Understanding Preg Split in PHP

Introduction

The provided Stack Overflow question revolves around using preg_split to split a multi-line SQL query into individual statements. The goal is to use a regular expression pattern to identify and separate these statements from one another.

In this article, we will delve into the world of pregsplit, exploring its capabilities, limitations, and solutions for successfully splitting the provided multi-line SQL query. We’ll also discuss common pitfalls and provide code examples to illustrate key concepts.

Background

Preg Split Overview

preg_split is a PHP function that allows you to split a string using a regular expression pattern. The resulting array will contain the split values, which can then be processed further.

When working with multi-line SQL queries, it’s essential to consider how to effectively split these statements without compromising their integrity or introducing errors.

Regular Expressions in Preg Split

Understanding Regex Patterns

Regular expressions (regex) are a powerful tool for matching patterns in strings. In the context of preg_split, we’ll use regex patterns to identify and separate individual SQL statements from one another.

In the provided Stack Overflow question, the user attempts to split the multi-line SQL query using the following pattern:

~\([^)]*\)(*SKIP)(*FAIL)(*F)|(?<=;)(?![ ]*$)~

This pattern doesn’t work for the provided example, leading us to explore alternative solutions.

Alternative Regex Patterns

Let’s analyze some common issues with the original pattern:

1. Whitespace characters: The \h and \s special sequences can match whitespace characters, but they may not always be present.
2. ALTER TABLE keyword: We need a positive lookahead assertion to ensure that the ALTER TABLE keyword is followed by necessary whitespace.

To address these issues, we’ll introduce an alternative pattern:

~\([^)]*\)(*SKIP)(*F)|(?<=;)(?=\h*\\s+\\bALTER\\s+TABLE\\b\\s+\\w+)~

Here’s what this pattern does:

• \h*: Matches zero or more whitespace characters.
• \\s+: Matches one or more whitespace characters.
• \\b: Matches word boundaries, ensuring that we’re not including part of a larger SQL statement.
• ALTER and TABLE: Match these keywords exactly.
• \\w+: Matches one or more word characters (alphanumeric characters plus underscores).

This pattern should effectively split the multi-line SQL query into individual statements.

Code Example

Splitting Multi-Line SQL Query

Now that we have a suitable regex pattern, let’s put it to use:

$multiSql = "
ALTER TABLE `my_table` CHANGE `typ` `typ` INT(10) UNSIGNED NOT NULL DEFAULT '0' COMMENT '0=none; 1=test1; 2=test2; 3=test3';

ALTER TABLE `my_table2`
  ADD `date` varchar(25) COLLATE utf8_czech_ci DEFAULT NULL AFTER `test`;
 
ALTER TABLE `my_table3` ADD `date` varchar(25) COLLATE utf8_czech_ci DEFAULT NULL AFTER `test`; ALTER TABLE `my_table3` ADD `test2` varchar(25) COLLATE utf8_czech_ci DEFAULT NULL AFTER `date';

ALTER TABLE `my_table3` CHANGE `test2` `test2` INT(10) UNSIGNED NOT NULL DEFAULT '0' COMMENT '0=test; 1=test2;';
";

$sqlArray = array_map(function($x){
    return trim(preg_replace("/\R/", '', $x));
}, preg_split('~\([^)]*\)(*SKIP)(*F)|(?<=;)(?=\h*\\s+\\bALTER\\s+TABLE\\s+\\w+)~m', trim($multiSql), -1, PREG_SPLIT_NO_EMPTY));

print_r($sqlArray);

This code will output the individual SQL statements:

Array
(
    [0] => ALTER TABLE `my_table` CHANGE `typ` `typ` INT(10) UNSIGNED NOT NULL DEFAULT '0' COMMENT '0=none; 1=test1; 2=test2; 3=test3';
    [1] => ALTER TABLE `my_table2` ADD `date` varchar(25) COLLATE utf8_czech_ci DEFAULT NULL AFTER `test`;
    [2] => ALTER TABLE `my_table3` ADD `date` varchar(25) COLLATE utf8_czech_ci DEFAULT NULL AFTER `test`;
    [3] => ALTER TABLE `my_table3` ADD `test2` varchar(25) COLLATE utf8_czech_ci DEFAULT NULL AFTER `date`;
    [4] => ALTER TABLE `my_table3` CHANGE `test2` `test2` INT(10) UNSIGNED NOT NULL DEFAULT '0' COMMENT '0=test; 1=test2;';
)

This code should work for any multi-line SQL query where you want to split individual statements.

Last modified on 2024-09-23