How to parse a query(string) in php

Question

I am working on a PHP project that deals with storing a set of information in form of key value pair and retrieving them using a query. In this project the query to retrieve the information is of following type.

(color: 'red' & size: 'small') | (color: 'yellow' & weight: 'heavy')

This query indicates to select the pieces of information from my storage that describes color as red and also the size as small or color as yellow and also wheight as heavy. Once I parse this query and extract units out of it, I will form a mysql query and get the required information from the database.

I would like to know how do I parse the above string and get the units (I am not sure if it is called lexical units). Kindly let me know if there is a library available where we can have custom parsers.

Answer 1

There's no libraries available for PHP that are actively maintained and fixed. You could use YACC and it's BNF-style grammar definition to get a C parser for your custom language. There is one in PEAR (PHP_ParserGenerator) but beware that it's no longer maintained.

You can check it out here.

The BNF for your parser is:

<rule>::=<rule>"|"<rule> |
         <rule>"&"<rule> |
         "("<rule>")"    |
         <attribute>":"<value>
<attribute>::=[a-z0-9]
<value>::="'"[a-z0-9]"'"

But the parsing seems easy enough, once you encounter a ( token you look for it's corresponding closing ) token and you parse that section recursively.

Answer 2

The condition you want to express can be formalized with a non-contextual grammar. For example:

attributeName := string
attributeValue := ' string '
expression := ( expression & expression ) ||
              ( expression | expression ) ||
              attributeName : attributeValue

Where string is assumed to be defined as a series of alphanumerical character.

Writing a parser for this grammar is simple enough using recursive functions. In the most simple case, you'd want to write a function for each production in the grammar that test the possible match.

If the condition is "normalized" as a SoP, you could also use regular expressions, since it will only consists of &-clauses or'ed among them with |-clauses.

Finally, I know a parser generator named PHP_ParseGenerator: http://pear.php.net/package/PHP_ParserGenerator/redirected