Another python, but recursive: def doit(expr): expr=''.join(expr.split()) if expr.startswith('('): (left,size) = doit(expr[1:]) size+

Another python, but recursive: def doit(expr): expr=''.join(expr.split()) if expr.startswith('('): (left,size) = doit(expr[1:]) size+=2 else: left = re.compile('(\w+)').match(expr).groups(0)[0] size = len(left) rest = expr[size:] if rest=='' or rest[0]==')': return left,size op = re.compile('([*/+-])').match(rest).groups(0)[0] right,size2 = doit(rest[1:]) return '(%s %s %s)'%(op, left, right) ,size+size2+1