Python program to find Indices of Overlapping Substrings

To count the number of overlapping sub strings in Python we can use the Re module. To get the indices we will use the re.finditer() method. But it returns the count of non-overlapping indices only.

Examples:

Input: String: “geeksforgeeksforgeeks” ; Pattern: “geeksforgeeks”
Output: [0, 8]
Explanation: The pattern is overlapping the string from 0th index to 12th index and again overlapping it from 8th index to 20th index. Hence, the output is the starting positions of overlapping i.e index 0 and index 8.  

Input: String: “barfoobarfoobarfoobarfoobarfoo” ;  Pattern: “foobarfoo”
Output: [3, 9,15, 21]
Explanation: The pattern is overlapping the string from index 3, 9 , 15 and 21.

This method returns the count of non-overlapping indices only from a string having multiple occurrences overlapping pattern. Below is a program depicting the use of finditer() method.

Python3

# Import required module
import re


# Function to depict use of finditer() method
def CntSubstr(pattern, string):

    # Array storing the indices
    a = [m.start() for m in re.finditer(pattern, string)]
    return a


# Driver Code
string = 'geeksforgeeksforgeeks'
pattern = 'geeksforgeeks'

# Printing index values of non-overlapping pattern
print(CntSubstr(pattern, string))

Output:

[0]

Therefore, to get the overlapping indices as well we need to do is escape out of the regular expressions in the pattern. The definition in the explicit function helps to select the characters in a partial way.

Approach:

1. re.finditer() helps in finding the indices where the match object occurs. As it returns an iterable object, the start() method helps in return the indices or else it would show that a match object has been found at some location.
2. The standard method in matching using re module is greedy which means the maximum number of characters are matched. Therefore, the ?={0} helps in minimum number of matches.
3. To match it so that partial characters are matched, the re.escape() helps in escaping out the special characters which have been added before such as the ?={0}.
4. The result is that by adding som modifications, the finditer() method returns a list of overlapping indices.

Below is the implementation of the above approach:

Python3

# Import required module
import re


# Explicit function to Count
# Indices of Overlapping Substrings
def CntSubstr(pattern, string):
    a = [m.start() for m in re.finditer(
        '(?={0})'.format(re.escape(pattern)), string)]
    return a


# Driver Code
string1 = 'geeksforgeeksforgeeks'
pattern1 = 'geeksforgeeks'

string2 = 'barfoobarfoobarfoobarfoobarfoo'
pattern2 = 'foobarfoo'


# Calling the function
print(CntSubstr(pattern1, string1))
print(CntSubstr(pattern2, string2))

Output:

[0, 8]
[3, 9, 15, 21]