A subclass of BreakIterator whose behavior is specified using a list of rules.

There are two kinds of rules, which are separated by semicolons: substitutions and regular expressions.

A substitution rule defines a name that can be used in place of an expression. It consists of a name, which is a string of characters contained in angle brackets, an equals sign, and an expression. (There can be no whitespace on either side of the equals sign.) To keep its syntactic meaning intact, the expression must be enclosed in parentheses or square brackets. A substitution is visible after its definition, and is filled in using simple textual substitution. Substitution definitions can contain other substitutions, as long as those substitutions have been defined first. Substitutions are generally used to make the regular expressions (which can get quite complex) shorted and easier to read. They typically define either character categories or commonly-used subexpressions.

There is one special substitution.  If the description defines a substitution called "<ignore>", the expression must be a [] expression, and the expression defines a set of characters (the "ignore characters") that will be transparent to the BreakIterator.  A sequence of characters will break the same way it would if any ignore characters it contains are taken out.  Break positions never occur before ignore characters.

A regular expression uses a subset of the normal Unix regular-expression syntax, and defines a sequence of characters to be kept together. With one significant exception, the iterator uses a longest-possible-match algorithm when matching text to regular expressions. The iterator also treats descriptions containing multiple regular expressions as if they were ORed together (i.e., as if they were separated by |).

The special characters recognized by the regular-expression parser are as follows:

*	Specifies that the expression preceding the asterisk may occur any number of times (including not at all).
{}	Encloses a sequence of characters that is optional.
()	Encloses a sequence of characters.  If followed by *, the sequence repeats.  Otherwise, the parentheses are just a grouping device and a way to delimit the ends of expressions containing |.
|	Separates two alternative sequences of characters.  Either one sequence or the other, but not both, matches this expression.  The | character can only occur inside ().
.	Matches any character.
*?	Specifies a non-greedy asterisk.  *? works the same way as *, except when there is overlap between the last group of characters in the expression preceding the * and the first group of characters following the *.  When there is this kind of overlap, * will match the longest sequence of characters that match the expression before the *, and *? will match the shortest sequence of characters matching the expression before the *?.  For example, if you have "xxyxyyyxyxyxxyxyxyy" in the text, "x[xy]*x" will match through to the last x (i.e., "xxyxyyyxyxyxxyxyxyy", but "x[xy]*?x" will only match the first two xes ("xxyxyyyxyxyxxyxyxyy").
[]	Specifies a group of alternative characters.  A [] expression will match any single character that is specified in the [] expression.  For more on the syntax of [] expressions, see below.
/	Specifies where the break position should go if text matches this expression.  (e.g., "[a-z]*/[:Zs:]*[1-0]" will match if the iterator sees a run of letters, followed by a run of whitespace, followed by a digit, but the break position will actually go before the whitespace).  Expressions that don't contain / put the break position at the end of the matching text.
\	Escape character.  The \ itself is ignored, but causes the next character to be treated as literal character.  This has no effect for many characters, but for the characters listed above, this deprives them of their special meaning.  (There are no special escape sequences for Unicode characters, or tabs and newlines; these are all handled by a higher-level protocol.  In a Java string, "\n" will be converted to a literal newline character by the time the regular-expression parser sees it.  Of course, this means that \ sequences that are visible to the regexp parser must be written as \\ when inside a Java string.)  All characters in the ASCII range except for letters, digits, and control characters are reserved characters to the parser and must be preceded by \ even if they currently don't mean anything.
!	If ! appears at the beginning of a regular expression, it tells the regexp parser that this expression specifies the backwards-iteration behavior of the iterator, and not its normal iteration behavior.  This is generally only used in situations where the automatically-generated backwards-iteration behavior doesn't produce satisfactory results and must be supplemented with extra client-specified rules.
(all others)	All other characters are treated as literal characters, which must match the corresponding character(s) in the text exactly.

Within a [] expression, a number of other special characters can be used to specify groups of characters:

-	Specifies a range of matching characters.  For example "[a-p]" matches all lowercase Latin letters from a to p (inclusive).  The - sign specifies ranges of continuous Unicode numeric values, not ranges of characters in a language's alphabetical order: "[a-z]" doesn't include capital letters, nor does it include accented letters such as a-umlaut.
::	A pair of colons containing a one- or two-letter code matches all characters in the corresponding Unicode category.  The two-letter codes are the same as the two-letter codes in the Unicode database (for example, "[:Sc::Sm:]" matches all currency symbols and all math symbols).  Specifying a one-letter code is the same as specifying all two-letter codes that begin with that letter (for example, "[:L:]" matches all letters, and is equivalent to "[:Lu::Ll::Lo::Lm::Lt:]").  Anything other than a valid two-letter Unicode category code or a single letter that begins a Unicode category code is illegal within colons.
[]	[] expressions can nest.  This has no effect, except when used in conjunction with the ^ token.
^	Excludes the character (or the characters in the [] expression) following it from the group of characters.  For example, "[a-z^p]" matches all Latin lowercase letters except p.  "[:L:^[\u4e00-\u9fff]]" matches all letters except the Han ideographs.
(all others)	All other characters are treated as literal characters.  (For example, "[aeiou]" specifies just the letters a, e, i, o, and u.)

For a more complete explanation, see Text Boundary Analysis in Java by Richard Gillam.   For examples, see the resource data (which is annotated).