Tags: regularexpression regex grep 


## Grep 1
![date](https://img.shields.io/badge/date-10.30.2020-brightgreen.svg) ![General category](https://img.shields.io/badge/Category-General-lightgrey.svg) ![score](https://img.shields.io/badge/score-200-blue.svg)

### Description
Elaine hid a REGULAR flag among more than 1,000,000 fake ones! The flag was an EXPRESSION of her love for nactf, so the first 10 characters after "nactf{" only have the characters 'n', 'a', 'c', and the last 14 characters only have the characters 'c', 't' and 'f'. There are 52 characters in total, including nactf{}.

### Attached files
- flag.zip

### Solution
Ok this challenge has a similar topic to Grep 0 except that this one will be a bit more complex. They gave us a 72.3 MB file that contains many counterfiet flags with the format "nactf{....}". Similar to grep 0, we are looking for a specific pattern to find the actual flag. They've given a hint about regular expressions when they ALL CAPS the "regular" and "expression" in the description. In this challenge, we have to formulate a regular expression to find patterns according to the description of the real flag.
- First 10 characters after nactf{ only have the characters 'n', 'a', 'c'
- Last 14 characters only have the characters 'c', 'f', and 'f'
- There are 52 characters in total including nactf{}

So this is what I did at first:
grep "nactf{[nac][nac][nac][nac][nac][nac][nac][nac][nac][nac]?????????????????????[ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf][ctf]}" flag.txt
After doing some research, I found this cheat sheet: [Link](https://www.rexegg.com/regex-quickstart.html) that helped me solve this problem (and make it look less messy)

- []: One of the characters in the brackets should match
- {}: How many times it should be iterated
- .: Literally any character except a line break

grep "nactf{[nac]{10}.{21}[ctf]{14}}" flag.txt
This was the correct pattern but I couldn't get it to work for some reason, so I decided to use sublime's regular expression search on the text file. After some debugging and playing around, I found the flag!

This took some time for me but it was worth it!

### Flag

Original writeup (https://github.com/JoshuEo/CTFs/tree/master/NACTF_2020).