Regex (or RegExp) is a way to match patterns in strings and is used for text search, validation, and replacement. PHP supports PCRE (Perl Compatible Regular Expressions) regex syntax by default using functions prefixed with "preg_". For example, preg_replace() searches for a pattern using a regex and replaces it, as shown when a space is removed from "Hello World" by replacing the \s character type with an empty string.

1. Regex Introduction

2.  Regex (or RegExp) stands for Regular Expressions,
which is fast and efficient way to match patterns inside
a string.
 Regex can be used for processes like text search, text
search and replace, input validation, etc.
 PHP supports the widely used syntax for regex: PCRE
(Perl Compatible Regular Expressions) by default.
 In PHP, PCRE functions are prefixed by preg_.

3.  Perl
 Perl is a family of two high-level, general-
purpose, interpreted, dynamic programming
languages. "Perl" refers to Perl 5, but from 2000 to 2019
it also referred to its redesigned "sister language", Perl
6, before the latter's name was officially changed
to Raku in October 2019.

4.  PHP Regex Replace Example
 <?php
 $str = 'Hello World';
 $regex = '/s/';
 echo preg_replace($regex, '', $str);

5.  In this example, the first white space in "Hello World"
is removed. So, it will output "HelloWorld". Let's see
what $regex and preg_replace() does.
 preg_replace() searches for a string (using regex
patterns) and replaces it with another string.
 $regex helps to search the string.
 / signs at the beginning and ending of $regex indicates
the start and the end of the regex. They are
called delimiters.
 s is a single expression. It matches any white space
characters. They are called character types.
 Then, the match is replaced with ''. So, the whitespace
gets removed.