<i18n dev> RFR: 8365675: Add String Unicode Case-Folding Support

[Naoto Sato]

On Fri, 3 Oct 2025 19:56:22 GMT, Xueming Shen <sherman at openjdk.org> wrote:

> ### Summary
> 
> Case folding is a key operation for case-insensitive matching (e.g., string equality, regex matching), where the goal is to eliminate case distinctions without applying locale or language specific conversions.
> 
> Currently, the JDK does not expose a direct API for Unicode-compliant case folding. Developers now rely on methods such as:
> 
> **String.equalsIgnoreCase(String)**
> 
> - Unicode-aware, locale-independent.
> - Implementation uses Character.toLowerCase(Character.toUpperCase(int)) per code point.
> - Limited: does not support 1:M mapping defined in Unicode case folding.
> 
> **Character.toLowerCase(int) / Character.toUpperCase(int)**
> 
> - Locale-independent, single code point only.
> - No support for 1:M mappings.
> 
> **String.toLowerCase(Locale.ROOT) / String.toUpperCase(Locale.ROOT)**
> 
> - Based on Unicode SpecialCasing.txt, supports 1:M mappings.
> - Intended primarily for presentation/display, not structural case-insensitive matching.
> - Requires full string conversion before comparison, which is less efficient and not intended for structural matching.
> 
> **1:M mapping example, U+00DF (ß)**
> 
> - String.toUpperCase(Locale.ROOT, "ß") → "SS"
> - Case folding produces "ss", matching Unicode caseless comparison rules.
> 
> 
> jshell> "\u00df".equalsIgnoreCase("ss")
> $22 ==> false
> 
> jshell> "\u00df".toUpperCase(Locale.ROOT).toLowerCase(Locale.ROOT).equals("ss")
> $24 ==> true
> 
> 
> ### Motivation & Direction
> 
> Add Unicode standard-compliant case-less comparison methods to the String class, enabling & improving reliable and efficient Unicode-aware/compliant case-insensitive matching.
> 
> - Unicode-compliant **full** case folding.
> - Simpler, stable and more efficient case-less matching without workarounds.
> - Brings Java's string comparison handling in line with other programming languages/libraries.
> 
> This PR proposes to introduce the following comparison methods in `String` class
> 
> - boolean equalsFoldCase(String anotherString)
> - int compareToFoldCase(String anotherString)
> - Comparator<String> UNICODE_CASEFOLD_ORDER
> 
> These methods are intended to be the preferred choice when Unicode-compliant case-less matching is required.
> 
> *Note: An early draft also proposed a String.toCaseFold() method returning a new case-folded string.
> However, during review this was considered error-prone, as the resulting string could easily be mistaken for a general transformation like toLowerCase() and then passed into APIs where case-folding semantics are not appropriate.
> 
> ### The New API
> 
> 
>    /**
>      * Compares thi...

While working on Unicode 17 upgrade, I noticed that they changed the example from "MASSE"/"Maße" to "FUSS"/"Fuß" (https://www.unicode.org/L2/L2025/25085.htm#183-A59), so you might want to switch them as well

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PR Comment: https://git.openjdk.org/jdk/pull/27628#issuecomment-3378153999