[FIXED] I'm stuck with regular expressions

Issue

I'm stuck with regular expressions in Python...

#!/usr/bin/python3
import re

combi="ABBAEAADCA"
one_a = len(re.findall('[^A](A)[^A]', combi))
print("A:"+str(one_a))

I try to make this variable (one_a) contain the number of A's that appear alone (3) but it does not count those at the beginning and end of lines so....

one_a = len(re.findall('\A(A)[^A]', combi))
print("A ini:"+str(one_a))
one_a += len(re.findall('[^A](A)[^A]', combi))
print("A_cen:"+str(one_a))
one_a += len(re.findall('[^A](A)\Z', combi))
print("A_end:"+str(one_a))

but it didn't work either when in this particular case the value that should stay in the variable should be 3. I would appreciate knowing what I am missing or what mistake I am making. Thank you very much

Solution

You can combine start-of-string (^) and end of string ($) with regular character classes through the or (|) operator.

re.findall(r'(?:^|[^A])A(?:$|[^A])', combi)

This gives you all substrings where A is either surrended by start of string and end of string, start of string and not-A, not-A or end of string or not-A and not-A.

>>> re.findall(r'(?:^|[^A])A(?:$|[^A])', combi)
['AB', 'BAE', 'CA']

Applying len to this list gives you the count of single A's.

Answered By - MatsLindh