Python RegEx Tutorial – How to use RegEx inside lambda Expression | digital daily

It’s possible to use RegEx inside a lambda function in Python. You can apply this to any Python method or function that takes a function as a parameter. Such functions and methods include filter(), map(), any(), sort(), and more.

Keep reading as I show you how to use regular expressions inside a lambda function.

What We’ll Cover

How to use RegEx inside the Expression of a Lambda Function

The syntax with which a lambda function can take a RegEx as its expression looks like this:

lambda x: re.method(pattern, x)

Be aware that you have to use the lambda function on something. And that’s where the likes of map(), sort(), filter(), and others come in.

How to use RegEx inside the Expression of a Lambda Function with the filter() Function

The first example I will show you use the filter() function:

import re

fruits = ['apple', 'mango', 'banana', 'cherry', 'apricot', 'raspberry', 'avocado']
filtered_fruits = filter(lambda fruit: re.match('^a', fruit), fruits)

# convert the new fruits to another list and print it
print(list(filtered_fruits)) # ['apple', 'apricot', 'avocado']

In the code above:

• the filter() takes the lambda function as the function to execute and the fruits list as the iterable
• for the expression of the lambda function, it uses the re.match() method of Python RegEx and uses the pattern ^a on the argument fruit
• the last thing I did was convert all items on the list that matches the pattern into a list

How to use RegEx inside the Expression of a Lambda Function with the map() Function

To use RegEx inside a lambda function with another function like map(), the syntax is similar:

import re

fruits2 = ['opple', 'bonono', 'cherry', 'dote', 'berry']
modified_fruits = map(lambda fruit: re.sub('o', 'a', fruit), fruits2)

# convert the new fruits to another list and print it
print(list(modified_fruits)) # ['apple', 'banana', 'cherry', 'date', 'berry']

In the code above:

• the modified_fruits is looping through the fruits2 list with a map() function
• uses the re.sub() method of Python RegEx as the expression of the lambda function.

The re.sub method lets you replace the first value with the second one. In the example, it switched all occurrences of o to a.

How to use RegEx inside the Expression of a Lambda Function with the sort() Method

The last example I will show you uses the sort() method of lists:

import re

fruits = [ 'banana', 'fig', 'grapefruit']

# sort fruits based on the number of vowels
fruits.sort(key=lambda x: len(re.findall('[aeiou]', x)))

print(fruits) #['fig', 'banana', 'grapefruit']

In the code, the lambda function sorts the list based on the number of vowels. It does it with the combination of the len() method, the findall() method of Python RegEx, and the pattern [aeiou].

The word fruit with the lowest number of vowels comes first. If you use reverse=True, it arranges the fruits based on those with the highest number of vowels – descending order:

import re

fruits = [ 'banana', 'fig', 'grapefruit']

# sort fruits based on the number of vowels
fruits.sort(key=lambda x: len(re.findall('[aeiou]', x)), reverse=True)

print(fruits) # ['grapefruit', 'banana', 'fig']

Conclusion

In this article, we looked at how you can pass in RegEx to a lambda function by showing you examples using the filter(), map() functions, and the sort() method.

I hope this article gives you the knowledge you need to use RegEx inside a lambda function.

Keep coding!