Extract substrings between brackets - Python

Extract substrings between bracket means identifying and retrieving portions of text that are enclosed within brackets. This can apply to different types of brackets like (), {}, [] or <>, depending on the context.

Using regular expressions

Regular expressions are the most efficient way to extract substrings from strings. re.findall() function allows us to match all substrings that are enclosed in brackets.

import re

s = "Welcome [GFG] to [Python]"

# Extract all substrings inside brackets
res = re.findall(r"\[(.*?)\]", s)
print(res) 

['GFG', 'Python']

Explanation:

• re.findall() function finds all matches of the given pattern in the string.
• pattern \[.*?\] matches text enclosed in square brackets, and ensures .*? non-greedy matching.
• This method is efficient, especially when dealing with multiple pairs of brackets.

Let’s explore some more methods and see how we can extract substrings between brackets.

Using string slicing

If we know the exact positions of the brackets, we can use string slicing to extract the substrings.

s = "Welcome [GFG] to [Python]"
# Find positions of the first pair of brackets
start = s.index("[") + 1
end = s.index("]")
# Extract the substring
res = s[start:end]
print(res)  

GFG

Explanation:

• index() function locates the opening and closing brackets.
• String slicing extracts the substring between the brackets.
• This method is suitable for a single pair of brackets but less efficient for multiple pairs.

Using a loop and stack

We can manually iterate through the string and use a stack to keep track of the bracket positions, extracting substrings as we encounter closing brackets.

s = "Welcome [GFG] to [Python]"
res = []  # List to store results
stk = []  # Stack to track opening brackets

for i, c in enumerate(s):
    if c == "[":  # Push the index of '[' to the stack
        stk.append(i)
    elif c == "]" and stk:  # When ']' is found, pop the last '['
        start = stk.pop()
        res.append(s[start + 1:i])  # Extract substring between brackets

print(res) 

['GFG', 'Python']

Explanation:

• for loop iterates through the string to find brackets.
• A stack is used to manage bracket positions, especially if brackets are nested.
• This method is more complex but useful for custom parsing requirements.

Using split() and list comprehension

This method splits the string by the opening bracket and uses list comprehension to extract substrings enclosed in brackets.

s = "Welcome [GFG] to [Python]"
# Split the string and extract parts between brackets
parts = s.split("[")
res = [p.split("]")[0] for p in parts if "]" in p]
print(res) 

['GFG', 'Python']

Explanation:

• string is split into parts using "[" as the delimiter.
• List comprehension extracts substrings by splitting each part at "]".
• This method is simple but less efficient than regular expressions for larger strings.

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Article Tags :

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• Python Programs
• Python string-programs

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