A regular expression (shortened as RegEx) is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are looking for.
A regular expression can be a single character, or a more complicated pattern. RegEx can be used to perform all types of text search and text replace operations, such as validating an email address or a strong password.
In PHP, regular expressions are strings composed of delimiters, a pattern, and optional modifiers.
$exp = "/intricate/i";
In the example above, / is the delimiter, intricate is the pattern that is being searched for, and i is a modifier that makes the search case-insensitive.
The delimiter can be any character that is not a letter, number, backslash, or space. The most common delimiter is the forward slash /, but when your pattern contains forward slashes, it is convenient to choose other delimiters such as # or ~.
Modifiers can change how a search is performed.
i - Performs a case-insensitive search.m - Performs a multiline search.u - Enables correct matching of UTF-8 encoded patterns.In the next chapter, we will see how to apply these patterns using PHP's built-in RegEx functions.