How to replace value of cell QGIS

Question

I have a column named foo and I want to replace certain values from the cells in this column. foo is a string column. The values are

1, 01, 2, 02, 3, 4, 010, 12, ...

I tried

replace("foo",'1','01')

But this replaces me all the ones 1 with 01, resulting in

01, 0101, 2, 02, 3, 4, 01010, 012, ...

I only want completly matching numbers, hence

01, 01, 2, 02, 3, 4, 010, 12, ...

How to do that?

Answer 1

You can use function lpad(): lpad( "foo", 2,'0')

From documentation:

lpad() returns a string padded on the left to the specified width, using a fill character.

Answer 2

I did it with

case 
when "foo" is '1' then '01' 
else "foo" end