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https://issues.apache.org/jira/browse/OPENNLP-1985?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kristian Rickert reassigned OPENNLP-1985:
-----------------------------------------
Assignee: Kristian Rickert
> StringUtil.toUpperCase does not uppercase supplementary Unicode letters
> -----------------------------------------------------------------------
>
> Key: OPENNLP-1985
> URL: https://issues.apache.org/jira/browse/OPENNLP-1985
> Project: OpenNLP
> Issue Type: Bug
> Reporter: Kristian Rickert
> Assignee: Kristian Rickert
> Priority: Major
> Fix For: 2.5.13, 3.0.0
>
>
> h2. Problem
> {{StringUtil.toUpperCase(CharSequence)}} iterates UTF-16 chars and calls
> {{Character.toUpperCase(char)}}. Supplementary letters occupy a surrogate
> pair,
> so lowercase letters outside the BMP are returned unchanged.
> h2. Reproduction
> {code:java}
> String input = new String(Character.toChars(0x10428));
> String result = StringUtil.toUpperCase(input);
> // Expected: U+10400 (DESERET CAPITAL LETTER LONG I)
> // Actual: U+10428 (DESERET SMALL LETTER LONG I)
> {code}
> The affected implementation is present on both branches checked on 2026-09-19:
> * {{opennlp-2.x}} at {{26f394a6ab01bf757334ba88f03bc9e2a42f6c27}}.
> * {{main}} at {{f2ab1ccd435c20e910008511a37467131683e9c7}}.
> The failure was reproduced against the exact 2.x source. These branch checks
> are not a determination of the earliest affected released version.
> h2. Proposed fix and validation
> Use a code-point cursor and {{Character.toUpperCase(int)}}. Preserve simple
> Unicode case mapping without multi-character expansions.
> Add regressions for supplementary letters, mixed BMP/supplementary text,
> unpaired surrogates and non-String CharSequence inputs. Verify that the source
> is not modified and that existing BMP behavior remains correct.
> Keep public signatures and null-input exception behavior unchanged. Changes to
> lowercase conversion or dependent callers are outside this issue.
> Prepare focused fixes for main and opennlp-2.x under this issue.
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