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https://issues.apache.org/jira/browse/LUCENE-1343?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12622607#action_12622607
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Robert Haschart commented on LUCENE-1343:
-----------------------------------------
The UnicodeNormalizationFilter does use the decompose normalization
portion of the icu4j library as a starting point. However even with
that there are several instances where the normalizer code does not
decompose a character into an unaccented character and a accent mark, a
notable one being ( Ł -> L ) so the UnicodeNormalizationFilter start
with the approach you outlined, perform a decompose normalization
followed by discarding all non-spacing modifier characters, and then can
go on from there to further normalize the data by folding the additional
characters that aren't handled by the decompose normalization onto their
Latin1 lookalikes.
-Robert
> A replacement for ISOLatin1AccentFilter that does a more thorough job of
> removing diacritical marks or non-spacing modifiers.
> -----------------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-1343
> URL: https://issues.apache.org/jira/browse/LUCENE-1343
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Analysis
> Reporter: Robert Haschart
> Priority: Minor
> Attachments: normalizer.jar, UnicodeCharUtil.java,
> UnicodeNormalizationFilter.java, UnicodeNormalizationFilterFactory.java
>
>
> The ISOLatin1AccentFilter takes Unicode characters that have diacritical
> marks and replaces them with a version of that character with the diacritical
> mark removed. For example é becomes e. However another equally valid way of
> representing an accented character in Unicode is to have the unaccented
> character followed by a non-spacing modifier character (like this: é )
> The ISOLatin1AccentFilter doesn't handle the accents in decomposed unicode
> characters at all. Additionally there are some instances where a word will
> contain what looks like an accented character, that is actually considered to
> be a separate unaccented character such as Ł but which to make searching
> easier you want to fold onto the latin1 lookalike version L .
> The UnicodeNormalizationFilter can filter out accents and diacritical marks
> whether they occur as composed characters or decomposed characters, it can
> also handle cases where as described above characters that look like they
> have diacritics (but don't) are to be folded onto the letter that they look
> like ( Ł -> L )
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