We highly recommend downloading the files for this Q&A and following along with us: 

wget https://courses.cs.washington.edu/courses/cse391/26au/lectures/6/questions6.zip
unzip questions6.zip

For many of these problems, you may find it helpful to refer to the Regex Syntax on our reference page.

  1. Suppose that we have a file named words.txt with the following contents 

    These are some words
a11 0f the5e w0rds c0n7ain number5
S0me 0f these w0rds do

    • Write a command that identifies all words which are exactly four characters long.

    • Write a command that identifies all words which are exactly four characters long and contain only letters (both upper and lowercase).

    • Write a command that identifies all words which are at least four characters long and contain only letters (both upper and lowercase).

  2. Suppose that we have the following file named vegetables.txt 🥦 

    broccoli
asparagus
potato
lettuce
zucchini
brocccccccoli

    • Come up with a grep command that correctly identifies all vegetables that have two or more consecutive c’s in their name.

    • Come up with a grep command that correctly identifies all vegetables that have two instances of c anywhere in their name.

  3. Using the file from Q2: Come up with a grep command that correctly identifies all vegetables that have two or more consecutive repeated letters in their name.

  4. Suppose we have a file kitkats.txt with the following contents: 

    kit kat
kat kit
my favorite part of the kit is the kat
cats do not like kit kats
this line only has kit
this line only has kat
    Write a command that finds all lines which contain kit and kat in any order.

  5. Suppose that we have the following file named emails.txt. This file contains a user’s first and last name, followed by a comma, and then their email address. What is a grep command that determines which users have exactly their last name as their email? 

    larry ruzzo, ruzzo@cs.washington.edu
zorah fung, zfung@yahoo.com
hunter schafer, hschafer@uw.edu
bennet goeckner, goeckner@math.uw.edu
ruth anderson, andersonr@gmail.com
    In other words, our command should correctly identify that Larry Ruzzo and Bennet Goeckner have their last names as their email address.

  6. The backend team at faang needs your help - we have lots of new products and they’re flying off the shelves like crazy (apparently you can sell happiness). In order to track all these transactions, each sale is assigned a unique ticket id. A ticket id is defined by the following properties:

    • It must contain exactly 16 letters (upper or lowercase) and numbers
    • To improve readability, the letters may optionally be grouped into segments that are multiples of length four delimited by dashes. However, the string may not end with a dash.

    The following are valid ticket ids: 

    1234567891011112
1234-4567-8910-1112
aBcD-Ef79-8122-fd01
aBcDEf798122-fd01
    The following are not valid ticket ids: 
    12345                            #too short
1233333333333333333333333333     #too long
1234-4567-8910-11?2              #illegal character
1234567891011112-                #ends with dash

    • Come up with a grep command that identifies valid ticket id’s in the file tickets.txt

    • Write a command that identifies how many unique valid ticket id’s are in the file ticket.txt.

    • Challenge: Come up with a grep command that identifies valid ticket id’s with the added constraint that if there is a single dash, all groups of four must be separated by a dash. (i.e. Now aBcDEf798122-fd01 is not a valid ticket id).