How do i various things using regular expressions?

hawksbill

Note: when using sed changes are only displayed to standard out only, but can be in-place. For demo only standard out.

Working with sample file, named ips & grepfile
ips:

10.50.20.3
10.50.20.4
10.50.20.5
10.50.20.6
10.50.20.7
10.50.20.8
10.50.20.9
10.50.20.10

grepfile

We're going to need a bigger boat
Go ahead make my day
The brothers in arms
The rain in spain stays mainly on the plain
aiin
aain

Change number 50 to 51.

$ sed 's/50/51/' ips
10.51.20.3
10.51.20.4
10.51.20.5
10.51.20.6
10.51.20.7
10.51.20.8
10.51.20.9
10.51.20.10

Change only line 3 from 50 to 51.

$ sed '3s/50/51/' ips
10.50.20.3
10.50.20.4
10.51.20.5
10.50.20.6
10.50.20.7
10.50.20.8
10.50.20.9
10.50.20.10

Delete all blank lines.

$ sed /^$/d  ips     
10.50.20.3
10.50.20.4
10.50.20.5
10.50.20.6
10.50.20.7
10.50.20.8
10.50.20.9
10.50.20.10

Delete blank lines, blank lines containing only tabs and spaces. Between the square brackets, the space and tab characters are generated using the Ctrl-V + Spacebar then Ctrl-V + Tab key.

$ sed '/^[      ]*$/d' ips

Change every first letter of a work to uppercase.

$ sed 's/\<./\u&/g' grepfile
We'Re Going To Need A Bigger Boat
Go Ahead Make My Day
The Brothers In Arms
The Rain In Spain Stays Mainly On The Plain
Aiin
Aain

Change the entire file to lowercase.

$ sed 's/.*/\L&/' grepfile 
we're going to need a bigger boat
go ahead make my day
the brothers in arms
the rain in spain stays mainly on the plain
aiin
aain

Add a word before the IP addresses.

$ sed  's/[0-9]/Item: &/' ips
Item: 10.50.20.3
Item: 10.50.20.4
Item: 10.50.20.5
Item: 10.50.20.6
Item: 10.50.20.7
Item: 10.50.20.8
Item: 10.50.20.9
Item: 10.50.20.10