Replacing string with placeholder and replacing them back after a function.

Question

Given a string and a list of substring that should be replaces as placeholders, e.g.

import re
from copy import copy 

phrases = ["'s morgen", "'s-Hertogenbosch", "depository financial institution"]
original_text = "Something, 's morgen, ik 's-Hertogenbosch im das depository financial institution gehen"

The first goal is to first replace the substrings from phrases in the original_text with indexed placeholders, e.g.

text = copy(original_text)
backplacement = {}
for i, phrase in enumerate(phrases):
    backplacement["MWEPHRASE{}".format(i)] = phrase.replace(' ', '_')
    text = re.sub(r"{}".format(phrase), "MWEPHRASE{}".format(i), text)
print(text)

[out]:

Something, MWEPHRASE0, ik MWEPHRASE1 im das MWEPHRASE2 gehen

Then there'll be some functions to manipulate the text with the placeholders, e.g.

cleaned_text = func('Something, MWEPHRASE0, ik MWEPHRASE1 im das MWEPHRASE2 gehen')
print(cleaned_text)

that outputs:

MWEPHRASE0 ik MWEPHRASE1 MWEPHRASE2

the last step is to do the replacement we did in a backwards manner and put back the original phrases, i.e.

' '.join([backplacement[tok] if tok in backplacement else tok for tok in clean_text.split()])

[out]:

"'s_morgen ik 's-Hertogenbosch depository_financial_institution"

The questions are:

1. If the list of substrngs in phrases is huge, the time to do the 1st replacement and the last backplacement would take very long.

Is there a way to do the replacement/backplacement with a regex?

2. using the re.sub(r"{}".format(phrase), "MWEPHRASE{}".format(i), text) regex substitution isn't very helpful esp. if there are substrings in the phrases that matches not the full word,

E.g.

phrases = ["org", "'s-Hertogenbosch", "depository financial institution"]
original_text = "Something, 's morgen, ik 's-Hertogenbosch im das depository financial institution gehen"
backplacement = {}
text = copy(original_text)
for i, phrase in enumerate(phrases):
    backplacement["MWEPHRASE{}".format(i)] = phrase.replace(' ', '_')
    text = re.sub(r"{}".format(phrase), "MWEPHRASE{}".format(i), text)
print(text)

we get an awkward output:

Something, 's mMWEPHRASE0en, ik MWEPHRASE1 im das MWEPHRASE2 gehen

I've tried using '\b{}\b'.format(phrase) but that'll didn't work for the phrases with punctuations, i.e.

phrases = ["'s morgen", "'s-Hertogenbosch", "depository financial institution"]
original_text = "Something, 's morgen, ik 's-Hertogenbosch im das depository financial institution gehen"
backplacement = {}
text = copy(original_text)
for i, phrase in enumerate(phrases):
    backplacement["MWEPHRASE{}".format(i)] = phrase.replace(' ', '_')
    text = re.sub(r"\b{}\b".format(phrase), "MWEPHRASE{}".format(i), text)
print(text)

[out]:

Something, 's morgen, ik 's-Hertogenbosch im das MWEPHRASE2 gehen

Is there some where to denote the word boundary for the phrases in the re.sub regex pattern?

Answer 1

Instead of using re.sub you can split it!

def do_something_with_str(string):
    # do something with string here.
    # for example let's wrap the string with "@" symbol if it's not empty
    return f"@{string}" if string else string


def get_replaced_list(string, words):
    result = [(string, True), ]

    # we take each word we want to replace
    for w in words:

        new_result = []

        # Getting each word in old result
        for r in result:

            # Now we split every string in results using our word.
            split_list = list((x, True) for x in r[0].split(w)) if r[1] else list([r, ])

            # If we replace successfully - add all the strings
            if len(split_list) > 1:

                # This one would be for [text, replaced, text, replaced...]
                sub_result = []
                ws = [(w, False), ] * (len(split_list) - 1)
                for x, replaced in zip(split_list, ws):
                    sub_result.append(x)
                    sub_result.append(replaced)
                sub_result.append(split_list[-1])

                # Add to new result
                new_result.extend(sub_result)

            # If not - just add it to results
            else:
                new_result.extend(split_list)
        result = new_result
    return result


if __name__ == '__main__':
    initial_string = 'acbbcbbcacbbcbbcacbbcbbca'
    words_to_replace = ('a', 'c')
    replaced_list = get_replaced_list(initial_string, words_to_replace)
    modified_list = [(do_something_with_str(x[0]), True) if x[1] else x for x in replaced_list]
    final_string = ''.join([x[0] for x in modified_list])

Here's variables values of the example above:

initial_string = 'acbbcbbcacbbcbbcacbbcbbca'
words_to_replace = ('a', 'c')
replaced_list = [('', True), ('a', False), ('', True), ('c', False), ('bb', True), ('c', False), ('bb', True), ('c', False), ('', True), ('a', False), ('', True), ('c', False), ('bb', True), ('c', False), ('bb', True), ('c', False), ('', True), ('a', False), ('', True), ('c', False), ('bb', True), ('c', False), ('bb', True), ('c', False), ('', True), ('a', False), ('', True)]
modified_list = [('', True), ('a', False), ('', True), ('c', False), ('@bb', True), ('c', False), ('@bb', True), ('c', False), ('', True), ('a', False), ('', True), ('c', False), ('@bb', True), ('c', False), ('@bb', True), ('c', False), ('', True), ('a', False), ('', True), ('c', False), ('@bb', True), ('c', False), ('@bb', True), ('c', False), ('', True), ('a', False), ('', True)]
final_string = 'ac@bbc@bbcac@bbc@bbcac@bbc@bbca'

As you can see the lists contain tuples. They contain two values - some string and boolean, representing whether it's a text or replaced value (True when text). After you get replaced list, you can modify it as in the example, checking if it's text value (if x[1] == True). Hope that helps!

P.S. String formatting like f"some string here {some_variable_here}" requires Python 3.6