# PHP Regular Expressions (RegEx)

A **Regular Expression** 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 searching for.

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## 1. Common Functions
PHP provides a set of functions that allow you to use regular expressions.

*   `preg_match()`: Returns 1 if the pattern was found in the string and 0 if not.
*   `preg_match_all()`: Returns the number of times the pattern was found in the string.
*   `preg_replace()`: Returns a new string where matched patterns have been replaced with another string.

## 2. Syntax
In PHP, regular expressions are strings composed of delimiters, a pattern, and optional modifiers.

```php
$exp = "/w3schools/i";
```
*   `/`: Delimiter
*   `w3schools`: Pattern
*   `i`: Modifier (case-insensitive)

## 3. Using preg_match()
The `preg_match()` function will tell you whether a string contains matches of a pattern.

```php
$str = "Visit W3Schools";
$pattern = "/w3schools/i";
echo preg_match($pattern, $str); // Outputs 1
```

## 4. Using preg_replace()
The `preg_replace()` function will replace all of the matches of the pattern in a string with another string.

```php
$str = "Visit Microsoft!";
$pattern = "/microsoft/i";
echo preg_replace($pattern, "W3Schools", $str); // Outputs "Visit W3Schools!"
```

## 5. Common Metacharacters
*   `^`: Start of string
*   `$`: End of string
*   `.`: Any character
*   `\d`: Digit
*   `\s`: Whitespace
*   `[abc]`: Find any character between the brackets

[[programming/php/php]]