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  1. Overview
    1. Instantiation
    2. Usage
    3. Rule Sets
    4. Rules


RuleBasedNumberFormat can format and parse numbers in spelled-out format, e.g. “one hundred and thirty-four”. For example:

"one hundred and thirty-four" // 134 using en_US spellout
"one hundred and thirty-fourth" // 134 using en_US ordinal
"hundertvierunddreissig" // 134 using de_DE spellout
"MCMLVIII" // custom, 1958 in roman numerals

RuleBasedNumberFormat is based on rules describing how to format a number. The rule syntax is designed primarily for formatting and parsing numbers as spelled-out text, though other kinds of formatting are possible. As a convenience, custom API is provided to allow selection from three predefined rule definitions, when available: SPELLOUT, ORDINAL, and DURATION. Users can request formatters either by providing a locale and one of these predefined rule selectors, or by specifying the rule definitions directly.

:point_right: Note: ICU provides number spellout rules for several locales, but not for all of the locales that ICU supports, and not all of the predefined rule types. Also, as of release 2.6, some of the provided rules are known to be incomplete.


Unlike the other standard number formats, there is no corresponding factory method on NumberFormat. Instead, RuleBasedNumberFormat objects are instantiated via constructors. Constructors come in two flavors, ones that take rule text, and ones that take one of the predefined selectors. Constructors that do not take a Locale parameter use the current default locale.

The following constructors are available:

  1. RuleBasedNumberFormat(int) Returns a format using predefined rules of the selected type from the current locale.

  2. RuleBasedNumberFormat(Locale, int) As above, but specifies locale.

  3. RuleBasedNumberFormat(String) Returns a format using the provided rules, and symbols (if required) from the current locale.

  4. RuleBasedNumberFormat(String, Locale) As above, but specifies locale.


RuleBasedNumberFormat can be used like other NumberFormats. For example, in Java:

double num = 2718.28;
NumberFormat formatter = 
    new RuleBasedNumberFormat(RuleBasedNumberFormat.SPELLOUT);
String result = formatter.format(num);

// output (in en_US locale):
// two thousand seven hundred and eighteen point two eight

Rule Sets

Rule descriptions can provide multiple named rule sets, for example, the rules for en_US spellout provides a ‘%simplified’ rule set that displays text without commas or the word ‘and’. Rule sets can be queried and set on a RuleBasedNumberFormat. This lets you customize a RuleBasedNumberFormat for use through its inherited NumberFormat API. For example, in Java:

You can also format a number specifying the ruleset directly, using an additional overload of format provided by RuleBasedNumberFormat. For example, in Java:

:point_right: Note: There is no standardization of rule set names, so you must either query the names, as in the first example above, or know the names that are defined in the rules for that formatter.


The following example provides a quick look at the RuleBasedNumberFormat rule syntax.

These rules format a number using standard decimal place-value notation, but using words instead of digits, e.g. 123.4 formats as ‘one two three point four’:

"-x: minus >>;\n"
+ "x.x: << point >>;\n"
+ "zero; one; two; three; four; five; six;\n"
+ "    seven; eight; nine;\n"
+ "10: << >>;\n"
+ "100: << >>>;\n"
+ "1000: <<, >>>;\n"
+ "1,000,000: <<, >>>;\n"
+ "1,000,000,000: <<, >>>;\n"
+ "1,000,000,000,000: <<, >>>;\n"
+ "1,000,000,000,000,000: =#,##0=;\n";

Rulesets are invoked by first applying negative and fractional rules, and then using a recursive process. It starts by finding the rule whose range includes the current value and applying that rule. If the rule so directs, it emits text, including text obtained by recursing on new values as directed by the rule. As you can see, the rules are designed to accomodate recursive processing of numbers, and so are best suited for formatting numbers in ways that are inherently recursive.

A full explanation of this example can be found in the RuleBasedNumberFormat examples. A complete description of the rule syntax can be found in the RuleBasedNumberFormat API Documentation.