How do i use pattern matching to remove all letters but leave numbers in sed?

[maltiger]

Man page for sed: https://www.lightnetics.com/post/3334

Sample file used:

$ cat linefile                            
This is line one
1000
This is line two
2000
This is line three

Use this: [a-Z] matching all ascii characters.

$ sed 's/[a-Z]/ /g' linefile

Result, including blank lines

1000

2000

To remove the blank line too

Use this: -e to specify expression or script. Here we used two expressions.

$ sed -e 's/[a-Z]/ /g' -e '/^\s*$/d' linefile

Result
1000
2000