How do I replace a substring only if it is a whole word?

Question

For example, if I want to replace the word "in" with "on" in the string "I was walking in Memphis.", and do this:

result = re.sub("in", "on", string)
# prints "I was walkong on Memphis." (Note "walkong".)

I tried using regular expressions like r"\Win\W" to only replace the expression if it is surrounded by non-word characters. But this will also replace the surrounding whitespaces, for example:

import re
pattern = r"\Win\W"
repl = "on"
string = "I was walking in Memphis."
result = re.sub(pattern, repl, string)
print(result)
# prints "I was walkingonMemphis."

Any ideas?

Update:
Sharing my progress. The following code works, but only as long as there is only one occurrence of "in". Maybe I could fix that, but it feels like an unnecessarily complicated way to go:

import re
old_word = "in"
pattern = r"\W" + old_word + r"\W"
repl = "on"
string = "I was walking in Memphis."

match = re.search(pattern, string).group()
splt = re.split(pattern, string)
new_word = re.sub(old_word, repl, match)
result = splt[0] + new_word + splt[1]
print(result)
# prints "I was walking on Memphis."

Answer 1

Thank you, everybody! You really helped me. I decided I liked my \b-solution best, so here is the full working code. I added a comma to the string to emphasize the solution doesn't only work for whitespace.

import re
s = "I was walking in, Memphis."
old_word = "in"
new_word = "on"
pattern = r"\b" + old_word + r"\b"
result = re.sub(pattern, new_word, s)
print(result)
# prints "I was walking on, Memphis."

Answer 2

Try this:
result = re.sub("(?<!\S)in(?!\S)", "on", string)

(?<!\S)in(?!\S) will match all ocurrences of "in" enclosed with withespaces or at the start/end of the string.

Check the following links to get more information on the pattern:
https://docs.python.org/3/library/re.html#index-20
https://docs.python.org/3/library/re.html#index-22
https://docs.python.org/3/library/re.html#index-30

Answer 3

Lennart Wisbar You're welcome! Always happy to help.

And feel free to use the answer you like the most.

Happy coding!

Answer 4

You don't even need regex for that 😜

print(' '.join([w if w != 'in' else 'on' for w in 'I was walking in Memphis'.split()]))

Or you can define a dict with arbitrary words and their replacements:

repl_dict = {
'in': 'on',
'I': 'You',
'was': 'were',
'Memphis': 'the sun',
}

s = 'I was walking in Memphis'

print(' '.join([repl_dict.get(w, w) for w in s.split()]))
# output: You were walking on the sun

Answer 5

Lennart Wisbar Thanks, I fixed it (#2)

https://code.sololearn.com/cerwjnasWmwB/?ref=app

Answer 6

Lennart Wisbar Right. I fixed it

Answer 7

Diego Acero Thank you! This is almost it. First, I modified your regex by replacing \S with \w, so it also worked if the word was followed by a comma, for example. Then I followed one of your links and found out about \b (again). It signifies word boundaries. This is much shorter:

pattern = r"\b" + old_word + r"\b"

Now I'm not sure if I should accept your answer or mine. Your answer missed non-whitespace characters like commas, but it was your answer that led me to mine, so it feels a bit unfair ...

Answer 8

Anna Nice, but it only works if you know exacly what seperates the words. In your example, that's " ".
If the sentence is "I was walking in Memphis.", then "Memphis" is not replaced, because it is directly followed by a fullstop. I guess you do need a regex to be that flexible.
Thank you anyways, still a helpful answer!

Answer 9

Anna I see it's possible without regex, but now you would have to do that for every non-word character like (, ), ", !, : etc.

Answer 10

Anna Nice! It works perfectly. The regex solution with \b seems easier and more readable to me, but it's nice to know that there are also other ways.