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https://issues.apache.org/jira/browse/LUCENE-9370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17107169#comment-17107169
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Jim Ferenczi commented on LUCENE-9370:
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+1
> RegExpQuery should error for inappropriate use of \ character in input
> ----------------------------------------------------------------------
>
> Key: LUCENE-9370
> URL: https://issues.apache.org/jira/browse/LUCENE-9370
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/search
> Affects Versions: master (9.0)
> Reporter: Mark Harwood
> Priority: Minor
>
> The RegExp class is too lenient in parsing user input which can confuse or
> mislead users and cause backwards compatibility issues as we enhance regex
> support.
> In normal regular expression syntax the backslash is used to:
> * escape a reserved character like \.
> * use certain unreserved characters in a shorthand context e.g. \d means
> digits [0-9]
>
> The leniency bug in RegExp is that it adds an extra rule to this list - any
> backslashed characters that don't satisfy the above rules are taken
> literally. For example, there's no reason to put a backslash in front of the
> letter "p" but we accept \p as the letter p.
> Java's Pattern class will throw a parse exception given a meaningless
> backslash like \p.
> We should too.
> In [Lucene-9336|https://issues.apache.org/jira/browse/LUCENE-9336] we added
> support for commonly supported regex expressions like `\d`. Sadly this is a
> breaking change because of the leniency that has allowed \d to be accepted as
> the letter d without an exception. Users were likely silently missing results
> they were hoping for and we made a BWC problem for ourselves in filling in
> the gaps.
> I propose we do like other RegEx parsers and error on inappropriate use of
> backslashes.
> This will be another breaking change so should target 9.0
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