Summary

Case folding is a key operation for case-insensitive matching (e.g., string comparison, 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:

(1) 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.

(2) Character.toLowerCase(int) / Character.toUpperCase(int)
    - Locale-independent, single code point only.
    - No support for 1:M mappings.

(3) 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.


Example. 1:M mapping, 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

Adding the unicode compliant comparison methods in JDK brings Java in line with other languages / libraries and makes Unicode-compliant case-less matching simpler and more efficient.

    - Unicode-compliant full case folding.
    - Simpler, stable and more efficient case-less matching without workarounds.
    - Consistency with other programming languages/libraries.

This enhancement proposes to introduce the following comparison APIs in String class,

    - boolean equalsCaseFold(String anotherString)
    - int compareToCaseFold(String anotherString)
    - Comparator 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 this {@code String} to another {@code String} for equality,
     * using <em>Unicode case folding</em>. Two strings are considered equal
     * by this method if their case-folded forms are identical.
     * <p>
     * Case folding is defined by the Unicode Standard in
     * <a href="https://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt">CaseFolding.txt</a>,
     * including 1:M mappings. For example, {@code "Maße".equalsFoldCase("MASSE")}
     * returns {@code true}, since the character {@code U+00DF} (sharp s) folds
     * to {@code "ss"}.
     * <p>
     * Case folding is locale-independent and language-neutral, unlike
     * locale-sensitive transformations such as {@link #toLowerCase()} or
     * {@link #toUpperCase()}. It is intended for caseless matching,
     * searching, and indexing.
     *
     * @apiNote
     * This method is the Unicode-compliant alternative to
     * {@link #equalsIgnoreCase(String)}. It implements full case folding as
     * defined by the Unicode Standard, which may differ from the simpler
     * per-character mapping performed by {@code equalsIgnoreCase}.
     * For example:
     * <pre>{@snippet lang=java :
     * String a = "Maße";
     * String b = "MASSE";
     * boolean equalsFoldCase = a.equalsFoldCase(b); // returns true
     * boolean equalsIgnoreCase = a.equalsIgnoreCase(b); // returns false
     * }</pre>
     *
     * @param anotherString
     * The {@code String} to compare this {@code String} against
     *
     * @return {@code true} if the given object is not {@code null} and represents
     * the same sequence of characters as this string under Unicode case
     * folding; {@code false} otherwise.
     *
     * @see #compareToFoldCase(String)
     * @see #equalsIgnoreCase(String)
     * @since 26
     */
    public boolean equalsFoldCase(String anotherString)

    /**
     * Compares two strings lexicographically using <em>Unicode case folding</em>.
     * This method returns an integer whose sign is that of calling {@code compareTo}
     * on the Unicode case folded version of the strings. Unicode Case folding
     * eliminates differences in case according to the Unicode Standard, using the
     * mappings defined in
     * <a href="https://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt">CaseFolding.txt</a>,
     * including 1:M mappings, such as {@code"ß"} → {@code }"ss"}.
     * <p>
     * Case folding is a locale-independent, language-neutral form of case mapping,
     * primarily intended for caseless matching. Unlike {@link #compareToIgnoreCase(String)},
     * which applies a simpler locale-insensitive uppercase mapping. This method
     * follows the Unicode <em>full</em> case folding, providing stable and
     * consistent results across all environments.
     * <p>
     * Note that this method does <em>not</em> take locale into account, and may
     * produce results that differ from locale-sensitive ordering. Use
     * {@link java.text.Collator} for locale-sensitive comparison.
     *
     * @apiNote
     * This method is the Unicode-compliant alternative to
     * {@link #compareToIgnoreCase(String)}. It implements the <em>full</em> case folding
     * as defined by the Unicode Standard, which may differ from the simpler
     * per-character mapping performed by {@code compareToIgnoreCase}.
     * For example:
     * <pre>{@snippet lang=java :
     * String a = "Maße";
     * String b = "MASSE";
     * int cmpFoldCase = a.compareToFoldCase(b); // returns 0
     * int cmpIgnoreCase = a.compareToIgnoreCase(b); // returns > 0
     * }</pre>
     *
     * @param str the {@code String} to be compared.
     * @return a negative integer, zero, or a positive integer as the specified
     * String is greater than, equal to, or less than this String,
     * ignoring case considerations by case folding.
     * @see #equalsFoldCase(String)
     * @see #compareToIgnoreCase(String)
     * @see java.text.Collator
     * @since 26
     */
    public int compareToFoldCase(String str)

    /**
     * A Comparator that orders {@code String} objects as by
     * {@link #compareToFoldCase(String) compareToFoldCase()}.
     *
     * @see #compareToFoldCase(String)
     * @since 26
     */
    public static final Comparator<String> UNICODE_CASEFOLD_ORDER;


Refs

 - Unicode Standard 5.18.4 Caseless Matching (https://www.unicode.org/versions/latest/core-spec/chapter-5/#G21790)
 - Unicode® Standard Annex #44: 5.6 Case and Case Mapping (https://www.unicode.org/reports/tr44/#Casemapping)
 - Unicode Technical Standard #18: Unicode Regular Expressions RL1.5: Simple Loose Matches (https://www.unicode.org/reports/tr18/#Simple_Loose_Matches)
 - Unicode SpecialCasing.txt (https://www.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt)
 - Unicode CaseFolding.txt (https://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt)

Other Languages

(1) Python string.casefold()

The str.casefold() method in Python returns a casefolded version of a string. Casefolding is a more aggressive form of lowercasing, designed to remove all case distinctions in a string, particularly for the purpose of caseless string comparisons.

(2) Perl’s fc()

Returns the casefolded version of EXPR. This is the internal function implementing the \F escape in double-quoted strings.
Casefolding is the process of mapping strings to a form where case differences are erased; comparing two strings in their casefolded form is effectively a way of asking if two strings are equal, regardless of case.
Perl only implements the full form of casefolding, but you can access the simple folds using "casefold()" in Unicode::UCD] ad "prop_invmap()" in Unicode::UCD].

(3) ICU4J UCharacter.foldCase (Java)

Purpose: Provides extensions to the standard Java Character class, including support for more Unicode properties and handling of supplementary characters (code points beyond U+FFFF).
Method Signature (String based): public static String foldCase(String str, int options)
Method Signature (CharSequence & Appendable based): public static A foldCase(CharSequence src, A dest, int options, Edits edits)
Key Features:
Case Folding: Converts a string to its case-folded equivalent.
Locale Independent: Case folding in UCharacter.foldCase is generally not dependent on locale settings.
Context Insensitive: The mapping of a character is not affected by surrounding characters.
Turkic Option: An option exists to include or exclude special mappings for Turkish/Azerbaijani text.
Result Length: The resulting string can be longer or shorter than the original.
Edits Recording: Allows for recording of edits for index mapping, styled text, and getting only changes.

(4) u_strFoldCase (C/C++)

A lower-level C API function for case folding a string.
Case Folding Options: Similar options as UCharacter.foldCase for controlling case folding behavior.
Availability: Found in the ustring.h and unistr.h headers in the ICU4C library.