Creating a regex to match the following scenario

I am a student working on a small research project where I need to scrape web pages which match the following requirement :-
If word X say "abc" is found anywhere in the text, look for pattern Y say "pqr" if it occurs within a 25 character window on either side of the occurrence of X.
Eg.

pqrxyz is valid.

pqrxyz

xyz is invalid.

xyz

xyzpqr is valid.

xyzpqr

pqr123456789123456789123456789xyz is invalid.

pqr123456789123456789123456789xyz

I can't figure this out. Any help will be greatly appreciated.

((?=pqr).20,abc) | (pqr20,(?!abc))


This is my attempt so far. I don't know how to incorporate the 20 character window constraint.

r'pqr.0,25xyz|xyz.0,25pqr'

2 Answers
2

. is the regular expression for "any single character."

.

n,m is the regular expression for "at least n, and no more than m, repetitions of the previous regular expression."

n,m

So, the regular expression xyz.0,25pqr means "xyz, followed by up to twenty-five characters, followed by pqr".

xyz.0,25pqr

xyz

pqr

So, accounting for the possibility of pqr occurring before or after xyz, we get this line of python code:

pqr

xyz

if re.search('pqr.0,25xyz', line) or re.search('xyz.0,25pqr', line):


Something like this should work, handling both cases:
pqr.,25?xyz|xyz.,25?pqr

pqr.,25?xyz|xyz.,25?pqr

I used Debuggex to test, and I think it's an easy way to show what how the regex is working.

John's answer gives more details on what the specific elements in the regex.

Regular expression visualization

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