How do you find all the occurrences of an element in a string Python?

Source Code

val = [1, 3, 4, 6, 5, 1]
a = 1
print ("Original list :" ,val)
c = val.count(a)
for i in range(c):
    val.remove(a)
print ("Remove operation :" , val)

Output

Original list : [1, 3, 4, 6, 5, 1]
Remove operation : [3, 4, 6, 5]

The function takes an iterable and returns an enumerate object containing tuples where the first element is the index and the second is the corresponding item.

We used a list comprehension to iterate over the enumerate() object.

List comprehensions are used to perform some operation for every element or select a subset of elements that meet a condition.

On each iteration, we check if the current character is equal to b and if the condition is met, we return the corresponding index.

The new list contains all of the indexes of the character in the string.

This approach is only suitable if you need to find the indexes of a single character in a string.

If you need to find all indexes of a substring in a string, use the re.finditer() method.

Find all indexes of a substring in a String in Python

To find all indexes of a substring in a string:

1. Use the re.finditer() to get an iterator object of the matches.
2. Use a list comprehension to iterate over the iterator.
3. Use the match.start() method to get the indexes of the substring in the string.

The method takes a regular expression and a string and returns an iterator object containing the matches for the pattern in the string.

The match.start() method returns the index of the first character of the match.

The new list contains the index of all occurrences of the substring in the string.

Alternatively, you can use a for loop.

Find all indexes of a substring in a String using a for loop

To find all indexes of a substring in a string:

1. Declare a new variable that stores an empty list.
2. Use the re.finditer() to get an iterator object of the matches.
3. Use a for loop to iterate over the object.
4. Append the index of each match to the list.

We used a for loop to iterate over the iterator object.

On each iteration, we use the match.start() method to get the index of the current match and append the result to the enumerate()3 list.

Problem Formulation: Given a longer string and a shorter string. How to find all occurrences of the shorter string in the longer one?

Consider the following example:

• Longer string: 'Finxters learn Python with Finxter'
• Shorter string pattern:

  Finxter matched from position 0 to 7
  Finxter matched from position 27 to 34

  0
• Result 1:

  Finxter matched from position 0 to 7
  Finxter matched from position 27 to 34

  1

Optionally, you may also want to get the positions where the shorter string arise in the longer string:

• Result 2:

  Finxter matched from position 0 to 7
  Finxter matched from position 27 to 34

  2

Method 1: Regex re.finditer()

To get all occurrences of a pattern in a given string, you can use the regular expression method

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 1: re.finditer
for m in re.finditer(pattern, s):
    print(pattern, 'matched from position', m.start(), 'to', m.end())

The output is:

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

🌍 Related Tutorial: Python Regex Finditer

Method 2: re.finditer() + List Comprehension

To get the pattern string, start index, and end index of the match into a list of tuples, you can use the following one-liner based on list comprehension:

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 2: re.finditer + list comprehension
l = [(pattern, m.start(), m.end()) for m in re.finditer(pattern, s)]
print(l)

The output is:

[('Finxter', 0, 7), ('Finxter', 27, 34)]

🌍 Related Tutorial: Python List Comprehension

A Simple Introduction to List Comprehension in Python

Method 3: Python String startswith()

The Python

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

Finxter matched from position 0 to 7
Finxter matched from position 27 to 34

We can use the

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 2: re.finditer + list comprehension
l = [(pattern, m.start(), m.end()) for m in re.finditer(pattern, s)]
print(l)

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 2: re.finditer + list comprehension
l = [(pattern, m.start(), m.end()) for m in re.finditer(pattern, s)]
print(l)

Here’s the full example using this approach:

s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 4: startswith() to find all occurrences of substring in string
l = [i for i in range(len(s)) if s.startswith(pattern, i)]

print(l)

The output shows a list of start indices where the substring (pattern) was found in the original string:

[0, 27]

It pays to learn the basics in Python—feel free to dive deeper into this method in the following Finxter blog tutorial only one click away:

🌍 Related Tutorial: Python String Startswith

Method 4: re.findall()

If you’re interested in only the matched substrings without their index location in the given string, you can use the following approach.

To find all substrings in a given string, use the

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 2: re.finditer + list comprehension
l = [(pattern, m.start(), m.end()) for m in re.finditer(pattern, s)]
print(l)

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 4: re.findall() to find all patterns in string
l = re.findall(pattern, s)
print(l)
# ['Finxter', 'Finxter']

In case you wonder how the

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 2: re.finditer + list comprehension
l = [(pattern, m.start(), m.end()) for m in re.finditer(pattern, s)]
print(l)

🌍 Related Tutorial: Python Regex Findall

Python Regex Findall()

Method 5: No-Regex, Recursive, Overlapping

The following method is based on recursion and it doesn’t require any external library.

The idea is to repeatedly find the next occurrence of the substring pattern in the string and call the same method recursively on a shorter string—moving the start position to the right until no match is found anymore.

All found substring matches are accumulated in a variable

import re
s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 2: re.finditer + list comprehension
l = [(pattern, m.start(), m.end()) for m in re.finditer(pattern, s)]
print(l)

s = 'Finxters learn Python with Finxter'
pattern = 'Finxter'

# Method 5: recursive, without regex
def find_all(pattern, # string pattern
             string, # string to be searched
             start=0, # ignore everything before start
             acc=[]): # All occurrences of string pattern in string

    # Find next occurrence of pattern in string
    i = string.find(pattern, start)
    
    if i == -1:
        # Pattern not found in remaining string
        return acc
    
    return find_all(pattern, string, start = i+1,
                    acc = acc + [(pattern, i)]) # Pass new list with found pattern

l = find_all(pattern, s)
print(l)

The output is:

[('Finxter', 0), ('Finxter', 27)]

Note that this method also finds overlapping matches—in contrast to the regex methods that consume all partially matched substrings.

Where to Go From Here?

Enough theory. Let’s get some practice!

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🚀 If your answer is YES!, consider becoming a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.

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Resources: https://stackoverflow.com/questions/3873361/finding-multiple-occurrences-of-a-string-within-a-string-in-python

Chris

While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.

To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.

His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.

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