Lecture 6 Discussion Questions

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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