Week 4

Question 1

What is POSIX Standard?

Answer 1

standard which defines regular expression into two types 1. Basic Regular Expression (BRE) 2. Extended Regular Expression (ERE)

Question 2

What is regex?

Answer 2

Regex is a pattern template to filter text

Question 3

How is grep command used?

Answer 3

1. grep ‘pattern’ filename
2. command| grep ‘pattern’
3. egrep ‘pattern’ filename
   grep -E ‘pattern’ filename

Question 4

what are special characters used in grep pattern?

Answer 4

. - any single char
* - zero or more of preceding char/expr

[] - Any of the enclosed character; hypen (-) indicates char range

^ Anchor for beginning of line or negation of enclosed characters

$ - Anchor for end of line

\ Escape special characters.

{n,m} - range of occurances of preceding pattern at least n and utmost m times

() - grouping of regular expressions

Question 5

What are some of the special character (ERE)?

Answer 5

{n,m} - Range of occurances of precing pattern at least n and utmost m times.
() grouping of regular expressions

+ one or more of preceding character/ expression

? Zero or one of preceding character / expression

logical OR over the patterns

Question 6

What are character classes?

Answer 6

[[:print:}} - Printable
[[:alnum:}} - alpha numeric

[[:alpha:}} - Alphabetic

[[:lower:}}- Lower case

[[:upper:}} - Upper case

[[:digit:}} - Decimal digits

[[:blank:}} - Space/ Tab

[[:space:}} - Whitespace

[[:punct:}} - Punctuation

[[:xdigit:}} - Hexadecimal

[[:graph:}} - Non - Space

[[:cntrl:}} - Control Characters

Question 7

What are Backreferences?

Answer 7

There are 9 backreferences
\1 through \9

\n matches whatever was matched by nth earlier paranthesized sub expression

A line with two occurances of hello will be matched using
(hello). *\1

Question 8

What is the BRE / ERE operator precedence?

Question 9

how to search for a string

Answer 9

grep <pattern> <filename></filename></pattern>

cat <filename> | grep <pattern></pattern></filename>

Question 10

how to use ‘.’ in grep command?

Answer 10

cat names.txt | grep ‘S.n’ - implies pattern ‘S*n’ here ‘.’ means any character available.

Suppose we use cat names.txt | grep ‘.am$’ - this pattern will look for pattern “*am” at the end of the line.

Question 11

How to check for a string with ‘.’

Answer 11

cat ‘names.txt’| grep ‘.’

Question 12

How to use anchors in grep?

Answer 12

cat ‘names.txt’ | grep ‘^M’ - Lines which begin with M.

cat names.txt |grep ‘^e’ does not return string which start with capital E. we need to use
cat names.txt | grep -i ‘^e’

Question 13

how to use word boundaries ?

Answer 13

cat names.txt | grep ‘am\b’ - end of the word boundary

cat names.txt | grep ‘am$’ - end of the line boundary

Question 14

How to use [] in grep?

Answer 14

[] are used to give options.

cat names.txt | grep ‘M[ME]’ - matches MM or ME

cat names.txt | grep ‘[aeiou][aeiou]’ - matches names which has 2 vowels side by side

cat names.txt| grep ‘B90[1-4]’ - matches B901 to B904

cat names.txt | grep ‘B90[^5-7] - matches everything except B905 till B907. Anchor inside square brackets act as negation.

Question 15

how to check frequency of occurance?

Answer 15

cat names.txt | grep ‘M{2} - matched MM

‘M{1,2} - either one or twice

Question 16

How to group patterns ?

Answer 16

cat names.txt | grep ‘{ma)’

’(ma).*\1’ - matches pattern ‘ma’ followed by any no of characters and then ends with ‘ma’

’{a.}{3}’ - matches presense of 3 a’s in a name followed by some other char.

Question 17

how to use egrep?

Answer 17

cat names.txt | egrep ‘M+’ - patterns containing M’

‘^M+’ - patterns which start with letter M

‘[ED][ME]’ - either ED or ME should occur.

Question 18

how to filter through files using egrep?

Answer 18

dpkg-query -w -f’${Section} ${Binary:Package}\n | egrep ‘^math’

lists all files which start with math

Question 19

How to use the character classes in practice?

Answer 19

cat chartypes.txt | grep ‘[[:alnum:]]’

Question 20

How to exclude a particular output using grep

Answer 20

cat chartypes.txt | grep -v ‘[[:cntrl:]]’

selects all lines which do not have a control character

Question 21

How to select all the non empty lines using egrep?

Answer 21

cat chartypes.txt | egrep -v ‘^$’

Question 22

How to select a pincode (which has exactly 6 digits using egrep command?

Answer 22

egrep ‘\b[[:digit:]]{6}\n patterns.txt

Question 23

how to select email addresses using egrep?

Answer 23

egrep ‘\b[[:alnum:]]+.[[:alnum:]]+\b’ patterns.txt

Question 24

What does cut command do?

Answer 24

It does horizontal trimming of sections.

cut -c 1-4 fields.txt

displays first 4 characters of each line of the file fields.txt

cat fields.txt | cut -d “ “ -f 1

displays text delimited by space and displays first column or field

Question 25

how is composition of cut operation used?

Answer 25

cat fields.txt | cut -d “;” -f 1 |cut -d “,” -f 1