Trey Jones created LUCENE-9754:
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Summary: 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
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.
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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