🧑‍🎓 Regex101

Category: Miscellaneous

100 points | 382 solves

Challenge Description

Our team stored a flag on our machine, however, we were hacked by someone, and he generated 2999 flags and hid our original flag in the .txt file. The flag consists of 5 uppercase letters, followed by 5 digits and another 6 uppercase letters. Can you find it for us?

Attached files

• flags.txt

Solution

flags.txt contains 3000 lines with a possible flag. Given the information in the description, we can construct a regular expression that we can use to find the correct flag.

The flag comprises several parts that we can translate into parts of a regular expression:

1. Start of the flag → SEETF{
2. 5 uppercase letters → [A-Z] (any uppercase letter) + {5} (5 of them)
3. 5 digits → [0-9] (any digit from 0-9) + {5} (5 of them)
4. 6 uppercase letters → [A-Z] (any uppercase letter) + {6} (6 of them)
5. End of the flag → }

Putting it together, we get this regular expression:

SEE{[A-Z]{5}[0-9]{5}[A-Z]{6}}

We can then use this regular expression with grep -E (to search using a regular expression) to search the file for the correct flag:

$ grep -E "SEE{[A-Z]{5}[0-9]{5}[A-Z]{6}}" flags.txt
SEE{RGSXG13841KLWIUO}

Flag

SEE{RGSXG13841KLWIUO}

Footnote

A useful website for creating and testing regular expressions is regexr.com.