Extract a substring from a string in Python

David Y.

The Problem

How do I extract a substring from a string in Python?

The Solution

We can extract a substring from a string using Python’s slice notation. The syntax is as follows:

substring = my_string[start:end]
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The variable substring will include all characters in the string, starting at the start index up to but not including the end index. Strings in Python are 0-indexed, so we must count characters from 0 to the length of the string minus 1. We can omit the start value to begin at index 0 or the end value to continue until the end of the string.

Consider the following examples:

my_string = "Hello world!"

substring = my_string[1:5]  # will be "ello"
substring = my_string[:5]  # will be "Hello"
substring = my_string[6:]  # will be "world!"
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We can also use regular expressions to extract a substring using the re module in Python. You can use the re.search() function to search the string for a specific pattern. The method returns a match object if a match is found. You can then use the .group() method on the match object to extract the matching substring. For example, to extract the price from a string:

import re

string = "The price is $20.45"
match = re.search(r'\d+(\.\d{1,2})?', string)
substring = match.group()
print(substring) # will output 20.45
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The regular expression pattern \d+(\.\d{1,2})? will match one or more digits followed by an optional decimal point and one or two digits, which corresponds to the price in the string. The match.group() method will return the matching substring.