Regular Expressions - regex

A regular expression (regex) is a string for describing a search pattern. Regular expression generally uses special characters called meta characters to represent something.

^  represents beginning of the line

$ represents end of the line

grep '^I'  test3.txt

Here the lines starting with 'I ' letter from the file 'test3.txt' will be displayed in the screen

grep '6$' marks.txt

Here the lines ending with '6'  from the file marks.txt will be displayed on the screen

To find the lines that starts with either letter 'a' or 'r' then use

grep -i '^[ar]' test.txt

To find the lines that doesn't starts with either letter 'a' or 'r' then use

grep -i '^[^[ar]]' test.txt

Also you can use '\<' and '\>' to search a particular word

Eg: If you want to replace the word 'in' with 'out' and if you use the following command

sed 's/in/out/gi' sample.txt

then this will replace all 'in' with 'out'. If the sample.txt has a word 'initial' it will be also replaced as 'outitial'. Here we wanted replace only 'in' word and so we have to alter the command

sed 's/\/out/gi' sample.txt

This will replace only the full words 'in' and not 'in' included in other words