python repeating regex

Question

I have a regex pattern like this

(\w)(..\1..\1|...\1...\1|....\1....\1|.....\1.....1)

I want to make it shorter. As we can see the number of dots is from 2 to 5. I want to write something like (\w)(.{2,5}\1){2} but in this case it will match some wrong strings q00q000q. Also I can try to use number reference but it repeats chars from a string instead of symbols .. Is it possible to repeat pattern .{2,5} as a specific count of .?

UPDATE It was a challange about tic-tac-toe game. I needed to write down a regexp of length <=50 which can be used to find out if somebody wins

In other words I need a reg exp for string like that QaaaQaaaQaaaQ (count of letter 'a' doesn't specified) — Leo
– Leo, Commented Jan 7, 2016 at 22:13
@Leo I don't think it's feasible. Not with a simple regex at least. You would need to "count" the dots and keep state. I don't know of any feature within python regex that can do this. — HamZa
– HamZa, Commented Jan 7, 2016 at 22:18
If I may ask, why do you want to make it shorter? Knowing that might help us to suggest alternatives. — Robᵩ
– Robᵩ, Commented Jan 7, 2016 at 22:33
not possible. what you want is a classical example of non-regular language. see en.wikipedia.org/wiki/Regular_language. it describes that regular expression is not capable of remembering how many characters it counted — Fabricator
– Fabricator, Commented Jan 7, 2016 at 22:49

Robᵩ · Accepted Answer · 2016-01-07 22:35:44Z

1

You could generate the regex with a Python expression:

In [13]: r'(\w)(%s)'%'|'.join(r'{dots}\1{dots}\1'.format(dots='.'*i) for i in range(2,6))
Out[13]: '(\\w)(..\\1..\\1|...\\1...\\1|....\\1....\\1|.....\\1.....\\1)'

answered Jan 7, 2016 at 22:35

Robᵩ

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2 Comments

HamZa Over a year ago

Cool but we're back to square one :)

Robᵩ Over a year ago

Maybe, maybe not. If OP's problem is "my regex is 16K characters long and I don't want to type it by hand", then my solution helps. If OP's problem is "I want to discover a cool new feature of regex", then I didn't help him at all.

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