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RegEx

A RegEx or Regular Expression, is a sequence of characters that forms a search pattern.
RegEx can be used to check if a string contains the specified search pattern.

Python has a built-in package called re, which can be used to work with Regular Expressions.
Import the re module.

# Search the string to see if it starts with "The" and ends with "India":
import re

txt = "The rain in India"
x = bool(re.search("^The.*India$", txt))
print(x)
# prints True

RegEx Functions

The re module offers a set of functions that allows us to search a string for a match.

findall() function

The findall() function returns a list containing all matches.

Note: If no matches are found, an empty list is returned.

import re

txt = "The rain in India"
x = re.findall("in", txt)
print(x)
# prints ['in', 'in']

search() function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned.

Note: If no matches are found, the value None is returned.

# Search for the first white-space character in the string
import re

txt = "The rain in India"
x = re.search("\s", txt)

print("The first white-space character is located in position:", x.start())
# prints The first white-space character is located in position: 3

split() function

The split() function returns a list where the string has been split at each match.

# Split at each white-space character:
import re

#Split the string at every white-space character:

txt = "The rain in India"
x = re.split("\s", txt)
print(x)
# prints ['The', 'rain', 'in', 'India']

You can control the number of occurrences by specifying the maxsplit parameter.

# Split the string only at the first occurrence
import re

txt = "The rain in India"
x = re.split("\s", txt, 1)
print(x)
# prints ['The', 'rain in India']

sub() function

The sub() function replaces the matches with the text of your choice.

# Replace every white-space character with the number 9:
import re

txt = "The rain in India"
x = re.sub("\s", "9", txt)
print(x)
# prints The9rain9in9India

You can control the number of replacements by specifying the count parameter.

import re

txt = "The rain in India"
x = re.sub("\s", "9", txt, 2)
print(x)
# prints The9rain9in India

You can learn more about RegEx from here

Try the following example in the editor below.

Given a string txt, perform the operations as defined in the comments.

Start solving RegEx on Interview Code Editor
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