Python: keep only letters in string

Question

What is the best way to remove all characters from a string that are not in the alphabet? I mean, remove all spaces, interpunction, brackets, numbers, mathematical operators..

For example:

input: 'as32{ vd"s k!+'
output: 'asvdsk'

Sashasashay

Answer 1

You could use re, but you don't really need to.

>>> s = 'as32{ vd"s k!+'
>>> ''.join(x for x in s if x.isalpha())
'asvdsk'    
>>> filter(str.isalpha, s) # works in python-2.7
'asvdsk'
>>> ''.join(filter(str.isalpha, s)) # works in python3
'asvdsk'

Roofing

Answer 2

If you want to use regular expression, this should be quicker:

import re
s = 'as32{ vd"s k!+'
print(re.sub('[^a-zA-Z]+', '', s))

prints 'asvdsk'

Mcphee

Answer 3

Here is a method that uses ASCII ranges to check whether an character is in the upper/lower case alphabet (and appends it to a string if it is):

s = 'as32{ vd"s k!+'
sfiltered = ''

for char in s:
    if((ord(char) >= 97 and ord(char) <= 122) or (ord(char) >= 65 and ord(char) <= 90)):
        sfiltered += char

The variable sfiltered will show the result, which is 'asvdsk' as expected.

Rinarinaldi

Answer 4

This simple expression get all letters, including non ASCII letters ok t áàãéèêçĉ... and many more used in several languages.

r"[^\W\d]+"

It means "get a sequence of one or more characters that are not either "non word characters" or a digit.

Nonsuch

Answer 5

If you'd like to preserve characters like áàãéèêçĉ that are used in many languages around thw world, try this:

import re
print re.sub('[\W\d_]+', '', yourString)

Agripinaagrippa

You're missing the argument to sub into the string: re.sub('[\W\d_]+', '', yourString) – Integumentary

Answer 6

As an alternative approach, the Alphabetic package can be used, which provides a function for this purpose.

First install the package via pip install alphabetic, then proceed as follows:

from alphabetic import WritingSystem

input_str = 'as32{ vd"s k!+'

ws = WritingSystem()
ws.strip_non_script_characters(input_str, 
                               ws.Language.English, 
                               process_token_wise=False)

In this way you will get the desired output:

'asvdsk'

Note: If ws.Language.English is not passed as an argument, all characters of all supported languages (>150) are taken into account instead.

Pathe