# Python Regular Expressions

Regular expressions (regex) are a powerful tool for matching patterns in text. Python provides support for regular expressions through the built-in `re` module.

## The `re` Module

To use regular expressions in Python, you need to import the `re` module.

```python
import re
```

## Common Functions

### `re.search()`

Searches the string for a match and returns a Match object if there is a match.

```python
txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)
if x:
  print("YES! We have a match!")
else:
  print("No match")
```

### `re.findall()`

Returns a list containing all matches.

```python
txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x) # Output: ['ai', 'ai']
```

### `re.split()`

Returns a list where the string has been split at each match.

```python
txt = "The rain in Spain"
x = re.split("\s", txt)
print(x) # Output: ['The', 'rain', 'in', 'Spain']
```

### `re.sub()`

Replaces one or many matches with a string.

```python
txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x) # Output: The9rain9in9Spain
```

## Metacharacters

*   `[]`: A set of characters (e.g., `[a-m]`)
*   `\`: Signals a special sequence (e.g., `\d`)
*   `.`: Any character (except newline character)
*   `^`: Starts with
*   `$`: Ends with
*   `*`: Zero or more occurrences
*   `+`: One or more occurrences
*   `?`: Zero or one occurrences
*   `{}`: Exactly the specified number of occurrences
*   `|`: Either or
*   `()`: Capture and group

## Special Sequences

*   `\d`: Returns a match where the string contains digits (numbers from 0-9).
*   `\w`: Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character).
*   `\s`: Returns a match where the string contains a white space character.

[[programming/python/python]]