pyparsing is pretty cool

interpolate.py

import pyparsing

class Word(object):
  def __init__(self, body):
    self.body = body

  def __repr__(self):
    return self.body[0]

class Tag(object):
  def __init__(self, body):
    self.body = body[1]

  def __repr__(self):
    return "{%s}"%self.body

class Newline(object):
  def __init__(self, body):
    self.body = body

  def __repr__(self):
    return "\n"

customword = pyparsing.Regex("[^<^>]+")
customword.setParseAction(Word)
tag="<"+customword+">"
tag.setParseAction(Tag)
newline = "<br" + pyparsing.Optional("/") + ">"
newline.setParseAction(Newline)

shit=pyparsing.OneOrMore(pyparsing.Or([newline, tag, customword]))
print "".join(map(repr, shit.parseString('foo <greet> <br/><br> baz <farewell>'))).format(greet='hi', farewell='bye')