Python – Extract String till Numeric

Given a string, extract all its content till first appearance of numeric character.

Input : test_str = “neveropen7 is best” 
Output : neveropen 
Explanation : All characters before 7 are extracted. 

Input : test_str = “2neveropen7 is best” 
Output : “” 
Explanation : No character extracted as 1st letter is numeric.

Method #1: Using isdigit() + index() + loop

The combination of above functions can be used to solve this problem. In this, we check for first occurrence of numeric using isdigit() and index() is used to get required index till which content needs to be extracted.

The original string is : Lazyroar4Lazyroar is best
Extracted String : Lazyroar

Time complexity: O(n), where n is the length of the input string. .
Auxiliary space: O(1), as only constant amount of extra space is used to store the temporary index variable.

Method #2: Using regex()

This is yet another way in which this task can be performed. Using appropriate regex(), one can get content before possible numerics.

The original string is : Lazyroar4Lazyroar is best
Extracted String : Lazyroar

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

Method #3: Without any builtin methods

Initialize a string with all digits from 0 to 9, an empty string.Iterate a for loop on given string and add  the characters to empty string, if you find first numeric character(using in operator) break from the for loop

The original string is : Lazyroar4Lazyroar is best
Extracted String : Lazyroar

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

Method 4: Using next() function and enumerate()

Uses the next() function and enumerate() to find the index of the first numeric character in the string. The next() function returns the next item in an iterator, which in this case is the index of the first numeric character. If no numeric character is found, len(test_str) is returned instead. Finally, the extracted string is printed

The original string is : Lazyroar4Lazyroar is best
Extracted String : Lazyroar

Time Complexity: O(n), where n is the length of the input string. The time complexity of the next() function is O(1) and the time complexity of the generator expression inside it is also O(1) in the worst case, so the overall time complexity of the program is O(n).
Auxiliary Space: O(1). The program uses constant auxiliary space to store the input string, index, and extracted string. It does not use any additional space that depends on the size of the input.

Method 5: Using list comprehension and isdigit() method. we can also use list comprehension along with the isdigit() method to extract the string until the first numeric character is encountered.

Extracted String : Lazyroar

Time complexity: O(n), where n is the length of the input string.
Auxiliary space: O(1), which is constant space. 

Method 6: Using re.match():

Steps:

1. Initiate a for loop to traverse the string test_str
2. If the character is a number then break out of the loop or else concatenate the characters to empty string res(checked using regular expression ^[0-9]+$ and re.match())
3. Display res which contains string till numeric

The original string is : Lazyroar4Lazyroar is best
Extracted String : Lazyroar

Time Complexity: O(N) N – length of string test_str
Auxiliary Space: O(1) because we used single variable res to store result