We were given a list with 10000 of flags out of which 1 was real.So according to question it clearly hints about **using GREP**.
As well as a hint is given in description that there are **7 random vowels at the end** of flag.
So the command for finding the real flag would be-
cat flag.txt | grep -e [aeiou][aeiou][aeiou][aeiou][aeiou][aeiou][aeiou]
This is **regular expression search(regex)**. [aeiou] this means that a letter from this.So our command means show content of flag.txt and then grep 7 letter(vowels) **each from [aeiou]**.
And that gave us our flag-** nactf{r3gul4r_3xpr3ss10ns_ar3_m0r3_th4n_r3gul4r_euaiooa}**