How to check if a string starts with a substring using regex in Python?

Prerequisite: Regular Expression in Python

Given a string str, the task is to check if a string starts with a given substring or not using regular expression in Python.

Examples:

Input: String: "geeks for geeks makes learning fun" 
       Substring: "geeks" 
Output: True 
Input: String: "geeks for geeks makes learning fun" 
       Substring: "makes" 
Output: False

Check if a string starts with a substring using regex

Here, we first check a given substring present in a string or not if yes then we use search() function of re library along with metacharacter “^”. This metacharacter checks for a given string starts with substring provided or not. 

Below is the implementation of the above approach:

Output:

string starts with the given substring
string doesn't start with the given substring

Time complexity : O(n), where n is the length of the input string.

Space complexity :  O(1) as it only uses a fixed amount of memory, regardless of the size of the input string.

if a string starts with a substring

Here, we first check a given substring present in a string or not if yes then we use search() function of re library along with metacharacter “\A”. This metacharacter checks for a given string starts with substring provided or not.

 
Below is the implementation of the above approach:

Output:

string starts with the given substring
string doesn't start with the given substring

Time complexity : O(n), where n is the length of the input string.

Space complexity :  O(1) as it only uses a fixed amount of memory, regardless of the size of the input string.