Python | Prefix extraction before specific character

Sometimes, we might have a use case in which we need to find a prefix in a string. But sometimes, the requirement can be something dynamic like a specific input character than a number of elements for the decision of getting prefix. Let’s discuss certain ways in which we can find the prefix of a string before a certain character.

Method #1: Using rsplit()

This method originally performs the task of splitting the string from the rear end rather than the conventional left-to-right fashion. This can though be limited to 1, for solving this particular problem. 

The original string is : Lazyroar
The prefix string is : Geeksfo

Time Complexity: O(n)
Space Complexity: O(n)

Method #2: Using rpartition() If we need to solve this particular problem, this inbuilt function is recommended to perform this particular task. This function performs the partition as required just once from the rear end. 

The original string is : Lazyroar
The prefix string is : Geeksfo

Time Complexity: O(n)
Space Complexity: O(n)

Method #3 : Using find() method.You can also use index() instead of find().

The original string is : Lazyroar
The prefix string is : Geeksfo

Time Complexity: O(n)
Space Complexity: O(n)

Method #4: Using regex

This method uses python regex to find the prefix of a string before a certain character. It uses the re.split() function to split the string at the specified character.

The original string is : Lazyroar
The prefix string is : Geeksfo

This method uses the re.split() function to split the string at the specified character, which takes O(n) time and the output prefix string is a subset of the input string, so the auxiliary space is O(n)

Method 5: Using split(): 

We can split the string based on the specific character and then extract the first element of the resulting list.

The original string is : Lazyroar
The prefix string is : Geeks

Time Complexity: O(n) (where n is the length of the string)
Space Complexity: O(n) (for the list created by split())

Method #6: Using rfind() and slicing

• Initialize a string variable test_str with the value “Lazyroar”.
• Initialize a string variable spl_char with the value “r”.
• Use the rfind() function on the test_str string to find the last occurrence of the spl_char character. Store this index in the last_index variable.
• Extract the prefix string from the test_str string up to the last_index index, and store it in the prefix variable using slicing. This means the prefix variable contains all the characters in test_str before the last occurrence of the spl_char character.
• Print out a message to the console, concatenating the string “The prefix string is : ” with the prefix variable to display the prefix string.

The prefix string is : Geeksfo

Time Complexity: O(n) time complexity, where n is the length of the string test_str.
Auxiliary Space: O(1).