regexbaby.txt

This was a terrific refresher of finer points of regex expression syntax. One catch may be the specific Python function to use when verifying your expressions - not all of them behave the same way in all cases. I asked the CTF admins and they recommended using re.findall() function, which helped me overcome some discrepancies I saw in testing expressions locally vs. testing against the server.

To test expressions locally I threw together a small script:

import re,sys

data = open("regexbaby.txt","r").read()

print sys.argv[1], "len: %d" % len(sys.argv[1])

m = re.findall(sys.argv[1], data)

for x in m:
    print x

After a lot of trial and error the following expressions worked (note that the expression with 38 characters has a trailing space):

$ nc 200.136.213.148 5000

Type the regex that capture: "from "Drivin" until the end of phrase, without 
using any letter, single quotes or wildcards, and capturing "Drivin'" in a 
group, and "blue." in another", with max. "16" chars: (.{7}).+-(.{5})$
Nice, next...
Type the regex that capture: "All "Open's", without using that word or [Ope-], 
and no more than one point", with max. "11" chars: (?i)(oPEn)
Nice, next...
Type the regex that capture: "(BONUS) What's the name of the big american 
television channel (current days) that matchs with this regex: .(.)\1", with 
max. "x" chars: CNN
Nice, next...
Type the regex that capture: "Chips" and "code.", and it is only allowed the 
letter "c" (insensitive)", with max. "15" chars: ([cC].{4})\n\n
Nice, next...
Type the regex that capture: "the follow words: "unfolds", "within" (just one 
time), "makes", "inclines" and "shows" (just one time), without using hyphen, 
a sequence of letters (two or more) or the words itself", with max. "38" chars: 
(?:t|d) ([^F]\w{2,5}(?:i|e|w|d)[^d]) 
Nice, next...
Type the regex that capture: "the only word that repeat itself in the same 
word, using a group called "a" (and use it!), and the group expression must 
have a maximum of 3 chars, without using wildcards, plus signal, the word 
itself or letters different than [Pa]", with max. "16" chars: (?P<a>..a)(?P=a)
Nice, next...
Type the regex that capture: "FLY until... Fly", without wildcards or the word 
"fly" and using backreference", with max. "14" chars: (FL.).+(?i)\1
Nice, next...
Type the regex that capture: "<knowing the truth. >, without using "line break"", 
with max. "8" chars: <[^z]*>
Nice, next...
CTF-BR{Counterintelligence_wants_you!}

The flag is CTF-BR{Counterintelligence_wants_you!}.