highlight pattern

@register.filter(name='highlightpattern')
def highlightpattern(string, pattern, points=None, opentag='<b>', 
                     closetag='</b>', autoescape=None):
    """
    Highlights the pattern in the text.
    """
    def findall(s, x):
        start, end = 0, len(s)
        while start<end:
            start = s.find(x, start, end)
            if start != -1:
                yield start
                start += len(x)
            else:
                start = end

    def esc(x, n=0):
        if n!=0:
            return x
        if autoescape:
            return conditional_escape(x)
        return x

    if string == None:
        return ''
    try:
        str(pattern)
    except:
        print 'Pattern must be a string. Failing silently.'
        return string 
 
    try:
        str(opentag)
        str(closetag)
    except:
        print ('Both `opentag` and `closetag` must be a string. '+
               ' Failing silently.')
        return string 
   
    if not points:
        points = list(findall(string, pattern))

    l = len(pattern)
    extra = len(opentag)+len(closetag)
    o = string
    for n, i in enumerate(points):
        o = (esc(o[:i+7*n], n) + opentag + esc(o[i+7*n:i+l+7*n]) + 
             closetag + esc(o[i+l+7*n:]))

    return mark_safe(o)
highlightpattern.needs_autoescape = True
highlightpattern.mark_safe = True