How to add a whitespace before "((VERB)" only if it is not preceded by a space or the beginning of the string?

Question

import re

#input string example:
input_text = "((VERB)ayudar a nosotros) ár((VERB)ayudar a nosotros) Los computadores pueden ((VERB)ayudar a nosotros)"


#this give me a raise error("look-behind requires fixed-width pattern") re.error: look-behind requires fixed-width pattern
#input_text = re.sub(r"(?<!^|s)((VERB)", " ((VERB)", input_text)

#and this other option simply places a space in front of all ((VERB) ) 
# without caring if there is a space or the beginning of the string in front 
input_text = re.sub(r"(^|s)((VERB)", lambda match: match.group(1) + "((VERB)", input_text)

print(repr(input_text)) # --> output

I have tried using (^|s) as it is a capturing group that looks for the start of the string ^ or a whitespace just before the pattern "((VERB)". Another pattern option could be with a non-capturing group (?:|) or better still using a context limiter like look-behind (?<!^|s)

This is the output you should be getting when running this script:

"((VERB)ayudar a nosotros) ár ((VERB)ayudar a nosotros) Los computadores pueden ((VERB)ayudar a nosotros)"

Asked By: Matt095

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Source

Answer 1

You can assert a non whitespace char to the left:

(?<=S)((VERB)

Regex demo | Python demo

In the replacement use a space followed by the full match r" g<0>"

import re

input_text = "((VERB)ayudar a nosotros) ár((VERB)ayudar a nosotros) Los computadores pueden ((VERB)ayudar a nosotros)"
input_text = re.sub(r"(?<=S)((VERB)", r" g<0>", input_text)
print(input_text)

Output

((VERB)ayudar a nosotros) ár ((VERB)ayudar a nosotros) Los computadores pueden ((VERB)ayudar a nosotros)

Answered By: The fourth bird

Answer 2

An alternative to using a lookbehind could be to match on any character except whitespaces before ((VERB):

([^s])(((VERB))

substitute with

1 2

( – start of capture group 1
- [^s] – match on a single character that is not a whitespace
) – end of capture group 1
( – start of capture group 2
- ((VERB) – literal match on ((VERB)
) – end of capture group 2

Demo

Answered By: Ted Lyngmo

How to add a whitespace before "((VERB)" only if it is not preceded by a space or the beginning of the string?

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