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From "Simon Willnauer (JIRA)" <>
Subject [jira] Commented: (LUCENE-2039) Regex support and beyond in JavaCC QueryParser
Date Fri, 13 Nov 2009 11:36:39 GMT


Simon Willnauer commented on LUCENE-2039:


    regexp:title:"/blah[a-z]+[0-9]+/" <- regexp extension, title index field
    complex_phrase:title:"(sun OR sunny) sky" <- complex_phrase extension, title index

    regexp_phrase::"/blah[a-z]+[0-9]+/" <- regexp extension, default field
    complex_phrase::"(sun OR sunny) sky" <- complex_phrase extension, default field

    title:"blah" <- regular field query

This is pretty much what I suggested above. We can extend the queryparser without breaking
the backwards compatibility just by adding some code which is aware of the fieldname scheme.
Even this could be extendable. FieldNames are terms and therefore they can not contain unescaped
special chars like : { ] ... I would not even hard code the separator into the query parser
but have the field name processed by something pluggable. So If somebody wants to have a regex
extension they could use re\:field: or re\:: or re_field:.... 
Escaping a field is easy, just like you would do it with a term. 
More interesting is that we do not change any syntax, no special character but we can add
a default implementation with a default implementation for extensions. This could be a whole
API which takes are of creating and escaping the field name, building the query once it is
passed to the extension etc. 
In a first step we can resolve the extension the second step calls the extension and build
the query. If no extension is registered the query parser works like in previous versions
so it is all up to the user.

The only part I disagree is when you pass the fieldname to the extension parser, I wouldn't
implement that on the contrib parser, because it assumes the syntax always has field names.
Anyway, for the core QP, I see the reason why you pass the fieldname

You need the field to create you query in the extension, the field will always be set to either
the default field or the explicitly defined field in the query. No reason why we should not
pass it.
I agree with you that we should wrap the information in a class so that we do not need to
change the method signature if something has to be changed in the future. Instead we just
add a new member to the wrapper though.

> Regex support and beyond in JavaCC QueryParser
> ----------------------------------------------
>                 Key: LUCENE-2039
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: QueryParser
>            Reporter: Simon Willnauer
>            Assignee: Grant Ingersoll
>            Priority: Minor
>             Fix For: 3.1
>         Attachments: LUCENE-2039.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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