Extract letters (and a specific number) from a string

Question

I have a list of strings similar to the one below:

l = ['ad2g3f234','4jafg32','fg23g523']

For each string in l, I want to delete every digit (except for 2 and 3 if they appear as 23). So in this case, I want the following outcome:

n = ['adgf23','jafg','fg23g23']

How do I go about this? I tried re.findall like:

w = [re.findall(r'[a-zA-Z]+',t) for t in l]

but it doesn't give my desired outcome.

score 0 · Answer 1 · answered Oct 25 '21 at 04:04

0

One way would be just to replace the string twice:

[re.sub("\d", "", i.replace("23", "*")).replace("*", "23") for i in l]

Output:

['adgf23', 'jafg', 'fg23g23']

answered Oct 25 '21 at 04:04

Chris

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score 0 · Answer 2 · answered Oct 25 '21 at 04:05

Use a placeholder with re.sub

l = ['ad2g3f234','4jafg32','fg23g523']
w = [re.sub('#','23',re.sub('\d','',re.sub('23','#',t))) for t in l]

['adgf23', 'jafg', 'fg23g23']

EDIT

As answered by Chris, the approach is the same although string replace will be a better alternative stack_comparison

score 0 · Answer 3 · answered Oct 25 '21 at 04:15

Using re.sub with function

import re

def replace(m):
    if m.group() == '23':
        return m.group()
    else:
        return ''
    
l = ['ad2g3f234','4jafg32','fg23g523']
w = [re.sub(r'23|\d', replace, x) for x in l]

#w: ['adgf23', 'jafg', 'fg23g23']

Explanation

re.sub(r'23|\d', replace, x)
- checks first for 23, next for a digit
- replace function leaves alone match with 23
- changes match with digit to null string.

The fourth bird · Accepted Answer · 2021-10-25T07:32:36.870

0

You can capture 23 in a group, and remove all other digits. In the replacement, use the group which holds 23 if it is there, else replace with an empty string.

import re

l = ['ad2g3f234', '4jafg32', 'fg23g523']

result = [
    re.sub(
        r"(23)|(?:(?!23)\d)+", 
        lambda m: m.group(1) if m.group(1) else "", s) for s in l
]
print(result)

Output

['adgf23', 'jafg', 'fg23g23']

Python demo

edited Oct 25 '21 at 07:32

answered Oct 25 '21 at 06:36

The fourth bird

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could you explain what the lambda function is doing? thanks – Oct 25 '21 at 20:02
@NewbieAF The lambda is the callback function for re.sub See https://docs.python.org/3/library/re.html#re.sub where is this code example `m` is the 'matchobj' – The fourth bird Oct 25 '21 at 20:26
1

The function checks if the match object has a capture group 1 which contains the value 23. If there is a capture group 1 present, then put back the 23. If there is no group 1 present, then the match contains all other digits except 23. That match is then replaced by an empty string. – The fourth bird Oct 25 '21 at 20:34
How do I exclude more characters? For example, if I want to exclude “-“, “+”, “{}” and “()” as well as any digit not making “23”? – Oct 25 '21 at 22:17
@NewbieAF The pattern does not match `-` or `+` so they will stay in the string. – The fourth bird Oct 25 '21 at 23:10

Extract letters (and a specific number) from a string

4 Answers4