How do i replace a work using pattern matching brackets in sed?

[maltiger]

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

Using the sample file:

cat linefile 
This is line one

This is line two

This is line three

We want to change the word three to 3.

Use this, this is more specific than using the dot to match any character.

$ sed 's/[Tt]hree/3/g' linefile

Result:
This is line one

This is line two

This is line 3

More accurate use of the dot.

$ sed 's/[Tt]..ee/3/g' linefile

Result:
This is line one

This is line two

This is line 3