Coding the AND Operator in PHP Regular Expression

Advanced PHP Regular Expressions - Part 8

Foreword: In this part of the series, I explain how to code the AND operation for a PHP regular expression.

By: Chrysanthus Date Published: 11 Jul 2019

Introduction

This is part 8 of my series, Advanced PHP Regular Expressions. In this part of the series, I explain how to code the AND operation for a PHP regular expression. The word, “subject” in this series, is the string in which the regular expression finds the match. Note: the abbreviation for regular expression in this series is, regex. You should have read the previous part of the series before reaching here, as this is the continuation.

Motivation
In this section I explain what motivated me to produce this article. As of today, if you want to match this or that in a subject string, you have to produce a regex like:

    /this|that/

where | is the regex OR operator. Is there an equivalent operator for AND? - No. If you want it, you have to code it. In this article I give you a simple solution that works with all systems.

The Solution
Above, you have two alternatives (this or that). You can actually have more than two alternatives. AND means all alternatives must be present in the subject string. The solution is surprisingly simple: Just make sure each of the alternatives is found in the subject, with a for-loop.

Algorithm
- Place all the alternatives in an array.
- Verify if each alternative is in the subject with regular expression technique in the for-loop.
- Any alternative not in the subject means false for the overall operation.

If all alternatives are in the subject, that means true, for the overall operation. So, that is my solution on how to code the AND operator for PHP regex, until PHP gives us an operator for AND.

Code Example
The following code illustrates this with two code segments. The first segment returns TRUE and the second one returns FALSE. The alternatives are: “pencil”, “book” and “pen”. Read and test the code:

<?php

        $subject = "This is a pencil. Is that a book? I use a pen.";
        $arr = Array("pencil", "book", "pen");
        $booll = 'false';
        for ($i=0;$i<count($arr);++$i)
            {
                $regex = '/' . $arr[$i] . '/';

                if (preg_match($regex, $subject) === 1)
                    {
                        $booll = 'true';
                    }
                else
                    {
                        $booll = 'false';
                        break;
                    }
            }
        echo $booll, '<br>';

        $subject =  "This is a pencil. I use a pen.";
        $arr = Array("pencil", "book", "pen");
        $booll = 'false';
        for ($i=0;$i<count($arr);++$i)
            {
                $regex = '/' . $arr[$i] . '/';

                if (preg_match($regex, $subject) === 1)
                    {
                        $booll = 'true';
                    }
                else
                    {
                        $booll = 'false';
                        break;
                    }
            }
        echo $booll;

?>

The output is:

    true
    false

Each of the code segments simulates the AND operator. In your own project, you can vary the AND code, but the algorithm stays the same.

That is it for this part of the series.

Chrys

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