Unicode and vulgar fractions

This little snippet will scan a block of text and replace fractions with the unicode vulgar fraction equivalent when possible, removing spaces if the fraction is immediately after another number. For example "1 1/2" will be replaced by "1½".

import re
FRACRE = re.compile(ur'(\d\ +)?(\d+/\d+)\b')

FRACTIONS = {
    u'1/2' : u'\u00BD',
    u'1/4' : u'\u00BC',
    u'3/4' : u'\u00BE',
    u'1/3' : u'\u2153',
    u'2/3' : u'\u2154',
    u'1/5' : u'\u2155',
    u'2/5' : u'\u2156',
    u'3/5' : u'\u2157',
    u'4/5' : u'\u2158',
    u'1/6' : u'\u2159',
    u'5/6' : u'\u215A',
    u'1/8' : u'\u215B',
    u'3/8' : u'\u215C',
    u'5/8' : u'\u215D',
    u'7/8' : u'\u215E',
}

def fractions(s):
    def subfrac(m):
        pre, post = m.groups()
        frac = FRACTIONS.get(post)
        if frac is None:
            start, end = m.span()
            return m.string[start:end]
        if pre is not None:
            frac = pre[0] + frac
        return frac
    return FRACRE.sub(subfrac, s)