What are the potential issues with using eregi functions in PHP and how can they be replaced with more modern alternatives?

The eregi functions in PHP are deprecated as of PHP 5.3.0 and removed in PHP 7. They were used for case-insensitive pattern matching but are now replaced by the preg functions, which offer more features and better performance. To replace eregi functions, you can simply use the preg functions with the 'i' modifier for case-insensitive matching.

// Using eregi_replace
$result = eregi_replace("[a-z]+", "replacement", $string);

// Replace with preg_replace
$result = preg_replace("/[a-z]+/i", "replacement", $string);