Comparing two .txt files in Python and saving exact and similar matches to .txt file

What i need is:

text_file_1.txt:
apple
orange
ice
icecream

text_file_2.txt:
apple
pear
ice

When i use "set", output will be:

apple
ice

("equivalent of re.match")

but I want to get:

apple
ice
icecream

("equivalent of re.search")

Is there any way how to do this? Files are large, so I can't just iterate over it and use regex.

you might want to check out difflib

If all you want is to extract from the files words which are one a substring of the other (including those that are identical) you could do:

fone = set(['apple', 'orange', 'ice', 'icecream'])
ftwo = set(['apple' ,'pear' ,'ice'])
# transforming to sets saves to check twice for the same combination

result = []
for wone in fone:
    for wtwo in ftwo:
        if wone.find(wtwo) != -1 or wtwo.find(wone) != -1:
            result.append(wone)
            result.append(wtwo)
for w in set(result):
    print w

Alternatively, if you want a similarity based on how strings are similar in the order of their letters, you could use as suggested by Paul in his answer one of the classes provided by difflib:

import difflib as dl

fone = set(['apple', 'orange', 'ice', 'icecream'])
ftwo = set(['apple' ,'pear' ,'ice'])

result = []
for wone in fone:
    for wtwo in ftwo:
        s = dl.SequenceMatcher(None, wone, wtwo)
        if s.ratio() > 0.6:  #0.6 is the conventional threshold to define "close matches"
            result.append(wone)
            result.append(wtwo)
for w in set(result):
    print w

I did not timed either of the two samples, but I would guess the second will run much slower, as for each couple you will have to instantiate an object...