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From "Simon Willnauer (JIRA)" <>
Subject [jira] Issue Comment Edited: (LUCENE-2039) Regex support and beyond in JavaCC QueryParser
Date Fri, 11 Dec 2009 16:41:18 GMT


Simon Willnauer edited comment on LUCENE-2039 at 12/11/09 4:41 PM:

Simon, it sounds like the right direction! But relying on ExtensionParser implementors to
manually copy all the parent settings to the child seems like a maintenance problem. We add
new settings relatively often. Unfortunately there's nothing like the token Attribute stuff
for QueryParser...
yeah that is the reason why I suggested passing the top level parser instance itself to the
extension and expose it there.

Umm, I don't quite follow. If the extension wraps an existing parser, the extension would
have to subclass it, override all the getters/setters to delegate to the parent, and trust
that everyone uses them? That's not currently true - for example QueryParser.getPrefixQuery()
directly accesses allowLeadingWildcard, without using the getter. There are probably other
cases too, that's just the first one I checked.

David here is an example of what I mean by wrapping the top level parser. This would give
you access to all the settings right away inside your extension.

      was (Author: simonw):
    David here is an example of what I mean by wrapping the top level parser. This would give
you access to all the settings right away inside your extension.
> Regex support and beyond in JavaCC QueryParser
> ----------------------------------------------
>                 Key: LUCENE-2039
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: QueryParser
>            Reporter: Simon Willnauer
>            Assignee: Simon Willnauer
>            Priority: Minor
>             Fix For: 3.1
>         Attachments: LUCENE-2039.patch, LUCENE-2039_field_ext.patch, LUCENE-2039_field_ext.patch,
LUCENE-2039_field_ext.patch, LUCENE-2039_field_ext.patch, LUCENE-2039_field_ext.patch, LUCENE-2039_wrap_master_parser.patch
> Since the early days the standard query parser was limited to the queries living in core,
adding other queries or extending the parser in any way always forced people to change the
grammar file and regenerate. Even if you change the grammar you have to be extremely careful
how you modify the parser so that other parts of the standard parser are affected by customisation
changes. Eventually you had to live with all the limitation the current parser has like tokenizing
on whitespaces before a tokenizer / analyzer has the chance to look at the tokens. 
> I was thinking about how to overcome the limitation and add regex support to the query
parser without introducing any dependency to core. I added a new special character that basically
prevents the parser from interpreting any of the characters enclosed in the new special characters.
I choose the forward slash  '/' as the delimiter so that everything in between two forward
slashes is basically escaped and ignored by the parser. All chars embedded within forward
slashes are treated as one token even if it contains other special chars like * []?{} or whitespaces.
This token is subsequently passed to a pluggable "parser extension" with builds a query from
the embedded string. I do not interpret the embedded string in any way but leave all the subsequent
work to the parser extension. Such an extension could be another full featured query parser
itself or simply a ctor call for regex query. The interface remains quiet simple but makes
the parser extendible in an easy way compared to modifying the javaCC sources.
> The downsides of this patch is clearly that I introduce a new special char into the syntax
but I guess that would not be that much of a deal as it is reflected in the escape method
though. It would truly be nice to have more than once extension an have this even more flexible
so treat this patch as a kickoff though.
> Another way of solving the problem with RegexQuery would be to move the JDK version of
regex into the core and simply have another method like:
> {code}
> protected Query newRegexQuery(Term t) {
>   ... 
> }
> {code}
> which I would like better as it would be more consistent with the idea of the query parser
to be a very strict and defined parser.
> I will upload a patch in a second which implements the extension based approach I guess
I will add a second patch with regex in core soon too.

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