![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](https://image.slidesharecdn.com/basicregularexressionextendedregularexression-200417131347/85/Basic-regular-expression-extended-regular-expression-3-320.jpg)
![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…](https://image.slidesharecdn.com/basicregularexressionextendedregularexression-200417131347/85/Basic-regular-expression-extended-regular-expression-4-320.jpg)
![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](https://image.slidesharecdn.com/basicregularexressionextendedregularexression-200417131347/85/Basic-regular-expression-extended-regular-expression-5-320.jpg)
![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](https://image.slidesharecdn.com/basicregularexressionextendedregularexression-200417131347/85/Basic-regular-expression-extended-regular-expression-6-320.jpg)
![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](https://image.slidesharecdn.com/basicregularexressionextendedregularexression-200417131347/85/Basic-regular-expression-extended-regular-expression-7-320.jpg)
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
Editor's Notes
- #2: We use commands that filter data to select only the portion of data that we wish to view or operate on. t can be used to process information in powerful ways such as restructuring output to generate useful reports, modifying text in files and many other system administration tasks.
- #3: If an expression uses metacharacters it is termed as regular exression
- #5: The above RE matches two names This pattern matches 3 patterns