Using regex to add leading zeroes

Question

I would like to add a certain number of leading zeroes (say up to 3) to all numbers of a string. For example:

Input: /2009/5/song 01 of 12

Outpu

Answer 1

Use something that supports a callback so you can process the match:

>>> r=re.compile(r'(?:^|(?<=[^0-9]))([0-9]{1,3})(?=$|[^0-9])')
>>> r.sub(lambda x: '%04d' % (int(x.group(1)),), 'dfbg345gf345', sys.maxint)
'dfbg0345gf0345'
>>> r.sub(lambda x: '%04d' % (int(x.group(1)),), '1x11x111x', sys.maxint)
'0001x0011x0111x'
>>> r.sub(lambda x: '%04d' % (int(x.group(1)),), 'x1x11x111x', sys.maxint)
'x0001x0011x0111x'