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

wget https://courses.cs.washington.edu/courses/cse391/24au/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).

    Solutions 

    grep -E "\<....\>" words.txt

    grep -E "\<[a-zA-Z]{4}\>" words.txt

    grep -E "\<[a-zA-Z]{4,}\>" words.txt

  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.

    Solutions 

    grep -E "cc+" vegetables.txt
# or
grep -E "c{2,}" vegetables.txt

    grep -E ".*c.*c.*" vegetables.txt

  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.

    Solutions 
    grep -E "([a-z])\1+" vegetables.txt

  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.

    Solutions 
    grep -E "kit" kitkats.txt | grep -E "kat"

  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.

    Solutions 
    grep -E "[a-z]+ ([a-z]+), \1@[a-z]+\.[a-z]+" emails.txt

  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).

    Solutions 

    grep -E "^([a-zA-Z0-9]{4}-?){3}[a-zA-Z0-9]{4}$" tickets.txt

    grep -E "^([a-zA-Z0-9]{4}-?){3}[a-zA-Z0-9]{4}$" tickets.txt | sort | uniq | wc -l

    grep -E "^[a-zA-Z0-9]{4}(-?)[a-zA-Z0-9]{4}\1[a-zA-Z0-9]{4}\1[a-zA-Z0-9]{4}$" tickets.txt