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:

Python3

# import library
import re

# define a function
def find(string, sample) :

  # check substring present
  # in a string or not
  if (sample in string):

      y = "^" + sample

      # check if string starts
      # with the substring
      x = re.search(y, string)

      if x :
          print("string starts with the given substring")

      else :
          print("string doesn't start with the given substring")

  else :
      print("entered string isn't a substring")


# Driver code
string = "geeks for geeks makes learning fun"
sample = "geeks"

# function call
find(string, sample)

sample = "makes"

# function call
find(string, sample)

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:

Python3

# import library
import re

# define a function
def find(string, sample) :

  # check substring present
  # in a string or not
  if (sample in string):

      y = "\A" + sample

      # check if string starts
      # with the substring
      x = re.search(y, string)

      if x :
          print("string starts with the given substring")

      else :
          print("string doesn't start with the given substring")

  else :
      print("entered string isn't a substring")


# Driver code
string = "geeks for geeks makes learning fun"
sample = "geeks"

# function call
find(string, sample)

sample = "makes"

# function call
find(string, sample)

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.