Kenneth Ölwing

NAME

Grep::Query - Query logic for lists of scalars/objects

VERSION

Version 1.005

SYNOPSIS

  use Grep::Query qw(qgrep);
  
  my @data = ( 'a' .. 'z' );
  my @result;

  # very simple query equal to a standard "grep(/[dkob]/, @data)"
  #
  @result = qgrep('REGEXP([dkob])', @data);
  #
  # @result contains ( 'd', 'k', 'o', 'b' )
  
  # go more wild
  #
  @result = qgrep('REGEXP([dkob]) AND ( REGEXP([yaxkz]) OR REGEXP([almn]) )', @data);
  #
  # @result contains ( 'k' )

  # or use it in OO fashion
  #
  my $gq = Grep::Query->new('REGEXP([dkob]) AND ( REGEXP([yaxkz]) OR REGEXP([almn]) )');
  @result = $gq->qgrep(@data);
  
  # also query a list of objects, and use numerical comparisons too
  #
  my @persons = ...; # assume person objects can respond to '->getName()' and '->calculateAge()'
  
  # create a query object - note that the syntax now references 'field' names of name/age in the query
  #
  my $personQuery = Grep::Query->new('name.REGEXP(^A) AND age.>=(42)');
  
  # set up a field accessor to teach G::Q how to match field names to whatever's needed to get data from the objects
  #
  my $fieldAccessor = Grep::Query::FieldAccessor->new();
  $fieldAccessor->add('name', sub { $_[0]->getName() });
  $fieldAccessor->add('age', sub { $_[0]->calculateAge() });
  
  # now execute the query by passing the field accessor before the person list
  #
  @result = $personQuery->qgrep($fieldAccessor, @persons);
  #
  # @result contains a list of person objects that has a name starting with 'A' and an age greater than or equal to 42
  
  # If what you have is a single hash (rather than a list of them) and you wish to query it and pick out key/values
  # that matches, the query is special cased for passing just a single hash.
  # A field accessor is necessary, and it will receive individual key/value pairs as small lists.
  # 
  # Assume a %videos hash, keyed by video name, and value is another hash with at least the key 'length' holding the video
  # length in seconds...:
  #
  my $fieldAccessor = Grep::Query::FieldAccessor->new();
  $fieldAccessor->add('key', sub { $_[0]->[0] });
  $fieldAccessor->add('length', sub { $_[0]->[1]->{length} });
  my $videoQuery = Grep::Query->new('key.REGEXP(^Alias) AND length.gt(2500)');
  @result = $videoQuery->qgrep($fieldAccessor, \%videos);
  #
  # $result[0] contains a hash ref with all videos with name starting with 'Alias' and at least 2500 seconds long
    

BACKGROUND

Why use this module when you could easily write a grep BLOCK or plain regexp EXPR to select things in a list using whatever criteria you desired?

The original use-case was this:

Given a number of commandline tools I provide to users in my workplace, quite frequently I wanted the user to be able to express, with some flag(s), a selection among a list of 'somethings' computed at runtime - the most common probably a list of file/directory names. It was also common to have this type of filtering defined in various configuration files and persistently apply them every time a command was run.

Example: the user gives the command:

  SomeCommand /some/path

The 'SomeCommand' may, for example, scan the given path and for all files it finds it will do something useful. So, I also wanted to provide flags for the command such that they can say...

  SomeCommand -exclude 'some_regexp' /some/path

...in order to filter the list of files that should be worked on.

Obviously not a problem, and I also provided the reverse if that was more convenient:

  SomeCommand -include 'another_regexp' /some/path

And the idea was extended so flags could be given multiple times and interweaved:

  SomeCommand -include 'rx1' -exclude 'rx2' -include 'rx3' ... /some/path

Thus, the original set was shrunk by first selecting only those matching the regexp rx1 and then shrink that by excluding those matching rx2 etc. - I think you get the idea.

What I found however is that it becomes hard to string together regexps to find the exact subset you want when the rules are a bit more complex. In fact, while regexps are powerful, they're not that suited to easily mix multiple of them (and some expressions are basically impossible, e.g. 'I want this but not this'), especially when you try to provide a commandline interface to them...

Thus, instead I'd wanted to provide a more capable way for a user to give a more complex query, i.e. where it'd be possible to use AND/OR/NOT as well as parenthesized groups, e.g. something like this (very contrived and structured on several lines for readability):

    (
      REGEXP/some_rx_1/ AND REGEXP/some_rx_2/
    )
  OR
    (
      REGEXP/some_rx_3/ AND NOT REGEXP/some_rx_4/
    )
  OR
    NOT
      (
        REGEXP/some_rx_5/ OR NOT REGEXP/some_rx_6/
      )

Basically, feed 'something' the query and a list of scalars and get back a list of the subset of scalars that fulfills the query. In short, behaving like a grep, you might say, but where the normal BLOCK or EXPR is a query decided by the user

As it turned out, once the basics above was functioning I added some other features, such as realizing that lists were not always just simple scalars, but could just as well be "objects" and also that it then was useful to use numerical comparisons rather than just regular expressions.

Hence, this module to encapsulate the mechanism.

Is it for you?

It may be comparatively slow and very memory-intensive depending on the complexity of the query and the size of the original data set.

If your needs can be met by a regular grep call, utilizing a regular expression directly, or using a block of code you can write beforehand, this module probably isn't necessary, although it might be convenient if your block is complex enough.

DESCRIPTION

The visible API is made to be simple but also compact - the single method/function qgrep, actually. For the slightly more complex scenarios a helper class is required, but generally a very simple one giving high flexibility in how to structure the query itself regardless of how the list itself is laid out.

It has a behavior similar to grep - give it a list and get back a list (or in scalar context, the number of matches). The main difference is that the matching stuff is a query expressed in a fairly simple language.

It can be used in both non-OO and OO styles. The latter obviously useful when the query will be used multiple times so as to avoid parsing the query every time.

The basic intent is to make it easy to do the easy stuff while still making it easy to move up to something more complex, without having a wide or wordy API. This is a two-edged sword - I hope this will not be confusing.

QUERY LANGUAGE

A query effectively have two slightly different "modes", depending on if the query is aimed at a list of ordinary scalars or if the list consists of objects (or plain hashes, which is regarded as a special case of objects). There is also a special case when you pass only a single hash ref - it can be treated as a list, and a new hash ref with matching key/value pairs passed back.

Scalars

In the first case, the query doesn't use "field" names - it is implicit that the comparison should be made directly on scalars in the list.

Note that is possible to use field names if desired - just make the accessors so that it properly extracts parts of each scalar.

Hashes/Objects

In the second case, the query uses field names for the comparisons and therefore a "field accessor" object is required when executing the query so as to provide the query engine with the mapping between a field name and the data.

A special case occurs when the list consists of hashes with keys being exactly the field names - if so, the query engine can transparently create the necessary field accessor if one is not passed in.

It's important to note that either the query uses field names everywhere, or not at all. Mixing comparisons with field names and others without is illegal.

For hashes/objects it's necessary to use field names - otherwise you will match against scalar representations of hashref values for example, e.g. 'HASH(0x12345678)'. Hardly useful.

SYNTAX

The query language syntax is fairly straightforward and can be divided in two main parts: the logical connectors and the comparison atoms.

In the tables below, note that case is irrelevant, i.e. 'AND' is equal to 'and' which is equal to 'And' and so on.

Comments

Comments can be used in the query using the begin/end style like '/* some comment */'.

Logical connectors

In this category we find the basic logic operators used to tie comparisons together, i.e AND/OR/NOT and parentheses to enforce order.

  • NOT or !

    Used to negate the list generated by an expression.

  • AND or &&

    Used to select the intersection of two lists formed by expressions before and after.

  • OR or ||

    Used to select the union of two lists formed by expressions before and after.

  • ()

    Used to enforce a grouping order.

Comparison atoms

A comparison atom is how to describe a match. It can be divided in string and numeric matches. A complete atom can contain the following:

fieldname.operatorstartdelimitervaluestopdelimiter

The fieldname is optional. If given, it is terminated with a period (.). It cannot contain a period or a space, but otherwise it can be any text that can be used as a hash key.

The rest of the expression consists of an operator and a value to be used by that operator delimited by startdelimiter and stopdelimiter. To accommodate values happening to use characters normally used in a delimiter, choice of character(s) is very flexible. The delimiters can be of two different kinds. Either common start/stop pairs like parentheses: (), braces: {}, brackets: [] or angles: <>. Or, it can be an arbitrary character except space, and the same character again after the value, e.g. /.

The operators are:

  • TRUE or FALSE

    These operators always evaluate to true and false respectively.

  • REGEXP or =~

    This operator expects to use the value as a regular expression for use in matching.

  • EQ, NE, LT, LE, GT, GE

    These are string based matches, i.e. equal, not equal, less than, less than or equal, greater than and greater than or equal.

    Don't confuse these with the numeric comparisons - results will likely be unexpected since using these means that "2" is greater than "19"...

  • ==, !=, <, <=, >, >=

    These are numerical matches.

EXAMPLES

  # in normal Perl code, we would for example write:
  #
  my $v = "abcdefgh";
  if ($v =~ /abc/)
  {
    ...
  }
  
  # equivalent ways to write the regexp in a query would be:
  #
  REGEXP(abc)
  regexp(abc)  # case doesn't matter
  =~(abc)      # in case you're more comfortable with the Perl operator
  =~{abc}      # braces as delimiters 
  =~[abc]      # brackets as delimiters 
  =~<abc>      # angles as delimiters 
  =~/abc/      # Perlish
  =~dabcd      # works, but quite confusing
  
  # a compound query with fields
  #
  name.REGEXP(^A) AND age.>=(42)  # field names before the operators

METHODS/FUNCTIONS

new( $query )

Constructor for a Grep::Query object if using the OO interface.

The argument query string is required.

Croaks if a problem is discovered.

EXAMPLE

  # create a G::Q object
  #
  my $gq = Grep::Query->new('==(42) OR >(100)');

getQuery()

Returns the original query text.

qgrep

Execute a query.

This method can be called in a few different ways, depending on if it's used in an OO fashion or not, or if the query contains field names or not.

Croaks if something is wrong.

Return value: Number of matches in the given data list if called in scalar context, the matching list otherwise. The return list will keep the relative order as the original data list. A notable exception: if called in void context, the query is skipped altogether - seems to be no point in spending a lot of work when no one's interested in the results, right?

  • Non-OO, no fields: qgrep( $query, @data )

    The given $query string will be parsed on the fly and executed against the @data.

  • Non-OO, with fields: qgrep( $query, $fieldAccessor, @data )

    The given $query string will be parsed on the fly and executed against the data, using the $fieldAccessor object to get values from @data objects.

    Note: In a certain case, the $fieldAccessor argument can be passed as undef and it will be auto-generated. See below for details.

  • OO, no fields: $obj->qgrep( @data )

    The $obj must first have been created using "new" and then it can be executed against the @data.

  • OO, with fields: $obj->qgrep( $fieldAccessor, @data )

    The $obj must first have been created using "new" and then it can be executed, using the $fieldAccessor object to get values from @data objects.

    Note: In a certain case, the $fieldAccessor argument can be passed as undef and it will be auto-generated. See below for details.

  • Passing a single hashref: qgrep($fieldAccessor, \%hash)

    In this case, the field accessor methods will be called with two-item arrayrefs, e.g. the key is in the first (0) slot, and the value is in the second (1) slot.

Autogenerated field accessor

If the @data holds plain hashes with keys exactly corresponding to the field names used in the query, the query engine can autogenerate a field accessor.

This is only a convenience, a manually constructed field accessor will be used if given. To take advantage of the convenience, simply pass undef as the $fieldAccessor argument.

EXAMPLES

  # sample data
  my @scalarData = ( 105, 3, 98, 100, 42, 101, 42 );

  # make sure to import the qgrep function
  #
  use Grep::Query qw(qgrep);
  
  # now call it directly
  #
  my $matches = qgrep('==(42) OR >(100)', @scalarData);
  #
  # $matches is now 4 (matching 105, 42, 101, 42)
  
  # or equivalently, create a G::E object and call the method on it
  #
  my $gq = Grep::Query->new('==(42) OR >(100)');
  $matches = $gq->qgrep(@scalarData);
  #
  # $matches again 4
  
  # some sample fielded data in a hash
  #
  my @hashData = 
        (
                { x => 52, y => 38 },
                { x => 94, y => 42 },
                { x => 25, y => 77 }
        );
  
  # autogenerate a field accessor since the query matches the fields
  #
  $matches = qgrep('x.>(20) AND y.>(40)', undef, @hashData);
  #
  # $matches is now 2 (matching last two entries)
  
  # but using different field names (or if it was opaque objects used)
  # we must provide an explicit field accessor
  #
  my $fieldAccessor = Grep::Query::FieldAccessor->new
                                               (
                                                 {
                                                   fieldY => sub { $_[0]->{y} },
                                                   fieldX => sub { $_[0]->{x} },
                                                 }
                                               );
  $matches = qgrep('fieldX.>(20) AND fieldY.>(40)', $fieldAccessor, @hashData);
  #
  # $matches again 2
  

AUTHOR

Kenneth Olwing, <knth at cpan.org>

BUGS

Please report any bugs or feature requests to bug-grep-query at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Grep-Query. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

SUPPORT

You can find documentation for this module with the perldoc command.

    perldoc Grep::Query

You can also look for information at:

ACKNOWLEDGEMENTS

First and foremost, I thank my family for putting up with me!

David Mertens, <dcmertens.perl(at)gmail.com> for the name.
Ron Savage, <ron(at)savage.net.au> for helping follow current best practices for modules.

REPOSITORY

https://github.com/kenneth-olwing/Grep-Query.

LICENSE AND COPYRIGHT

Copyright 2016 Kenneth Olwing.

This program is free software; you can redistribute it and/or modify it under the terms of the the Artistic License (2.0). You may obtain a copy of the full license at:

http://www.perlfoundation.org/artistic_license_2_0

Any use, modification, and distribution of the Standard or Modified Versions is governed by this Artistic License. By using, modifying or distributing the Package, you accept this license. Do not use, modify, or distribute the Package, if you do not accept this license.

If your Modified Version has been derived from a Modified Version made by someone other than you, you are nevertheless required to ensure that your Modified Version complies with the requirements of this license.

This license does not grant you the right to use any trademark, service mark, tradename, or logo of the Copyright Holder.

This license includes the non-exclusive, worldwide, free-of-charge patent license to make, have made, use, offer to sell, sell, import and otherwise transfer the Package with respect to any patent claims licensable by the Copyright Holder that are necessarily infringed by the Package. If you institute patent litigation (including a cross-claim or counterclaim) against any party alleging that the Package constitutes direct or contributory patent infringement, then this Artistic License to you shall terminate on the date that such litigation is filed.

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