Negated Character Classes in Python Regular Expressions

 While working with Python regex, if we want to match everything except certain characters, then we can use negated character classes by placing a caret (^) as the first character inside square brackets. The pattern [^abdfgh] will match any character not in that set.

What is a Negated Character Class?

A character class like [abc] matches any single character except 'a', 'b', or 'c'. But if we use a ^ symbol at the beginning, like [abc], it will match any character except 'a', 'b', or 'c'. This allows us to eliminate certain characters from the match quickly.

The position of the ^ is important, it must come right after the opening square bracket to indicate negation. If we place it somewhere else, it will be treated as a literal character.

Basic Negation of Specific Characters 

Let's see how to exclude specific alphabets from matching using a negated character class and re.findall() method to find all occurrences in the given string.

Example

In the following program, we want to find all characters in a string that are not in the group. The regex [^abdfgh] matches any character that is not 'a', 'b', 'd', 'f', 'g', or 'h'. As a result, it matches and prints all the remaining characters.

import re

text = "abcdefghi123"
matches = re.findall(r'[^abdfgh]', text)
print(matches)

Following is the output of the above code -

['c', 'e', 'i', '1', '2', '3']

Negated Character Classes with Word Boundaries

Another common method is using negated classes to find characters that are not part of certain words or patterns. For instance, the regex [^a-zA-Z] will locate all characters that aren't letters, such as spaces, punctuation, or numbers.

Example

In the following, we want to extract all non-alphabetic characters from a string using negated character classes.

import re

text = "Hello, World! 123"
matches = re.findall(r'[^a-zA-Z]', text)
print(matches)  

Following is the output of the above code -

[',', ' ', '!', ' ', ' ', '1', '2', '3']

Excluding Special Characters

We can also exclude special characters by using a negated character class of all special characters.

Example

Here, the regex [^@!#$%] excludes these five special characters. As a result, the program prints all remaining characters.

import re

text = "Hello@World! This#is$Python%"
matches = re.findall(r'[^@!#$%]', text)
print(''.join(matches))

Following is the output of the above code -

HelloWorld ThisisPython