How to Find Index of a Substring in Python

In Python, sometimes we need to know where a certain substring appears in a string. To do this, we can find the index (or position) of that substring. The index tells us where the substring is located. Let’s look at some simple ways to find the index of a substring in a string.

Using find() method

The easiest way to find the index of a substring is by using Python’s built-in find() method. This method returns the index of the first occurrence of the substring. If the substring is not found, it returns -1.

s = "hello world"
idx = s.find("world")
print(idx)

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Let's see more methods to Find substring in given Python string.

Using index() method

Another method to find the index is using the index() method. It works like find(), but it raises an error if the substring is not found.

s = "hello world"
idx = s.index("hello")
print(idx)

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Find substring from a specific position

Sometimes you may want to start searching from a specific position in the string. You can use find() method with a second argument that specifies the starting index.

s = "hello world"
idx = s.find("o", 5)  # Start searching from index 5
print(idx)

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Using Regular Expressions - re.search() method

Using RegEx allows us for advance pattern matching. We can search substring index using re.search() method.

import re
s = "Hello, Geeks for geeks"
# searching for string 'geeks'
res = re.search(r"geeks", s)
# printing starting index of string
print(res.start())

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Using rfind() - find index of last occurrence

If you want to find the index of the last occurrence of a substring, you can use the rfind() method. It searches from the end of the string to the beginning.

s = "hello hello world"
idx = s.rfind("hello")
print(idx)

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