5 Best Ways to Filter Strings within ASCII Range in Python

💡 Problem Formulation: In Python programming, it’s common to need filtering of strings to ensure they consist of ASCII characters only. Given an input string, the desired output is a new string containing only those characters within the ASCII range (0-127). For example, from the input ‘Pythön! is füñ 🚀’, the output should be ‘Python! is fn ‘.

Method 1: Using Regular Expressions

The first method utilizes the re module in Python, which provides support for regular expressions. The method involves compiling a pattern that matches non-ASCII characters and replacing them with an empty string. This effectively filters out any characters that fall outside of the ASCII range.

Here’s an example:

import re

def filter_ascii(string):
    ascii_pattern = re.compile(r'[^\x00-\x7F]')
    return ascii_pattern.sub('', string)

example_string = 'Pythön! is füñ 🚀'
filtered_string = filter_ascii(example_string)
print(filtered_string)

Output:

Python! is fn 

The provided code defines a function filter_ascii() that takes a string and filters out non-ASCII characters using a regular expression pattern. When passed the example_string, the function returns the string without any non-ASCII characters, as shown in the output.

Method 2: Using String Methods

This approach relies on the built-in string method isascii() available in Python. It checks if the string only contains ASCII characters. By iterating over the input string and concatenating only the characters that pass this check, we can filter out non-ASCII characters.

Here’s an example:

def filter_ascii(string):
    return ''.join(char for char in string if char.isascii())

example_string = 'Pythön! is füñ 🚀'
filtered_string = filter_ascii(example_string)
print(filtered_string)

Output:

Python! is fn 

In this snippet, the function filter_ascii() iterates over each character in the string and builds a new string using list comprehension and the join() method. Only characters that return True for isascii() are included in the result.

Method 3: Using The ‘filter’ Function

Python’s built-in filter function can be applied here to iterate over the input string and apply a function that returns only ASCII characters. It is a more functional programming oriented solution.

Here’s an example:

example_string = 'Pythön! is füñ 🚀'
filtered_string = filter(lambda x: x.isascii(), example_string)
print(''.join(filtered_string))

Output:

Python! is fn 

The snippet illustrates the use of the filter function with a lambda expression to check if each character is ASCII. The result is a filter object that’s then turned back into a string with the join() method, producing the filtered string.

Method 4: Using ASCII Encoding

An encoding-based approach involves attempting to encode the input string to ASCII format, replacing or ignoring characters that cannot be encoded. Python’s encode() method is used here with the 'ignore' option to discard non-ASCII characters.

Here’s an example:

example_string = 'Pythön! is füñ 🚀'
filtered_string = example_string.encode('ascii', 'ignore').decode()
print(filtered_string)

Output:

Python! is fn 

The code converts the input string into bytes, ignoring characters during ASCII encoding that cannot be included. It then decodes the byte string back into a regular string, yielding the ASCII-only result.

Bonus One-Liner Method 5: Using List Comprehension

This concise one-liner uses list comprehension to filter non-ASCII characters, relying on the ord() function to check the character code.

Here’s an example:

example_string = 'Pythön! is füñ 🚀'
filtered_string = ''.join(char for char in example_string if ord(char) < 128)
print(filtered_string)

Output:

Python! is fn 

This compact line of code loops through each character in the input string and includes it in a new string if its ordinal value is less than 128, which signifies that it’s part of the ASCII character set.

Summary/Discussion

• Method 1: Regular Expressions. Strengths: Precise control over filtering conditions. Weaknesses: Can be slower for larger text data.
• Method 2: String Methods. Strengths: Clear and concise code, easy to understand. Weaknesses: Availability of isascii() only in Python 3.7 or newer.
• Method 3: filter Function. Strengths: Functional programming style, concise syntax. Weaknesses: Slightly less straightforward for beginners.
• Method 4: ASCII Encoding. Strengths: Fast and efficient for large strings. Weaknesses: Encoding handling can be confusing to some users.
• Method 5: List Comprehension. Strengths: One-liner, straightforward. Weaknesses: Direct use of ASCII value might be less clear.