CaseConversion.java

import java.util.Locale;

/// Utilities for case conversion
public class CaseConversion {
    private CaseConversion() {
    }

    public static String convertPascalCaseToSnakeCase(String pascalCase) {
        if (pascalCase.indexOf('_') >= 0) return pascalCase; // already converted
        StringBuilder res = new StringBuilder();
        int currentIndex = 0;
        while (currentIndex < pascalCase.length()) {
            int regionStart = currentIndex;
            int regionEnd = regionStart;
            int leadingUpperChars = 0;
            // skip over uppercase characters at the beginning of the region
            for (int cp; regionEnd < pascalCase.length() && Character.isUpperCase(cp = pascalCase.codePointAt(regionEnd)); ) {
                regionEnd += Character.charCount(cp);
                leadingUpperChars += 1;
            }
            // if the region ends at the end of the string, we don't want to shrink or extend it
            if (regionEnd < pascalCase.length()) {
                // if we have multiple uppercase characters at the start,
                // they are treated as their own region,
                // with the final char removed
                if (leadingUpperChars > 1) {
                    regionEnd = pascalCase.offsetByCodePoints(regionEnd, -1);
                } else {
                    // expand region to include all remaining lowercase characters
                    int cp;
                    while (regionEnd < pascalCase.length() && Character.isLowerCase(cp = pascalCase.codePointAt(regionEnd))) {
                        regionEnd += Character.charCount(cp);
                    }
                }
            }
            String region = pascalCase.substring(regionStart, regionEnd);
            if (!res.isEmpty()) res.append("_");
            res.append(region.toLowerCase(Locale.ROOT));
            currentIndex = regionEnd;
        }
        return res.toString();
    }
}