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https://issues.apache.org/jira/browse/LUCENE-1029?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12534839
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DM Smith commented on LUCENE-1029:
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Transliteration rules are language dependent. I suggest that the documentation 
for the ISOLatin1AccentFilter be adjusted to match it's behavior, stating that 
it strips diacritics from characters and does further substitutions (giving the 
precise list) and that it does not do transliteration. Further give examples as 
stated in the above comments that the results for such a stripping may result 
in examples that are entirely inappropriate.

ICU4J can be used to do per language transliteration.  IIRC, dependency on 
third party code is allowed in contrib. So, it would be appropriate for such 
filters to be in contrib.


> Illegal character replacements in ISOLatin1AccentFilter
> -------------------------------------------------------
>
>                 Key: LUCENE-1029
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1029
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Analysis
>    Affects Versions: 2.2
>            Reporter: Marko Asplund
>
> The ISOLatin1AccentFilter class is responsible for replacing "accented 
> characters in the ISO Latin 1 character set by their unaccented equivalent".
> Some of the replacements performed for scandinavian characters (used e.g. in 
> the finnish, swedish, danish languages etc.) are illegal. The scandinavian 
> characters are different from the accented characters used e.g. in latin 
> based languages such as french in that these characters (ä, ö, å) represent 
> entirely independent sounds in the language and therefore cannot be 
> represented with any other sound without change of meaning. It is therefore 
> illegal to replace these characters with any other character.
> This means for example that you can't change the finnish word sää (weather) 
> to saa (will have) because these are two entirely different words with 
> different meaning. The same applies to scandinavian languages as well.
> There's no connection between the sounds represented by ä and a; ö and o or å 
> and a. 
> In addition to the three characters mentioned above danish and norwegian use 
> other special characters such as ø and æ. It should be checked if the 
> replacement is legal for these characters.

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