**Regular Expression** or **`RegEx`** is a sequence of characters that forms a search pattern. `RegEx` is used to search for and replace specific patterns. 

Python provides a built-in module, `re`, which supports regular expressions. The module can be imported as follows:

# Metacharacters
**Metacharacters** are characters which are interpreted in a particular way. The following table lists down all the metacharacters used in `RegEx`, along with their functionality:

# Special sequences
**Special sequences** are a `\` followed by any one of the following characters, based on their particular functionality:


# RegEx functions
The `re` module provides users a variety of functions to search for a pattern in a particular string. Below are some of the most frequently used functions in detail:

## 1. re.findall()
The `re.findall()` function returns a list of strings containing all matches of the specified pattern. 

The function takes as input the following:
 *  a character pattern
*  the string from which to search

### Example
The following example will return a list of all the instances of the substring *at* in the given string:

import re

string = "at what time?"
match = re.findall('at',string)
print (match)

## 2. re.search()
The `re.search()` function returns a match object in case a match is found. 

> **Note:** 
> * In case of more than one match, the first occurrence of the match is returned. 
> * If no occurrence is found, `None` is returned.

### Example
Suppose you wish to look for the occurrence of a particular sub-string in a string:

import re

string = "at what time?"
match = re.search('at',string)
if (match):
    print "String found at: " ,match.start()
else:
    print "String not found!"

StringFound

import re

string = "at what time?"
match = re.search('of',string)
if (match):
    print "String found at: " ,match.start()
else:
    print "String not found!"








StringNotFound

> **Note:** The `start()` function returns the start index of the matched string. 

## 3. re.split()
The `re.split()` function splits the string at every occurrence of the sub-string and returns a list of strings which have been split.

Consider the following example to get a better idea of what this function does:

### Example

Suppose we wish to split a string wherever there is an occurrence of `a` 

import re

string = "at what time?"
match = re.split('a',string)
print (match)

> **Note:** In case there is no match, the string will be returned as it is, in a list. 

## 4. re.sub()
The `re.sub()` function is used to replace occurrences of a particular sub-string with another sub-string. 

This function takes as input the following:
1. The sub-string to replace
2. The sub-string to replace with
3. The actual string

### Example
Suppose you wish to insert **!!!** instead of a **white-space character** in a string. This can be done via the `re.sub()` function as follows:

import re

string = "at what time?"
match = re.sub("\s","!!!",string)
print (match)

How to use RegEx in Python

Metacharacters

Special sequences

RegEx functions

1. re.findall()

Example

2. re.search()

Example

3. re.split()

Example

4. re.sub()

Example