Recently in a Python webinar, the was the topic of discussion was how to find all occurrences of a substring in a string using Python. After researching and discussing I found three effective ways to accomplish this task. In this tutorial, Let us learn all the methods with suitable examples and screenshots of executed example code.
Find All Occurrences of a Substring in a String Using Python
Python provides various ways to find all occurrences of a substring in a string. Let us learn all the important methods.
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Method 1. Use re.finditer() Function
The re.finditer() function in Python from the re module finds all occurrences of a substring, including overlapping matches.
Example: Find “New” in USA city names
import re
text = "New York, New Jersey, Newport, New Orleans"
pattern = r"New" # Substring to search for
matches = [match.start() for match in re.finditer(pattern, text)]
print("Occurrences of 'New':", matches)Output:
Occurrences of 'New': [0, 10, 22, 31]In the screenshot below you can see the executed example code with the output.
"New" appears at index 0 (New York), 10 (New Jersey), 22 (Newport), and 30 (New Orleans).re.finditer() returns iterators with match positions.
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Method 2. Use str.find() Method
The str.find(substring, start) method in Python finds the first occurrence of a substring after start. We use a loop to find all occurrences.
Example: Find “San” in USA city names
text = "San Francisco, San Diego, Santa Monica, San Jose"
substring = "San"
positions = []
index = text.find(substring) # Find first occurrence
while index != -1:
positions.append(index)
index = text.find(substring, index + 1) # Find next occurrence
print("Occurrences of 'San':", positions)Output:
Occurrences of 'San': [0, 15, 26, 40]In the screenshot below you can see the executed example code with the output.
"San" appears at index 0 (San Francisco), 15 (San Diego), 26 (Santa Monica), and 40 (San Jose).find() returns -1 when no more occurrences are found.
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Method 3. Use List Comprehension with range()
This method searches through all possible start positions in the Python string and collects matches.
Example: Find “ton” in USA city names
text = "Houston, Boston, Charleston, Trenton"
substring = "ton"
matches = [i for i in range(len(text)) if text.startswith(substring, i)]
print("Occurrences of 'ton':", matches)Output:
Occurrences of 'ton': [4, 12, 24, 33]In the screenshot below you can see the executed example code with the output.
"ton" appears at index 4 (Houston), 13 (Boston), 24 (Charleston), and 32 (Trenton).text.startswith(substring, i) checks if the substring starts at the index i.
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Conclusion
In this tutorial, I have explained how to find all occurrences of a substring in a string using Python. I have discussed some important methods such as using the re.finditer() function, str.find() method, and list comprehension.
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I am Bijay Kumar, a Microsoft MVP in SharePoint. Apart from SharePoint, I started working on Python, Machine learning, and artificial intelligence for the last 5 years. During this time I got expertise in various Python libraries also like Tkinter, Pandas, NumPy, Turtle, Django, Matplotlib, Tensorflow, Scipy, Scikit-Learn, etc… for various clients in the United States, Canada, the United Kingdom, Australia, New Zealand, etc. Check out my profile.
