Question

I have a string which could be arbitrary long say

s = 'Choose from millions of possibilities on Shaadi.com. Create your profile, search&contact; your special one.RegisterFree\xa0\xa0\xa0unsubscribing reply to this mail\xa0\n and 09times and this is limited time offer! and this is For free so you are saving cash'

I have a list of spam words which could be like

p_words = ['cash', 'for free', 'limited time offer']

All I want to know if there pattern exists in the input text and how many times?

It becomes simple when it has just one word

import re
p = re.compile(''.join[p_words])  # correct me if I am wrong here
m = p.match(s)  

but it could be a bi-gram, tri-gram or n-gram

How do we approach this?

Answer 1

If the text and number of words is not very large you could start with, example:

d = {w: s.count(w) for w in p_words if w in s}
# -> {'cash': 1, 'limited time offer': 1}

You could compare its performance with:

import re
from collections import Counter

p = re.compile('|'.join(map(re.escape, p_words)))
d = Counter(p.findall(s))
# -> Counter({'limited time offer': 2, 'cash': 2})

For reference compare its speed with fgrep. It should be fast at matching multiple strings in the input stream:

$ grep -F -o -f  patternlist.txt largetextfile.txt  | sort | uniq -c

Output

  2 cash
  2 limited time offer