[DOCS] Clarify regex character range case insensitivity limitations (elastic#125412)

Author: leemthompo

docs/reference/query-dsl/regexp-syntax.asciidoc

@@ -181,6 +181,15 @@ example:
 [^-abc]     # matches any character except '-', 'a', 'b', or 'c'
 [^abc\-]    # matches any character except 'a', 'b', 'c', or '-'
 ....
+
+[NOTE]
+====
+Character range classes such as `[a-c]` do not behave as expected when using `case_insensitive: true` — they remain case sensitive.
+For example, `[a-c]+` with `case_insensitive: true` will match strings containing only the characters 'a', 'b', and 'c', but not 'A', 'B', or 'C'. 
+Use `[a-zA-Z]` to match both uppercase and lowercase characters.
+This is due to a known limitation in Lucene's regular expression engine.
+See https://github.com/apache/lucene/issues/14378[Lucene issue #14378] for details.
+====
 --
 
 [discrete]