Python re.findall() is not working as expected

Question

I have the code:

import re
sequence="aabbaa"
rexp=re.compile("(aa|bb)+")
rexp.findall(sequence)

This returns ['aa']

If we have

import re
sequence="aabbaa"
rexp=re.compile("(aa|cc)+")
rexp.findall(sequence)

we get ['aa','aa']

Why is there a difference and why (for the first) do we not get ['aa','bb','aa']?

Thanks!

Answer 1

The unwanted behaviour comes down to the way you formulate regualar expression:

rexp=re.compile("(aa|bb)+")

Parentheses (aa|bb) forms a group.

And if we look at the docs of findall we will see this:

Return all non-overlapping matches of pattern in string, as a list of strings. The string is scanned left-to-right, and matches are returned in the order found. If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result unless they touch the beginning of another match.**

As you formed a group, it mathced first aa, then bb, then aa again (because of + quantifier). So this group holds aa in the end. And findall returns this value in the list ['aa'] (as there is only one match aabbaa of the whole expression, the list contains only one element aa which is saved in the group).

From the code you gave, you seemed to want to do this:

>>> rexp=re.compile("(?:aa|bb)+")
>>> rexp.findall(sequence)
['aabbaa']

(?: ...) doesnt create any group, so findall returns the match of the whole expression.

In the end of your question you show the desired output. This is achieved by just looking for aa or bb. No quantifiers (+ or *) are needed. Just do it the way is in the Inbar Rose's answer:

>>> rexp=re.compile("aa|bb")
>>> rexp.findall(sequence)
['aa', 'bb', 'aa']

Answer 2

let me explain what you are doing:

regex = re.compile("(aa|bb)+")

you are creating a regex which will look for aa or bb and then will try to find if there are more aa or bb after that, and it will keep looking for aa or bb until it doesnt find. since you want your capturing group to return only the aa or bb then you only get the last captured/found group.

however, if you have a string like this: aaxaabbxaa you will get aa,bb,aa because you first look at the string and find aa, then you look for more, and find only an x, so you have 1 group. then you find another aa, but then you find a bb, and then an x so you stop and you have your second group which is bb. then you find another aa. and so your final result is aa,bb,aa

i hope this explains what you are DOING. and it is as expected. to get ANY group of aa or bb you need to remove the + which is telling the regex to seek multiple groups before returning a match. and just have regex return each match of aa or bb...

so your regex should be:

regex = re.compile("(aa|bb)")

cheers.