How to format labels with leading zeroes?

We have the rpad function for that:

rpad('1', 2, '0') 
rpad('10', 2, '0')

result

01
10

rpad("yourcolumn", 2, '0')

rpad() function

Returns a string with supplied width padded using the fill character.

Syntax rpad(string, width, fill)

Arguments

string - is string. The string.
width - is int. The length of the new string.
fill - is char. The character to padd the remaining space with.

Example

rpad('Hello', 10, 'x') → 'xxxxxHello'

Working on QGIS 2.18.20 I see a different behavior of the one described by @NathanW:

lpad("id",2,'0')-> '02'

rpad("id",2,'0')-> '20'

Where I think r stands for right and l for left.

Quoting the man page of the function rpad:

Returns a string padded to supplied width using a fill character.

Syntax

rpad(string, width, fill)

Arguments

string string to pad

width length of new string

fill character to pad the remaining space with

Examples

rpad('Hello', 10, 'x') → 'Helloxxxxx'