Break Rules

ICU locates boundary positions within text by means of rules, which are a form of regular expressions. The form of the rules is similar, but not identical, to the boundary rules from the Unicode specifications [UAX-14, UAX-29], and there is a reasonably close correspondence between the two.

Taken as a set, the ICU rules describe how to move forward to the next boundary, starting from a known boundary.

ICU includes rules for the standard boundary types (word, line, etc.). Applications may also create customized break iterators from their own rules.

ICU's built-in rules are located at icu/icu4c/source/data/brkitr/rules/. These can serve as examples when writing your own, and as starting point for customizations.

Rule Tutorial

Rules most commonly describe a range of text that should remain together, unbroken. For example, this rule


matches a run of one or more letters, and would cause them to remain unbroken.

The part within [brackets] follows normal ICU UnicodeSet pattern syntax.

The qualifier, '+' in this case, can be one of
 empty       Match exactly once
 ? Match zero or one time
 + Match  one or more times
 * Match zero or more times


A variable names a set or rule sub-expression. They are useful for documenting what something represents, and for simplifying complex expressions by breaking them up.
"Variable" is something if a misnomer; they cannot be reassigned, but are more of a constant expression.

They start with a '$', both in the definition and use.

# Variable Definition
$ASCIILetNum = [A-Za-z0-9];

# Variable Use

Comments and Semicolons

'#' begins a comment, which extends to the end of a line.
Comments may stand alone, or appear after another statement on a line.

All rule statements or expressions are terminated by semicolons.

Chained Matching

Most ICU rule sets use the concept of "chained matching". The idea is that complete match can be composed from multiple pieces, with each piece coming from an individual rule of a rule set.

This idea is unique to ICU break rules, it is not a concept found in other regular expression based matchers. Some of the Unicode standard break rules would be difficult to implement without it.

Starting with an example,

word_char = [\p{Letter}];
word_joiner = [_-];

$word_char $word_joiner $word_char;

These rules will match "abc",   "hello_world",   "hi-there",   "a-bunch_of-joiners-here".
They will not match "-abc",  "multiple__joiners",   "tail-"

A full match is composed of pieces or submatches, possibly from different rules, with adjacent submatches linked by at least one overlapping character.

In the example below, matching "hello_world", 
'^' shows matches of the first rule, word_char+
'!' shows matches of the second rule, $word_char $word_joiner $word_char

^^^^^ ^^^^^

There is an overlap of the matched regions, which causes the chaining mechanism to join them into a single overall match.

The mechanism is a good match to, for example, Unicode's word break rules, where rules WB5 through WB13 combine to piece together longer words from multiple short segments.

!!chain; enables chaining in a rule set. It is disabled by default for back compatibility - very old versions of ICU did not support it, and it was originally introduced as an option.

Parentheses and Alternation

Rule expressions can contain parentheses and '|' operators, representing alternation or  "or" operations. This follows conventional regular expression behavior.

For example, the following would match a simplified identifier:

$Letter ($Letter | $Digit)*;

String and Character Literals

Similarly to common regular expressions, literal characters that do not have other special meaning represent themselves. So the rule


would match the literal input "Hello".

In practice, nearly all break rules are composed from [sets] based on Unicode character properties; literal characters in rules are very rare.
To prevent random typos in rules from being treated as literals, use this option:


With the option, the naked Hello becomes a rule syntax error while a quoted "hello" still matches a literal hello.

!!quoted_literals_only is strongly recommended for all rule sets. The random typo problem is very real, and surprisingly hard to recognize and debug.

Explicit Break Rules

A rule containing a slash (/) will force a boundary when it matches, even when other rules or chaining would otherwise lead to a longer match. Also called Hard Break Rules, these have the form

pre-context / post-context;

where the pre and post-context look like normal break rules. Both the pre and post context are required, and must not allow a zero-length match. There should be no overlap between characters that end a match of the pre-context and those that begin a match of the post-context.

Chaining into a hard break rule operates normally. There is no chaining out of a hard break rule; when the post-context matches a break is forced immediately.

Note: future versions of ICU may loosen the restrictions on explicit break rules. The behavior of rules with missing or overlapping contexts is subject to change.

Chaining Control

Chaining into a rule can be dis-allowed by beginning that rule with a '^'. Rules so marked can begin a match after a preceding boundary or at the start of text, but cannot extend a match via chaining from another rule. 

The !!LBCMNoChain; statement modifies chaining behavior by preventing chaining from one rule to another from occurring on any character whose Line Break property is Combining Mark. This option is subject to change or removal, and should not be used in general. Within ICU, it is used only with the line break rules. We hope to replace it with something more general.

!!LBCMNoChain is deprecated, and will be removed completely from a future version of ICU.

Rule Status Values

Break rules can be tagged with a number, which is called the rule status. After a boundary has been located, the status number of the specific rule that determined the boundary position is available to the application through the function getRuleStatus().

For the predefined word boundary rules, status values are available to distinguish between boundaries associated with words, numbers, and those around spaces or punctuation. Similarly for line break boundaries, status values distinguish between mandatory line endings (new line characters) and break opportunities that are appropriate points for line wrapping. Refer to the ICU API documentation for the C header file ubrk.h or to Java class RuleBasedBreakIterator for a complete list of the predefined boundary classifications.

When creating custom sets of break rules, integer status values can be associated with boundary rules in whatever way will be convenient for the application. There is no need to remain restricted to the predefined values and classifications from the standard rules.

It is possible for a set of break rules to contain more than a single rule that produces some boundary in an input text. In this event, getRuleStatus() will return the numerically largest status value from the matching rules, and the alternate function getRuleStatusVec() will return a vector of the values from all of the matching rules.

In the source form of the break rules, status numbers appear at end of a rule, and are enclosed in {braces}.

Hard break rules that also have a status value place the status at the end, for example

pre-context / post-context {1234};

Word Dictionaries

For some languages that don't normally use spaces between words, break iterators are able to supplement the rules with dictionary based breaking. Some languages, Thai or Lao, for example, use a dictionary for both word and line breaking. Others, such as Japanese, use a dictionary for word breaking, but not for line breaking.

To enable dictionary use,
  1. The break rules must select, as unbroken chunks, ranges of text to be passed off to the word dictionary for further subdivision.
  2. The break rules must define a character class named $dictionary that contains the characters (letters) to be handled by the dictionary.
The dictionary implementation, on receiving a range of text, will map it to a specific dictionary based on script, and then delegate to that dictionary for subdividing the range into words.

See, for example, this snippet from the line break rules:

#   Dictionary character set, for triggering language-based break engines. Currently
#   limited to LineBreak=Complex_Context (SA).

$dictionary = [$SA];

Rule Options

 Option    Description
 !!chain Enable rule chaining. Default is no chaining.
 !!forwardThe rules that follow are for forward iteration. Forward rules are now the only type of rules needed or used.
 !!reverse[deprecated] The rules that follow are for reverse iteration. No longer needed; any rules in a Reverse rule section are ignored.
 !!safe_forward[deprecated] The rules that follow are for safe forward iteration. No longer needed; any rules in such a section are ignored.
 !!safe_reverse[deprecated] The rules that follow are for safe reverse iteration. No longer needed; any rules in such a section are ignored.
 !!LBCMNoChain[deprecated] Disable chaining when the overlap character matches \p{Line_Break=Combining_Mark}

Rule Syntax

Here is the syntax for the boundary rules.

Rule NameRule ValuesNotes
statementassignment | rule | control
control(“!!forward” | “!!reverse” | “!!safe_forward” | “!!safe_reverse” | “!!chain”) ';'
assignmentvariable '=' expr ';'5
rule'^'? expr ('{'number'}')? ';'8, 9
exprexpr-q | expr '|' expr | expr expr3
expr-qterm | term '*' | term '?' | term '+'
termrule-char | unicode-set | variable | quoted-sequence | '(' expr ')' | break-point
rule-specialany printing ascii character except letters or numbers | white-space
rule-charany non-escaped character that is not rule-special | '.' | any escaped character except '\p' or '\P'
variable'$' name-start-char name-char*7
name-start-char'_' | \p{L}
name-charname-start-char | \p{N}
quoted-sequence''' (any char except single quote or line terminator or two adjacent single quotes)+ '''
escaped-charSee “Character Quoting and Escaping” in the UnicodeSet chapter

Unicode setSee UnicodeSet4
commentunescaped '#' [any char except new-line]* new-line2
sunescaped \p{Z}, tab, LF, FF, CR, NEL6
new-lineLF, CR, NEL2


  1. The number associated with a rule that actually determined a break position is available to the application after the break has been returned. These numbers are not Perl regular expression repeat counts.

  2. Comments are recognized and removed separately from otherwise parsing the rules. They may appear wherever a space would be allowed (and ignored.)

  3. The implicit concatenation of adjacent terms has higher precedence than the '|' operation. "ab|cd" is interpreted as "(ab)|(cd)", not as "a(b|c)d" or "(((ab)|c)d)"

  4. The syntax for UnicodeSet is defined (and parsed) by the UnicodeSet class. It is not repeated here.

  5. For $variables that will be referenced from inside of a UnicodeSet, the definition must consist only of a Unicode Set. For example, when variable $a is used in a rule like [$a$b$c], then this definition of $a is ok “$a=[:Lu:];” while this one “$a=abcd;” would cause an error when $a was used.

  6. Spaces are allowed nearly anywhere, and are not significant unless escaped. Exceptions to this are noted.

  7. No spaces are allowed within a variable name. The variable name $dictionary is special. If defined, it must be a Unicode Set, the characters of which will trigger the use of word dictionary based boundaries.

  8. A leading '^' on a rule prevents chaining into that rule. It can only match immediately after a preceding boundary, or at the start of text.

  9. {nnn} appearing at the end of a rule is a Rule Status number, not a repeat count as it would be with conventional regular expression syntax.

  10. A '/' in a rule specifies a hard break point. If the rule matches, a boundary will be forced at the position of the '/' within the match.

EBNF Syntax used for the RBBI rules syntax description

a?zero or one instance of a
a+one or more instances of a
a*zero or more instances of a
a | beither a or b, but not both
'a' "a"the literal string between the quotes

Planned Changes and Removed or Deprecated Rule Features

  1. Reverse rules could formerly be indicated by beginning them with an exclamation '!'. This syntax is deprecated, and will be removed from a future version of ICU.

  2. !!LBCMNoChain was a global option that specified that characters with the line break property of "Combining Character" would not participate in rule chaining. This option was always considered internal, is deprecated and will be removed from a future version of ICU.

  3. Naked rule characters. Plain text, in the context of a rule, is treated as literal text to be matched, much like normal regular expressions. This turns out to be very error prone, has been the source of bugs in released versions of ICU, and is not useful in implementing normal text boundary rules. A future version will reject literal text that is not escaped. 

  4. Exact reverse rules and safe forward rules: planned changes to the break engine implementation will remove the need for exact reverse rules and safe forward rules.

  5. {bof} and {eof}, appearing within [sets], match the beginning or ending of the input text, respectively. This is an internal (not documented) feature that will probably be removed in a future version of ICU. They are currently used by the standard rules for word, line and sentence breaking. An alternative is probably needed. The existing implementation is incomplete. 

Additional Sample Code

C/C++: See icu/source/samples/break/ in the ICU source distribution for code samples showing the use of ICU boundary analysis.

Details about Dictionary-Based Break Iteration

(This section originally from August 2012.)

Certain Unicode characters have a "dictionary" bit set in the break iteration rules, and text made up of these characters cannot be handled by the rules-based break iteration code for lines or words. Rather, they must be handled by a dictionary-based approach. The ICU approach is as follows:

Once the Dictionary bit is detected, the set of characters with that bit is handed off to "dictionary code." This code then inspects the characters more carefully, and splits them by script (Thai, Khmer, Chinese, Japanese, Korean). If text in this script has not yet been handled, it loads the appropriate dictionary from disk, and initializes a specialized "BreakEngine" class for that script.

There are three such specialized classes: Thai, Khmer and CJK.

Thai and Khmer use very similar approaches. They look through a dictionary that is not weighted by word frequency, and attempt to find the longest total "match" that can be made in the text.

For Chinese and Japanese text, on the other hand, we have a unified dictionary (due to the fact that both use some of the same characters, it is difficult to distinguish them) that contains information about word frequencies. The algorithm to match text then uses dynamic programming to find the set of breaks it considers "most likely" based on the frequency of the words created by the breaks. This algorithm could also be used for Thai and Khmer, but we do not have sufficient data to do so. This algorithm could also be used for Korean, but once again we do not have the data to do so.

Code of interest is in source/common/dictbe.{h, cpp}, source/common/brkeng.{h, cpp}, source/common/dictionarydata.{h, cpp}. The dictionaries use the BytesTrie and  UCharsTrie as their data store. The binary form of these dictionaries is produced by the gendict tool, which has source in source/tools/gendict.

In order to add new dictionary implementations, a few changes have to be made. First, you should create a new subclass of DictionaryBreakEngine or LanguageBreakEngine in dictbe.cpp that implements your algorithm. Then, in brkeng.cpp, you should add logic to create this dictionary break engine if we strike the appropriate script - which should only be 3 or so lines of code at the most. Lastly, you should add the correct data file. If your data is to be represented as a .dict file - as is recommended, and in fact required if you don't want to make substantial code changes to the engine loader - you need to simply add a file in the correct format for gendict to the source/data/brkitr directory, and add its name to the list of BRK_DICT_SOURCE in source/data/brkitr/ This will cause your dictionary (say, foo.txt) to be added as a UCharsTrie dictionary with the name foo.dict. If you want your dictionary to be a BytesTrie dictionary, you will need to specify a transform within the Makefile. To do so, find the part of source/data/ and source/data/makedata.mak that deals with thaidict.dict and khmerdict.dict and add a similar set of lines for your script. Lastly, in source/data/brkitr/root.txt, add a line to the dictionaries {} section of the form:


(for example, for Katakana: Kata:process(dependency){“cjdict.dict”} )

Make sure to add appropriate tests for the new implementation.