Using regular expression to match and replace

Question

There is a list of string A which is some how matching with another list of string B. I wanted to replace string A with list of matching string B using regular expression. However I am not getting the correct result.

The solution should be A == ["Yogesh","Numita","Hero","Yogesh"].

import re

A = ["yogeshgovindan","TNumita","Herohonda","Yogeshkumar"]
B=["Yogesh","Numita","Hero"]

for i in A:
    for j in B:
        replaced=re.sub('i','j',i)
        
print(replaced)

Asked By: yocodefreak

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Source

Answer 1

this one works to me:

lst=[]
for a in A:
    lst.append([b for b in B if b.lower() in a.lower()][0])

This returns element from list B if it is found at A list. It’s necessary to compare lowercased words. The [0] is added for getting string instead of list from comprehension list.

Answered By: Lucie H

Answer 2

If looping over B, you don’t need a regular expression; you can simply use membership testing.

A regex might result in better performance, as membership testing will scan each string in A for every string in B, resulting in O(len(A) * len(B) performance).

As long as the individual terms don’t contain any metacharacters and can appear in any context, the simplest way to form the regex is to join the entries of B with the alternation operation:

reTerms = re.compile('|'.join(B), re.I)

However, to be safe, the entries should first be escaped, in case any contains a metacharacter:

# map-based
reTerms = re.compile('|'.join(map(re.escape, B)), re.I)
# comprehension-based
reTerms = re.compile('|'.join([re.escape(b) for b in B]), re.I)

If there is any restrictions on the context the terms appear in, sub-patterns for the restrictions would need to be prepended and appended to the pattern. For example, if the terms must appear as full words:

reTerms = re.compile(f"b(?:{'|'.join(map(re.escape, B))})b", re.I)

This regex can be applied to each item of A to get the matching text:

replaced = [reTerms.search(name).group(0) for name in A]
# result: ['yogesh', 'Numita', 'Hero', 'Yogesh']

Since the terms in the regex are straight string matches, the content will be correct, but the case may not. This could be corrected by a normalization step, passing the matched text through a dict:

normed = {term.lower():term for term in B}

replaced = [normed[reTerms.search(name).group(0).lower()] for name in A]
# result: ['Yogesh', 'Numita', 'Hero', 'Yogesh']

One issue remains: what if an item of A doesn’t match? Then reTerms.search returns None, which doesn’t have a group attribute. If None-propagating attribute access is added to Python (such as suggested by PEP 505), this would be easily addressed by using such:

names = ["yogeshgovindan","TNumita","Herohonda","Yogeshkumar", "hrithikroshan"]
normed[None] = None
replaced = [normed[reTerms.search(name)?.group(0).lower()] for name in names]

In the absence of such a feature, there are various approaches, such as using a ternary expression and walrus assignment. In the sample below, a list is used as a stand-in to provide a default value for the match:

import re

names = ["yogeshgovindan","TNumita","Herohonda","Yogeshkumar", "hrithikroshan"]
terms = ["Yogesh","Numita","Hero"]
normed = {term.lower():term for term in terms}
normed[''] = None

reTerms = re.compile('|'.join(map(re.escape, terms)), re.I)

# index may need to be changed if `reTerms` includes any context
[normed[(reTerms.search(sentence) or [''])[0].lower()] for sentence in sentences]

Answered By: outis

Using regular expression to match and replace

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