alhudz opened a new pull request, #1725:
URL: https://github.com/apache/commons-lang/pull/1725
`CharSequenceUtils.regionMatches` is the green stand-in for
`String.regionMatches` used whenever an argument is not a `String`, but its
`ignoreCase` branch folds case one UTF-16 `char` at a time. The two halves of a
supplementary code point are surrogates with no case mapping, so a
case-insensitive compare of a supplementary letter never matches. Repro with
`U+10400` (`𐐀`, Deseret capital long I) and its lowercase `U+10428` (`𐐨`):
`StringUtils.equalsIgnoreCase("𐐀", "𐐨")` returns `true` through the `String`
fast path, yet `equalsIgnoreCase(new StringBuilder("𐐀"), "𐐨")` returns `false`.
`startsWithIgnoreCase`, `endsWithIgnoreCase`, `containsIgnoreCase` and
`Strings.CI.indexOf` go the same way, so the answer depends on the argument's
runtime type.
On a `char`-level mismatch the fix rebuilds the full code point from the
adjacent surrogate and folds at the code point level, which is what
`String.regionMatches` itself does, so the green path lines back up with the
`String`/`String` path. Cross-checked against `String.regionMatches` over a few
million randomised BMP, supplementary and lone-surrogate inputs with no
remaining divergence; the added `CharSequenceUtilsTest` cases fail before the
change and pass after.
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