unix- Basic regular expression, extended regular expression

1. Introduction to Unix and OS
Module 3
grep and Regular Expression
Dr. Girisha G S
Dept. of CSE
SoE,DSU, Bengaluru
1

2. Basic Regular Expression
What is regular expression?
- Regular expression is a pattern which contains meta characters for a complex
search/match
Classification of regular expression
1. Basic Regular Expression (BRE)
2. Extended Regular Expression (ERE)
- grep command supports
-> BRE by default
-> ERE with the –E option
- Basic Regular Expression make it possible to match a group of similar patterns

3. The BRE character subset
Meta character /
character
Matches
* Zero or more occurrences of preceding expression
E.g. a* matches no ‘a’, a, aa, aaa, …….
. A single character
E.g. .* matches any number of characters
[ ] Matches single character with in the brackets
E.g. [pqr] matches a single character p, q, or r
[1-4] matches digit between 1 & 3
^ Match a pattern at the beginning of line
E.g. ^The match any line which begin with the word
The
$ Match a pattern at the end of line
E.g. ab$ matches any stream that end with b

4. The character class ( [ ] )
- matches one of the character in the brackets
E.g. To match Agarwal or agrawal, the RE is
[Aa]g[ar][ra]wal
The *
Matches zero or more occurrences of the preceding expression
E.g.1 g* matches single character g or any no of g’s
i.e. no ‘g’, g, gg, ggg, …….
E.g.2 To match the pattern Agarwal, agrawal, aggarwal, the RE is
[Aa]gg*[ar][ra]wal
The Dot(.)
- A dot(.) matches single character
- .* sigifies any no of character or none
E.g. Match all 4 character words beginning with 2
2…

5. Specifying pattern location (^ & $)
E.g. 1. Match lines starting with 2
^2
E.g. 2 match lines where salary between no 7000 to 7999
7…$
grep & RE
- You can also use regular expression with grep command
Example1: search files Agarwal and agrawal from emp.lst
$ grep ‘[aA]g[ar[ar]wal emp.lst
Example2: search for the lines Agarwal, agrawal, aggarwal from emp.lst
$ gepr ‘[aA]gg*[ar[ar]wal’ emp.lst
Example3: print all lines that begin with letter ‘a ‘ followed by one
character followed by letter sequence ‘ple’ from fruitlist.txt
$ grep ‘^a.ple’ fruitlist.txt
Example4: print all the lines ending with letter ‘e’ from sample.txt
$ grep ‘e$’ sample.txt

6. Extended Regular Expression
- Extended RE make it possible to match disimilar patterns with
a single expression
- This set uses additional characters
- grep command use them with –E option
The ERE set
+ matches one or more occurrence of the previous character
E.g a+ matches a,aa,aaa,……..
[ab]+ matches ab,abaab, ababab,…..
[a-az]+ matches all string of lower characters
? Matches zero or q occurrence of preceding expression
E.g. ab?c matches either ac or abc, here b is optiona
| Indicates alternation
E.g . (ab|cd) matches either ab or cd
( ) groups a series of REs together into a new RE

7. grep and ERE
- Use grep’s -E option to use ERE
Example1: search for the lines Agarwal, aggarwal from the file
emp.lst
$ gepr -E ‘[aA]gg?arwal’ emp.lst
Example2: search for the lines segupta and dasgupta from
the file emp.lst
$ grep –E ‘(sen|das)gupta’ emp.lst