Laurent Dami

NAME

Search::QueryParser - parses a query string into a data structure suitable for external search engines

SYNOPSIS

  my $qp = new Search::QueryParser;
  my $s = '+mandatoryWord -excludedWord +field:word "exact phrase"';
  my $query = $qp->parse($s)  or die "Error in query : " . $qp->err;
  $someIndexer->search($query);

  # query with comparison operators and implicit plus (second arg is true)
  $query = $qp->parse("txt~'^foo.*' date>='01.01.2001' date<='02.02.2002'", 1);

  # boolean operators (example below is equivalent to "+a +(b c) -d")
  $query = $qp->parse("a AND (b OR c) AND NOT d");

  # subset of rows
  $query = $qp->parse("Id#123,444,555,666 AND (b OR c)");

DESCRIPTION

This module parses a query string into a data structure to be handled by external search engines. For examples of such engines, see File::Tabular and Search::Indexer.

The query string can contain simple terms, "exact phrases", field names and comparison operators, '+/-' prefixes, parentheses, and boolean connectors.

The parser can be parameterized by regular expressions for specific notions of "term", "field name" or "operator" ; see the new method. The parser has no support for lemmatization or other term transformations : these should be done externally, before passing the query data structure to the search engine.

The data structure resulting from a parsed query is a tree of terms and operators, as described below in the parse method. The interpretation of the structure is up to the external search engine that will receive the parsed query ; the present module does not make any assumption about what it means to be "equal" or to "contain" a term.

QUERY STRING

The query string is decomposed into "items", where each item has an optional sign prefix, an optional field name and comparison operator, and a mandatory value.

Sign prefix

Prefix '+' means that the item is mandatory. Prefix '-' means that the item must be excluded. No prefix means that the item will be searched for, but is not mandatory.

As far as the result set is concerned, +a +b c is strictly equivalent to +a +b : the search engine will return documents containing both terms 'a' and 'b', and possibly also term 'c'. However, if the search engine also returns relevance scores, query +a +b c might give a better score to documents containing also term 'c'.

See also section "Boolean connectors" below, which is another way to combine items into a query.

Field name and comparison operator

Internally, each query item has a field name and comparison operator; if not written explicitly in the query, these take default values '' (empty field name) and ':' (colon operator).

Operators have a left operand (the field name) and a right operand (the value to be compared with); for example, foo:bar means "search documents containing term 'bar' in field 'foo'", whereas foo=bar means "search documents where field 'foo' has exact value 'bar'".

Here is the list of admitted operators with their intended meaning :

:

treat value as a term to be searched within field. This is the default operator.

~ or =~

treat value as a regex; match field against the regex.

!~

negation of above

== or =, <=, >=, !=, <, >

classical relational operators

#

Inclusion in the set of comma-separated integers supplied on the right-hand side.

Operators :, ~, =~, !~ and # admit an empty left operand (so the field name will be ''). Search engines will usually interpret this as "any field" or "the whole data record".

Value

A value (right operand to a comparison operator) can be

  • just a term (as recognized by regex rxTerm, see new method below)

  • A quoted phrase, i.e. a collection of terms within single or double quotes.

    Quotes can be used not only for "exact phrases", but also to prevent misinterpretation of some values : for example -2 would mean "value '2' with prefix '-'", in other words "exclude term '2'", so if you want to search for value -2, you should write "-2" instead. In the last example of the synopsis, quotes were used to prevent splitting of dates into several search terms.

  • a subquery within parentheses. Field names and operators distribute over parentheses, so for example foo:(bar bie) is equivalent to foo:bar foo:bie. Nested field names such as foo:(bar:bie) are not allowed. Sign prefixes do not distribute : +(foo bar) +bie is not equivalent to +foo +bar +bie.

Boolean connectors

Queries can contain boolean connectors 'AND', 'OR', 'NOT' (or their equivalent in some other languages). This is mere syntactic sugar for the '+' and '-' prefixes : a AND b is translated into +a +b; a OR b is translated into (a b); NOT a is translated into -a. +a OR b does not make sense, but it is translated into (a b), under the assumption that the user understands "OR" better than a '+' prefix. -a OR b does not make sense either, but has no meaningful approximation, so it is rejected.

Combinations of AND/OR clauses must be surrounded by parentheses, i.e. (a AND b) OR c or a AND (b OR c) are allowed, but a AND b OR c is not.

METHODS

new
  new(rxTerm   => qr/.../, rxOp => qr/.../, ...)

Creates a new query parser, initialized with (optional) regular expressions :

rxTerm

Regular expression for matching a term. Of course it should not match the empty string. Default value is qr/[^\s()]+/. A term should not be allowed to include parenthesis, otherwise the parser might get into trouble.

rxField

Regular expression for matching a field name. Default value is qr/\w+/ (meaning of \w according to use locale).

rxOp

Regular expression for matching an operator. Default value is qr/==|<=|>=|!=|=~|!~|:|=|<|>|~/. Note that the longest operators come first in the regex, because "alternatives are tried from left to right" (see "Version 8 Regular Expressions" in perlre) : this is to avoid a<=3 being parsed as a < '=3'.

rxOpNoField

Regular expression for a subset of the operators which admit an empty left operand (no field name). Default value is qr/=~|!~|~|:/. Such operators can be meaningful for comparisons with "any field" or with "the whole record" ; the precise interpretation depends on the search engine.

rxAnd

Regular expression for boolean connector AND. Default value is qr/AND|ET|UND|E/.

rxOr

Regular expression for boolean connector OR. Default value is qr/OR|OU|ODER|O/.

rxNot

Regular expression for boolean connector NOT. Default value is qr/NOT|PAS|NICHT|NON/.

defField

If no field is specified in the query, use defField. The default is the empty string "".

parse
  $q = $queryParser->parse($queryString, $implicitPlus);

Returns a data structure corresponding to the parsed string. The second argument is optional; if true, it adds an implicit '+' in front of each term without prefix, so parse("+a b c -d", 1) is equivalent to parse("+a +b +c -d"). This is often seen in common WWW search engines as an option "match all words".

The return value has following structure :

  { '+' => [{field=>'f1', op=>':', value=>'v1', quote=>'q1'}, 
            {field=>'f2', op=>':', value=>'v2', quote=>'q2'}, ...],
    ''  => [...],
    '-' => [...]
  }

In other words, it is a hash ref with 3 keys '+', '' and '-', corresponding to the 3 sign prefixes (mandatory, ordinary or excluded items). Each key holds either a ref to an array of items, or undef (no items with this prefix in the query).

An item is a hash ref containing

field

scalar, field name (may be the empty string)

op

scalar, operator

quote

scalar, character that was used for quoting the value ('"', "'" or undef)

value

Either

  • a scalar (simple term), or

  • a recursive ref to another query structure. In that case, op is necessarily '()' ; this corresponds to a subquery in parentheses.

In case of a parsing error, parse returns undef; method err can be called to get an explanatory message.

err
  $msg = $queryParser->err;

Message describing the last parse error

unparse
  $s = $queryParser->unparse($query);

Returns a string representation of the $query data structure.

AUTHOR

Laurent Dami, <laurent.dami AT etat ge ch>

COPYRIGHT AND LICENSE

Copyright (C) 2005, 2007 by Laurent Dami.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.




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