How to detect identical part(s) inside string?

Question

I try to break down the decoding algorithm wanted question into smaller questions. This is Part I.

Question:

• two strings: s1 and s2
• part of s1 is identical to part of s2
• space is separator
• how to extract the identical part(s)?

example 1:

s1 = "12 November 2010 - 1 visitor"
s2 = "6 July 2010 - 100 visitors"

the identical parts are "2010", "-", "1" and "visitor"

example 2:

s1 = "Welcome, John!"
s2 = "Welcome, Peter!"

the identical parts are "Welcome," and "!"

example 3: (to clarify the "!" example)

s1 = "Welcome, Sam!"
s2 = "Welcome, Tom!"

the identical parts are "Welcome," and "m!"

Python and Ruby preferred. Thanks

Answer 1

EDIT: Updated this example to work with all the examples, including #1:

def scan(s1, s2):
    # Find the longest match where s1 starts with s2
    # Returns None if no matches
    l = len(s1)
    while 1:
        if not l:
            return None
        elif s1[:l] == s2[:l]:
            return s1[:l]
        else:
            l -= 1

def contains(s1, s2):
    D = {} # Remove duplicates using a dict
    L1 = s1.split(' ')
    L2 = s2.split(' ')

    # Don't add results which have already 
    # been processed to satisfy example #1!
    DProcessed = {}

    for x in L1:
        yy = 0
        for y in L2:
            if yy in DProcessed:
                yy += 1
                continue

            # Scan from the start to the end of the words
            a = scan(x, y)
            if a: 
                DProcessed[yy] = None
                D[a] = None
                break

            # Scan from the end to the start of the words
            a = scan(x[::-1], y[::-1])
            if a: 
                DProcessed[yy] = None
                D[a[::-1]] = None
                break
            yy += 1

    return list(D.keys())

print contains("12 November 2010 - 1 visitor",
               "6 July 2010 - 100 visitors")
print contains("Welcome, John!",
               "Welcome, Peter!")
print contains("Welcome, Sam!",
               "Welcome, Tom!")

Which outputs:

['1', 'visitor', '-', '2010']
['Welcome,', '!']
['Welcome,', 'm!']