Python - Extract range characters from String

Given a String, extract characters only which lie between given letters.

Input : test_str = 'geekforgeeks is best', strt, end = "g", "s" 

Output : gkorgksiss 

Explanation : All characters after g and before s are retained. 

Input : test_str = 'geekforgeeks is best', strt, end = "g", "r" 

Output : gkorgki 

Explanation : All characters after g and before r are retained.

Method #1 : Using list comprehension

In this, we check for character in range using comparison operation and list comprehension does task of iteration and creation of new list. Then join() can be employed to reconvert to string.

# Python3 code to demonstrate working of 
# Extract range  characters from String
# Using list comprehension

# initializing string
test_str = 'geekforgeeks is best'

# printing original string
print("The original string is : " + str(test_str))

# initializing range letters 
strt, end = "f", "s"

# join() to get result in string 
res = ''.join([chr for chr in test_str if chr >= strt and chr <= end])

# printing result 
print("Extracted String : " + str(res)) 

The original string is : geekforgeeks is best
Extracted String : gkforgksiss

Method #2 : Using filter() + lambda + join()

Another way to solve this, in this, we perform task of  comparison using lambda, and filter() used lambda function to get required characters.

# Python3 code to demonstrate working of 
# Extract range  characters from String
# Using filter() + lambda + join()

# initializing string
test_str = 'geekforgeeks is best'

# printing original string
print("The original string is : " + str(test_str))

# initializing range letters 
strt, end = "f", "s"

# join() to get result in string 
res = ''.join(list(filter(lambda chr : chr >= strt and chr <= end, test_str)))

# printing result 
print("Extracted String : " + str(res)) 

The original string is : geekforgeeks is best
Extracted String : gkforgksiss

The Time and Space Complexity for all the methods are the same:

Time Complexity: O(n)

Space Complexity: O(n)

METHOD 3:Using re 

APPROACH:

This program extracts a range of characters from a given string using regular expression and the findall() function.

ALGORITHM:

Initialize the original string and print it. Define the range of characters to extract. Define a regular expression pattern to match the characters within the given range. Use the findall() function with the pattern and the original string to extract all matches. Join the matches to form the extracted string. Print the extracted string.

import re

# initializing string
test_str = 'geekforgeeks is best'

# printing original string
print("The original string is : " + str(test_str))

# initializing range letters
strt, end = "f", "s"

# pattern to match characters within range
pattern = re.compile(f'[{strt}-{end}]')

# using findall() to extract all matches
matches = pattern.findall(test_str)

# joining the matches to get the extracted string
res = ''.join(matches)

# printing result
print("Extracted String : " + str(res))

The original string is : geekforgeeks is best
Extracted String : gkforgksiss

Time Complexity: The time complexity of this program depends on the length of the input string and the size of the character range. The findall() function has a time complexity of O(n), where n is the length of the input string. The join() function also has a time complexity of O(n), where n is the length of the resulting string. Overall, the time complexity of this program can be considered as O(n).

Space Complexity: The space complexity of this program depends on the length of the input string and the size of the character range. The program uses a regular expression pattern and a list to store the matches. The space complexity of the regular expression pattern is constant. The space complexity of the list is O(m), where m is the number of matches. Therefore, the space complexity of this program can be considered as O(m).