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Full Text Search Parser for TSearch2 and PostgreSQL

Latest News

March 31, 2008: Version 2.0 Release
Version 2.0 released after a full rewrite to correct some bugs found in version 1, in addition to making it possible to add new features. I only wrote version 2 for PHP5, so if you are still running PHP4 (you should really consider upgrading) then you will have to make the appropriate modifications yourself. Really the only thing non-PHP4 is the use of PHP5's class constants instead of global defines.

July 14, 2006: Initial Release
The Full Text Search Parser for TSearch2 and PostgreSQL is officially released with little fanfare.

Description

This is a full text search parser written in PHP for use with the TSearch2 extension to PostgreSQL. TSearch2 adds a full-featured, integrated, and fast full text search for any Postgres database, but the search syntax uses & for AND, | for OR, and ! for NOT, which are more suited to programmers than the users who will generally be using the search. Most people just want to type words, in which case there is generally an assumed "AND" operation between the words. Last, when the search is complete, users generally like to see the search words highlighted in the result set. I wrote this parser to solve all of the above problems and it is currently deployed in a real-world application. I also wanted to get into writing languages and parsers, so this was a good way to introduce myself to the topic without getting overly complex.

What does it cost?

I'm releasing the PHP source under the BSD Open Source License, so basically it's free. However, if you use this code in a commercial project, a donation would be greatly appreciated.

Requirements

System requirements are as follows:

1. A PostgreSQL database installation with the TSearch2 extension.
2. PHP 5.x

Features

These are the primary features of the parser:

• Simple to use.
• Entirely written in PHP with no dependencies.
• Allows the user to enter search strings in a natural way and can apply a default "AND" or "OR" between keywords.
• Provides error information suitable for user consumption.
• Supports unlimited depth of expression nesting using parenthesis.
• Supports exact phrase searches using double or single quotes.
• Will generate results for use as a normal SQL query or for creating prepared statements.
• Returns an array of individual keywords for highlighting the result sets.

Installation and Configuration

Install

Extract the .tgz and you will have three files:

• parse_model.php - Main parser code. I design and write my code using a MVC (model, view, control) paradigm, and the parser is a model. In this case it simply defines a class so it should work as a drop-in for any project.
• parse_test.php - A simple test program that can be run from a browser.
• parse_test_cmd.php - Same as the test program above, but runs from the command line.

Put the parse_model.php file in your project directory and include it where ever you need the parser. Here is a very simple example:

Configuration

Make sure the array of stop-words in the stop_words() function matches the same list defined for your installation of TSearch2, usually found at: /usr/local/pgsql/share/contrib/english.stop

Using the Parser

Include the parse_model.php file and create a parse object:

Class parse_model API:

Input Variable Members:

• $debug is a boolean to enable or disable debug information. Defaults to false. You will certainly want to keep this set to false in a production environment. When in debug mode the parser outputs debug information with echo statements which will certainly cause output to the browser or command shell.

• $upper_op_only is a boolean that determines if the word-based operators "AND", "OR", and "NOT" must be typed in uppercase by the user to be considered as operators. Defaults to false.

• $use_prepared_sql is a boolean that determines if the generated ILIKE statments will be in Prepared Statement format (recommended) or regular SQL format. When false and output is regular SQL, any single quotes are escaped in the PostgreSQL specific way of doubling the single quote. Defaults to true.

• set_default_op($op) is a boolean that determines if the generated ILIKE statments will be in Prepared Statement format (recommended) or regular SQL format. When false (output is regular SQL) any single quotes are escaped in the PostgreSQL specific way of doubling the single quote. Defaults to true.

Function Members:

• set_default_op($op) is used to set the default operator to be injected between search words and quoted phrases that do not have an operator between them. Can be set to "and", "or", or "none". Defaults to "and".

• parse($text, $db_field) parses the input $text into TSearch2 and ILIKE parts and returns false if there was an error, otherwise true. If the return value is false the $error_msg variable will contain a human readable error message, and the contents of $keywords, $tsearch, $ilike, and $db_ilike_data are undefined and should not be used. The $db_field parameter should be set to the database table field name to be used in the TSsearch2 and ILIKE parts of the results.

Output Variable Members:

• $keywords is an array that is filled with all keywords found.

• $tsearch is a string containing the TSearch2 part of the SQL WHERE clause.

• $ilike is a string containing the ILIKE part of the SQL WHERE clause in either Prepared Statement format or regular SQL.

• $db_ilike_data is an array containing the ILIKE Prepared Statement references as keys, and the search strings as values.

• $error_msg is a string containing a human readable error message if parse() returned false.

• $error_tok is a string containing the token where the error occurred.

• $error_pos is an integer of the character offset in the original string where the error occurred.

Notes

TSearch2 looks hard on the surface, but is really very easy to install, set up, and even add to an existing database! I suggest reading through the introduction page to get started with TSearch2 if you have not already.

One of the main reasons for writing a full blown parser for this, instead of trying a bunch of fancy regex, has to do with dealing with searching on phrases (text in double or single quotes). TSearch2 is a word indexer, and therefore cannot perform true phrase searches. For example, if you searched on 'computer & programmer', TSearch2 would return all matches that contained both words, but they would not necessarily be next to each other, which may not matter. However, if you searched for '"computer programmer"', indicating you wanted that phrases, i.e. those two words have to appear in the text and they must be consecutive, TSearch2 cannot guarantee such a match.

The way to include phrase searches with TSearch2 is to include a LIKE or ILIKE (case insensitive LIKE) in the WHERE clause of the SQL statement. The problem gets a little complex though, when trying to extract any phrases from the user supplied search string since full nested parenthesis are supported. The ILIKE clause also has to retain the logic (AND, OR, and NOT) between possible multiple phrases in a single search. For example, is someone searched on: 'computer ("c++" or "visual basic")' then the ILIKE would be: 'AND (text ILIKE '%c++%' OR text ILIKE '%visual basic%')' The attachment of the ILIKE expression to the WHERE clause is with AND, but the two phrases are an OR condition which must be retained.

In light of the complexity, I decided early on that this would be much easier to do with a real parser instead of trying to use regex. However, it could also be that my regex knowledge is not up to the task since I know regex does support recursive back-references, but I never dug into regex that deeply. I wanted to write a parser anyway... :-)

I have a few more notes about highlighting the keywords and phrases in the result set, but it's late and I need to sleep. Those notes will be forth coming, but briefly it has to do with the fact that TSearch2 "stemms" the words it indexes and searches with.