can be handled by handling the special entries @ and # in the driving loop, as explained in Section 7.5, Combining Marks. Conjoining jamos are considered separately in Section 7.6, Conjoining Jamos. In Table 2, the rows are labeled with the B class and the columns are labeled with the A class.

Resolved outside the pair table:  AI, BK, CB, CR,  LF, NL, SA, SG, SP, XX

Table 2 uses the following notation:

^ denotes a prohibited break: B ^ A is equivalent to B SP* × A; in other words, never break before A and after B, even if one or more spaces intervene.

% denotes an indirect break opportunity: B % A is equivalent to B × A and  B SP+ ÷ A; in other words, do not break before A, unless one or more spaces follow B.

@ denotes a prohibited break for combining marks: B @ A is equivalent to B SP* × A, where A is of class CM. For more details, see Section 7.5, Combining Marks.

# denotes an indirect break opportunity for combining marks following a space: B # A is equivalent to (B × A and  B SP+ ÷  A), where A is of class CM.

_ denotes a direct break opportunity (equivalent to ÷ as defined above).

7.4 Sample Code

The following two sections provide sample code [Code14] that demonstrates how the pair table is used. For a complete implementation of the line breaking algorithm, if statements to handle the line breaking classes CR, LF, and NL need to be added. They have been omitted here for brevity, but see Section 7.7, Explicit Breaks.

The sample code assumes that the line breaking classes AI, CB, SG, and XX have been resolved according to rule LB1 as part of initializing the pcls array. The code further assumes that the complex line break analysis for characters with line break class SA is handled in function findComplexBreak, for which the following placeholder is given:

    // placeholder function for complex break analysis
    // cls - resolved line break class, may differ from pcls[0]
    // pcls - pointer to array of line breaking classes (input)
    // pbrk - pointer to array of line breaking opportunities (output)
    // cch - remaining length of input 
    int 
   findComplexBreak(enum break_class cls, enum break_class *pcls, 
                             enum break_action *pbrk, int cch)
    {
            if (!cch)
                return 0;
            for (int ich = 1; ich < cch; ich++) {
    
                // .. do complex break analysis here
                // and report any break opportunities in pbrk ..
               
                pbrk[ich-1] = PROHIBITED_BRK; // by default, no break
               
                if (pcls[ich] != SA)
                    break;
            }
            return ich;
    }

The entries in the example pair table correspond to the following enumeration. For diagnostic purposes, the sample code returns these value to indicate not only the location but also the type of rule that triggered a given break opportunity.

    enum break_action {
           DIRECT_BRK = 0,             // _ in table
           INDIRECT_BRK,               // % in table
           COMBINING_INDIRECT_BRK,     // # in table
           COMBINING_PROHIBITED_BRK,   // @ in table
           PROHIBITED_BRK,             // ^ in table
           EXPLICTI_BRK };             // ! in rules    

Because the contexts involved in indirect breaks of the form B SP* A are of indefinite length, they need to be handled explicitly in the driver code. The sample implementation of a findLineBrk function below remembers the line break class for the last characters seen, but skips any occurrence of SP without resetting this value. Once character A is encountered, a simple lookback is used to see if it is preceded by a SP. This lookback is necessary only if B % A.

    // handle spaces separately, all others by table
    // pcls - pointer to array of line breaking classes (input)
    // pbrk - pointer to array of line break opportunities (output)
    // cch - number of elements in the arrays (“count of characters”) (input)
    // ich - current index into the arrays (variable) (returned value)
    // cls - current resolved line break class for 'before' character (variable)

    int 
    findLineBrk(enum break_class *pcls, enum break_action *pbrk, int cch)
    {
        if (!cch) return 0;
    
        enum break_class cls = pcls[0];   // class of 'before' character
      
        // loop over all pairs in the string up to a hard break
        for (int ich = 1; (ich < cch) &amp;&amp; (cls != BK); ich++) {
    
            // handle explicit breaks here (see Section 7.7)

            // handle spaces explicitly
            if (pcls[ich] == SP) {
                pbrk[ich-1] = PROHIBITED_BRK;   // apply rule LB7: × SP
                continue;                       // do not update cls
            }
    
            // handle complex scripts in a separate function
            if (pcls[ich] == SA) {
                ich += findComplexBreak(cls, &pcls[ich-1], &pbrk[ich-1], 
                           cch - (ich-1));
                if (ich < cch)
                    cls = pcls[ich];
                continue;
            }
        
            // lookup pair table information in brkPairs[before, after];
            enum break_action brk = brkPairs[cls][pcls[ich]];
    
            pbrk[ich-1] = brk;                     // save break action in output array
    
            if (brk == INDIRECT_BRK) {             // resolve indirect break
                if (pcls[ich - 1] == SP)           // if context is A SP * B
                    pbrk[ich-1] = INDIRECT_BRK;    //       break opportunity
                else                               // else
                    pbrk[ich-1] = PROHIBITED_BRK;  //       no break opportunity
            } 
            
            // handle breaks involving a combining mark (see Section 7.5)

            // save cls of 'before' character (unless bypassed by 'continue')
            cls = pcls[ich];                                
        }
        pbrk[ich-1] = EXPLICIT_BRK;                                      // always break at the end
    
        return ich;
    }

The function returns all of the break opportunities in the array pointed to by pbrk, using the values in the table. On return, pbrk[ich] is the type of break after the character at index ich.

A common optimization in implementation is to determine only the nearest line break opportunity prior to the position of the first character that would cause the line to become overfull. Such an optimization requires backward traversal of the string instead of forward traversal as shown in the sample code.

7.5 Combining Marks

The implementation of combining marks in the pair table presents an additional complication because rule LB9 defines a context X CM* that is of arbitrary length. There are some similarities to the way contexts of the form B SP* A that are involved in indirect breaks are evaluated. However, contexts of the form SP CM* or CM* SP also need to be handled, while rule LB10 requires some CM* to be treated like AL.

Implementing LB10. This rule can be reflected directly in the example pair table in Table 2 by assigning the same values in the row marked CM as in the row marked AL. Incidentally, this is equivalent to rewriting the rules LB11–LB31 by duplicating any expression that contains an AL on its lefthand side with another expression that contains a CM. For example, in LB22

AL × IN

would become

AL × IN

CM × IN

Rewriting these rules as indicated here (and then deleting LB10) is fully equivalent to the original rules because rule LB9 already accounts for all CMs that are not supposed to be treated like AL. For complete prescription see Example 9 in Section 8.2, Examples of Customization.

Implementing LB9. Rule LB9 is implemented in the example pair table in Table 2 by assigning a special # entry in the column marked CM for all rows referring to a line break class that allows a direct or indirect break after itself. (Note that the intersection between the row for class ZW and the column for class CM must be assigned “_” because of rule LB8.) The # corresponds to a break_action value of COMBINING_INDIRECT_BREAK, which triggers the following code in the sample implementation:

    else if (brk == COMBINING_INDIRECT_BRK) {    // resolve combining mark break
        pbrk[ich-1] = PROHIBITED_BRK;             // do not break before CM
        if (pcls[ich-1] == SP){
            #ifndef LEGACY_CM                    // new: space is not a base
                pbrk[ich-1] = COMBINING_INDIRECT_BRK;    // apply rule SP ÷ 
            #else                                 
                pbrk[ich-1] = PROHIBITED_BRK;      // legacy: keep SP CM together
                if (ich > 1)
                    pbrk[ich-2] = ((pcls[ich - 2] == SP) ? 
                                                  INDIRECT_BRK : DIRECT_BRK);
            #endif
        } else                                   // apply rule LB9: X CM * -> X
            continue;                            // do not update cls
    }

When handling a COMBINING_INDIRECT_BREAK, the last remembered line break class in variable cls is not updated, except for those cases covered by rule LB10. A tailoring of rule LB9 that keeps the last SPACE character preceding a combining mark, if any, and therefore breaks before that SPACE character can easily be implemented as shown in the sample code. (See Section 9.2, Legacy Support for Space Character as Base for Combining Marks.)

Any rows in Table 2 for line break classes that prohibit breaks after must be handled explicitly. In the example pair table, these are assigned a special entry “@”, which corresponds to a special break action of COMBINING_PROHIBITED_BREAK that triggers the following code:

    else if (brk == COMBINING_PROHIBITED_BRK) { // this is the case OP SP* CM
        pbrk[ich-1] = COMBINING_PROHIBITED_BRK;  // no break allowed
        if (pcls[ich-1] != SP)
            continue;                          // apply rule LB9: X CM* -> X
    } 

The only line break class that unconditionally prevents breaks across a following SP is OP. The preceding code fragment ensures that OP CM is handled according to rule LB9 and OP SP CM is handled as OP SP AL according to rule LB10.

7.6 Conjoining Jamos

For Korean Syllable Blocks, the information in rule LB26 is represented by a simple pair table shown in Table 3.

Table 3. Korean Syllable Block Pair Table

	H2	H3	JL	JV	JT
H2	_	_	_	%	%
H3	_	_	_	_	%
JL	%	%	%	%	_
JV	_	_	_	%	%
JT	_	_	_	_	%

When constructing a pair table such as Table 2, this pair table for Korean syllable blocks in Table 3 is merged with the main pair table for all other line break classes by adding the cells from Table 3 beyond the lower-right corner of the main pair table. Next, according to rule LB27, any empty cells in the new rows are filled with the same values as in the existing row for class ID, and any empty cells for the new columns are filled with the same values as in the existing column for class ID. The resulting merged table is shown in Table 2.

7.7 Explicit Breaks

Handling explicit breaks is straightforward in the driver code, although it does clutter up the loop condition and body of the loop a bit. For completeness, the following sample shows how to change the loop condition and add if statements to the loop that handle BK, NL, CR, and LF. Because NL and BK behave identically by default, this code can be simplified in implementations where the character classification is changed so that BK will always be substituted for NL when assigning the line break class. Because this optimization does not change the result, it is not considered a tailoring and does not affect conformance.

    // handle case where input starts with an LF
    if (cls == LF)
         cls = BK;

    // treat NL like BK
    if (cls == NL)
         cls = BK;

    // loop over all pairs in the string up to a hard break or CRLF pair
    for (int ich = 1; (ich < cch) && (cls != BK) && (cls != CR || pcls[ich] == LF); ich++) {
    
        // handle BK, NL and LF explicitly
        if (pcls[ich] == BK ||pcls[ich] == NL ||  pcls[ich] == LF) 
        {
            pbrk[ich-1] = PROHIBITED_BRK;
            cls = BK;
            continue;
        }
    
        // handle CR explicitly
        if(pcls[ich] == CR)
        {
            pbrk[ich-1] = PROHIBITED_BRK;
            cls = CR;
            continue;
        }
            
        // handle spaces explicitly...
            

8 Customization

A real-world line breaking algorithm must be tailorable to some degree to meet user or document requirements.

In Korean, for example, two distinct line breaking modes occur, which can be summarized as breaking after each character or breaking after spaces (as in Latin text). The former tends to occur when text is set justified; the latter, when ragged margins are used. In that case, even ideographs are broken only at space characters. In Japanese, for example, tighter and looser specifications of prohibited line breaks may be used.

Specialized text or specialized text constructs may need specific line breaking behavior that differs from the default line breaking rules given in this annex. This may require additional tailorings beyond those considered in this section. For example, the rules given here are insufficient for mathematical equations, whether inline or in display format. Likewise, text that commonly contains lengthy URLs might benefit from special tailoring that suppresses SY × NU from rule LB25 within the scope of a URL to allow breaks after a “/” separated segment in the URL regardless of whether the next segment starts with a digit.

The remainder of this section gives an overview of common types of tailorings and examples of how to customize the pair table implementation of the line breaking algorithm for these tailorings.

8.1 Types of Tailoring

There are three principal ways of tailoring the sample pair table implementation of the line breaking algorithm:

1. Changing the line breaking class assignment for some characters
   This is useful in cases where the line breaking properties of one class of characters are occasionally lumped together with the properties of another class to achieve a less restrictive line breaking behavior. 
   
2. Changing the table value assigned to a pair of character classes
   This is particularly useful if the behavior can be expressed by a change at a limited number of pair intersections. This form of customization is equivalent to permanently overriding some of the rules in Section 6, Line Breaking Algorithm.
   
3. Changing the interpretation of the line breaking actions
   This is a dynamic equivalent of the preceding. Instead of changing the values for the pair intersection directly in the table, they are labeled with special values that cause different actions for different customizations. This is most suitable when customizations need to be enabled at run time.

Beyond these three straightforward customization steps, it is always possible to augment the algorithm itself—for example, by providing specialized rules to recognize and break common constructs, such as URLs, numeric expressions, and so on. Such open-ended customizations place no limits on possible changes, other than the requirement that characters with normative line breaking properties be correctly implemented.

Reference [Cedar97] reports on a real-world implementation of a pair table-based implementation of a line breaking algorithm substantially similar to the one presented here, and including the types of customizations presented in this section. That implementation was able to simultaneously meet the requirements of customers in many European and East Asian countries with a single implementation of the algorithm.

8.2 Examples of Customization

Example 1. The exact method of resolving the line break class for characters with class SA is not specified in the default algorithm. One method of implementing line breaks for complex scripts is to invoke context-based classification for all runs of characters with class SA. For example, a dictionary-based algorithm could return different classes for Thai letters depending on their context: letters at the start of Thai words would become BB and other Thai letters would become AL. Alternatively, for text consisting of or predominantly containing characters with line breaking class SA, it may be useful to instead defer the determination of line breaks to a different algorithm entirely. Section 7.4, Sample Code, sketches such approach in which the interface to the dictionary-based algorithm directly reports break opportunities.

Example 2. To implement terminal style line breaks, it would be necessary to allow breaks inside a run of spaces. This requires a change in the way the driver loop handles spaces and, therefore, cannot be simply done by customizing the pair table. However, the additional task of line wrapping runs of spaces could also be performed after the fact at the layout system level while leaving unchanged the actual line breaking algorithm.

Example 3.  Depending on the nature of the document, Korean either uses implicit breaking around characters (type 2 as defined above in Section 3, Introduction) or uses spaces (type 1). Space-based layout is common in magazines and other informal documents with ragged margins, while books, with both margins justified, use the other type, as it affords more line break opportunities and therefore leads to better justification. Reference [Suign98] shows how the necessary customizations can be elegantly handled by selectively altering the interpretation of the pair entries. Only the intersections of ID/ID, AL/ID, and ID/AL are affected. For alphabetic style line breaking, breaks for these cases require space; for ideographic style line breaking, these cases do not require spaces. Therefore, one defines a pseudo-action, which is then resolved into either direct or indirect break action based on user selection of the preferred behavior for a given text.

Example 4.  Sometimes in a Far Eastern context it is necessary to allow alphabetic characters and digit strings to break anywhere. According to reference [Suign98], this can again be done in the same way as Korean. In this case the intersections of NU/NU, NU/AL, AL/AL, and AL/NU are affected.

Example 5.  Some users prefer to relax the requirement that Kana syllables be kept together. For example, the syllable kyu, spelled with the two kanas KI and “small yu”, would no longer be kept together as if KI and yu were atomic. This customization can be handled via the first method by changing the classification of the Kana small characters from NS to ID as needed.

Example 6. Some implementations may wish to tailor the line breaking algorithm to resolve grapheme clusters according to Unicode Standard Annex #29, “Text Boundaries” [Boundaries], as a first stage. Generally, the line breaking algorithm does not create line break opportunities within default grapheme clusters; therefore such a tailoring would be expected to produce results that for most practical cases are close to what are defined by the default algorithm. However, if such a tailoring is chosen, characters that are members of line break class CM but not part of the definition of default grapheme clusters must still be handled by rules LB9 and LB10, or by some additional tailoring.

Example 7. Regular expression-based line breaking engines might get better results using a tailoring that directly implements the following regular expression for numeric expressions:

( PR | PO ) ? ( OP | HY ) ? NU (NU | SY | IS) * CL ?  ( PR | PO ) ?

This is equivalent to replacing the rule LB25 by the following tailored rule:

Regex-Number: Do not break numbers.

(PR | PO) × ( OP | HY )? NU

( OP | HY ) × NU

NU × (NU | SY | IS)

NU (NU | SY | IS)* × (NU | SY | IS | CL)

NU (NU | SY | IS)* CL? × (PO | PR)

This customized rule uses extended contexts that cannot be represented in a pair table.

When the tailored rule is used, LB13 must also be tailored as follows:

[^NU] × CL

× EX

[^NU] × IS

[^NU] × SY

Otherwise, single digits may be handled by rule LB13 before being handled in the regular expression. In these tailored rules, [^NU] designates any line break class other than NU. The symbol ^ is used, instead of !, to avoid confusion with the use of ! to indicate an explicit break.

Example 8. For some implementations it may be difficult to implement LB9 due to the added complexity of its indefinite length context. Because combining marks are most commonly applied to characters of class AL, rule LB10 by itself generally produces acceptable results for such implementations, but such an approximation is not a conformant tailoring.

9 Implementation Notes

This section provides additional notes on implementation issues.

9.1 Combining Marks in Regular Expression-Based Implementations

For implementations that use regular expressions, it is not possible to directly express rules LB9 and LB10. However, it is possible to make these rules unnecessary by rewriting all the rules from LB11 on down so that the overall result of the algorithm is unchanged. This restatement of the rules is therefore not a tailoring, but rather an equivalent statement of the algorithm that can be directly expressed as regular expressions.

To replace rule LB9, terms of the form

B # A

B SP* # A

B #

B SP* #

  are replaced by terms of the form

B CM* # A

B CM* SP* # A

B CM* #

B CM* SP* #

where B  and A are any line break class or set of alternate line break classes, such as (X |Y), and where # is any of the three operators !, ÷, or ×.

Note that because sot, BK, CR, LF, NL, and ZW are all handled by rules above LB9, these classes cannot occur in position B in any rule that is rewritten as shown here.

Replace LB10 by the following rule:

×  CM

For each rule containing AL on its left side, add a rule that is identical except for the replacement of AL by CM, but taking care of correctly handling sets of alternate line break classes. For example, for rule

(AL | NU) × OP

add another rule

CM × OP.

These prescriptions for rewriting the rules are, in principle, valid even where the rules have been tailored as permitted in Section 4, Conformance. However, for extended context rules such as in Example 7, additional considerations apply. These are described in Section 6.2, Replacing Ignore Rules, of Unicode Standard Annex #29, “Text Boundaries” [Boundaries].

9.2 Legacy Support for Space Character as Base for Combining Marks

As stated in [Unicode5.0], Section 7.9, Combining Marks, combining characters are shown in isolation by applying them to U+00A0 NO-BREAK SPACE (NBSP). In earlier versions, this recommendation included the use of U+0020 SPACE. This use of SPACE for this purpose is now deprecated because it has been found to lead to many complications in text processing. When using either NBSP or SPACE as the base character, the visual appearance is the same, but the line breaking behavior is different. Under the current rules, SP CM* will allow a break between SP and CM*, which could result in a new line starting with a combining mark. Previously, whenever the base character was SP, the sequences CM* and SP CM* were defined to act like indivisible clusters, allowing breaks on either side like ID.

Where backward compatibility with documents created under the prior practice is desired, the following tailoring should be applied to those CM characters that have a General_Category value of Combining_Mark (M):

Legacy-CM: In all of the rules following rule LB8, if a space is the base character for a combining mark, the space is changed to type ID. In other words, break before SP  in the same cases as one would break before an ID.

Treat SP CM* as if it were ID.

While this tailoring changes the location of the line break opportunities in the string, it should ordinarily not affect the display of the text. That is because spaces at the end of the line are normally invisible and the recommended display for isolated combining marks is the same as if they were applied to a preceding SPACE or NBSP.

References

For references for this annex, see Unicode Standard Annex #41, “Common References for Unicode Standard Annexes.”

Acknowledgments

The initial assignments of properties are based on input by Michel Suignard. Mark Davis provided algorithmic verification and formulation of the rules, and detailed suggestions on the algorithm and text. Ken Whistler, Rick McGowan and other members of the editorial committee provided valuable feedback. Tim Partridge enlarged the information on dictionary usage. Sun Gi Hong reviewed the information on Korean and provided copious printed samples. Eric Muller reanalyzed the behavior of the soft hyphen and collected the samples. Christopher Fynn provided the background information on Tibetan line breaking. Andrew West, Kamal Mansour, Andrew Glass, Daniel Yacob, and Peter Kirk suggested improvements for Mongolian, Arabic, Kharoshthi, Ethiopic, and Hebrew punctuation characters, respectively. Kent Karlsson reviewed the line break properties for consistency. Andy Heninger reviewed the rules and provided input on regular expression-based implementations. Many others provided additional review of the rules and property assignments.

Modifications

Rule Numbering Across Versions

Table 4 documents changes in the numbering of line breaking rules. A duplicate number indicates that a rule was subsequently split. (In each version, the rules are applied in their numerical order, not in the order they appear in this table.) Versions prior to 3.0.1 are not documented here.

Table 4. Rule Numbering Across Versions

5.0.0	4.1.0	4.0.1	4.0.0	3.2.0	3.1.0	3.0.1
LB1	1	1	1	1	1	1
LB2	2a	2a	2a	2a	2a	2a
LB3	2b	2b	2b	2b	2b	3b
LB4	3a	3a	3a	3a	3a	3a
LB5	3b	3b	3b	3a	3a	3a
LB6	3c	3c	3c	3b	3b	3b
LB7	4	4	4	4	4	4
LB8	5	5	5	5	5	5
	deprecated	7a	7a	7	7	7
LB9	7b	7b	7b	6	6	6
LB10	7c	7c	7c
LB11	11b	11b	11b	13	13	13
LB12	13	11b	11b	13	13	13
LB13	8	8	8	8	8	8
LB14	9	9	9	9	9	9
LB15	10	10	10	10	10	10
LB16	11	11	11	11	11	11
LB17	11a	11a	11a	11a	11a
LB18	12	12	12	12	12	12
LB19	14	14	14	14	14	14
LB20	14a	14a	14a
LB21	15	15	15	15	15	15
LB22	16	16	16	16	16	16
LB23	17	17	17	17	17	17
LB24	18	18	18	18	18	18
LB25	18	18	18	18	18	18
	removed	18b	18b	15b	15b	15b
LB26	18b	6	6	6	6	6
LB27	18c	6	6	6	6	6
LB28	19	19	19	19	19	19
LB29	19b	19b
LB30
LB31	20	20	20	20	20	20