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:
Optionally, you may also want to print the positions where the shorter string arise in the longer string:
- Result 2:
[(0, 'Finxter'), (27, 'Finxter')]
Method 1: Regex re.finditer()
To get all occurrences of a pattern in a given string, you can use the regular expression method
re.finditer(pattern, string). The result is an iterable of match objects—you can retrieve the indices of the match using the
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
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:
[(pattern, m.start(), m.end())for m in re.finditer(pattern, s)].
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)]
Method 3: 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
acc as you go through the recursion calls.
s = 'Finxters learn Python with Finxter' pattern = 'Finxter' # Method 3: 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!
To become successful in coding, you need to get out there and solve real problems for real people. That’s how you can become a six-figure earner easily. And that’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?
Practice projects is how you sharpen your saw in coding!
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Then become 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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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.