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https://issues.apache.org/jira/browse/LUCENE-9754?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17295633#comment-17295633
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Robert Muir commented on LUCENE-9754:
-------------------------------------
This tokenizer splits on scripts because it lets you customize the tokenization
per-script by design.
The reason is some writing systems need different approaches... or even
choices. See LUCENE-7393 for a great example.
So it is just like the "notes" section says, and I will quote:
{quote}
Normally word breaking does not require breaking between different scripts.
However, adding that capability may be useful in combination with other
extensions of word segmentation.
{quote}
And that is what we do, for that exact reason. I am guessing it confuses you
because it seems to break all kinds of "rules" (e.g. don't break between
letters).
If you want a simple state-machine based on those rules without fancy stuff,
again I recommend using StandardTokenizer instead.
This tokenizer is quite different, it will do entirely different algorithms
depending on the writing system. And you can customize that with rules and
options (e.g. break Myanmar with ICU word dictionary or with syllables)
> ICU Tokenizer: letter-space-number-letter tokenized inconsistently
> ------------------------------------------------------------------
>
> Key: LUCENE-9754
> URL: https://issues.apache.org/jira/browse/LUCENE-9754
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/search
> Affects Versions: 7.5
> Environment: Tested most recently on Elasticsearch 6.5.4.
> Reporter: Trey Jones
> Priority: Major
> Attachments: LUCENE-9754_prototype.patch
>
>
> The tokenization of strings like _14th_ with the ICU tokenizer is affected by
> the character that comes before preceeding whitespace.
> For example, _x 14th_ is tokenized as x | 14th; _ァ 14th_ is tokenized as ァ |
> 14 | th.
> In general, in a letter-space-number-letter sequence, if the writing system
> before the space is the same as the writing system after the number, then you
> get two tokens. If the writing systems differ, you get three tokens.
> -If the conditions are just right, the chunking that the ICU tokenizer does
> (trying to split on spaces to create <4k chunks) can create an artificial
> boundary between the tokens (e.g., between _ァ_ and _14th_) and prevent the
> unexpected split of the second token (_14th_). Because chunking changes can
> ripple through a long document, editing text or the effects of a character
> filter can cause changes in tokenization thousands of lines later in a
> document.- _(This inconsistency was included as a side issue that I thought
> might add more weight to the main problem I am concerned with, but it seems
> to be more of a distraction. Chunking issues should perhaps be addressed in a
> different ticket, so I'm striking it out.)_
> My guess is that some "previous character set" flag is not reset at the
> space, and numbers are not in a character set, so _t_ is compared to _ァ_ and
> they are not the same—causing a token split at the character set change—but
> I'm not sure.
>
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