What is the purpose of preg_replace in PHP?

In PHP, the preg_replace function is used to search for parts of a string that match a specific pattern and replace them with specified content. It effectively replaces parts of a string that match a specified pattern.

The syntax of preg_replace is:

preg_replace($pattern, $replacement, $subject);

Among them:

1. $pattern is a regular expression pattern used to match the portion that needs to be replaced.
2. $replacement is the content to be replaced.
3. $subject is the string that will be undergoing a replacement operation.

The preg_replace function searches for parts of a string that match a pattern and replaces them with specified content. It can perform either a single replacement or multiple replacements.

For example, the following example replaces all numbers in a string with the character ‘X’:

$str = "12345abcde";
$newStr = preg_replace("/\d/", "X", $str);
echo $newStr; // 输出：XXXXXabcde

In the example above, the regular expression pattern /\d/ is used to match any digit. The preg_replace function replaces all numbers in the string $str with the character ‘X’, and stores the result in $newStr. Finally, $newStr outputs as “XXXXXabcde”.

In addition to simple replacement, the preg_replace function also supports using a callback function as replacement content to achieve more complex replacement operations.