How to add tags to negated words in strings that follow “not”, “no” and “never”

Question

How do I add the tag NEG_ to all words that follow not, no and never until the next punctuation mark in a string(used for

Answer 1

To make up for Python's re regex engine's lack of some Perl abilities, you can use a lambda expression in a re.sub function to create a dynamic replacement:

import re
string = "It was never going to work, he thought. He did not play so well, so he had to practice some more. Not foobar !"
transformed = re.sub(r'\b(?:not|never|no)\b[\w\s]+[^\w\s]', 
       lambda match: re.sub(r'(\s+)(\w+)', r'\1NEG_\2', match.group(0)), 
       string,
       flags=re.IGNORECASE)

Will print (demo here)

It was never NEG_going NEG_to NEG_work, he thought. He did not NEG_play NEG_so NEG_well, so he had to practice some more. Not NEG_foobar !

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Explanation

• The first step is to select the parts of your string you're interested in. This is done with

  \b(?:not|never|no)\b[\w\s]+[^\w\s]
  

  Your negative keyword (\b is a word boundary, (?:...) a non capturing group), followed by alpahnum and spaces (\w is [0-9a-zA-Z_], \s is all kind of whitespaces), up until something that's neither an alphanum nor a space (acting as punctuation).

  Note that the punctuation is mandatory here, but you could safely remove [^\w\s] to match end of string as well.

• Now you're dealing with never going to work, kind of strings. Just select the words preceded by spaces with

  (\s+)(\w+)
  

  And replace them with what you want

  \1NEG_\2