Regex contains words if no negative words are before it

Question

I want to grab phrases that say good or great, but are not negated by the words not or isn't before it.

sents= ["good words",                   # Words after phrase
        "not good words",
        "isn't good words",

        "great words",
        "not great words",
        "isn't great words",



        "words good",                   # Words before phrase
        "words not good",
        "words isn't good"

        "words great",
        "words not great",
        "words isn't great"


        
        "words good words",             # Words before and after phrase
        "words not good words",
        "words isn't good words",

        "words great words",
        "words not great words",
        "words isn't great words",
]

I want to return

good words
words good
words good words

great words
words great
words great words

What is the regular expression that will let me do this? In theory, I want to be able to have a list of words that are only found if the string does not contain any from a list of negatives precede it.

anubhava · Accepted Answer · 2020-09-08T15:57:41.050

You may use this regex with 2 negative lookbehind assertions in python:

(?<!isn't )(?<!not )\b(?:good|great)\b

RegEx Demo

RegEx Details:

(?<!isn't ): Negative lookbehind to fail the match if we have isn't followed by a single space behind us
(?<!not ): Negative lookbehind to fail the match if we have not followed by a single space behind us
\b: Word boundary
(?:good|great): Match good or great
\b: Word boundary

Code:

>>> sents= ["good words",                   # Words after phrase
...         "not good words",
...         "isn't good words",
...         "great words",
...         "not great words",
...         "isn't great words",
...         "words good",                   # Words before phrase
...         "words not good",
...         "words isn't good",
...         "words great",
...         "words not great",
...         "words isn't great",
...         "words good words",             # Words before and after phrase
...         "words not good words",
...         "words isn't good words",
...         "words great words",
...         "words not great words",
...         "words isn't great words",
... ]
>>> reg = re.compile(r"(?<!isn't )(?<!not )\b(?:good|great)\b")
>>> for s in sents:
...     if reg.search(s):
...             print(s)
...
good words
great words
words good
words great
words good words
words great words

Dalorzo · Answer 2 · 2020-09-08T15:55:57.220

0

You need to use look behind, in this case negative since there is a version for positive as well. And you can use it as simple as:

(?<!not\s)great

In this example the word not cannot exists before great.

Here is a how it may look like:

(?<!not\s)(?<!isn't\s)(great|good)

Online Demo

edited Sep 08 '20 at 15:55

answered Sep 08 '20 at 15:46

Dalorzo

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Regex contains words if no negative words are before it

2 Answers2