Regex for number check below a value

Regex is not suited to dealing with ranges like this, but anyway this should work:

/^(0?[1-9]|1[012])$/

Description:

^       match start of the string
(0?     optional zero
[1-9]       any number between 1 and 9
|1[012])    or match 1 followed by either a 0, 1, 2 i.e. 10, 11, 12.
$           match end of string