Number Format (format and parse)

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Formatter types: URBNF_SPELLOUT, which creates a formatter that spells out a value in words in the desired language, URBNF_ORDINAL, which attaches an ordinal suffix from the desired language to the end of a number (e.g. "123rd"), URBNF_DURATION, which formats a duration in seconds as hours, minutes, and seconds, and URBNF_NUMBERING_SYSTEM, which is used to invoke rules for alternate numbering systems such as the Hebrew numbering system, or for Roman Numerals, etc.

icu::RuleBasedNumberFormat::RuleBasedNumberFormat   (   URBNFRuleSetTag     tag,
        const Locale &      locale,
        UErrorCode &    status 
    )   

virtual UnicodeString &     format (double number, UnicodeString &toAppendTo, FieldPosition &pos) const

parse: lenient...

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or maybe just a MessageForrmat?

http://userguide.icu-project.org/formatparse/messages

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By the way, number formatting is not currently used, but setting -DUCONFIG_NO_FORMATTING would also disable TimeZone, date/time formatting, etc.

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https://unicode-org.github.io/icu/userguide/format_parse/numbers/

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https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/numberformatter_8h.html#details