Match exact phrase within a string in Python

Question

I'm trying to determine whether a substring is in a string. The issue I'm running into is that I don't want my function to return True if the substring is found within another word in the string.

For example: if the substring is; "Purple cow" and the string is; "Purple cows make the best pets." This should return False. Since cow isn't plural in the substring.

And if the substring was; "Purple cow" and the string was; "Your purple cow trampled my hedge!" would return True

My code looks something like this:

def is_phrase_in(phrase, text):
    phrase = phrase.lower()
    text = text.lower()

    return phrase in text


text = "Purple cows make the best pets!"
phrase = "Purple cow"
print(is_phrase_in(phrase, text)

In my actual code I clean up unnecessary punctuation and spaces in 'text' before comparing it to phrase, but otherwise this is the same. I've tried using re.search, but I don't understand regular expressions very well yet and have only gotten the same functionality from them as in my example.

Thanks for any help you can provide!

Answer 1

Since your phrase can have multiple words, doing a simple split and intersect won't work. I'd go with regex for this one:

import re

def is_phrase_in(phrase, text):
    return re.search(r"\b{}\b".format(phrase), text, re.IGNORECASE) is not None

phrase = "Purple cow"

print(is_phrase_in(phrase, "Purple cows make the best pets!"))   # False
print(is_phrase_in(phrase, "Your purple cow trampled my hedge!"))  # True

Answer 2

Using PyParsing:

import pyparsing as pp

def is_phrase_in(phrase, text):
    phrase = phrase.lower()
    text = text.lower()

    rule = pp.ZeroOrMore(pp.Keyword(phrase))
    for t, s, e in rule.scanString(text):
      if t:
        return True
    return False

text = "Your purple cow trampled my hedge!"
phrase = "Purple cow"
print(is_phrase_in(phrase, text))

Which yields:

True