Python regular expression findall using Lookahead

Question

I'm new to regular expressions and would like to understand how findall() and lookahead can be used to find all occurrences of a given pattern within a string. I am having problems with alternating characters. Here is an example of what I want:

s = 'ababa4abaab'
p = 'aba'
print([ s[i:i+len(p)] for i in range(len(s)) if s[i:i+len(p)]==p])
['aba', 'aba', 'aba']

Here is my attempt with findall():

import re
re.findall('aba', 'ababa4abaab')
['aba', 'aba']

It only returns 2 matches but I want all three. I read this tutorial but did not quite understand. I tried

re.findall('(?=aba)', 'ababa4abaab')
['', '', '']

Can someone please tell me how to use this lookahead concept in this case and provide a brief explanation of how it works?

What is your question about ? findall(), lookahead or finding recurring patterns ? — Rahul, Dec 27 '17 at 08:30
So, you want overlapping patterns. Use `import regex as re; re.findall('aba', 'ababa4abaab', overlapped=True)`. — cs95, Dec 27 '17 at 08:33
Possible duplicate of [Python regex find all overlapping matches?](https://stackoverflow.com/questions/5616822/python-regex-find-all-overlapping-matches) — Sebastian Proske, Dec 27 '17 at 09:09
Your regex quite literally means, find every empty string which is followed by "aba". — tripleee, Dec 27 '17 at 09:39
Thank you all. I'm still trying to understand things but your comments were helpful. — Valery Marcel, Dec 29 '17 at 02:43

score 0 · Answer 1 · answered Dec 27 '17 at 08:40

I think you just need to search either there is an 'ab' and 'a' right after, You don't need to catch it as 'aba', you can use this look ahead:

ab(?=a)

which gives you 3 matches.

you can also capture it inside a group and then iterate each one of them and concatenate 'a' so you'll end with the desired text 'aba' for each match

 (ab(?=a))

score 0 · Answer 2 · answered Dec 27 '17 at 09:29

0

Official doc about findall says it

"Return a list of all non-overlapping matches in the string."

answered Dec 27 '17 at 09:29

Ratnesh Kushwaha

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Python regular expression findall using Lookahead

2 Answers2