list - Basic indexing recurrences of a substring within a string (python)

Question

I'm working on teaching myself basic programming.
One simple project is to find the index of recurrences of a substring within a string. So for example, in string "abcdefdef" and substring "def", I would like the output to be 3 and 6. I have some code written, but I'm not getting the answers I want. Following is what I have written

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Note:I'm aware that there may be easier way to produce the result, leveraging built-in features/packages of the language, such as Regular Expressions. I'm also aware that my approach is probably not an optimal algorithm. Never the less, at this time, I'm only seeking advice on fixing the following logic, rather than using more idiomatic approaches.

import string

def MIT(String, substring): # "String" is the main string I'm searching within
    String_list = list(String)
    substring_list = list(substring)
    i = 0
    j = 0
    counter = 0
    results = []
    while i < (len(String)-1):
        if [j] == [i]:
            j = j + 1
            i = i + 1
            counter  = counter + 1
            if counter == len(substring):
                results.append([i - len(substring)+1])
                counter = 0
                j = 0
                i = i+1
        else:
            counter = 0
            j = 0
            i = i+1
    print results
    return

My line of reasoning is as such. I turn the String and substring into a list. That allows for indexing of each letter in the string. I set i and j = 0--these will be my first values in the String and substring index, respectively. I also have a new variable, counter, which I set = to 0. Basically, I'm using counter to count how many times the letter in position [i] is equal to the element in position [j]. If counter equals the length of substring, then I know that [i - len(substring) + 1] is a position where my substring starts, so I add it to a list called results. Then I reset counter and j and continue searching for more substrings.

I know the code is awkward, but I thought that I should still be able to get the answer. Instead I get:

>>> MIT("abcdefghi", "def")
[[3]]
>>> MIT("abcdefghi", "efg")
[[3]]
>>> MIT("abcdefghi", "b")
[[1]]
>>> MIT("abcdefghi", "k")
[[1]]

Any thoughts?

Answer 1

The regular expressions module (re) is much more suited for this task.

Good reference: http://docs.python.org/howto/regex.html

Also: http://docs.python.org/library/re.html

EDIT: A more 'manual' way may be to use slicing

s = len(String)
l = len(substring)
for i in range(s-l+1):
    if String[i:i+l] == substring:
        pass #add to results or whatever