Operating Systems and Systems Integration

Regular Expressions, sed and awk 1 Background

For the background information you need to answer these questions, please refer to the shell programming lecture notes. The program egrep stands for extended grep. It supports so-called extended regular expressions. Well, since I have taught you the syntax of extended regular expressions in the lecture, I suggest that you use egrep rather than grep with regular expressions. Both grep and egrep support an option -o to print only the part of the line that matches the expression. The gnu sed program supports the -r option that tells sed to use regular expressions like egrep. So when you use sed, use it with sed -r. The gnu awk program has an option --posix that makes awk behave like egrep so that it understands the use of {n}, {n,m} without having to put in extra backslashes ‘\’.
You might wonder what is the difference between “extended regular expressions” and the regular expressions that grep uses? The difference is explained at the end of the info page: $ info ’(grep)Regular Expressions’ where it says, “In basic regular expressions the metacharacters ‘?’, ‘+’, ‘{’, ‘|’, ‘(’, and ‘)’ lose their special meaning; instead use the backslashed versions ‘\?’, ‘\+’, ‘\{’, ‘\|’, ‘\(’, and ‘\)’.”

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2.1

Questions
egrepping through the dictionary

Your dictionary is a file /usr/share/dict/words. Use egrep to: 1. Find all words containing three letter ‘a’s. 2. Find all words containing no vowels. (A vowel is one of the letters ‘a’, ‘e’, ‘i’, ‘o’ and ‘u’.) 3. Find all words containing at least 5 vowels. Count the number of matching words. 4. Find all words containing exactly 5 vowels. Count the number of matching words.

2.2

egrep: Selecting data from student records

1. Save the file http://ictlab.tyict.vtc.edu.hk/snm/lab/regular-expressions/ artificial-student-data.txt to your local directory. For this data, write a regular expression that will select each of the following. Test it on the data using egrep -o

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Regular Expressions, sed and awk Operating Systems and Systems Integration

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2. student number i

3. Hong Kong ID. Count the number of Hong Kong ids. i

4. the course code. Count the number of courses.
The course and year are shown in this case on the sixth line: 2241/2. The course is 2241; this is the second year of study.

i

5. the year of study i

6. The company the student works for i

7. The home telephone number i

8. The gender of the student i

9. The student’s name i

2.3

Using sed

Write a sed expression to output only the data for which you wrote each of the regular expressions above. For example, write a sed command that will print only the HK ids and all the HK ids from the file, using the regular expression you wrote for question 3. You should write eight sed expressions. 2. student number 3. Hong Kong ID. 4. the course code. 5. the year of study 6. The company the student works for
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Regular Expressions, sed and awk Operating Systems and Systems Integration

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7. The home telephone number 8. The gender of the student 9. The student’s name

2.4

Using awk

Use awk and ls to add up the size of all the files in your current directory. i

Nick Urbanik nicku(at)vtc.edu.hk

ver. 1.3