Python – Check whether a string starts and ends with the same character or not (using Regular Expression)

Given a string. The task is to write a regular expression to check whether a string starts and ends with the same character.

Examples:

Input :  
abba
Output :  
Valid

Input :  
a
Output :  
Valid

Input :  
abc
Output :  
Invalid

Solution:

The input can be divide into 2 cases:  

• Single character string: All single character strings satisfies the condition that they start and end with the same character. The regex for a string with only 1 character will be- 

'^[a-z]$'

• Multiple character string: Here we need to check whether the first and the last character is same or not. We do this using \1. The regex will be-

'^([a-z]).*\1$'

The two regular expressions can be combined using |  

'^[a-z]$|^([a-z]).*\1$'

In this program, we will use search() method of re module.

Below is the implementation. 

Python3

# Python program to check if a string starts
# and ends with the same character

# import re module as it provides
# support for regular expressions
import re

# the regular  expression
regex = r'^[a-z]$|^([a-z]).*\1$'

# function for checking the string
def check(string):

    # pass the regular expression
    # and the string in the search() method
    if(re.search(regex, string)):
        print("Valid")
    else:
        print("Invalid")

if __name__ == '__main__' :

    sample1 = "abba"
    sample2 = "a"
    sample3 = "abcd"

    check(sample1)
    check(sample2)
    check(sample3)

Output :

Valid
Valid
Invalid

Time Complexity : O(n), where n is length of string

Auxiliary Space : O(1)