Regular Expression syntax in Python with the either or operator [duplicate]

Question

I am trying to find amino acid sequences that start with either L or A and then end in either L or A with two amino acids in between each instance of L or A.

This is what I have:

re.findall("A|L.{2}A|L", string1)

output:

['A', 'L', 'A', 'L', 'A', 'A', 'L', 'L', 'L', 'L', 'L', 'L', 'L', 'L', 'LNLA', 'A', 'L', 'L', 'L', 'L', 'A', 'L', 'A', 'L', 'L', 'L', 'A', 'A', 'A', 'A', 'LACA', 'A', 'L', 'L', 'L', 'A', 'A', 'A', 'A', 'A', 'A', 'L', 'LPYA', 'A', 'A', 'A', 'A', 'L', 'L', 'A', 'A', 'A']

I assume the extra L's and A's have something to do with the syntax, but I'm not sure what this | is exactly doing.

It would help the question a lot if you provided sample input, with the desired output. Without seeing your input, I agree with @kay3 that it sounds like you want [AL].{2}[AL]. — CryptoFool
– CryptoFool, Commented Sep 26, 2022 at 0:47

Paul Wang · Accepted Answer · 2022-09-26 01:54:20Z

| means "or". So "A|L" means to match a char "A" or "L".

So "A|L.{2}A|L" means to match "A" or "L.{2}A" or "L".

Perhaps what you want is: "[AL].{2}[AL]"

where [ ] is a set of characters, re will match any char in the [ ]. For example "[AaLy]" will match 'A' or 'a' or 'L' or 'y'.

import re string1="AALLLNLALPYAL" out0=re.findall("[A|L].{2}[A|L]", string1) print(out0)

Output

['AALL', 'LNLA', 'LPYA']

However, "(A|L).{2}(A|L)" means search by first group in first parenthesis which is A or L, then after arbitrary 2 characters, search the 2nd group by 2nd parenthesis which is A or L. So the output would look different:

out1=re.findall("(A|L).{2}(A|L)", string1) print(out1)

Output

[('A', 'L'), ('L', 'A'), ('L', 'A')]

Well this may not be what you want, right?

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