Renaming attributes with regular expressions

If you have an attribute with this name

Firstpart_Secondpart_Thirdpart

and you want to rename it to

Thirdpart_Secondpart_Firstpart

Use the 'Rename By Replacing' operator with this

(.*)_(.*)_(.*)

in the 'replace what' field

and

$3_$2_$1

in the 'replace by' field.

The brackets denote what is known as a capturing group so that everything inside that matches can be used later on by the use of the $1, $2 and $3 entries. The '.*' means match 0 or more characters and will continue until the '_' is found. The capturing group brackets mean that everything from the beginning of the string up to the character before the '_' will be placed into capturing group 1.