lex: Regular expression for rule 't_PLUS' matches empty string.
Traceback (most recent call last):
  File "./lex_re2.py", line 25, in <module>
    lex.lex()
  File "../ply/lex.py", line 759, in lex
    raise SyntaxError,"lex: Unable to build lexer."
SyntaxError: lex: Unable to build lexer.